Introducing The Spring Street Passive House: A Healthy Home Project By Kristof & Diane Irwin
For two decades, our work at Positive Energy has been driven by a single, powerful question: why aren’t buildings created to better support the people inside them? We’ve dedicated our careers to answering that question, moving from hands-on custom home building to the forefront of building science and MEP engineering. Now, we’re bringing that journey full circle by taking on our most personal project yet: our own family home, the Spring Street Passive House.
Kristof and Diane Irwin, Co-Founders of the niche building science and MEP firm, Positive Energy.
For two decades, our work at Positive Energy has been driven by a single, powerful question: why aren’t buildings created to better support the people inside them? We’ve dedicated our careers to answering that question, moving from hands-on custom home building to the forefront of building science and MEP engineering. Now, we’re bringing that journey full circle by taking on our most personal project yet: our own family home, the Spring Street Passive House.
This project is more than just a structure of wood and glass; it's a physical manifesto. It’s our chance to apply everything we’ve learned about creating healthy, comfortable, resilient, and durable buildings to the place we will raise our family and welcome our community.
A Dream Site with a Challenge
Our story begins in the dramatic landscape of the Columbia River Gorge, a place we’ve dreamed of calling home for decades. When a steep, rocky, and seemingly unbuildable lot became available, we saw not obstacles, but potential. The site’s defining feature is its dramatic slope, a constraint that has fundamentally shaped the home’s design. Instead of fighting gravity, we are working with it, designing a multi-level home that nests into the hillside and culminates in a surprise, panoramic view of Wy’east (Mt. Hood).
Walking the Walk with Passive House (Phius)
From the start, we knew this home had to align with our professional values. That's why the decision to pursue Phius (Passive House Institute US) certification was an easy one. For us, Passive House represents the fruition of the building science perspective, a holistic, performance-based approach that guarantees exceptional results.
So, what does this mean in practice? It means we are prioritizing the "fabric" of the home first:
Airtight Construction: Creating a meticulously sealed building envelope to eliminate drafts, save energy, and block out wildfire smoke, a critical resilience feature in the Gorge.
Continuous Insulation: Wrapping the home in a thick thermal blanket, free of weak spots, to ensure stable, comfortable indoor temperatures year-round, no matter the weather outside.
High-Performance Windows: Using triple-glazed windows that prevent heat loss and eliminate the feeling of radiant cold, allowing us to frame the stunning landscape without compromising comfort.
Filtered Fresh Air: Employing an Energy Recovery Ventilator (ERV) to act as the "lungs of the house," continuously supplying fresh, filtered air while exhausting pollutants and stale air.
By investing in a superior envelope, we drastically reduce the energy needed for heating and cooling, paving a clear path for our all-electric home to become net-zero with the future addition of solar panels.
A Place for Community
While the technical details are exciting, our ultimate goal is human-centered. We are designing this house to be a sanctuary of health, quiet, and comfort. Above all, we envision it as a welcoming hub for friends and family, with a kitchen at its heart and a seamless connection to the outdoors.
This project is an opportunity for us to live our values and share the process. It’s a chance to answer the tough questions about cost, materials, and complexity we’ve helped so many of our clients navigate. We invite you to follow along as we build not just a house, but a home that embodies the future of resilient, human-centered design.
The Collaborative Legacy of Lake|Flato Architects and Positive Energy
The landscape of contemporary architecture is increasingly defined by the synergy between visionary design and rigorous building science. At the forefront of this evolution stands the enduring partnership between San Antonio based Lake|Flato Architects, renowned for their distinctive, context-responsive designs, and Positive Energy, an Austin, TX-based residential MEP engineering and building science firm. For over a decade, our collaboration has consistently yielded award-winning projects, particularly within the challenging environmental contexts of the Texas Hill Country and beyond. This blog post explores how our integrated approach to design has not only created beautiful and award winning architecture, but also offers invaluable lessons for the broader architectural community.
The landscape of contemporary architecture is increasingly defined by the synergy between visionary design and rigorous building science. At the forefront of this evolution stands the enduring partnership between San Antonio based Lake|Flato Architects, renowned for their distinctive, context-responsive designs, and Positive Energy, an Austin, TX-based residential MEP engineering and building science firm. For over a decade, our collaboration has consistently yielded award-winning projects, particularly within the challenging environmental contexts of the Texas Hill Country and beyond. This blog post explores how our integrated approach to design has not only created beautiful and award winning architecture, but also offers invaluable lessons for the broader architectural community.
The Power of Partnership: Lake|Flato and Positive Energy's Collaborative Legacy
The collaboration between Lake|Flato and Positive Energy transcends a typical client-consultant relationship; it represents a deep, integrated design collaboration. This partnership is founded on a shared commitment to creating buildings that are not only aesthetically remarkable but also inherently healthy, durable, and environmentally responsive.
We at Positive Energy have endeavored to clearly articulate our mission to leverage "building science and human-centered design to engineer healthy, comfortable, and resilient spaces". This commitment practically means that we work with architecture teams to create healthier indoor environments and electrify those homes, leveraging resilient systems that move our society forward and away from fossil fuel based solutions. This forward-thinking approach aligns with Lake|Flato's architectural ethos, which is rooted in fostering "meaningful connections to the landscape that inspire positive change and environmental conservation". Lake|Flato consistently aims to design “buildings that conserve water and other resources, use less energy, and reduce operational and embodied carbon". This shared philosophy forms the bedrock of our highly successful project history together.
When architectural vision, as exemplified by Lake|Flato, and engineering expertise, as provided by Positive Energy, are driven by a fundamental commitment to human well-being and resilience, it creates a dynamic wherein collaboration can occur on a deep level. In this model, the engineering team does not merely fulfill a design brief; it becomes an active partner in shaping the design itself from the earliest stages. This deep integration allows for proactive problem-solving, the selection of innovative materials and systems, and a holistic approach to building performance. Such comprehensive outcomes are significantly more challenging to achieve when the underlying philosophies of an architectural firm and our engineering team are disparate. For architects, selecting engineering partners whose values and approach to design are in strong alignment with their own is paramount. This can lead to more cohesive, higher-performing, and ultimately more impactful architectural outcomes. A shared vision is just as crucial as technical competence.
Collaborative Excellence in Action: Award-Winning Projects
The following case studies illustrate the practical application of building science principles and the profound benefits of integrated design.
Marfa Ranch: Rammed Earth, Thermal Mass, and Healthy Interiors
Situated in the remote and climatically challenging Chihuahuan Desert, the Marfa Ranch is a low-profile residential compound comprising eight structures organized around a central courtyard. This design consciously "borrows from the area's earliest structures", creating a cool respite from the sun-drenched desert. The defining feature of its architectural response to climate is its construction with two-foot-thick rammed earth walls , specifically chosen to protect its inhabitants from the extremes of the region, heat, cold, and wind. Lightweight breezeways and porches made of recycled oil field pipe connect the structures, allowing inhabitants to connect with the vast landscape.
Photography: Casey Dunn
Positive Energy served as both MEP Engineer and Building Envelope consultant for this project. This dual responsibility for an MEP firm is unusual compared to traditional project structures where an independent waterproofing consultant is also onboarded. It was helpful to the integrated design approach for us as the MEP engineer to have a deep understanding of the unique wall assembly behavior. This building-science-forward approach to MEP engineering led to a high quality experience for the occupants of the home.
The massive rammed earth walls at Marfa Ranch function as a passive heating and cooling system, a practical application of building science principles. In climates with high diurnal swings, like Marfa, TX, the thermal mass effect can be particularly useful. During the hot desert days, the walls slowly absorb and store heat. As external temperatures decline at night, this stored heat is gradually released back into the interior, contributing to a warmer indoor environment. Conversely, during cool nights, the walls release heat, and can be "regenerated" by absorbing cooler night air. This strategic use of thermal mass can significantly reduce the reliance on active heating and cooling systems, with some studies showing 20% to 52% reductions in heating and cooling loads compared to conventional buildings. The heavy thermal mass of the rammed earth walls can act as a natural, passive climate control system. Instead of relying solely on mechanical HVAC equipment to maintain indoor temperatures, the walls themselves temper the internal environment by buffering the large external temperature swings in the desert. This reduces the peak heating and cooling demands, allowing for smaller, more efficient mechanical systems. This is a fundamental principle of passive design in high desert climates that directly impacts energy consumption and resilience. Architects should view high-thermal-mass materials, when appropriate for the climate, as primary design elements that can dramatically reduce a building's energy footprint and enhance occupant comfort. This approach moves beyond simply insulating walls to actively engaging the building envelope in climate regulation, offering a key lesson in practical building science.
Beyond thermal performance, the crucial role of moisture management was addressed. For instance, maintaining a 75mm exposed slab edge above finished grade helps protect against moisture ingress. This detail highlights that even high-performing walls like rammed earth require careful attention to moisture, as even high-R walls can be susceptible to moisture problems. Every wall needs robust moisture management and rammed earth is no exception to the rule.
Marfa Ranch has garnered significant recognition, including the 2022 Texas Society of Architects Design Award, 2022 Dezeen’s Top 10 Houses of 2022, and featured in publications like Dwell and Architectural Digest.
The Prow: Off-Grid Resilience and Integrated Systems
The Prow is Lake|Flato’s first off-the-grid Porch House, nestled against a secluded bluff in the Davis Mountains of far west Texas. Its simple design is protected by a long-gable roof with a porch running the length of the building, offering expansive views. Positive Energy provided crucial Building Envelope and Energy Modeling/Consulting services for this net-zero project.
Photography: Casey Dunn
The Prow achieves net-zero energy consumption through a combination of active and passive systems. It utilizes a photovoltaic array for electricity generation, battery storage for energy independence, and solar thermal collectors for a radiant flooring heating system. A large cistern collects rainwater, which is used for potable purposes and fire protection, showcasing comprehensive resource management. The exterior is clad in rusting steel, chosen for its durability to withstand the harsh West Texas environment and its inherent fire resistance, a critical consideration in remote areas.
Energy modeling can be a powerful tool that allows engineers and architects to see the effects of design changes on a building's energy consumption. For an off-grid project like The Prow, this capability is paramount because the demand for energy cannot exceed the building’s ability to provide it. There is no energy grid to lean on if the home’s energy systems reach their limit. Positive Energy's modeling was used to inform how Lake|Flato would meticulously optimize the orientation, window-to-wall ratio, and insulation levels to reduce energy demand before sizing the renewable energy systems. A highly efficient building envelope is the foundation for achieving net-zero, as it minimizes the energy load that the solar array needs to meet, ensuring the off-grid system is robust and reliable. Energy modeling is not merely a compliance check; it can be used as a dynamic, predictive design tool. It allows architects and engineers to virtually simulate the building's performance under various conditions and with different design choices. This iterative process enables informed decision-making early in the design phase, identifying the most effective and cost-efficient strategies to achieve ambitious energy targets like net-zero. For an off-grid project, this predictive capability is critical for ensuring that the renewable energy systems are appropriately sized and the building can reliably meet its own energy demands. Architects should proactively integrate energy modeling into their design workflow from the conceptual stage. This empowers them to make evidence-based decisions that optimize building performance, reduce operational costs, and confidently pursue advanced sustainability goals, transforming theoretical ambitions into tangible realities.
The Prow received the 2016 AIA San Antonio Design Award.
Verde Creek Ranch: Self-Sustaining Design and Energy Independence
Verde Creek Ranch is a private family retreat nestled within a large creek bend, designed to evoke a "camp experience" with separate structures spaced apart to maintain the feeling of a hidden clearing. Positive Energy served as the MEP Engineer for this project.
Photography: Casey Dunn
The ranch features a 12.8 kW solar array on the carport roof and two Tesla batteries. This system is designed to allow the house to sustain itself through power outages and offset its energy use. This integration of solar and battery storage provides significant energy independence, a crucial feature in rural settings where grid reliability can be a concern. It ensures continuous comfort and functionality even during power disruptions. In an era of increasing climate variability, extreme weather events, and potential grid instability, designing for resilience is no longer a niche concern but a fundamental necessity. Integrating on-site renewable energy generation with battery storage directly addresses this by providing energy independence and ensuring critical systems remain operational during power outages. This moves beyond simply reducing environmental impact to actively safeguarding occupant well-being and property value in the face of external disruptions. Architects should increasingly consider resilience as a core design parameter, integrating passive and active strategies to ensure buildings can perform effectively and safely even under adverse conditions. This proactive approach adds significant long-term value for clients.
Confluence Park: A Living Laboratory of Sustainable Design
Located along the San Antonio River, Confluence Park is a public amenity transformed from a blighted industrial yard. It serves as a living laboratory designed to educate visitors on south Texas ecotypes and the impact of urban development on local watersheds. The design features a central pavilion with unique concrete petal structures and a multi-purpose education center. Positive Energy took a step outside of its conventional residential project typology to provide Energy Modeling and Consulting services for this ambitious public project.
Photography: Casey Dunn
The park showcases an innovative biomimetic rainwater harvesting system: the central pavilion's concrete "petal" structures are "inspired by plants that funnel rainwater to their roots". These petals are formed to collect and funnel rainwater into a central underground catchment basin, predicted to collect around 825,000 gallons annually and capable of holding up to 100,000 gallons. The collected rainwater is filtered through alluvial soils, preventing contaminated runoff from entering the San Antonio River, and is then used for sewage conveyance and irrigation within the park. Instead of imposing purely technological or conventional solutions, the design team at Confluence Park looked to natural systems for elegant and efficient blueprints. This biomimetic approach resulted in a rainwater harvesting system that is not only highly functional but also aesthetically integrated and deeply meaningful to the park's educational mission. Building science and civil engineering expertise is crucial here to translate these natural inspirations into quantifiable performance, ensuring the system's efficiency, capacity, and durability. Architects should explore biomimicry as a powerful source of sustainable design inspiration. By studying how nature solves problems, they can uncover innovative, context-responsive solutions that are both environmentally effective and architecturally compelling. Collaboration with building science experts is key to translating these natural principles into engineered realities.
The Estela Avery Education Center features a green roof and a solar photovoltaic array intended to produce 100% of the park’s energy needs. Confluence Park transformed a blighted industrial site into a vibrant public amenity, welcoming over 32,000 students and registrants since its opening, serving as a powerful example of sustainable urban regeneration.
The park has received significant accolades, including the 2023 AIA Committee on the Environment Top Ten Award and the 2022 Metropolis Planet Positive Award Honoree.
Other Distinctive Projects: A Glimpse into Diverse Collaborations
The breadth of successful collaborations between Lake|Flato and Positive Energy demonstrates the universal applicability and necessity of building science expertise in architectural practice. These projects span diverse geographies (desert, rural Texas, urban Austin, San Antonio), project types (residential, public park), and scales. Positive Energy's scope also varies, from full MEP engineering to specialized building envelope and energy modeling. This diversity demonstrates that building science principles and integrated engineering are not niche disciplines applicable only to extreme climates or highly specialized projects. Instead, they are universally valuable tools for enhancing performance, comfort, durability, and sustainability across virtually any architectural challenge. Positive Energy's ability to adapt its deep expertise to the specific needs of each project—whether it is optimizing complex mechanical systems, fine-tuning a building envelope, or modeling energy flows—underscores the fundamental role of building science in achieving design excellence in varied contexts. Architects should recognize that engaging building science expertise is beneficial for all projects aiming for high performance, occupant well-being, and long-term value. It is not an optional add-on but an integral part of modern, responsible architectural practice, regardless of project type or location.
Madrone Mesa Ranch, for instance, is a multi-building family compound in the Texas Hill Country, designed as a retreat and later a full-time residence. Positive Energy provided MEP Engineering for this project, which is centered around a party barn and courtyard, thoughtfully integrated with large mature oak trees.
Photography: Casey Dunn
The Fall Creek Residence, for which Positive Energy also provided MEP Engineering, comprises a series of humble shed-roofed structures perched on a bluff. It features limestone walls and weathered steel, with a large porch designed to capture the sound of the falls and interiors using a "rich, truly native palette" of local materials. This project received the 2025 Residential Design Architecture Award.
Photography: Casey Dunn
The River Bend Residence, with MEP Engineering by Positive Energy, was designed to "sit lightly upon the land" overlooking the Guadalupe River, composed of multiple structures. Its orientation strategically takes advantage of prevailing winds for natural ventilation, and large skylights capture Northern daylight. The landscape is intentionally minimal and indigenous to reduce maintenance and environmental impact.
Finally, the Hog Pen Creek Residence, where Positive Energy provided Enclosure & Energy Modeling/Consulting, is situated at the confluence of Hog Pen Creek and Lake Austin. This residence emphasizes exterior living space. Its L-shaped footprint and orientation thoughtfully address challenging site constraints like towering oak trees and a steeply sloping site, featuring a boardwalk connecting structures down to a screened pavilion by the water's edge.
Inspiring the Next Generation of Architecture
The decade-plus-long collaboration between Lake|Flato Architects and Positive Energy stands as a powerful model for the architecture and construction industry. Their joint portfolio of distinctive, award-winning projects demonstrates that high-performance, durable, and healthy buildings are not abstract ideals but achievable realities. These buildings are realized through thoughtful, context-responsive design, the practical application of rigorous building science principles, and, most importantly, deep, early, and integrated collaboration between architectural visionaries and building science experts.
This partnership illustrates that by embracing building science and fostering similar integrated design relationships, architects can create buildings that not only stand the test of time but also profoundly respond to their environment, enhance the lives of their occupants, and inspire the next generation of truly sustainable and resilient architecture.
The Theresa Passive House: A Blueprint for High-Performance Design in Hot-Humid Climates
The Theresa Passive House, nestled in Austin's historic Clarksville neighborhood, stands as a remarkable example of how architectural preservation can harmoniously merge with modern sustainable design. This 2100 square foot residence, completed in 2020, is not merely a renovation and addition to a 1914 Craftsman bungalow; it is a meticulously engineered dwelling that embodies rigorous targets in energy efficiency, indoor air quality (IAQ), thermal comfort, embodied carbon, and responsible materials sourcing.[1] These ambitious goals were established by the Passive House Institute U.S. (Phius), a leading authority in high-performance building standards.
by Positive Energy staff. Photography by Casey Dunn
Redefining Residential Performance
A Historic Blend with Cutting-Edge Sustainability
The Theresa Passive House, nestled in Austin's historic Clarksville neighborhood, stands as a remarkable example of how architectural preservation can harmoniously merge with modern sustainable design. This 2100 square foot residence, completed in 2020, is not merely a renovation and addition to a 1914 Craftsman bungalow; it is a meticulously engineered dwelling that embodies rigorous targets in energy efficiency, indoor air quality (IAQ), thermal comfort, embodied carbon, and responsible materials sourcing.[1] These ambitious goals were established by the Passive House Institute U.S. (Phius), a leading authority in high-performance building standards.
The project achieved full Passive House certification and served as a pilot for the groundbreaking PHIUS 2018+ Source Zero standard.[1] This distinction is particularly significant as it marks the Theresa Passive House as one of the first PHIUS-certified, source-zero projects in a challenging hot and humid climate, specifically ASHRAE Climate Zone 2A.[1] The commitment to these principles has yielded exceptional energy performance, with the home consuming approximately 75% less energy than typical new constructions.[1] This impressive efficiency also earned it the highest rating by Austin Energy Green Building to date.[1] Beyond its reduced energy consumption, the Theresa Passive House functions as its own energy hub, integrating photovoltaic panels and battery backup systems. This provides unparalleled self-sufficiency and resilience, ensuring peace of mind even during extreme weather events and power outages.[1]
Forge Craft, Hugh Jefferson Randolph, and the Pursuit of Passive House Excellence
The creation of the Theresa Passive House was a deeply collaborative endeavor, bringing together the expertise of Forge Craft Architecture + Design (led by Trey Farmer, AIA), Hugh Jefferson Randolph Architects, and Studio Ferme (with Adrienne Farmer contributing to interior design).[1] The homeowners themselves, an architect and a designer, envisioned the house as more than just a personal residence. They conceived it as a "forum for learning" and a tangible "proof point" for the feasibility and benefits of Passive House construction in challenging contexts, such as a modest-sized renovation on a small, urban lot within a hot, humid climate.[1]
This deliberate approach to the project, viewing it as a public demonstration, highlights a critical trend in high-performance building: successful outcomes in challenging climates necessitate a truly integrated design process. Architects, engineers, and specialized consultants must work synergistically from the very inception of a project, rather than operating in isolation. The "proof point" aspect of the Theresa Passive House suggests a broader objective of normalizing Passive House principles in the Southern United States, actively addressing and overcoming perceived barriers like cost and climate suitability through demonstrated success. The design team's commitment to health and sustainability was evident in their financial prioritization; rather than maximizing square footage, they strategically invested in a robust building envelope, a high-performance HVAC system, and on-site solar panels.[2]
Positive Energy's Role as MEP Engineer
Positive Energy, an MEP (Mechanical, Electrical, and Plumbing) engineering firm renowned for its specialization in high-end residential architecture, was a proud partner on this project.[1] Positive Energy's fundamental mission—to transform the way homes are delivered to society by leveraging building science and human-centered design—aligns deeply with core tenets of the Passive House standard.[6] Our expertise is dedicated to engineering spaces that are not only healthy and comfortable but also inherently resilient.
For the Theresa Passive House, Positive Energy's scope of involvement was comprehensive MEP engineering.[1] This deep engagement was instrumental in ensuring the precise integration and optimal performance of the advanced mechanical systems. In a hot and humid climate like Austin, where managing moisture and achieving efficient cooling are paramount, the specialized knowledge and meticulous execution provided by an experienced MEP firm are indispensable for reaching Passive House performance benchmarks. Their involvement from design through construction ensured that the ambitious performance targets were not just theoretical but were realized in the built environment.
Passive House Goes Beyond Energy Savings
The Core Principles of Passive House
Passive House represents a building design standard rooted in extreme energy efficiency and sustainable living, engineered to slash energy consumption by up to 90% compared to conventional structures.[8] It offers a direct pathway to achieving net-zero energy buildings that are also significantly more comfortable, durable, healthy, and predictable in their performance.[10] Originating in Germany in the 1990s, the Passive House concept has undergone substantial evolution, particularly with the Passive House Institute U.S. (Phius) developing climate-specific standards, such as PHIUS+ 2015 and 2018.[3] This adaptation was crucial to make the standard practically feasible across the diverse climates of North America, including the challenging hot and humid regions like Austin.
The PHIUS standard operates on a performance-based framework, underpinned by three primary pillars: stringent limits on annual and peak heating and cooling loads, a cap on overall source energy use, and demanding airtightness requirements.[11] Compliance with these criteria is rigorously verified through energy modeling, ensuring that design intent translates into real-world performance.[12]
Continuous Insulation: Eliminating Thermal Bridges
The principle of continuous insulation dictates that a building must be completely wrapped with insulation to minimize heat flow through its entire envelope.[10] This strategy directly addresses thermal bridging, which occurs where structural elements, such as framing members, possess lower R-values than the surrounding insulation. These interruptions create pathways that allow heat to escape in cold conditions or penetrate in warm conditions, undermining the overall thermal performance of the enclosure. The application of continuous, thick insulation on the exterior of a building is fundamental to maintaining stable indoor temperatures and significantly reducing energy demand.[10]Airtight Construction: The Foundation of Performance
Passive Houses are meticulously designed for extreme airtightness, typically targeting 0.6 air changes per hour at 50 Pascals (ACH@50 Pa) or less.[10] This stringent requirement aims to prevent uncontrolled air leakage, which is a significant vector for both heat and moisture transfer. Air leaks can account for up to 40% of total heat loss even in otherwise well-insulated structures.[15] More critically, in hot-humid climates, warm, moist outdoor air leaking into cooler interior wall cavities can condense, leading to moisture accumulation, potential mold growth, and long-term durability issues within the building fabric itself.[10] Airtightness is empirically verified through a Blower Door Test, a diagnostic tool that measures the rate of air changes per hour under a controlled pressure difference.[14]High-Performance Windows: Balancing Solar Gain and Heat Loss
Windows are inherently complex components of the building envelope, tasked with managing air, water, and heat flow while also providing views and daylight.[10] Passive Houses typically employ triple-glazing and specialized low-emissivity (low-e) coatings to effectively block radiant heat transfer.[10] In a hot climate, the Solar Heat Gain Coefficient (SHGC) of windows is particularly crucial. Windows with a high SHGC are desirable on facades where passive solar heating is beneficial in winter (e.g., east and south orientations), while those with a low SHGC are essential on facades exposed to intense summer sun (e.g., west-facing windows) to prevent unwanted solar heat gain and subsequent overheating.[10]Balanced Ventilation with Heat/Energy Recovery
Given the exceptional airtightness of Passive Houses, controlled mechanical ventilation becomes indispensable to ensure a continuous supply of fresh air and to effectively manage indoor air quality.[10] Energy Recovery Ventilators (ERVs) are commonly employed for this purpose. These systems continuously pull in fresh outdoor air and exhaust stale indoor air, simultaneously transferring heat and moisture between the two airstreams.[10] This process minimizes energy loss while managing latent loads, ensuring a constant flow of fresh, filtered air without compromising the building's thermal comfort or energy efficiency.Dedicated Dehumidification
Relying on the heating/cooling system alone is insufficient to create the necessary drying potential in a building, especially when an air tight envelope and ERV create both interior and exterior latent loads that need to be handled by mechanical means. Dedicated dehumidifiers are critical to decouple the drying function from the heating and cooling systems.Right-Sizing Mechanical Systems for Efficiency
One of the significant advantages of a highly insulated and airtight Passive House envelope is the drastic reduction in heating and cooling loads, which eliminates the need for oversized HVAC systems.[10] This allows for the specification of smaller, less expensive, and inherently more efficient mechanical systems. The upfront investment in a robust building envelope can be partially offset by the savings realized from reduced mechanical equipment costs.[10] The focus shifts to precisely right-sizing and selecting systems that can efficiently handle the minimal and precise loads of the building.
Why Passive House Matters
The benefits of Passive House design extend far beyond mere energy savings, encompassing a holistic improvement in the living environment.
Comfort: Passive Houses are engineered to maintain a remarkably stable indoor temperature, eliminating drafts and cold spots that often plague conventional buildings and ensuring superior thermal comfort for occupants.[2]
Health: The meticulous control over indoor air quality, achieved through continuous mechanical ventilation and advanced filtration, significantly reduces the presence of indoor pollutants and allergens. This proactive management minimizes the risk of respiratory problems and contributes to a healthier living environment.[2]
Durability: The emphasis on high-quality building materials and exacting construction practices, particularly concerning moisture control within the building envelope, contributes to structures that are inherently more durable and capable of withstanding extreme weather conditions over their lifespan.[8]
Resilience: Perhaps one of the most compelling advantages in an era of increasing climate volatility is the inherent resilience of Passive House design. The robust building envelope and energy-efficient systems provide "passive survivability," allowing homes to maintain habitable temperatures for extended periods even during power outages or severe weather events.[1] The Theresa Passive House notably demonstrated this capability during both the extreme cold of Winter Storm Uri and intense summer heat events, as validated by research from the University of Texas.[3]
The evolution of the Passive House standard from its European origins, which primarily focused on heating loads, to the climate-specific PHIUS+ 2015 and 2018 standards for North America, represents a strategic adaptation crucial for broader market penetration. This adaptation acknowledges the unique challenges presented by diverse climates, particularly the significant cooling and dehumidification demands of hot and humid regions like Austin.[3] Without this climate-specific optimization, the standard's applicability in many parts of the United States would be severely limited. The Theresa Passive House's designation as a pilot project for PHIUS 2018+ Source Zero in a hot, humid climate underscores the importance of this ongoing evolution, positioning PHIUS as a leader in making passive building principles effective and accessible across varied environmental contexts.[1]
The relationship among the five Passive House principles is a cornerstone of their effectiveness. For instance, the extreme airtightness achieved in a Passive House fundamentally changes how the building interacts with its environment. This virtual elimination of uncontrolled air infiltration, a major pathway for heat, moisture, and pollutants, then mandates the integration of sophisticated mechanical ventilation systems to introduce fresh air and manage humidity.[10] Conversely, the superior performance of the envelope—through continuous insulation, high-performance windows, and airtight construction—allows for significantly downsized and optimized MEP systems, leading to both cost savings and increased efficiency. This highlights that envelope and mechanical systems are not independent elements but rather an interdependent entity, requiring an integrated design approach for optimal performance.
Key Performance Metrics of Theresa Passive House (vs. Typical Code-Built)
The following table provides a quantitative overview of the Theresa Passive House's performance, contrasting it with typical code-built homes to illustrate the tangible advantages of Passive House design. These metrics demonstrate the practical application of building science principles and the level of performance achievable in real-world projects.
Passive House Principles and Their Practical Application
The following table illustrates how the core principles of Passive House are translated into tangible design and construction elements, using the Theresa Passive House as a concrete example. This breakdown aims to demystify complex concepts by showing their real-world implementation and benefits.
Walls and Roofs in a Hot-Humid Climate
Understanding Wall Assemblies: The Four Control Layers in Practice
Designing a durable and high-performing building enclosure, especially in challenging climates, requires a nuanced understanding of how its various components interact with environmental loads such as rain, temperature, and humidity. Building science principles emphasize the importance of four principal control layers within a wall assembly, each addressing a critical function for long-term durability and performance.[17] These layers, listed in their order of importance for preventing building failure, are:
Water Control Layer: This is the primary defense against liquid water—whether from rain, surface water, or groundwater—from entering the building.[18] Its continuous and robust application is paramount, as a failure in this layer can lead to rapid and catastrophic system failure, including mold, decay, and corrosion.
Air Control Layer: This layer prevents uncontrolled air movement through the building envelope.[22] Air leakage is not merely an energy drain; it carries significant heat and, critically, moisture. In hot-humid climates, warm, humid outdoor air infiltrating cooler interior wall cavities can condense, leading to moisture accumulation, reduced effective R-value of insulation, and potential mold or decay.[10] A continuous, strong, and durable air barrier is essential to mitigate these risks.[18]
Thermal Control Layer: This is the insulation, designed to minimize heat transfer through conduction.[22] While often the most visible component of a high-performance wall, its effectiveness is severely compromised if the air and moisture control layers are not adequately addressed and integrated.[10]
Vapor Control Layer: This layer manages the movement of moisture vapor through building materials via diffusion.[22] Its precise placement and permeability are highly dependent on the specific climate zone and interior conditions. In hot-humid climates, the strategy often involves allowing for "inward drying" or utilizing semi-vapor permeable materials on the exterior to prevent moisture from becoming trapped and accumulating within the assembly.[22]
Theresa Passive House Wall and Roof Design: Strategies for Austin's Climate
Austin, Texas, is classified as ASHRAE Climate Zone 2A – Hot-Humid.[4] This climate presents distinct challenges for building enclosures, primarily characterized by high humidity levels and substantial cooling loads, alongside the potential for inward moisture drive caused by solar heating of exterior surfaces.[10] The Theresa Passive House's envelope design directly addresses these challenges through thoughtful material selection and assembly configuration.
Specific R-Values and Insulation Types: The Theresa Passive House is constructed with a wood frame system.[4] Its walls are designed as framing with continuous insulation, achieving an R-value of 26 and utilizing mineral wool with cavity fill as the insulation material.[4] This approach of combining cavity insulation with continuous exterior insulation is crucial for minimizing thermal bridging and achieving robust thermal performance. The roof is an unvented assembly with an R-value of 33.[4] Unvented roofs are frequently favored in hot-humid climates because they offer superior control over interior moisture and effectively prevent solar-driven moisture from entering the roof deck.[24] The floor sits above a crawlspace and is insulated to an R-value of 14.[4] For fenestration, Marvin windows were selected, featuring a Whole Window U-Value of 0.17 and a Solar Heat Gain Coefficient (SHGC) of 0.26.[4] This low SHGC is particularly vital for mitigating unwanted solar heat gain in a climate dominated by cooling needs.[10]
The Blower Door Test and Its Significance
A hallmark of the Theresa Passive House's performance is its extraordinary airtightness, measured at 0.036 ACH@50 Pa.[4] This figure is remarkably lower, indicating a far more airtight enclosure, than the PHIUS certification requirement of 0.6 ACH@50 Pa.[12] The Blower Door Test, a crucial diagnostic tool, quantifies the airflow between the interior and exterior of a structure, pinpointing areas of air leakage.[15] The test creates a controlled pressure difference, typically 50 Pascals, to simulate wind conditions, and then measures the resulting air changes per hour.[15] This extreme level of airtightness is a fundamental cornerstone of Passive House design, as it prevents significant energy loss and uncontrolled moisture movement. However, it simultaneously necessitates the integration of controlled mechanical ventilation to ensure a continuous supply of fresh air.[10] The extremely low ACH@50 achieved by the Theresa Passive House powerfully demonstrates that airtightness is not merely an energy-saving measure but a foundational prerequisite for creating a truly controlled indoor environment. For architects, this means recognizing that embracing airtightness as a design priority shifts the responsibility for air exchange from random leaks to precisely engineered mechanical systems, enabling superior indoor air quality and humidity control.Moisture Management in Unvented Roofs with Asphalt Shingles
In hot-humid climates, unvented roof assemblies, particularly those utilizing asphalt shingles, demand a specific and critical moisture management strategy: the installation of a vapor barrier between the asphalt shingles and the roof deck.[24] This is due to the nature of asphalt shingles, which, similar to traditional wood shingles, can act as a reservoir for water from dew and rain.[24] When these shingles are heated by solar radiation, the stored moisture can be driven inward through permeable roofing felts into the underlying roof deck (typically plywood or OSB), potentially leading to moisture accumulation and material degradation such as buckling.[24] The solution involves using an impermeable roofing underlayment, which functions as a vapor barrier. This layer effectively prevents this inward moisture drive, thereby controlling moisture transmission through the roof assembly and eliminating shingle buckling and moisture issues within the roof deck.[24] This detail is paramount for ensuring the long-term durability of the roof in hot, humid environments and maintaining the integrity of the roof deck.[25]
Practical Takeaways for Durable Wall Assemblies
For architects, a deep understanding of the climate-specific behavior of wall assemblies is paramount. In hot-humid climates, the primary focus shifts from preventing outward moisture drive (as is common in cold climates) to meticulously managing inward moisture drive and preventing condensation within the assembly, which occurs when humid outdoor air encounters cooler interior surfaces.[10] The Theresa Passive House serves as a compelling demonstration that robust thermal control, exemplified by its R-26 walls and R-33 roof [4], combined with exceptional air control (0.036 ACH@50 Pa [4]) and precise vapor control (such as the specific vapor barrier in its unvented roof [24]), is not only achievable but essential for ensuring both durability and high performance in such challenging climates.
The selection of materials like mineral wool for the walls and the specific unvented roof assembly reflects a sophisticated understanding of hygrothermal performance in Austin's climate. The design prioritizes assemblies that can effectively "dry" in the appropriate direction, preventing moisture accumulation within the building fabric.[4] This approach aligns with the "perfect wall" concept, which, in hot-humid climates, often implies placing the primary thermal and vapor control layers on the exterior side of the structure. This strategy helps keep the sheathing warm and minimizes the risk of condensation, or it effectively manages inward vapor drive. This illustrates that achieving high performance while maintaining durability in a challenging climate requires that "more insulation" be accompanied by "smarter assembly design."
Theresa Passive House Envelope Specifications
The following table provides a detailed overview of the Theresa Passive House's key envelope specifications, offering concrete examples of the components and performance metrics that contribute to its high-performance status in a hot-humid climate.
Positive Energy's MEP Solutions
The Imperative of Indoor Air Quality in Airtight Homes
In highly airtight Passive Houses, the focus on indoor air quality (IAQ) becomes paramount. Because natural infiltration, or uncontrolled air leakage, is virtually eliminated, pollutants can accumulate within the living space if not properly managed through mechanical means.[21]
Common indoor pollutants and their sources are diverse and pervasive in residential settings. These include combustion products from unvented stoves, furnaces, or tobacco; off-gassing from building materials like insulation, wet carpet, or pressed wood products; chemicals from furnishings and household cleaning products; and emissions from human activities such as cooking and cleaning.[21] These sources can introduce a range of contaminants, including carbon dioxide (CO2), Volatile Organic Compounds (VOCs), and fine particulate matter (PM2.5).[21]
To define and ensure "acceptable indoor air quality," the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) developed Standard 62.2, "Ventilation and Acceptable Indoor Air Quality in Residential Buildings".[27] This standard serves as the recognized benchmark for residential ventilation design, specifying minimum ventilation rates and other measures to minimize adverse health effects for occupants.27 ASHRAE 62.2 defines "Whole Building" Mechanical Ventilation using the formula: Q fan = 0.03A floor + 7.5 (BR + 1).[26] In this equation, A floor represents the conditioned floor area, serving as a proxy for material sources that might off-gas pollutants, while BR (Bedrooms) acts as a surrogate for the number of occupants and their activities. The standard also provides "Source Control" Exhaust Ventilation requirements for specific areas. For instance, kitchens require 100 cfm (cubic feet per minute) of on-demand ventilation or 5 ACH (air changes per hour) continuously, while full bathrooms require 50 cfm on-demand or 20 cfm continuously.[26] The development of ASHRAE 62.2 was instrumental in overcoming initial builder resistance to constructing airtight homes by providing a clear and accepted method for ensuring proper IAQ.[27]
Theresa Passive House's Integrated MEP System
Positive Energy's MEP engineering for the Theresa Passive House exemplifies a highly sophisticated and integrated approach to environmental control. This level of integration is particularly critical for a building that is not only located in a hot and humid climate but also boasts an exceptionally airtight envelope.[1] The comprehensive system is aptly described as the "workhorse" that enables much of the Theresa Passive House's performance.3
Variable Refrigerant Flow (VRF) Heat Pump AC: Efficient Heating and Cooling
The Theresa Passive House employs a Mitsubishi Variable Refrigerant Flow (VRF) heat pump AC unit for its primary heating and cooling needs.[3] VRF systems are highly advantageous in high-performance homes because their variable capacity allows them to precisely match the significantly reduced heating and cooling loads. Unlike oversized conventional units that cycle frequently and inefficiently, VRF systems can operate for longer durations at lower capacities, which is crucial for effective latent heat (moisture) removal.[19] This precise control enhances both energy efficiency and occupant comfort.Energy Recovery Ventilation (ERV): Delivering Fresh Air and Managing Latent Loads
A Panasonic Intellibalance 1000 ERV system is integral to delivering continuous fresh air throughout the Theresa Passive House.[3] The fundamental function of an ERV is to exchange both sensible heat and latent heat (moisture) between the incoming fresh outdoor air and the outgoing stale indoor air.[10] In a hot, humid climate, this is particularly vital: the ERV transfers moisture from the wetter incoming outdoor air to the drier exhaust air, thereby significantly reducing the latent load that the cooling system would otherwise have to handle.[19] This mechanism is crucial for maintaining excellent indoor air quality in an airtight home by continuously flushing out pollutants while simultaneously minimizing the energy penalty associated with conditioning untreated outdoor air.[10]Dedicated Dehumidification: The Key to Comfort in Humidity
Complementing the VRF and ERV systems, the Theresa Passive House incorporates a dedicated dehumidifier.[3] Even with an efficient VRF system and an ERV managing the latent load from ventilation air, a dedicated dehumidifier is often indispensable in hot, humid climates like Austin. This component allows for precise control of indoor humidity levels without the need to overcool the space to achieve dehumidification.[19] While ERVs are effective at reducing the moisture burden from incoming ventilation air, they do not fully dehumidify the entire indoor air volume.[19] The dedicated dehumidifier ensures optimal thermal comfort by maintaining desired humidity levels (typically 50-55% Relative Humidity), which is critical for occupant well-being and preventing potential mold growth within the building.[20] This focus on latent load management is a critical consideration in hot-humid climates, as a standard AC system alone is often insufficient for optimal comfort and durability in a high-performance, airtight home. A dedicated strategy for latent load management, typically involving an ERV for ventilation air and a separate dehumidifier for internal moisture, is not merely a luxury but a fundamental requirement for preventing mold, ensuring comfort, and protecting the building fabric.Hospital-Grade Air Filtration: Ensuring Clean Air (MERV Ratings Explained)
The Theresa Passive House integrates a MERV16 filtration system [3], a commitment to indoor air quality beyond typical residential standards. Air filter effectiveness is quantified by its MERV (Minimum Efficiency Reporting Value) rating, which measures a filter's ability to trap particles ranging from 0.3 to 10 microns in size.32 Higher MERV ratings indicate superior filtration capabilities.[32]
MERV 1-4: Offer minimal filtration, capturing larger particles like dust and pollen.[32]
MERV 5-8: Common in residential and commercial settings, capable of capturing mold spores, dust mites, and household lint.[32]
MERV 9-12: Provide improved IAQ, trapping finer dust, pet dander, some bacteria, and mold spores. Filters in this range are often used in hospitals, although not in surgical settings.[32]
MERV 13-16: Recommended for environments demanding high air quality, capable of capturing particles as small as 0.3 microns, including bacteria, viruses, smoke, and smog. These are frequently used in commercial buildings, hospitals, and clean rooms.[32]
MERV 17-20 (HEPA): Represent the highest level of filtration, typically used in specialized settings like surgical rooms and cleanrooms, capable of removing 99.97% of 0.3-micron particles, including viruses and combustion smoke. These are generally not suitable for standard residential HVAC systems due to significant airflow restriction, [32] but do provide superior protection against a wide spectrum of airborne contaminants, including allergens, pollutants, and even some viruses and bacteria.[32] This level of filtration offers substantial benefits, particularly in regions with high allergen counts or during public health concerns.[3] This commitment to high-level filtration signifies a growing trend where high-performance homes are not merely about energy efficiency but also about creating inherently healthier indoor environments. In airtight homes, filtration becomes the primary defense mechanism against both outdoor and indoor airborne contaminants.
Heat Pump Hot Water Heater: Energy-Efficient Domestic Hot Water
The MEP system further includes a heat pump hot water heater.[3] Heat pump water heaters are considerably more energy-efficient than traditional electric resistance models, contributing significantly to the overall low energy consumption profile of the Passive House.[14]
How Positive Energy Ensures Optimal Performance
Positive Energy's approach to the Theresa Passive House demonstrates how individual MEP components are meticulously integrated to function as a cohesive, high-performing system. The extreme airtightness of the Passive House envelope, measured at an impressive 0.036 ACH@50 Pa [4], allows the mechanical systems to operate with unparalleled precision, as uncontrolled air leakage, which would otherwise introduce unpredictable loads, is virtually eliminated.[10]
The combination of a VRF system, an ERV, and a dedicated dehumidifier represents a highly targeted strategy for hot-humid climates. This trifecta effectively addresses both sensible (temperature) and latent (humidity) loads.[19] The ERV efficiently handles the latent load introduced by incoming fresh air, while the dedicated dehumidifier precisely manages internal latent loads, preventing the AC system from overcooling the space in an attempt to remove excess moisture.[19]
A critical aspect of Positive Energy's involvement was collaboration with the means/methods team during construction to ensure design intent was met.[3] This process is essential to verify that all complex systems are installed correctly, calibrated precisely, and operate as designed to achieve the rigorous Passive House performance targets.[21] Construction phase collaboration ensures that the theoretical design performance translates into real-world operational excellence, maximizing the comfort, health, and efficiency benefits for the occupants.
Indoor Air Quality Parameters and ASHRAE 62.2 Requirements
For architects seeking to understand the intricacies of indoor air quality, the following table outlines key parameters, their significance, health implications, and how ASHRAE 62.2 provides a framework for achieving acceptable indoor air quality.
Theresa Passive House MEP System Components and Functions
This table details the specific MEP system components engineered by Positive Energy for the Theresa Passive House, highlighting their functions and benefits within the context of a high-performance home in a hot-humid climate.
Lessons from the Theresa Passive House
Passive Survivability: Performance During Extreme Weather Events
The Theresa Passive House stands as a powerful demonstration of climate resilience, a core benefit of Passive House design that extends beyond daily energy savings.[1] Its performance during extreme weather events provides compelling evidence of its robust design.
During the unprecedented Winter Storm Uri, which brought single-digit temperatures to Austin and caused widespread power outages and burst pipes in many conventional homes, the Theresa Passive House maintained an indoor temperature of approximately 47 degrees Fahrenheit after three days without power.[3] This remarkable passive survivability demonstrates a significant "cushion of time" for occupants, ensuring safety and comfort even when the grid fails.[3]
Similarly, researchers at the University of Texas (UT Austin) conducted studies on the home's ability to tolerate extreme heat, comparing its performance to a code-built house. After 12 hours on a sweltering summer day, the code-built house reached a stifling 98 degrees Fahrenheit, while the Passive House registered a much more comfortable 83 degrees.[1] This highlights the effectiveness of its robust envelope and design strategies in mitigating heat gain, even without active cooling. This performance during both extreme cold and heat showcases that high-performance homes are not just energy-efficient but also robust climate adaptation tools, shifting the value proposition from purely operational cost savings to essential safety and quality of life benefits in an era of increasing climate volatility. Further enhancing its resilience, the home operates as its own energy hub, generating electricity through photovoltaic panels and utilizing battery backup to provide full backup power and self-sufficiency during grid outages.[1]
Source Zero Certification: Producing More Energy Than Consumed
A crowning achievement for the Theresa Passive House is its PHIUS 2018+ Source Zero certification.[1] This designation signifies that the building produces more energy than it consumes on an annual basis, specifically accounting for "source energy".[1] Source energy is a more comprehensive metric than site energy, as it includes all energy consumed from generation at the power plant through transmission and delivery to the building, providing a more accurate measure of environmental impact.[11]
As the only PHIUS-certified, source-zero project in the Southern United States, the Theresa Passive House sets a new benchmark for energy efficiency and serves as a pioneering model for climate action in residential construction.[1] This achievement underscores that true sustainability in building extends beyond merely reducing energy consumption. It involves actively contributing to the energy grid's decarbonization by producing clean, renewable energy. For architects, aiming for Source Zero means integrating on-site renewables, such as photovoltaic panels and battery storage, as an intrinsic part of the design, working in tandem with the super-efficient envelope and MEP systems. This elevates the goal from simply "doing less harm" to "actively doing good" for the environment and the grid, establishing a higher standard for future projects.
The Theresa Passive House as a Case Study for Future Builds and Community Education
The homeowners of the Theresa Passive House actively embraced its role as a "proof point" and a learning opportunity. They engaged extensively with the community, hosting events for product companies and welcoming students from the University of Texas at Austin to visit, openly sharing data and designs as a living case study.[1] This commitment to knowledge dissemination has been instrumental in demystifying Passive House principles and showcasing their practical application.
The impact extends beyond this single project. Trey Farmer of Forge Craft is actively applying Passive House principles to affordable multifamily housing projects, demonstrating the scalability and broader applicability of these crucial benefits to a wider range of communities.[3] The project's excellence and influence have been widely recognized, garnering numerous accolades, including the prestigious 2024 AIA Housing Award, PHIUS' Passive Project of the Year – Retrofit, and Austin Green Awards.[1] These awards underscore its significant impact and recognition within the architectural and building science industries, further cementing its status as an inspiring blueprint for future high-performance construction.
Empowering Architects for High-Performance Futures
The Theresa Passive House stands as a compelling testament to the transformative potential of high-performance building design, particularly in challenging hot and humid climates. Its success demonstrates that achieving superior energy efficiency, indoor air quality, thermal comfort, and resilience is not merely a collection of disparate technologies but an integrated science.
For architects seeking to design durable, healthy, and efficient homes, several key principles emerge from this project:
Prioritize the Building Envelope: A robust, continuous, and airtight building envelope—encompassing walls, roofs, and high-performance windows—is the fundamental prerequisite for energy efficiency, effective moisture control, and consistent thermal comfort. This demands a meticulous understanding and implementation of all four control layers: water, air, vapor, and thermal, with careful consideration of their climate-specific interactions.
Embrace Controlled Mechanical Ventilation: In highly airtight structures like Passive Houses, mechanical ventilation with energy recovery (ERV) is not optional; it is essential for maintaining superior indoor air quality and effectively managing latent loads. This controlled approach ensures a continuous supply of fresh, filtered air while preserving energy efficiency.
Right-Size and Integrate MEP Systems: The inherent efficiency of the high-performance envelope allows for significantly smaller, more efficient mechanical systems, such as Variable Refrigerant Flow (VRF) heat pumps. Furthermore, in hot and humid climates, dedicated dehumidification is crucial for achieving optimal comfort and preventing moisture-related durability issues, as it addresses latent loads precisely without overcooling.
Invest in Advanced Air Filtration: Implementing high-MERV filtration is vital for ensuring a healthy indoor environment. This protects occupants from a wide range of airborne pollutants, allergens, and even some pathogens, a benefit that has gained increasing importance in public health considerations.
Design for Resilience: Beyond the immediate benefits of energy savings, architects must consider passive survivability and active energy independence (through integrated photovoltaics and battery storage). These features are critical for ensuring occupant safety and comfort during increasingly frequent extreme weather events and power outages, making homes truly future-proof.
The profound success of the Theresa Passive House is a powerful endorsement of the value of an integrated design process. This project clearly illustrates that when architects, building science consultants, and MEP engineers collaborate from the earliest stages of conception, the full potential of high-performance design can be unlocked. Positive Energy's pivotal role as MEP Engineer and Commissioning Agent was indispensable in translating the ambitious performance targets into a functional, resilient, and healthy home. Their specialized expertise in climate-specific MEP solutions, particularly tailored for hot and humid environments, underscores the critical contribution of specialized engineering in achieving Passive House certification and pushing beyond it to Source Zero. For architects, partnering with experienced MEP engineers and building science consultants is not just about achieving compliance; it is about empowering the creation of homes that are healthier, more comfortable, more durable, and genuinely climate-resilient for their occupants, setting an inspiring blueprint for the future of residential architecture.
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Marfa Ranch
The Marfa Ranch is a distinguished residential project by Lake Flato Architects, is thoughtfully situated on a low rise within the expansive, pristine desert grasslands of Marfa, Texas. This unique location, nestled between the Chihuahuan Desert and the majestic Davis Mountains, presents a challenging yet profoundly beautiful environment. The architectural design of the ranch consciously adopts a low profile, comprising eight distinct structures meticulously organized around a central courtyard. This layout, shaded by native mesquite trees, serves as a cool respite from the sun-drenched desert beyond its walls, drawing inspiration from the area's earliest regional architectural traditions. Architect Bob Harris of Lake Flato articulated that the design embodies a "deliberate quality of spareness that matches the qualities of the land," emphasizing the importance of the house maintaining a low profile to merge seamlessly with the terrain while simultaneously opening to distant views and providing crucial protection from the region's harsh winds and intense sun. This project has garnered significant recognition, including the 2022 Texas Society of Architects Design Award and its inclusion in Dezeen's Top 10 Houses of 2022.
By Positive Energy staff. Photography by Casey Dunn
Architecture Meets Applied Building Science in the Chihuahuan Desert
The Marfa Ranch is a distinguished residential project by Lake Flato Architects, is thoughtfully situated on a low rise within the expansive, pristine desert grasslands of Marfa, Texas. This unique location, nestled between the Chihuahuan Desert and the majestic Davis Mountains, presents a challenging yet profoundly beautiful environment.[1] The architectural design of the ranch consciously adopts a low profile, comprising eight distinct structures meticulously organized around a central courtyard. This layout, shaded by native mesquite trees, serves as a cool respite from the sun-drenched desert beyond its walls, drawing inspiration from the area's earliest regional architectural traditions.[1] Architect Bob Harris of Lake Flato articulated that the design embodies a "deliberate quality of spareness that matches the qualities of the land," emphasizing the importance of the house maintaining a low profile to merge seamlessly with the terrain while simultaneously opening to distant views and providing crucial protection from the region's harsh winds and intense sun.[2] This project has garnered significant recognition, including the 2022 Texas Society of Architects Design Award and its inclusion in Dezeen's Top 10 Houses of 2022.[1]
The design approach at Marfa Ranch exemplifies a profound synergy between traditional and modern climate-responsive architecture. The repeated emphasis on the design "borrowing from the area's earliest structures" [1] and utilizing a courtyard plan with thick rammed earth walls to combat the "extremes of the region — heat, cold, and wind" [1] is not merely a stylistic choice. It represents a deliberate reinterpretation of vernacular architecture, where ancient wisdom regarding thermal mass and passive cooling through courtyards is integrated with contemporary building science and engineering. The project, therefore, is not simply a modern house in the desert; it is a modern house of the desert, demonstrating how historical climate-adapted strategies remain highly relevant and effective when enhanced by modern technical expertise. This integrated perspective suggests that successful high-performance design often finds its roots in time-tested, climate-specific principles.
Positive Energy played a pivotal role as both Mechanical Engineers and Building Envelope consultants for the Marfa Ranch project, collaborating closely with Lake Flato Architects.[1] This dual responsibility is a significant departure from traditional project structures, where these critical roles are often separated. As an MEP engineering firm specializing in high-end residential architecture, Positive Energy is committed to leveraging building science and human-centered design to engineer healthy, comfortable, and resilient spaces.[10] Our overarching vision is to create buildings that are healthy, comfortable, durable, efficient, resilient, sustainable, and regenerative, all while maintaining architectural excellence.[12] The building envelope (comprising walls, roof, and windows) and the MEP systems (including heating, cooling, and ventilation) are intrinsically linked in determining a building's overall energy performance, occupant comfort, and indoor air quality. Positive Energy's comprehensive involvement across both mechanical systems and the building enclosure was part of an integrated design approach where these interconnected elements are considered holistically from the project's inception. This collaborative model leads to optimized performance outcomes that would be challenging to achieve if these critical aspects were addressed in isolation or sequentially, representing a hallmark of advanced building science practices.
The Rammed Earth Building Envelope
Harnessing Thermal Mass in Arid Climates
The concept of thermal mass refers to a material's inherent ability to absorb, store, and subsequently release heat.[13] Materials characterized by high density and a high specific heat capacity are ideally suited for this purpose, with rammed earth being a prime example.[13] The Marfa Ranch prominently features two-foot-thick (approximately 600mm) rammed earth walls, constructed using an impressive three million pounds of earth, some of which was sourced directly from the local site.1 These substantial walls are fundamental to the home's passive heating and cooling strategy.[1]
In arid climates such as Marfa, which are defined by significant diurnal temperature ranges—hot days followed by cool nights—thermal mass proves exceptionally effective.[14] During the intense heat of the day, the thick rammed earth walls absorb thermal energy from direct sunlight and the ambient air, effectively preventing this heat from immediately penetrating the interior spaces. As external temperatures decline during the night, the stored heat is gradually released back into the interior, contributing to a warmer indoor environment.[13] Conversely, during cool nights, the walls release their stored heat, and if the building is strategically ventilated, they can be "regenerated" by absorbing the cooler night air. This process prepares the walls to absorb heat again during the subsequent day, thereby maintaining a comfortable indoor climate.[13]
The effectiveness of rammed earth's thermal mass is directly tied to the diurnal temperature range of the Marfa climate. While insulation (R-value) is commonly understood for its thermal resistance, research consistently highlights that rammed earth's primary thermal benefit in arid climates is its thermal mass and the resulting thermal lag.[13] Studies indicate that rammed earth is "especially beneficial in high diurnal temperature ranges," capable of both moderating indoor temperatures and shifting peak temperatures, with reported time lags ranging from 6 to 9 hours, or even up to 10 hours.[16] This means the wall actively buffers temperature swings rather than simply resisting heat flow. For architects, this distinction is crucial: in climates with significant day-night temperature differences, designing for thermal lag—effectively matching the building's thermal response time to the climate's daily cycle—can provide a powerful impact on occupant comfort and energy efficiency than solely maximizing R-value, particularly given that uninsulated rammed earth typically has a lower thermal resistance.[16] This approach, however, requires a deep understanding of climate-specific building science principles.
The strategic use of rammed earth at Marfa Ranch significantly reduces the reliance on active heating and cooling systems, but does not eliminate the need entirely.[13] Studies on rammed earth buildings demonstrate substantial reductions in heating and cooling loads, ranging from 20% to 52% compared to conventional building assemblies depending on their context.[16] They can contribute to a more stable and comfortable indoor environment throughout the year, minimizing the need for large mechanical cooling systems in favor of smaller, more efficient ones.[13]
Ensuring Durability and Moisture Resilience
To enhance the structural integrity and resistance to weathering, particularly against water and wind driven erosion, rammed earth can be stabilized with additives such as Portland cement, however this does represent additional embodied carbon to an assembly that is otherwise very low embodied carbon.[8] The Marfa Ranch project utilized a stabilized mixture, initially experimenting with 7% Portland cement and ultimately settling on a 9% mixture for the majority of the construction.8 This stabilization process was crucial for achieving high compressive strengths, often comparable to concrete, and contributes to an extended lifespan of the rammed earth, with some stabilized rammed earth structures modeled to endure for more than 1,000 years.[17] This longevity is a key performance metric for sustainability when cement is added - the lifespan is required to offset the upfront carbon. While energy efficiency is a common focus in high-performance buildings, the exceptional durability and long lifespan of properly constructed rammed earth walls suggest that for a "non-disposable" building [22], the enduring quality and low maintenance requirements of the material also become a critical performance metric. This expands the definition of "good" building performance to include reduced future resource consumption and a lower lifecycle environmental impact.
Despite its inherent robustness, effective moisture management is vital for the long-term performance and durability of rammed earth. While rammed earth can naturally regulate indoor humidity if unclad walls containing clay are exposed to the interior [17], external protection is essential. Strategies employed include incorporating hydrophobic (water-repellent) additives during the mixing process [15] and ensuring proper drainage around the foundation. For instance, maintaining a 75mm exposed slab edge above finished grade helps protect against moisture ingress, such as rising damp.[15] Research from Building Science Corporation highlights that even high-R walls can be susceptible to moisture problems, underscoring the necessity of robust moisture management, particularly for wall assemblies relying solely on cavity insulation.[24]
A common assumption might be that a material's thermal properties are static. However, research indicates that the "thermal physical parameters of the rammed earth... increased with an increase in moisture content" [20], and that conductivity "varies enormously" with moisture content.25 This highlights a crucial point: effective moisture management for rammed earth walls is not solely about preventing degradation or mold; it is fundamental to maintaining the intended thermal performance of the wall assembly. If the walls become damp, their ability to store and release heat efficiently is compromised, directly impacting the building's energy consumption and occupant comfort. This demonstrates the interconnectedness of moisture control and thermal design in building science.
Rammed earth walls also exhibit a valuable moisture-buffering capacity (hygric buffering). This means they can absorb and desorb significant amounts of water vapor from the indoor environment, which helps to maintain a stable indoor relative humidity, typically within the comfortable range of 40-60%.17 This hygric mass effect can effectively reduce the demands on mechanical systems for humidification and dehumidification, depending on climate specifics.[25]
Table 1: Rammed Earth Wall Performance Attributes. This table provides a holistic view of rammed earth's performance, moving beyond the singular metric of R-value to emphasize its unique benefits such as thermal mass, moisture buffering, and exceptional durability. It directly addresses the need to understand how walls interact with the physical environment by presenting a multi-faceted performance profile, thereby enabling more informed design decisions for climate-appropriate and durable wall assemblies. It visually reinforces that rammed earth functions as a dynamic system with multiple interacting properties, rather than merely a static barrier.
The Imperative of an Airtight Enclosure
An air barrier is a meticulously designed system of materials intended to control airflow within a building enclosure, effectively resisting air pressure differences.[26] It precisely defines the pressure boundary that separates conditioned indoor air from unconditioned outdoor air.[26] For high-performance buildings like Marfa Ranch, establishing an airtight enclosure is paramount, as it serves multiple critical functions:
Firstly, it prevents significant energy loss. Uncontrolled air leakage, whether through infiltration (outdoor air entering) or exfiltration (conditioned indoor air escaping), can substantially compromise energy efficiency, leading to considerable heat gain in summer or heat loss in winter.[26]
Secondly, airtightness is crucial for preventing moisture issues. Air leakage can transport moisture-laden air into the hidden cavities of wall assemblies. When this warm, humid air encounters cooler surfaces within the wall, it can condense, leading to interstitial condensation, mold growth, and potential long-term structural damage. This is particularly prevalent in humid climates or during heating seasons when indoor air is warmer and more humid than the wall cavity.[24]
Thirdly, a robust air barrier is essential for maintaining superior indoor air quality. An uncontrolled air path allows unfiltered outdoor pollutants—such as dust, pollen, and allergens—to infiltrate the building. Simultaneously, it permits indoor contaminants to circulate freely, undermining the effectiveness of any efforts to maintain a healthy indoor environment.[27]
The outdated concept of "homes needing to breathe" is a common misconception, as highlighted by contemporary building science principles.[27] Instead, the prevailing understanding is that healthy, efficient buildings shouldn't leak and that air sealed walls, ceilings, and floors are fundamental for achieving healthy indoor air quality.[27] This is a foundational principle in building science: an airtight enclosure (the air barrier) is not merely about preventing drafts, but about enabling controlled ventilation. Without an effective air barrier, mechanical ventilation systems cannot efficiently dilute pollutants or recover energy, as uncontrolled air leakage bypasses filters and heat recovery mechanisms. This also exacerbates moisture issues due to uncontrolled air movement.[24] Therefore, the airtightness of the wall assembly is directly linked to the optimal performance of the MEP systems and, consequently, to the health and comfort of the occupants.
Finally, an airtight enclosure is vital for complementing both the thermal mass of the rammed earth walls and the mechanical ventilation systems. It ensures that the thermal mass can perform optimally by preventing unintended heat transfer via uncontrolled air movement. Crucially, it allows mechanical ventilation systems, such as Energy Recovery Ventilators (ERVs) or Heat Recovery Ventilators (HRVs), to operate effectively. This ensures that fresh, filtered, and conditioned outdoor air is delivered precisely where and when needed, without being diluted or contaminated by uncontrolled infiltration.[27]
Engineering for Superior Indoor Air Quality (IAQ)
Defining and Prioritizing IAQ
Indoor Air Quality (IAQ) refers to the overall quality of the air within and immediately surrounding buildings, directly influencing the health, comfort, and productivity of its occupants.[28] It is a critical, yet often underestimated, aspect of building design with significant implications for human well-being and functional performance.[28]
Substandard IAQ can manifest in various adverse health outcomes, including respiratory problems, exacerbated allergies, and chronic fatigue. Beyond physical health, poor IAQ has been shown to negatively affect cognitive function and overall well-being.[28] Common indoor air pollutants that contribute to these issues include particulate matter (such as dust, pollen, and mold spores), volatile organic compounds (VOCs) off-gassing from building materials, and combustion byproducts like carbon monoxide (CO) and nitrogen dioxide (NO2).[29]
High-performance buildings inherently prioritize IAQ as a fundamental component of occupant health and comfort to a large degree.[10] This emphasis aligns with the comprehensive guidelines and best practices established by organizations such as ASHRAE for the design, construction, and commissioning of buildings with excellent indoor air quality.[35]
The importance of IAQ extends far beyond mere comfort. Research explicitly links improved IAQ in green-certified buildings (which homes like the Marfa Ranch embody) to "reduced incidence of respiratory problems, allergies, and other health issues," as well as "higher cognitive function scores and better decision-making abilities".[33] Moreover, it has been observed that passive building strategies, which inherently emphasize superior IAQ, can provide a "cushion of time" during power outages, thereby enhancing a building's resilience.31 This elevates IAQ from a "nice-to-have" feature to a critical component of occupant health, productivity, and a building's overall resilience, providing a robust, data-backed justification for architects to prioritize it in their designs.
MEP Strategies for Clean Indoor Air
Achieving superior indoor air quality is a multi-faceted endeavor that requires a comprehensive and integrated approach to MEP system design. The following strategies are crucial for ensuring clean and healthy indoor environments:
1. Ventilation: Bringing in Fresh Air
Adequate ventilation is fundamental for effectively diluting indoor air pollutants and continuously replenishing indoor air with fresh, filtered outdoor air.[28] High-performance homes frequently incorporate mechanical whole-house fresh air systems, such as Energy Recovery Ventilators (ERVs) or Heat Recovery Ventilators (HRVs).[29] These systems are designed to continuously deliver a consistent volume of fresh, filtered outdoor air while simultaneously exhausting stale indoor air. A key benefit of ERVs and HRVs is their ability to recover energy from the outgoing exhaust air to pre-condition the incoming fresh air, significantly reducing the thermal load on the building's heating and cooling systems.[30] ASHRAE Standard 62.2 provides the recognized minimum ventilation rates and other measures for acceptable indoor air quality in residential buildings, serving as a critical guide for engineers in designing effective systems.[27] Local exhaust systems, particularly high-performing kitchen and bath fans vented directly to the outdoors, are essential for removing source-specific pollutants like cooking fumes (which can include particulates, carbon monoxide, and nitrogen dioxide) and excess humidity at their point of origin.[29]
2. Filtration: Removing Contaminants
High-efficiency air filters are indispensable for effectively removing airborne contaminants such as dust, pollen, and other fine particulates from the air stream.[28] Filters are rated by their Minimum Efficiency Reporting Value (MERV), with higher MERV ratings indicating a greater capacity to capture smaller particles.[29] Positive Energy, in its designs, typically specifies MERV 6+ filters for ducted systems, ensuring that air passes efficiently through the filter rather than bypassing it.[29] Some advanced high-performance projects, such as the Theresa Passive House in Texas (also involving Positive Energy), integrate even more robust, hospital-grade filtration systems to achieve superior air purity.[31]
3. Humidity Control: Preventing Mold and Enhancing Comfort
Excessive indoor humidity creates an environment conducive to mold growth, which can lead to various health issues and potential damage to building materials.[27] Consequently, MEP systems must incorporate measures for precise humidity control, such as dedicated dehumidifiers or properly sized HVAC systems, to maintain optimal indoor humidity levels, typically within the comfortable and healthy range of 40-60% relative humidity.[27] This is particularly crucial in climates that, while generally arid, may experience periods of elevated humidity or have internal moisture sources. For instance, the Marfa Ranch courtyard features a water fountain [8], which, while aesthetically pleasing and providing a connection to water, necessitates careful coordination to prevent adverse effects.
While Marfa is a desert environment, leading one to assume humidity is not a primary concern, the presence of the Marfa Ranch courtyard's "water feature that provides much-needed humidity in the dry climate" [8] introduces a localized moisture source. Our indoor air quality guidance always emphasizes the importance of humidity control to prevent mold, even in a dry climate like Marfa, TX.[27] This reveals a nuanced challenge: even when the outdoor climate is predominantly dry, internal moisture generation (from cooking, bathing, or intentional water features) can create localized humidity issues that require careful MEP design to prevent mold growth and maintain occupant comfort. Architects must consider both the macro-climate and any micro-climates created within or immediately adjacent to the building.
4. Source Control: Minimizing Emissions
The most effective strategy for ensuring good IAQ is to proactively minimize the introduction of pollutants at their source.27 This involves several key practices:
Material Selection: Specifying low-VOC (Volatile Organic Compound) or VOC-free building materials, finishes, furnishings, and cleaning products is paramount.[27] VOCs are chemical compounds that can off-gas into the indoor environment, contributing to air pollution and potential health issues.[28]
Combustion Safety: Ensuring that all combustion appliances (e.g., gas stoves, water heaters, fireplaces) are properly vented to the outdoors prevents dangerous gases like carbon monoxide and nitrogen dioxide from accumulating within the living spaces.[29]
Architects might view ventilation, filtration, and humidity control as separate components. However, the available information consistently presents these as interconnected strategies.[27] The emphasis on an "integrated design approach" for optimal IAQ [28] and the description of a comprehensive "environmental control system" that includes hospital-grade filtration and a dedicated dehumidifier [31] demonstrate that achieving truly superior IAQ requires a holistic MEP design. In this approach, ventilation, advanced filtration, precise humidity control, and source reduction work synergistically. It is not merely about adding an ERV; it is about designing a complete system where each component plays a specific, complementary role in ensuring the highest quality indoor air.
Table 2: Key Indoor Air Quality (IAQ) Parameters and MEP Strategies. This table serves as a practical guide for architects, directly addressing the need to understand "what constitutes indoor air quality" and how to achieve it through specific MEP design interventions. By linking common IAQ concerns to actionable strategies and relevant MEP components, it translates abstract concepts into concrete design considerations, fostering a deeper understanding of the interplay between building science and occupant well-being.
Positive Energy's Holistic MEP Design at Marfa Ranch
Integrated Systems for Comfort and Efficiency
Positive Energy is an MEP engineering firm dedicated to leveraging building science and human-centered design to create spaces that are not only healthy and comfortable but also resilient.[10] Our mission extends beyond conventional engineering, aiming to transform the way buildings are created to improve lives and cultivate meaningful relationships with project partners.[40] Kristof Irwin, one of the principals and visionary co-founder of Positive Energy, often articulates a comprehensive philosophy where buildings are envisioned to be healthy, comfortable, durable, efficient, resilient, sustainable, and regenerative, all while maintaining architectural distinction.[12] That vision is brought to life in each project for which we are fortunate enough to collaborate with great partners. This project was no exception.
As both Mechanical Engineers and Building Envelope consultants for Marfa Ranch, our involvement was instrumental in ensuring the seamless integration of the project's passive design strategies—such as the thermal mass of the rammed earth walls and the cooling effects of the central courtyard—with the active mechanical systems. This home features a hydronic heating system, as well as a VRF heating/cooling system. The home’s mechanical systems also featured humidity control, makeup air, and ventilation components. Positive Energy's commitment to resilient design means creating homes that are capable of adapting to changing climate conditions and future needs.[11] This focus is particularly pertinent in a remote, high-desert environment like Marfa, where extreme temperature swings, wind, and occasional intense rain events present significant environmental challenges.[1] This approach moves beyond merely designing functional mechanical systems to actively shaping the occupant's well-being and their interaction with the built environment. For architects, this redefines the value proposition of MEP consultants, highlighting their integral role in delivering holistic, life-enhancing spaces, rather than simply providing infrastructure.
Sustainable Water Management
The Marfa region, situated within the Chihuahuan Desert, is characterized by sparse rainfall and inherent water scarcity.[3] This environmental reality makes thoughtful water conservation a critical design consideration for any project in the area. Furthermore, concerns regarding groundwater contamination from industrial activities in the nearby Permian Basin underscore the broader importance of both water quality and self-sufficiency in the region.[45]
Lake Flato’s water stewardship ambitions for this project aimed at a 97% reduction in water draw from the local utility compared to typical office buildings.[46] The strategies to achieve this included extensive greywater capture and reuse for irrigation purposes.[46] Complementing this, the property also features substantial onsite water storage capacity: 100,000 gallons stored below grade and an additional 20,000 gallons above ground.[46]
A notable example of adaptive reuse and resourcefulness at Marfa Ranch is the conversion of an old water tank, the only pre-existing structure on the site, into the property's swimming pool.[2] This innovative approach minimizes the consumption of new resources. Additionally, the central courtyard features a fountain that is replenished by collected rainwater, further showcasing the project's commitment to water capture and contributing to the oasis-like quality of the outdoor space.[1]
Designing for Performance and Well-being
The Marfa Ranch serves as a compelling case study for climate-responsive, high-performance residential architecture. It vividly demonstrates how a deep understanding and strategic application of building science principles, combined with thoughtful architectural design, can transform a challenging desert environment into a sanctuary of comfort, health, and sustainability.
The project offers invaluable lessons for architects aiming to design for superior performance and occupant well-being.
Practical Application of Building Science for Durable Wall Assemblies:
Marfa Ranch illustrates that truly durable and high-performing wall assemblies, such as stabilized rammed earth, are not merely a result of selecting a particular material. Their success stems from a comprehensive understanding of how multiple building science principles interact. This includes leveraging the inherent thermal mass of the material, meticulously managing moisture through features like hydrophobic additives and proper drainage, and ensuring the continuous integrity of the air barrier. These elements must work in concert to create a robust enclosure that effectively shields inhabitants from environmental extremes—be it heat, cold, or wind—and guarantees the building's longevity.[8]
Strategies for Good Indoor Air Quality:
Marfa Ranch exemplifies that superior indoor air quality is not an accidental outcome but a deliberate product of multi-faceted MEP strategies. This encompasses controlled ventilation, achieved through Energy Recovery Ventilators (ERVs), ensure a continuous supply of fresh, filtered air while recovering energy. It also involves high-efficiency filtration to remove particulates, precise humidity control to prevent mold growth and enhance comfort, and diligent source control, which includes specifying low-VOC materials and ensuring proper exhaust for pollutant-generating areas like kitchens and bathrooms.[27] These integrated elements collectively ensure a healthy, comfortable, and productive indoor environment, highlighting that IAQ is a proactive design outcome, not a reactive fix.
The Cornerstone of Early and Integrated Collaboration:
The successful execution of Marfa Ranch's complex rammed earth construction and integrated MEP systems underscores the immense value of early and deep collaboration between architects and building science/MEP engineering experts. Positive Energy's unique dual role in both mechanical engineering and building envelope consulting on this project is a clear demonstration of the benefits derived from an integrated design process. This approach allows for performance goals to be established and addressed from the earliest design phases, leading to optimized outcomes across energy efficiency, occupant comfort, health, and durability.[1] For architects aiming to deliver truly high-performance, resilient, and healthy buildings, early and continuous collaboration with building science and MEP experts is not merely beneficial; it is essential. This partnership enables the identification of synergies, the navigation of trade-offs, and the development of optimized solutions that seamlessly integrate architectural vision with scientific principles from the foundational design stages, rather than attempting to retrofit performance later in the project lifecycle.
Building a Healthier, More Resilient Future
The Marfa Ranch project, designed by Lake Flato Architects and engineered by Positive Energy's integral MEP and building envelope consulting, is a compelling benchmark for climate-responsive, high-performance residential architecture. It illustrates how a deep understanding and strategic application of building science can transform a challenging natural environment into a sanctuary of comfort, health, and sustainability.
This project exemplifies Positive Energy's unwavering commitment to delivering buildings that not only meet but consistently exceed expectations for occupant health, comfort, and environmental stewardship. Their specialized expertise in seamlessly integrating passive design strategies with advanced mechanical systems, coupled with a steadfast human-centered approach, illuminates a clear and actionable path forward for the Architecture, Engineering, and Construction (AEC) industry.
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The 5 Principles of a Healthy Home
This blog post will present a foundational framework for architectural practice, emphasizing the profound impact of building design decisions on human health and well-being. Moving beyond conventional priorities of aesthetics and initial construction costs, which are unfortunately all too common and mundane in our modern era, this post introduces and explores "5 Principles of a Healthy Home." These principles offer a holistic approach to achieving superior indoor environmental quality (IEQ) and long-term building durability. By understanding and integrating these foundational building science concepts, architects are empowered to design spaces that actively promote the health, cognitive function, and restorative sleep of occupants, thereby elevating their role to advocates for human thriving.
By Positive Energy staff
This blog post will present a foundational framework for architectural practice, emphasizing the profound impact of building design decisions on human health and well-being. Moving beyond conventional priorities of aesthetics and initial construction costs, which are unfortunately all too common and mundane in our modern era, this post introduces and explores "5 Principles of a Healthy Home." These principles offer a holistic approach to achieving superior indoor environmental quality (IEQ) and long-term building durability. By understanding and integrating these foundational building science concepts, architects are empowered to design spaces that actively promote the health, cognitive function, and restorative sleep of occupants, thereby elevating their role to advocates for human thriving.
Architects as Advocates for Human Thriving
Beyond Aesthetics and First Cost
Historically, the evaluation of a "good" building has often been narrowly defined by its visual appeal and the initial financial outlay required for its construction. Terms such as "builder grade" and "developer driven" frequently signify projects where quality, particularly in residential settings, may be compromised in favor of sales volume and cost efficiency.1 This historical prioritization of "eyeballs, egos, and first cost" has inadvertently led to a systemic undervaluation of fundamental building science principles that directly influence both occupant health and the long-term durability of structures.1
This prevailing bias means that critical aspects like indoor air quality and structural resilience are often merely assumed to be adequate, rather than being meticulously designed and verified as are, let’s say, the integration of milled cabinetry in a kitchen. The consequence is a pervasive disconnect between market drivers and true building performance. For architects, this necessitates a proactive stance, challenging these entrenched norms and educating clients on the intrinsic value of healthy, durable design. This shift positions the architect not merely as a fulfiller of aesthetic and budgetary requirements, but as a crucial advocate for occupant well-being, embodying a deeper ethical responsibility to foster human thriving within the built environment.
Indoor Environments and Human Health
The indoor environment is a primary determinant of human health, given that individuals spend approximately 87% of their lives indoors, with nearly 70% of that time within their residence and a significant 30% in their bedroom.1 Within these spaces, invisible threats such as particles, gas-phase pollutants, and bioaerosols are ubiquitous and often undetectable by human senses, yet they exert a profound influence on physiological and cognitive functions.1
This pervasive and often invisible nature of indoor air pollutants, coupled with the vast amount of time spent indoors, transforms the home from a mere shelter into a primary determinant of long-term human health. This influence extends to fundamental biological processes and daily functions. For instance, environmental exposures, including indoor air pollutants like fine particulate matter (PM2.5), have been shown to induce changes in gene expression within a single lifetime.1 This phenomenon, known as epigenetics, impacts prenatal gene regulation and can lead to negative health outcomes for future generations, a concern highlighted by the American Council of Obstetricians and Gynecologists.1 The implications are significant: the very air a pregnant mother breathes can introduce pollutants into the baby's bloodstream, affecting methylation and gene regulation.1
Beyond biological impacts, indoor air quality profoundly affects cognitive function. Research from institutions such as the Harvard T.H. Chan School of Public Health, particularly their CogFX study, demonstrates that better indoor air quality can sharpen decision-making, enhance cognitive abilities, and improve various metrics associated with decision-making, including basic and focused activity, task organization, crisis response, and information processing.1 Elevated carbon dioxide (CO2) levels, often a proxy for inadequate ventilation and increased pollutant concentrations, have been correlated with decreased cognitive performance.1
Furthermore, the quality of indoor air directly impacts sleep. Studies indicate a strong correlation between poor indoor air quality, specifically exposure to particulate matter and nitrogen dioxide, and increased sleep disturbances and decreased sleep efficiency.1 Considering that approximately 30% of an average human life is spent in the bedroom, this "sleep zone" becomes a critical microenvironment for exposure science, demanding careful consideration of what is present in the air, bedding, and surrounding materials.1 The cumulative effect of these influences elevates the architect's role to that of a public health professional, designing not just spaces, but tangible health interventions.
The 5 Principles of a Healthy Home
The following five principles, distilled from peer-reviewed medical and environmental chemistry research, provide a robust framework for designing homes that prioritize occupant health and well-being.
Principle 1: Start with a Good Building Enclosure
Defining the Enclosure and its Foundational Role
A "good" building enclosure is functional, durable, and reliable, performing its intended purpose over a long lifespan.1 It serves as the primary environmental separator, defining the conditioned space and mediating the interaction between the indoor and outdoor environments.1 This six-sided box, comprising the foundation, walls, and roof, is the critical element that creates the "indoors".1 Its design, including massing, shape, orientation, and the placement of apertures, has a lasting impact on the building's performance.1 The enclosure is a passive, durable, and functional assembly, representing a singular opportunity to achieve correct installation, as rectifying issues later can be inconvenient and costly.1
The enclosure plays a vital role in indoor environmental quality in several ways. Firstly, it defines the breathing zone of the conditioned space, directly influencing the volume and quality of air occupants inhale.1 Secondly, it mediates moisture transport processes, either preventing or allowing water ingress from rain, groundwater, air-transported moisture, or diffusion through materials.1 This control is paramount for preventing dampness and subsequent issues like mold growth. Thirdly, the very materials chosen for the enclosure can be a permanent source of toxic air pollutants, highlighting the need for careful material selection.1
Mediating Moisture Transport: The 3 Ds and Control Layers
Effective moisture control within the building envelope is critical, as water is a universal solvent capable of degrading building materials and fostering biological growth.1 Building science principles emphasize the "3 Ds" for water management: Deflect, Drain, and Dry.10
Deflect: This involves preventing water from entering the building in the first place, primarily through the exterior cladding.10
Drain: A crucial safety net involves creating a drainage plane behind the cladding to direct any water that bypasses the deflection layer away from the wall assembly.10 This often involves a water-resistive barrier (WRB) that can also function as a drainage plane.10 Proper flashing details at windows, doors, and roof-to-wall intersections are essential to direct water "down and out" over the cladding or drainage plane.13 Kick-out flashings, for example, are critical to prevent water concentration at wall surfaces.13
Dry: Should any moisture penetrate the system, the assembly must have the capacity to dry out, either to the interior or exterior.10 Highly permeable materials for the WRB can facilitate this drying process by allowing moisture vapor to pass through the wall assembly.10
Beyond water barriers, the building envelope incorporates other control layers:
Air Barrier: This layer is paramount for energy efficiency and indoor air quality, as air leakage can transport unwanted heat, cool air, pollutants, odors, and, crucially, water vapor into the building cavity.10
Insulation Layer: Continuous insulation on the building's exterior significantly reduces heating and cooling needs, improving energy efficiency and occupant comfort.10 Thermal bridge elimination is also critical to prevent "cold corners" and minimize mold growth risk.15
Vapor Barrier: This layer manages water vapor diffusion, preventing condensation within the wall assembly at the dew point.10 The design should allow the wall assembly to dry if liquid water forms.10
The Critical Air Barrier: Preventing Uncontrolled Air and Moisture Movement
An effective air barrier is a cornerstone of a high-performance enclosure, essential for both durability and energy savings.15 It is a continuous system of interconnected materials, assemblies, and sealed joints that minimizes air leakage into or out of the building's thermal envelope.16 Codes, such as the International Energy Conservation Code (IECC) and ASHRAE Standard 90.1, mandate continuous air barriers for new commercial construction in certain climate zones.17
The air barrier's significance extends beyond energy efficiency. By preventing uncontrolled air movement, it mitigates the transport of water vapor, which can lead to moisture accumulation and material degradation within the wall cavity.10 Even with a robust water-resistive barrier, an air leak can introduce water vapor at a much higher rate than diffusion, causing internal damage.10 The air barrier must be impermeable, continuous, structurally supported, and durable.17 Its continuity is achieved by meticulously detailing transitions between different materials and assemblies, ensuring a seamless barrier across the entire building enclosure, including below-grade components.16 This meticulous design and installation, often guided by manufacturer instructions and prescriptive requirements, are critical for the long-term performance of the building.16
Material Selection and Avoiding Enclosure-Based Pollutants
The choice of materials for the building enclosure directly impacts indoor air quality, as many common construction products can be permanent sources of toxic air pollutants.1 This concern is particularly acute given the historical tendency to use occupants as "science experiments," introducing materials with unknown long-term health outcomes.1 For example, flame retardants, once commonly found in children's pajamas, are also present in spray foam insulation and various textiles used in buildings.1 These chemicals do not easily break down and can leach into dust, food, and water, posing risks such as endocrine and thyroid disruption, immunotoxicity, reproductive toxicity, and adverse effects on fetal and child development.18
Other hazardous chemicals found in building materials include formaldehyde, a known carcinogen present in pressed wood products, insulation, glues, and paints; chromated copper arsenate (CCA) in pressure-treated wood; lead in older paints and plumbing; polyvinyl chloride (PVC) in pipes, window frames, and flooring, which contains phthalates and dioxins linked to hormone disruption and cancer; and isocyanates in spray foam insulation.11 Crystalline silica, when pulverized during construction, can also lead to severe respiratory issues.11 These substances can lead to a range of health effects, from eye and respiratory irritation to neurological problems and cancer.11 Architects must advocate for the selection of low-emitting and non-toxic materials, understanding that the enclosure is not merely a structural element but a critical determinant of indoor chemical exposure.
Integrating Air Distribution Systems as Part of the "Enclosure"
While typically considered part of mechanical systems, the air distribution system of a home—its "lungs"—functions as a passive, durable, and highly functional component that should be treated with the same design rigor as the building enclosure itself.1 The common practice of using flex duct and duct board, often installed with "origami-like" distortions, leads to significant energy waste due to needless friction and fluid dynamic inefficiencies.1 This neglect, often driven by "low first cost" and an "out of sight, out of mind, out of budget" mentality, compromises the entire system's performance.1
The air distribution system is intimately connected to indoor air quality, as it is responsible for delivering conditioned air deep into occupants' lungs.1 The time it takes for air to move from the room to the alveoli in the lungs, where gas exchange occurs, is on the same timescale as the exchange from alveoli to blood.1 Therefore, the quality of air within the ducts directly impacts occupant health. Architects have a critical role in integrating the building's "lungs" into the architectural design, insisting on robust, well-designed systems, such as metal ductwork, that ensure proper air mixing and efficient pollutant removal.1 This involves thinking about fluid dynamics and collaborating with engineers to ensure that air enters the room with sufficient energy to entrain particles and gases, facilitating their capture by filters and promoting thermal and humidity comfort.1 This approach recognizes that the air distribution system is not an aesthetic inconvenience but a functional necessity for human thriving.
Principle 2: Minimize Indoor Pollutants/Emissions
Understanding Indoor Pollutants: Particles, Gases, and Bioaerosols
The "fishbowl strategy" of our indoor environments means we are immersed in air containing a complex mixture of pollutants, often without our awareness.1 These can be broadly categorized into three main types:
Particles: These include particulate matter (PM) of various sizes, such as coarse particles (PM10), fine particles (PM2.5), and ultrafine particles (PM0.1 or PM0.5).1 PM2.5, with a diameter of less than 2.5 micrometers, is particularly dangerous as it can penetrate deep into the lungs and enter the bloodstream, causing cardiovascular and respiratory diseases, neurodegenerative diseases, and cancers.3 These particles are often "candy-coated" with chemical gases, making them a rich chemical mixture.1
Gas-Phase Pollutants: This category includes volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs).1 VOCs are chemicals that easily vaporize at room temperature, releasing fumes into the air, and are found in thousands of household products and building materials.20 They can cause eye, nose, and throat irritation, headaches, dizziness, and damage to the liver, kidneys, and central nervous system, with some being suspected or known carcinogens.20 SVOCs can outgas for decades.1
Bioaerosols: This growing area of study encompasses a rich ecosystem of bacteria, viruses, protozoa, fungal spores, archaea, and dust mites suspended in the air.1 These microorganisms interact with surfaces and occupants, and their populations are significantly influenced by indoor environmental conditions, particularly humidity.1
These pollutants, whether of outdoor origin infiltrating indoors or emitted from indoor sources, lead to concentrations that result in exposure, and ultimately, intake and dose, which can have toxic health effects.1 The science of indoor chemistry focuses on emissions and concentrations, while health science investigates exposure, intake, dose, and health outcomes.1
Active (Anthropogenic) Sources and Mitigation Strategies
Active sources of indoor pollutants are those derived from human activity, and many are controllable through design and occupant behavior.1
Cooking: This is a major indoor source of PM2.5 and other combustion byproducts, including nitrogen and sulfur oxides, and unburned gases, especially when cooking with gas.1 Effective mitigation requires a well-designed range hood with a deep sump and adequate exhaust ventilation.1 Downdraft range hoods are generally ineffective at capturing upward-flowing pollutants and are not recommended for comprehensive pollutant capture.1
Showering: Steamy showers introduce significant water vapor, which, if not removed, can linger and contribute to dampness.1 Moisture-sensing bath fans and proper material selection in bathrooms are essential to manage this moisture.1
Indoor Combustion: Unvented combustion appliances, such as decorative gas fireplaces, are a significant health hazard, releasing pollutants like carbon monoxide and PM2.5.1 These should be avoided indoors or properly vented to the outdoors.1
Air Fresheners and Personal Care Products: Many air fresheners, creams, lotions, cosmetics, and scented laundry detergents contain endocrine-disrupting chemicals, highly fluorinated chemicals, plasticizers, and antimicrobials that are emitted into the indoor air.1 Educating clients about these sources and advocating for their avoidance is crucial.1
Occupants and Pets: Humans and animals are continuous sources of particles and gases, contributing to the indoor chemical spectrum.1
These active sources represent categories where direct action can be taken through design choices, equipment selection, and educating homeowners on operational best practices.1
Passive Emissions: Persistent Chemical Contaminants in Building Materials and Products
Beyond active, human-driven sources, indoor environments are also affected by passive emissions from building materials and consumer products that off-gas pollutants over time.
Flame Retardants: These chemicals, often found in furniture foam, textiles, carpets, and even spray foam insulation, do not easily break down and can continuously leach into the environment.1 They are linked to endocrine and thyroid disruption, immunotoxicity, reproductive toxicity, cancer, and adverse effects on fetal and child development, with children being particularly vulnerable due to their developing organs and hand-to-mouth behaviors.18
Phthalates and Plasticizers: Found in vinyl blinds, flooring, and many plastics, plasticizers are added to make materials supple but off-gas over time, making the material brittle.1 Phthalates are hormone-disrupting chemicals widely used as plasticizers in food contact materials and construction plastics.27 They can enter the human body through inhalation, ingestion, or dermal absorption and are associated with endocrine and reproductive dysregulation, early puberty, asthma, and allergies.27
Perfluorinated Chemicals (PFAS): Used for non-stick coatings and water/stain repellency in carpets and other textiles, these "forever chemicals" pose long-lasting health threats.1
Antimicrobials: Found in hand soaps, laundry detergents, and some building products, these chemicals have limited benefits and can cause adverse health effects.1
Volatile Organic Compounds (VOCs): Beyond formaldehyde, other VOCs like acetone, benzene, toluene, and xylene are emitted from paints, varnishes, wax, cleaning products, and stored fuels.1 These can cause a range of health issues, including respiratory irritation, headaches, and damage to various organ systems.20
These passive emissions highlight the need for careful material specification during design and client education regarding product choices within the home.
The "Six Classes of Harmful Chemicals" and Their Pervasiveness
To simplify the complex landscape of chemical pollutants, the "Six Classes of Harmful Chemicals" framework provides a useful categorization for architects and clients to understand and mitigate exposure.1 These classes represent toxic substances commonly found in everyday products that contribute to serious health problems:
PFAS (Per- and Polyfluoroalkyl Substances): "Forever chemicals" with long-lasting environmental and health threats.29
Antimicrobials: Chemicals with limited health benefits but adverse health effects.29
Flame Retardants: Chemicals that do not provide a fire safety benefit and can damage health.29
Bisphenols & Phthalates: Hormone-disrupting chemicals with widespread use leading to constant exposure.29
Some Solvents: Linked to neurological problems and increased cancer risk.29
Certain Metals: Toxic metals like mercury, arsenic, cadmium, and lead that should be avoided.29
These classes underscore the pervasive nature of chemical exposure in indoor environments, emphasizing that many common products and materials contribute to the overall chemical load. Understanding these categories empowers architects to make informed material selections and advocate for healthier product choices, thereby reducing occupant exposure to these harmful substances.29
The Role of Dust as a Pollutant Reservoir
Indoor dust is not merely innocuous debris; it is a complex chemical mixture.1 Particles in dust can be likened to "candy-coated M&Ms," where the particulate core is coated with various chemical gases.1 Studies indicate that the constituent molecules found in human blood from indoor environments often correlate in relative concentrations to those found on the floor, suggesting that whatever is on the floor is likely already in the body.1 This highlights dust as a significant reservoir for semi-volatile organic compounds (SVOCs) that can off-gas for decades, as well as VOCs.1 Effective strategies for minimizing indoor emissions must therefore consider not only source reduction but also the management of dust as a chemical sink.
Principle 3: Properly Ventilate
Distinguishing True Ventilation from Air Leakage
Effective ventilation is the controlled movement of air into and out of a building, typically achieved through mechanical means and deliberately placed openings in the building envelope.30 It is crucial to differentiate this from uncontrolled air leakage, often mistakenly referred to as a "building breathing".1 Buildings themselves do not need to breathe; rather, the occupants require fresh air.1 Air leakage, where air infiltrates from random spaces like crawl spaces or wall cavities, is not ventilation and can introduce pollutants and moisture into the conditioned space.1 True ventilation, conversely, ensures that clean air is supplied and stale, polluted air is exhausted in a controlled manner.30
The Dual Purpose of Ventilation: Exhausting Pollutants and Supplying Fresh Air
Ventilation serves a dual purpose: to remove polluted indoor air and to introduce clean outdoor air.1 This process is analogous to a car's engine pulling in clean air for combustion and an exhaust pipe expelling polluted air.1 The priority is first to get the "bad stuff out," and then to bring "clean air in".1 This requires a systems-based approach, where professionals, rather than homeowners, determine the appropriate climate-zone-specific enclosure and mechanical systems to deliver conditions that support human thriving.1 ASHRAE Standard 62.1 provides guidelines for ventilation rates, contaminant control, and air distribution to ensure acceptable indoor air quality in commercial and institutional buildings, while ASHRAE 62.2 addresses residential applications.31
Effective Local Exhaust: Kitchen and Bathroom Ventilation
Local exhaust systems are designed to remove high concentrations of contaminants at their source, primarily in kitchens and bathrooms.1
Kitchens: Cooking is a significant source of indoor air pollution, including particulate matter and combustion gases.1 An effective range hood is essential for capturing these pollutants at the source.1 ASHRAE guidelines emphasize "capture and containment" and specify minimum exhaust flow rates based on cooking appliance type and hood configuration.23 Flat-bottomed or downdraft range hoods are generally less effective at capturing upward-flowing cooking effluents compared to deep-sump, overhead models.1 ASHRAE 62.2 recommends a minimum of 100 CFM for kitchen exhaust, or 5 air changes per hour for continuous ventilation.33
Bathrooms: Showers generate substantial moisture, which must be removed to prevent dampness and mold growth.1 ASHRAE 62.2 recommends a minimum of 50 CFM of intermittent ventilation or 20 CFM of continuous ventilation for bathrooms, typically 1 CFM per square foot.33
For both kitchen and bathroom exhaust fans, ASHRAE 62.2 mandates certified sound levels of 3.0 sones or less to ensure they are actually used by occupants, rather than being turned off due to noise.35 Automated ventilation, such as humidity or motion sensing fans, is also encouraged to ensure consistent operation.35
Whole-Building Fresh Air: The Role of ERVs & HRVs
Beyond local exhaust, whole-building ventilation introduces fresh outdoor air to dilute unavoidable contaminants from people, pets, and off-gassing.33 For airtight, energy-efficient homes, this requires mechanical ventilation systems that can recover energy and moisture.15
Heat Recovery Ventilators (HRVs): These systems recover sensible heat from the outgoing exhaust airstream and transfer it to the incoming fresh air, reducing heating and cooling demands.36 HRVs are most often suitable for colder, drier climates where sensible heat transfer is the primary concern, although with a changing climate with hotter and more humid summers, more climate zones are becoming ERV territory.38
Energy Recovery Ventilators (ERVs): ERVs are "total enthalpic devices" that transfer both sensible and latent heat (moisture) between air streams.37 In warmer seasons, ERVs pre-cool and dehumidify incoming air, while in cooler seasons, they humidify and pre-heat.37 This helps maintain indoor relative humidity within comfortable ranges (e.g., 40-50%) and reduces the overall HVAC equipment capacity needed.37 ERVs are highly beneficial ventilation devices, where they help prevent a certain percentage of unwanted outdoor humidity from entering the indoor environment (although they do require dedicated dehumidification in order to properly work), and in very dry climates, where they can help retain desired indoor humidity conditions.38
ASHRAE 62.2 provides formulas for calculating whole-house ventilation rates based on floor area and the number of bedrooms.33 Despite their significant benefits for indoor air quality and energy efficiency, ERVs and HRVs are adopted in a very small percentage of American homes, estimated at 1-2%.1 This low adoption rate reflects a lag behind Europe and Asia, partly due to misaligned cost-benefit relationships and a general lack of awareness regarding the overlap of building science and health sciences.1 Architects are instrumental in advocating for the inclusion of these systems to ensure continuous, balanced ventilation and superior indoor air quality.
Principle 4: Keep the Air in Proper Humidity Ranges
The Detrimental Effects of Excess Moisture: Promoting Biological Growth and Material Degradation
Maintaining proper humidity levels is paramount for a healthy home. Water, often referred to as the "universal solvent," inexorably works to break down materials and facilitate chemical changes, leading to the emission of substances into the air.1 Excess moisture creates conditions conducive to the growth of undesirable biological organisms, particularly mold and bacteria.1 Mold, a decomposer essential outdoors, is highly detrimental indoors, producing allergens, irritants, and potentially toxic substances.1 Fungal growth is significantly promoted by high humidity levels.42
Beyond biological growth, high humidity can cause dimensional instability in wood products, leading to issues like cupping in hardwood floors.1 It can also lead to condensation on windows and absorption into sheetrock and wood, initiating rot and decay.1 Furthermore, high humidity can increase the emission rates of volatile organic compounds (VOCs) from building materials through hydrolysis.1
Health Impacts of Damp Environments: Respiratory Issues and Beyond
The presence of dampness and mold in homes has well-documented associations with adverse health outcomes.1 Meta-studies on dampness and health have established sufficient evidence for relationships between exposure to damp indoor environments and various respiratory issues.1 These include upper respiratory tract infections, wheezing, coughing, exacerbation or development of asthma, chronic bronchitis, and other respiratory infections.1 Allergic rhinitis and eczema are also correlated with dampness.1 For instance, there is a 20-50% increased risk of asthma in damp houses.41 The indoor microbiome, which is heavily influenced by environmental conditions, directly impacts the human microbiome, further underscoring the importance of moisture control.1
Maintaining Optimal Humidity Levels: The 40-60% RH Range
To mitigate these risks, maintaining indoor relative humidity within an optimal range is crucial. While specific set points can be debated, a range between 40% and 60% relative humidity (RH) at normal room temperatures is widely recommended by professional bodies, including ASHRAE and the Danish Technical University.1 This range is considered ideal for minimizing the growth of bacteria, viruses, and fungi, as well as reducing the incidence of respiratory infections.42 Humidity levels below this range can cause dryness of skin and mucous membranes, leading to irritation and potentially impairing the respiratory immune system, while levels above can promote microbial growth and hinder evaporative cooling.42 For individuals with chemical sensitivities, even lower humidity levels may be advised.1
The Impact of Energy Codes on Latent Loads and Dehumidification Needs
A significant challenge in modern home design stems from the evolution of energy codes. These codes have drastically improved building thermal envelopes, leading to substantial reductions in sensible cooling loads through increased insulation, better windows, and improved airtightness.1 While this reduces overall energy consumption for cooling, it also means that conventional air conditioning systems, which traditionally handled both sensible (temperature) and latent (humidity) loads, run less frequently.1
However, internal humidity loads from occupants and their activities remain persistent.1 As sensible loads decrease, the ratio of sensible to latent loads shifts, making standard air conditioners less effective at maintaining comfortable humidity levels.44 This creates a situation where homes may be thermally comfortable but excessively humid, leading to issues like mold growth and poor indoor air quality, even in energy-efficient designs.1 This is not a sudden problem but one that has grown over years as buildings have become tighter, and it necessitates a dedicated approach to dehumidification.1
Strategies for Effective Dehumidification
Given the limitations of traditional air conditioning in low-load homes, supplemental or dedicated dehumidification is increasingly necessary to maintain healthy indoor humidity levels.44 There are two primary methods for drying air:
Vapor Compression (Refrigerant-Based) Dehumidifiers: These systems draw air over a cold coil, causing moisture to condense and be collected.1 They are generally more energy-efficient and cost less for residential applications, working best in warmer climates (above 16°C).1
Desiccant Dehumidifiers: These draw air over a desiccant chemical that absorbs moisture.1 While they typically have higher energy consumption, they perform consistently across a wider temperature range, including colder environments, and can even release warmth, which can be beneficial in winter.45
For most residential applications, vapor compression systems are currently the more practical and energy-efficient choice.1 The cost of operating dedicated dehumidification in humid climates can be surprisingly low, often just cents per day, making it a highly cost-effective intervention for health and durability.1 Architects should integrate dedicated dehumidification systems into their designs, recognizing that they are a critical component for maintaining a healthy indoor environment in modern, energy-efficient homes.
Principle 5: Use Robust Filtration to Capture Indoor Pollutants
The Ubiquity and Harm of Particulate Matter
Particulate matter pollution is pervasive in homes, generated both mechanically (e.g., dust, pet dander) and chemically (e.g., cooking, off-gassing).1 These particles, particularly fine (PM2.5) and ultrafine (PM0.1), represent the majority of sources for indoor air-related sickness.1 PM2.5 can penetrate deep into the lungs, enter the bloodstream, and lead to serious health outcomes, including neurodegenerative diseases, neurodevelopmental disorders, and cardiovascular diseases.3 Exposure to PM2.5 has been linked to epigenetic alterations and cognitive impairment, even hours after exposure.3 Given that particles can also enter the body through the skin, robust filtration is essential for overall health.1
Understanding Filtration Efficacy: MERV Ratings and HEPA Filters
The effectiveness of air filters is quantified by their Minimum Efficiency Reporting Value (MERV) rating, which indicates a filter's ability to capture particles between 0.3 and 10 microns.48 A higher MERV rating signifies better particle capture efficiency.48
MERV 13: This is generally considered a minimum for effective particulate capture in homes, capable of capturing at least 50% of particles between 0.3-1.0 microns, and 85% or more of particles between 1.0-3.0 microns.1 ASHRAE has recommended MERV-13 or better filtration for infectious aerosol exposure reduction.47
HEPA Filters: High-Efficiency Particulate Air (HEPA) filters are mechanical filters designed to remove at least 99.97% of airborne particles with a size of 0.3 microns, which represents the Most Penetrating Particle Size (MPPS).48 Particles larger or smaller than 0.3 microns are captured with even higher efficiency.49 HEPA filtration is considered the gold standard for capturing dust, pollen, mold, bacteria, and other airborne particles.48
Architects should specify mechanical systems capable of accommodating high-efficiency filters (e.g., MERV 13 or higher) and ensure that ductwork design minimizes pressure drop to allow for proper airflow through these denser filters.1 Regular filter replacement is crucial for maintaining performance.48
The Economic Benefits of Effective Filtration
Investing in effective particulate capture systems yields significant economic benefits that consistently exceed costs.1 Studies from Lawrence Berkeley National Laboratory, for example, estimate annual economic benefits ranging from $0.2 billion to $1.1 billion from improved particle filtration in U.S. homes and commercial buildings.50 These benefits stem from reduced respiratory diseases, allergies, asthma, and symptoms of sick building syndrome, as well as increased productivity and reduced absenteeism.50 For some interventions, the predicted annual mortality-related economic benefits can exceed $1000 per person, with benefit-to-cost ratios ranging from approximately 3.9 to 133.51 The largest reductions in mortality and highest economic benefits are often observed with continuously operating portable air cleaners equipped with HEPA filters.51 This evidence strongly supports the integration of robust filtration as a cost-effective strategy for improving public health within buildings.
Caution Regarding Active Air Cleaning Technologies
While mechanical filtration (like MERV and HEPA) is highly effective and generally safe, caution is advised regarding certain "active" air cleaning technologies, such as plasma-based, ion-based, or ozone-generating devices.1 Many ionizers, for instance, produce ozone as a byproduct.52 Ozone, a molecule composed of three oxygen atoms, can damage the lungs even at relatively low concentrations, causing chest pain, coughing, shortness of breath, and throat irritation.53 It can also worsen chronic respiratory diseases like asthma and compromise the body's ability to fight infections.52 Furthermore, ozone can react with other chemicals in the indoor environment to form harmful or irritating by-products, potentially increasing the total concentration of organic chemicals in the air.53 While some manufacturers claim these devices "purify" the air, scientific research suggests that for many common indoor chemicals, the reaction with ozone may take months or years, or produce new harmful compounds.53 Therefore, more research is needed on these active systems, and architects should prioritize proven, passive filtration methods for occupant safety.
Home as Health Intervention
The traditional paradigm of home design, often driven by visual aesthetics and initial cost, has overlooked the profound and lasting impact of indoor environments on human health. This report underscores that the home is not merely a structure but a critical health intervention, capable of influencing fundamental biological processes, cognitive function, and restorative sleep. The pervasive and often invisible nature of indoor air pollutants, coupled with the vast amount of time spent indoors, elevates the architect's role from a designer of spaces to an advocate for public health.
By embracing the "5 Principles of a Healthy Home"—starting with a good building enclosure, minimizing indoor pollutants, properly ventilating, maintaining optimal humidity, and employing robust filtration—architects can proactively design environments that foster human thriving. This requires a shift in priorities, challenging the "eyeballs, egos, and and first cost" mentality and instead prioritizing durability, moisture control, air quality, and non-toxic material selection. Integrating robust air distribution systems, dedicated dehumidification, and high-efficiency filtration are not mere conveniences but essential components of a health-centric design strategy.
The evidence from leading institutions like Lawrence Berkeley National Labs, Harvard T.H. Chan School of Public Health, and ASHRAE consistently demonstrates the tangible health benefits and economic advantages of these principles. Architects are uniquely positioned to lead this transformation, educating clients and project teams on the long-term value of healthy homes. The path forward demands a commitment to building science, a systems-thinking approach, and an unwavering dedication to the well-being of building occupants. This is the new normal: home as health intervention, and architect as advocate.
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Six Classes of Harmful Chemicals - Green Science Policy Institute, accessed May 27, 2025, https://greensciencepolicy.org/harmful-chemicals/
BALANCED VENTILATION DESIGN PRINCIPLES - Phius, accessed May 27, 2025, https://www.phius.org/sites/default/files/2022-06/Ryan%20Abendroth%20-%20Ryan%20Abendroth%20Mechanical%20Summit.pdf
Understanding ASHRAE Ventilation Standard 62.1 | Sanalife, accessed May 27, 2025, https://www.sanalifeenergy.com/blog/understanding-ashrae-ventilation-standard-62-1
Standards 62.1 & 62.2 - ASHRAE, accessed May 27, 2025, https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2
ASHRAE 62.2 for New Construction - How to Calculate the Required Amount of Ventilation, accessed May 27, 2025, https://hvacdesignpros.com/ashrae-62-2-construction-calculate-required-amount-ventilation/
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Designing Healthier Homes by Eliminating Fossil Gas Appliance Emissions
Architects, as the primary designers of our built environment, hold a profoundly influential position in shaping the health and well-being of building occupants. Beyond the critical considerations of aesthetics, structural integrity, and energy performance, a deep understanding of the invisible forces at play within a building's envelope is increasingly paramount. This report aims to equip architects with the essential knowledge to proactively design for superior indoor air quality (IAQ), particularly concerning emissions from common household gas appliances. The decisions made during the design phase, from material selection to mechanical system integration, directly influence the indoor environment and, by extension, the health outcomes of those who inhabit these spaces. This effectively positions architects as critical guardians of public well-being within the built space, expanding their traditional role to encompass a vital public health responsibility.
By Positive Energy staff
The Architect's Role in Indoor Environmental Quality
Architects, as the primary designers of our built environment, hold a profoundly influential position in shaping the health and well-being of building occupants. Beyond the critical considerations of aesthetics, structural integrity, and energy performance, a deep understanding of the invisible forces at play within a building's envelope is increasingly paramount. This report aims to equip architects with the essential knowledge to proactively design for superior indoor air quality (IAQ), particularly concerning emissions from common household gas appliances. The decisions made during the design phase, from material selection to mechanical system integration, directly influence the indoor environment and, by extension, the health outcomes of those who inhabit these spaces. This effectively positions architects as critical guardians of public well-being within the built space, expanding their traditional role to encompass a vital public health responsibility.
Unmasking the Impact of Gas Appliances on Home Health
While gas appliances, such as stoves and heaters, are ubiquitous in modern homes due to their convenience and efficiency, their combustion byproducts and even unburned gas can significantly degrade indoor air quality. This degradation poses documented health risks that have been the subject of extensive scientific inquiry over the past two decades.1 These appliances release a complex cocktail of pollutants that, when confined within residential structures, can lead to a range of adverse health effects. The presence of these combustion products and hazardous air pollutants (HAPs) in indoor environments warrants a re-evaluation of their widespread use and the design strategies employed to mitigate their impact.2
Bridging Science and Design for Healthier Buildings
This post synthesizes complex scientific findings from leading institutions, including the Rocky Mountain Institute (RMI) 1, the U.S. Environmental Protection Agency (EPA) 3, ASHRAE 2, and Lawrence Berkeley National Laboratory (LBNL).14 The goal is to translate these technical insights into actionable strategies for architectural practice. The report will detail specific pollutants emitted by gas appliances, their associated health effects, and, crucially, how thoughtful design and engineering solutions can effectively mitigate these risks, fostering truly healthier indoor environments.
Fundamentals of Indoor Air Quality (IAQ) for Architects
Defining Good IAQ: Source Control, Ventilation, and Filtration
Good indoor air quality management is fundamentally built upon three interconnected principles: controlling airborne pollutants at their source, ensuring adequate ventilation through the introduction of outdoor air and removal of indoor air, and employing effective filtration to remove contaminants from the air.9 Beyond these, maintaining acceptable temperature and relative humidity levels is also critical for overall IAQ and occupant comfort.10 These principles are not isolated but rather form a synergistic approach to managing indoor air. For example, while ventilation dilutes pollutants, it can also introduce outdoor contaminants, highlighting the need for a comprehensive strategy.22 It is particularly important to control pollutant sources, as IAQ problems can persist even with a properly operating HVAC system if the sources themselves are not addressed.10 This interconnectedness means architects must consider these elements holistically, recognizing that optimizing one pillar without considering the others can lead to suboptimal or even detrimental IAQ outcomes.
The Building as a Dynamic System: How Structure, Systems, and Occupants Shape IAQ
A building's indoor environment is not a static entity but a complex, dynamic system. Its IAQ is profoundly influenced by the intricate interactions among various factors, including the building's geographic site, local climate, physical structure, mechanical systems (HVAC), construction techniques, the array of internal and external contaminant sources, and the activities and behaviors of its occupants.10 Pollutants can originate from within the building itself, such as combustion byproducts from appliances or off-gassing from materials, or they can be drawn in from the outdoors, including vehicle emissions or pollen.10
Air exchange, a critical process for maintaining healthy IAQ, occurs through multiple pathways. These include designed mechanical ventilation systems utilizing fans, uncontrolled infiltration (the leakage of air through cracks and myriad openings in the building envelope), and the intentional opening of windows and doors.11 Air pressure differences, both within and around the building, act as driving forces that can move airborne pollutants through any available openings in walls, ceilings, floors, doors, windows, and even HVAC systems.10 This perspective underscores the importance of viewing the building envelope not as a passive barrier, but as an active, permeable interface that constantly mediates the exchange of air and pollutants between the interior and exterior. This dynamic interplay necessitates a design approach that manages these exchanges intentionally to promote health.
The "Building Tight, Ventilate Right" Imperative and Its IAQ Implications
Modern energy-efficient construction frequently adopts the strategy of "Building Tight, Ventilate Right".21 This approach is primarily driven by the goal of reducing energy consumption by minimizing uncontrolled air leakage, or infiltration, through the building envelope.20 By creating a tighter building, less energy is required for heating and cooling, which is a significant step towards sustainable design.
However, a crucial implication of this strategy is that reduced infiltration and ventilation rates in tightly sealed buildings can lead to a significant increase in the concentration of indoor-generated contaminants.10 The very measures taken to enhance energy efficiency, such as improved insulation and sealing, can inadvertently trap pollutants indoors if not accompanied by compensatory measures. This creates a fundamental tension for architects: while energy efficiency is a vital design objective, it must be meticulously balanced with robust, intentional mechanical ventilation strategies. Without such integrated planning, the unintended consequence can be elevated pollutant levels and compromised indoor air quality, undermining the overall health performance of the building.10 This highlights the necessity of designing for controlled air exchange rather than relying on uncontrolled leakage.
Why Indoor Air Pollutants Often Exceed Outdoor Levels
It is a common, yet often mistaken, assumption that indoor air is inherently cleaner than outdoor air. However, studies conducted by the EPA and other research institutions consistently demonstrate that indoor levels of many air pollutants can be 2 to 5 times, and occasionally more than 100 times, higher than outdoor levels.6 This phenomenon is particularly concerning given that people spend approximately 90% of their time indoors.9
The primary reason for this disparity is the presence of numerous pollutant sources located within the building itself.11 These internal sources include combustion from appliances, off-gassing from building materials and furnishings, and emissions from cleaning products, among many others.6 When these internally generated pollutants are released into a relatively confined space and then trapped by a tighter building envelope—a characteristic of modern, energy-efficient construction—their concentrations can rapidly accumulate and surpass outdoor levels.6 This situation, sometimes referred to as the "concentration trap," means that the primary challenge for architects is not merely preventing outdoor pollutants from entering, but effectively managing and removing the contaminants generated within the home. This understanding underscores the critical need for proactive IAQ design that addresses internal pollutant generation.
Key Pollutants from Gas Appliances and Their Health Implications
Gas appliances, particularly those used for cooking and heating, are significant indoor sources of a variety of pollutants. The combustion process, and even the unburned fuel itself, can release substances that pose substantial risks to human health. Understanding these specific pollutants and their impacts is crucial for architects aiming to design healthier homes.
Nitrogen Dioxide (NO2): A Respiratory Concern
Nitrogen dioxide (NO2) and nitric oxide (NO) are toxic gases, with NO2 being particularly hazardous as a highly reactive oxidant and corrosive agent.3 The primary indoor sources of NO2 are combustion processes, especially from unvented gas stoves, kerosene heaters, and defective vented appliances.2 While electric coil burners also emit NO2, their emission rates are significantly lower than those from gas burners, making gas combustion the predominant concern for this pollutant in residential settings.18
The health effects of NO2 exposure range from immediate irritation to more severe, long-term respiratory conditions. NO2 acts mainly as an irritant, affecting the mucous membranes of the eyes, nose, throat, and respiratory tract.3 Even low-level exposure can significantly impact sensitive individuals, leading to increased bronchial reactivity in asthmatics, decreased lung function in patients with chronic obstructive pulmonary disease (COPD), and a heightened risk of respiratory infections, particularly in young children.3 Extremely high-dose exposure, such as might occur in a building fire, can result in severe outcomes like pulmonary edema and diffuse lung injury.3 Continued exposure to elevated NO2 levels can also contribute to the development of acute or chronic bronchitis.3 ASHRAE identifies NO2 as a potential cause of respiratory disease, underscoring its importance in IAQ considerations.2
Indoor NO2 levels in homes with gas stoves frequently surpass outdoor concentrations.3 Studies by LBNL have consistently shown that NO2 levels in indoor environments where gas appliances are used often approach or exceed ambient air quality standards.14 For example, in an experimental kitchen, NO2 concentrations reached as high as 2500 µg/m3 when there was no stove vent and low air exchange.14 Further research in energy-efficient homes revealed that NO2 levels in both kitchens and living rooms frequently exceeded the EPA's proposed one-hour ambient air quality standard of 470 µg/m3 (equivalent to 100 ppb) following typical gas stove use.14 A study of nine Northern California homes found that four of them had kitchen 1-hour NO2 concentrations exceeding the national ambient air quality standard (100 ppb), with elevated levels also observed throughout the home, including bedrooms.17 This demonstrates that homes with gas stoves are actively creating an indoor environment that disproportionately impacts sensitive individuals, particularly children, placing them at higher risk for respiratory illness and infection.
Carbon Monoxide (CO): The Silent, Deadly Gas
Carbon monoxide (CO) is a particularly insidious pollutant because it is an odorless, colorless, and toxic gas, making it impossible to detect without specialized alarms.4 It is a primary product of the incomplete combustion of natural gas.2 Key indoor sources from gas appliances include unvented gas space heaters, gas stoves, and back-drafting from other combustion appliances such as furnaces, gas water heaters, wood stoves, and fireplaces.3 The risk of CO emissions significantly increases with poorly adjusted or inadequately maintained combustion devices.4
The health effects of CO exposure vary widely based on the concentration, duration of exposure, and the individual's age and overall health.4 Acute effects are primarily due to the formation of carboxyhemoglobin in the blood, which severely inhibits the body's ability to absorb and transport oxygen.4 At low concentrations, CO can cause fatigue in healthy individuals and chest pain in those with pre-existing heart disease. Moderate concentrations may lead to symptoms such as angina, impaired vision, and reduced brain function. At higher concentrations, individuals may experience impaired vision and coordination, headaches, dizziness, confusion, nausea, and flu-like symptoms that typically resolve upon leaving the affected area. At very high concentrations, CO exposure is fatal.4 Given these severe risks, ASHRAE strongly recommends the installation of carbon monoxide alarms in all homes, regardless of the heating fuel type used.2
Typical CO levels in homes without combustion appliances generally range from 0.5 to 5 parts per million (ppm). In homes with properly adjusted gas stoves, levels are often between 5 and 15 ppm, but near poorly adjusted stoves, these levels can escalate to 30 ppm or higher.4 While an LBNL study in an energy-efficient house did not find CO levels exceeding the EPA one-hour standard (40 mg/m3) 14, it is important to acknowledge that the U.S. Consumer Product Safety Commission (CPSC) reports approximately 170 deaths annually from CO produced by non-automotive consumer products, including malfunctioning fuel-burning appliances.2 A critical architectural and engineering concern arises from the interaction of ventilation systems with the building envelope. High airflow range hoods, intended to improve IAQ, can inadvertently create negative pressure within a home, potentially causing other combustion appliances (like furnaces or water heaters) to backdraft, drawing harmful carbon monoxide into living areas.8 This highlights the complex, interconnected nature of building physics, where ventilation design must be carefully integrated with the overall airtightness of the building and the presence of other combustion appliances.
Particulate Matter (PM2.5 & Ultrafine Particles): Microscopic Threats
Particulate matter (PM) found indoors originates from both outdoor air and a variety of indoor activities.8 Key indoor sources include cooking, certain cleaning activities, and combustion processes such as burning candles, using fireplaces, unvented space heaters, kerosene heaters, and tobacco products.8 Gas appliances, particularly unvented ones, are significant sources of ultrafine particles (less than 100 nm in diameter) and respirable particulate matter (PM10 and PM2.5).2 Cooking activities, especially frying, broiling, and grilling, are major contributors to indoor PM2.5 emissions, with the rapid production of large quantities of PM when food is burned.8
The health effects of exposure to airborne particles, particularly fine particles (PM2.5) and ultrafine particles, have been recognized for millennia.13 PM2.5 is especially concerning because its minute size allows it to penetrate deeply into the respiratory system, leading to increased short- and long-term adverse health effects.13 Ultrafine particles have been specifically linked to oxidative damage to DNA and increased mortality.2 The aggregate harm to the population in the indoor environment, measured in Disability Adjusted Life Years (DALY), is overwhelmingly dominated by exposure to particulate matter, surpassing other contaminants by a factor of five.13 This makes PM the single most significant indoor air quality health burden. Furthermore, airborne pathogens, including SARS-CoV-2, are transmitted via respiratory aerosols that are predominantly fine particles.13
Despite the migration of outdoor pollution indoors, particles generated from indoor sources often constitute the majority of an individual's personal exposure.13 LBNL studies confirmed this, showing that natural gas cooking burner use led to very high 1-hour kitchen particle number (PN) concentrations (exceeding 2x10^5 cm-3-h) in all homes studied.17 While ventilation is important for overall IAQ, LBNL research explicitly states that PM2.5-related health burdens are not very sensitive to changes in ventilation rates, and that filtration is significantly more effective at controlling PM2.5 concentrations and their associated health effects.15 This finding is crucial for architects, as it highlights that while ventilation plays a role, filtration is the superior and necessary strategy for mitigating the predominant indoor health risk posed by particulate matter.
Volatile Organic Compounds (VOCs): Formaldehyde, Benzene, and Beyond
Volatile Organic Compounds (VOCs) are emitted as gases from a vast array of indoor products and materials, with their concentrations consistently found to be higher indoors—often 2 to 10 times higher—than outdoors.6 Gas appliances are identified as sources of formaldehyde.14 Beyond combustion, unburned natural gas itself contains hazardous air pollutants (HAPs), notably benzene, which is detected in a high percentage (99%) of residential natural gas samples.23 Benzene is also a known byproduct of combustion processes 2, and other common indoor sources include environmental tobacco smoke and automobile exhaust from attached garages.6
Exposure to VOCs can induce a range of immediate symptoms, including irritation of the eyes, nose, and throat, headaches, dizziness, loss of coordination, and nausea.5 More severe or long-term exposure can lead to damage to the liver, kidneys, and central nervous system.5 Critically, some organic chemicals are known to cause cancer in animals, and several are suspected or confirmed human carcinogens.5 Formaldehyde is particularly well-documented as a cause of sensory irritation and is identified as the primary risk driver for cancer health effects in studies of offices and schools.15 Benzene is unequivocally classified by the EPA as a Group A, known human carcinogen for all routes of exposure, with occupational exposure linked to an increased incidence of leukemia.7
A significant and often overlooked finding is that benzene is detected in 99% of unburned natural gas samples from residential stoves.23 Furthermore, leakage from gas stoves and ovens while they are not in use (i.e., when they are off) can result in indoor benzene concentrations that exceed health reference levels established by the California Office of Environmental Health Hazard Assessment (OEHHA). These concentrations can be comparable to those found in environmental tobacco smoke.23 Such exceedances are particularly likely when there are elevated leakage rates combined with low ventilation rates.23 This finding is particularly important because it means the carcinogenic risk from benzene is not limited to cooking times but is continuous, even when appliances are idle. This significantly strengthens the argument for addressing the source of the fuel itself, as ventilation alone is not highly effective in reducing airborne concentrations of semivolatile organic compounds (SVOCs), which are higher molecular weight VOCs that tend to reside mostly on indoor surfaces.16 This has broad implications for architectural specifications and policy regarding gas appliances.
The Unseen Byproduct with Health and Durability Consequences
Water vapor is a primary product of natural gas combustion.2 Unvented combustion appliances can produce a substantial amount of moisture, contributing significantly to the overall internal moisture load of a home.2 Other internal moisture sources include human respiration and perspiration, cooking, bathing, washing, plants, and pets.24
The presence of dampness in buildings, even in the absence of visible mold growth, has been consistently linked to adverse health outcomes, particularly respiratory problems.2 Mold growth, a common biological contaminant, thrives in high humidity environments, specifically when relative humidity is consistently above 50%.10 Mold is a known trigger for asthma symptoms and allergic reactions.10 A critical interplay exists between energy-efficient design and moisture management. Modern, tightly sealed building envelopes, while beneficial for energy efficiency by reducing sensible cooling loads, can inadvertently reduce the incidental dehumidification provided by cooling systems.24 This means that the moisture generated indoors by gas appliances and other activities is more likely to be trapped, leading to elevated indoor humidity levels if not properly managed. Elevated humidity, in turn, is a primary catalyst for mold growth, creating a feedback loop where energy-efficient design, if not coupled with deliberate moisture control and ventilation strategies, can inadvertently create conditions conducive to mold and associated health problems. This highlights the necessity of integrated design thinking that accounts for moisture balance.
Architectural Strategies for Mitigating Gas Appliance Health Risks
Prioritizing Source Control in Design
Effective indoor air quality management begins with source control—the elimination or reduction of pollutant emissions at their origin. This is often the most impactful strategy for safeguarding occupant health.
Appliance Selection: Embracing All-Electric and Electronic Ignitions
Source control is identified as the primary and most effective method for limiting indoor exposure to volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs).16 ASHRAE explicitly advises consumers who wish to reduce the risk of adverse health effects from combustion products to avoid using unvented appliances.2 When specifying gas cooking appliances, selecting models with electronic ignitions is recommended where possible.2 A profound understanding of the risks associated with gas appliances extends beyond their operational use. The discovery that unburned natural gas leaks from stoves, even when they are off, can continuously release carcinogenic benzene 23, provides a compelling health-based rationale for architects to advocate for and design all-electric homes. This moves beyond solely energy efficiency arguments to directly address a pervasive, continuous, and carcinogenic exposure that cannot be fully mitigated by ventilation alone, offering a significant health benefit to occupants.
Proper Appliance Installation and Maintenance Considerations
For any permanently mounted unvented combustion appliances, strict adherence to manufacturer installation instructions and local codes is paramount, with installation performed by a qualified professional.2 Regular, annual inspections by a qualified service technician are also strongly recommended to ensure proper function and minimize emissions.2 For example, poorly adjusted gas stoves can lead to significantly elevated carbon monoxide levels, potentially reaching 30 ppm or higher.4 The proper installation and ongoing maintenance are critical to preventing dangerous pollutant accumulation in the home.
Designing for Effective Ventilation
Ventilation is a cornerstone of good indoor air quality, essential for diluting and removing pollutants that cannot be entirely eliminated through source control.
The Critical Role of Ducted Range Hoods: Capture Efficiency and Airflow Requirements
Venting nitrogen dioxide (NO2) sources to the outdoors and installing a ducted exhaust fan over gas stoves are among the most effective measures to reduce exposure to combustion pollutants.3 Studies by LBNL demonstrate that operating a venting range hood can substantially reduce cooking burner pollutant concentrations, achieving reductions in the range of 80-95% for well-designed hoods.17 LBNL simulations specifically recommend a minimum capture efficiency of at least 70% for range hoods to avoid unacceptably high 1-hour average NO2 concentrations (100 ppb or higher) and at least 60% capture efficiency to avoid unacceptably high 24-hour average PM2.5 concentrations (25 µg/m3 or higher).18 These targets are particularly crucial for multi-family homes, which have smaller air volumes for pollutant dilution, leading to higher concentrations if not properly managed.18 Range hoods should be operated during cooking and for an additional 10-20 minutes afterward to ensure effective pollutant removal.8 In contrast, recirculating (non-venting) range hoods are largely ineffective for NO2 and CO2, offering only small net reductions, though they may achieve modest PM reductions (~30%).17 This highlights that architects must look beyond raw airflow numbers (CFM) and prioritize the design, geometry, and placement of the hood relative to the cooking surface and the overall kitchen layout to ensure effective pollutant capture, rather than just air movement.
Beyond the Kitchen: Whole-House Ventilation Strategies for Tighter Envelopes
While kitchen-specific ventilation is crucial, whole-house ventilation strategies are also necessary, especially in tighter building envelopes. Increased outdoor air ventilation can effectively reduce indoor concentrations of many VOCs.16 However, it is important to note that ventilation typically increases building energy use 22 and is not highly effective for reducing semivolatile organic compounds (SVOCs), which tend to adsorb onto indoor surfaces rather than remain airborne.16 ASHRAE recommends that when air-sealing measures are implemented in a building containing unvented appliances, ventilation should be reassessed and augmented if necessary to maintain adequate indoor air quality.2
Addressing Backdrafting Risks in High-Performance Homes
A critical design consideration for architects is the risk of backdrafting. High airflow range hoods, while effective at removing cooking pollutants, can create negative pressure within a tightly sealed home. This negative pressure can potentially draw harmful carbon monoxide from other combustion appliances (e.g., furnaces, water heaters, fireplaces) into the living space through their flues or chimneys.8 This complex interaction between powerful exhaust systems and the building envelope's airtightness necessitates careful planning. Architects must consult with qualified MEP engineers and other professionals during the design and installation phases to properly size and integrate ventilation systems, ensuring that backdrafting is prevented, potentially through the incorporation of make-up air systems.8
Table 2: Recommended Ventilation Strategies for Gas Appliance Pollutant Control
This table provides concrete, quantitative design targets for architects, translating scientific recommendations into actionable performance metrics. It offers specific guidance that can be incorporated into design specifications and discussions with mechanical engineers, helping to bridge the technical depth gap for architects.
Integrating Filtration for Enhanced IAQ
While ventilation plays a crucial role in diluting pollutants, filtration serves as a distinct and highly effective strategy for actively removing contaminants from the air.
The Role of High-Efficiency Filtration for Particulate Matter
LBNL research explicitly states that filtration is significantly more effective than ventilation at controlling PM2.5 concentrations and their associated health effects.15 This is a critical distinction, as it means architects cannot rely solely on increased ventilation to address all indoor air pollution problems, particularly for particulate matter, which constitutes the most significant indoor health burden. ASHRAE recommends MERV-13 or better filtration for reducing infectious aerosol exposure, a standard increasingly adopted as a new baseline in building codes and guidelines.13 Cost-benefit analyses consistently demonstrate that air cleaning for PM2.5 control is highly cost-effective, offering substantial health benefits.13 ASHRAE is actively working to incorporate requirements for controlling indoor particle concentrations into its standards for all building types and climatic conditions, further emphasizing the importance of this strategy.13 This highlights the necessity of integrating robust filtration systems as a complementary, rather than substitutable, strategy for comprehensive IAQ.
Limitations of Ventilation Alone for Certain Pollutants
It is critical for architects to understand that ventilation alone has inherent limitations in addressing the full spectrum of indoor air pollutants. While increased ventilation helps dilute many volatile organic compounds (VOCs), it is significantly less effective for semivolatile organic compounds (SVOCs), which primarily reside on indoor surfaces rather than remaining airborne.16 Moreover, as previously highlighted, PM2.5-related health burdens are not highly sensitive to changes in ventilation rates.15 This means architects must recognize that simply increasing airflow will not solve all indoor air pollution problems, particularly for persistent particulates and certain surface-bound VOCs. This understanding mandates the inclusion of high-efficiency filtration as a distinct, necessary layer of protection, especially in tightly built homes where internally generated particulates and surface-bound VOCs can accumulate.
Monitoring and Alarms: Essential Safeguards
Beyond proactive design, equipping homes with appropriate monitoring and alarm systems provides essential safeguards and empowers occupants to manage their indoor environment.
Mandatory Carbon Monoxide Alarms
The installation of carbon monoxide (CO) alarms is a non-negotiable safety measure, strongly recommended by ASHRAE for all homes, irrespective of the heating fuel type used.2 These alarms provide critical early warning for a colorless, odorless, and potentially fatal gas, serving as a last line of defense against acute CO poisoning.
Considering Advanced IAQ Monitors for Comprehensive Protection
Beyond mandatory safety alarms, architects should consider integrating advanced indoor air quality monitors into their designs. While consumer IAQ monitors may not always detect ultrafine particles, they have proven useful in alerting occupants to significant PM2.5 sources, such as cooking events.19 These monitors can provide real-time data, empowering occupants to make informed decisions about ventilation and source control, and offering a proactive approach to maintaining healthy indoor environments. This approach moves beyond mere code compliance to a continuous, performance-based assessment of IAQ, enhancing the building's value and occupant well-being.
Collaboration with MEP Engineers and Qualified Professionals
The successful implementation of healthy building strategies, particularly concerning gas appliance emissions, necessitates close and early collaboration between architects, mechanical, electrical, and plumbing (MEP) engineers, and other qualified building professionals. Professional installation and annual maintenance by certified technicians are crucial for the safe and efficient operation of gas appliances.2 Furthermore, the selection and installation of high-airflow range hoods, essential for pollutant removal, requires expert consultation to prevent the dangerous phenomenon of backdrafting, which can draw carbon monoxide into living spaces.8 ASHRAE advocates for installer certification to ensure competence in these critical areas.2 The complex interactions between the building envelope, mechanical systems, and pollutant pathways underscore that architects cannot address indoor air quality in isolation. While architects lead the overall design, their ability to foster and integrate expert collaboration is paramount to achieving truly healthy indoor environments.
Building a Healthier Future
This report has illuminated the significant, often unseen, health impacts of fossil fuel combustion gas appliances in homes. The analysis has detailed how these appliances contribute to a complex array of indoor air pollutants, including nitrogen dioxide (NO2) and particulate matter (PM2.5), which exacerbate respiratory illnesses like asthma. Furthermore, the report highlighted the carcinogenic risks posed by volatile organic compounds such as benzene, notably from the continuous leakage of unburned natural gas, even when appliances are off. The critical role of moisture management was also underscored, revealing how the moisture byproduct of combustion, combined with tighter building envelopes, can create conditions conducive to mold growth and associated health problems.
Architects are uniquely positioned to mitigate these risks through informed design choices that prioritize occupant health. This includes advocating for and specifying source control measures, such as the transition to all-electric homes, thereby eliminating the continuous release of hazardous air pollutants. It also involves implementing robust ducted ventilation systems with high capture efficiency for kitchen exhaust, integrating advanced filtration for particulate matter throughout the home, and specifying essential monitoring and alarm systems to provide continuous oversight of indoor air quality.
By understanding the intricate dynamics of indoor air quality and the specific hazards associated with gas appliances, architects can move beyond conventional design to become leaders in creating truly healthy, high-performance homes. This leadership demands a commitment to continuous learning, fostering interdisciplinary collaboration with MEP engineers and building science specialists, and adopting a proactive approach to safeguarding occupant well-being. The future of residential design necessitates buildings that are not only energy-efficient and aesthetically pleasing but are fundamentally engineered and designed for optimal human health.
Works cited
Gas Stoves: Health and Air Quality Impacts and Solutions - RMI, accessed May 22, 2025, https://rmi.org/insight/gas-stoves-pollution-health
UNVENTED COMBUSTION DEVICES AND INDOOR AIR QUALITY - ASHRAE, accessed May 22, 2025, https://www.ashrae.org/file%20library/about/position%20documents/unvented-combustion-devices-and-iaq-pd-6.28.2023.pdf
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Ductwork for a Retrofit ERV
We have had a number of customers ask for energy recovery ventilation (ERV) in their existing homes. Can we use the existing furnace ductwork? If not, what size and type of ducts can be used?
By Miguel Walker, originally published in The Journal of Light Construction, March 9, 2023
Q: We have had a number of customers ask for energy recovery ventilation (ERV) in their existing homes. Can we use the existing furnace ductwork? If not, what size and type of ducts can be used?
A: M. Walker of Positive Energy, an MEP engineering services firm based in Austin, Texas, responds: The short answer is yes, you can use the ductwork for the furnace, but you may not want to. The full answer has a number of annoying and important caveats and considerations. Let’s take a moment to remember the goal: Adding ventilation to a building is all about introducing fresh air for improved indoor air quality. The strategies you use to implement such a system should align with the goal of providing healthy indoor air in the building for people to breathe. If you can manage to retrofit this into an existing system for little cost, you’re very lucky.
Photo: David TrelevenMost ERVs like this one (upper piece of equipment) have ports for standard 4- or 5-inch ductwork. Note that this system includes the indoor coil (lower piece) for a dedicated dehumidifier, which will be needed in most locations to address latent loads.
It is also crucial that the hygrothermal gradient—meaning how hot/humid it is on either side of the ERV core—be considered in your approach. An ERV will not effectively exchange enthalpy if there is not a dry/cool air mass on one side of the core media. In humid climates, this means you’ll need a dedicated dehumidifier to handle latent load, especially in shoulder seasons where there isn’t much dry-bulb load to deal with. Beyond the obvious implications for poor indoor air quality and material durability, I’ll explain why else this is important later.
Best Overall Strategy
The best distribution strategy for a balanced ventilation system with enthalpy (heat and moisture) recovery (such as an ERV) is an independently dedicated duct system that meets the equipment manufacturer’s installation requirements. This setup allows you to leverage efficiencies of the ventilation device’s designed fan performance, ensure ventilation is delivered to every room, and control locations from which your system returns. If you’re trying to be careful about how much new ductwork you’re adding, focus on getting fresh air supply in bedrooms and living spaces (den, kitchen, and such).
How Big Are Ventilation Ducts
As far as duct sizing, generally ERV/HRV collars are designed for standard 5- or 4-inch ducts. You may find it difficult to reduce the size from these diameters for several reasons (supply-house inventory, product availability for the American market, among others). You could technically reduce the standard-diameter ERV/HRV ducts down to 3 inches and run those in a wall cavity, but you have to be careful not to undersize the ductwork. There are ERV/HRV manufacturers who make flexible ductwork at this smaller scale and have some pretty slick multiport terminal devices. If you’re not a mechanical engineer with calculations fresh in hand, I recommend leaning on the manufacturer for support.
When New Ducts Won't Work, Use Existing Ones
New ductwork is not always an option in retrofit situations, and it is possible to leverage existing ductwork as long as you’re careful about how new equipment will impact the overall system performance. Positive Energy’s general approach to retroactively adding an ERV/HRV into an existing system is to supply ventilation air into the air handler’s return plenum (a caveat is that you’ll need to move a return-air temperature sensor upstream).
Generally, ERV/HRVs aren’t moving a tremendous volume of air (50 to 100 cfm is common), so adding this volume into an HVAC system’s return doesn’t necessarily cause fan-to-fan issues or significantly increase system pressure. However, to deliver that air to the existing diffusers, you’ll need to rely on the air handler’s fan, which is much larger than an ERV/HRV fan and will use more energy even when you don’t need heating/cooling. There is plenty of nuance we could get into regarding operational strategies in this configuration that can greatly impact energy use, but that’s beyond the scope of this article. You’ll also need to figure out where you’re going to get return air from for the ventilation system and ideally return from foul-smelling areas like bathrooms and the kitchen. This is where the challenges of coordinating with other trades really kick in.
Remember the dehumidifier consideration from earlier? If you need a dedicated dehumidifier (and in most places, you probably do) and are trying to leverage existing ductwork for both the humidity control and the ventilation, it is crucial to understand the pressure that will be created in the system. Adding too much pressure to a duct system will prevent it from delivering the needed airflows to the terminal devices (registers) and can cause serious comfort issues, among other performance deficiencies.
There are other potential cost-saving strategies. Leaving existing in-line bathroom fans in place may be a convenient way to repurpose existing opportunities for an ERV/HRV return where foul-smelling and high-humidity events regularly occur (that’s the stuff we want to get rid of the quickest) if you can intercept that exhaust ductwork. Obviously, this is not an easy thing to do if you’re up against spatial constraints with inaccessible existing ductwork. If you’re clever about it, decommissioned flues may also be repurposed for ERV/HRV exhaust out of the building, but please be discerning about their condition before doing so. Again, refer to the ERV/HRV manufacturer specifications for specific details.
It's About Health
Ventilation is a necessary function of buildings and can make a tremendous impact on health outcomes in the spaces where we spend time. Ventilation is crucial to good indoor air quality. When buildings are constructed with more airtight assemblies, we need to reliably introduce outdoor air—filtered and within a reasonable temperature and humidity range—via mechanical means, and we want to exhaust old, fouled air. Compared with the old method of random ventilation or exhaust-only, this strategy adds cost and requires architectural accommodation. It’s unfortunate, but most often, it’ll be costly and inconvenient to retrofit buildings with existing equipment that wasn’t originally designed to meet our new goals. We can’t walk into the next 50 years of construction with the same budget expectations and practices that existed in the previous half-century. As our knowledge of building performance evolves with research, especially health research, so too should our approach to every aspect of creating new living space.
What Have We Learned About Air Conditioning & The Coronavirus
In an effort to broadly provide resources to our clientele and audience, we’ve written articles on the topics of health precautions for construction job sites and designing for healthy environments while reducing pathogen spread. We’ve released podcast episodes on the impact of ventilation and filtration on virus transmission. But now it’s time to talk about a serious elephant in the room as it pertains to coronavirus spread - air conditioning.
by Kristof Irwin and M. Walker
There’s an unprecedented unifying force in the world today and it’s the SARS-CoV-2 pandemic. If you’ve been paying attention to the news at all lately, you’ve likely been inundated with articles, news and facts (both real and alternative) about COVID. Young or old, rich or poor, we are all in this together. The virus has intersected with everyone’s daily life in myriad, unexpected ways and continues to do so. Never before in the history of our company have we heard from so many existing clients, potential new clients, and podcast listeners telling their pandemic stories, expressing concerns about their indoor air quality, and asking what they can do to create healthier home and office environments.
In an effort to broadly provide resources to our clientele and audience, we’ve written articles on the topics of health precautions for construction job sites and designing for healthy environments while reducing pathogen spread. We’ve released podcast episodes on the impact of ventilation and filtration on virus transmission. But now it’s time to talk about a serious elephant in the room as it pertains to coronavirus spread - air conditioning.
Those two words appear together so commonly that we scarcely think about them. Air is a relatively simple concept, but conditioning is worth exploring. Conditioning means to condition something for a purpose. You condition leather to make shoes, you condition limestone to make Portland cement. When it comes to air, you condition it for human occupancy. Conditioning is far involved more than simply cooling, it includes humidity control, ventilation, and filtration.
Setting The Stage - The Starbucks Case
Recently, we came across an eye opening case study in South Korea that took place at the beginning of August (note the link is to a Korean site, but Google Chrome’s automatic translation tool works quite well and is, as far as we can tell, accurate). The situation presented in the Starbucks case illustrates the interdependent relationship of air conditioning systems and protective masks with the spread and prevention of the SARS-CoV-2 virus. The basics of the situation are straight forward enough - 27 people in a 2 level Starbucks in Paju, South Korea tested positive for COVID-19 after 1 unknowingly infected patient visited the store and stayed for a couple of hours. However, each of the workers on shift during this time all tested negative.
For context, South Korea is not under stay-at-home-orders, as much of the US and other countries are, so cafes, restaurants, and stores are all open, and people can eat or drink inside. Like much of the southern United States, South Korea is also steaming hot and humid in the summers, so air conditioners are always on during this time of the year. Also like the U.S., it is common in South Korea to see minimal levels of effective filtration and ventilation in public spaces (although the mileage varies greatly from place-to-place and building-to-building).
Two things immediately stood out in the story that will lead us to our discussion on air conditioning:
The employees, who tested negative, were wearing protective KF94* masks for the duration of the infected person’s stay at the coffee shop, while the infected visitors either did not wear them or removed them at some point.
There are notable differences in the space conditioning equipment between the 1st floor, where the employees were working, and the 2nd floor, where the majority of infections occurred.
*Quick side note to clarify a term: if you’re unfamiliar with KF94 masks, or Korea Filter masks, don’t worry. Essentially, they’re a Korean made version of their American counterpart, the N95 mask, with a few minor differences in performance and testing protocols. They look similar, and they filter a nearly identical percentage of particles—95% versus 94%. See the chart below from 3M for more specifics regarding the differences between these two types of mask or check out this link to learn more about other masks and their function.
So based on the data we have at hand in the Starbucks case, backed by the growing body of evidence suggesting masks’ effectiveness at preventing transmission, we can reasonably infer that the masks were indeed effective in protecting the employees from infection. But what exactly happened with the rest of the store? How is it that 1 infected individual was able to transmit the virus to 27 other people in just a 2 hour period? Let’s take a look at some of the highlights from the Insight article (translated from Korean, of course) and use their reporting as a launching pad to look more critically at the science behind virus spread inside buildings:
“A woman in her 30s stayed at the store on the second floor [of the starbucks] for about two hours…”
“On the second floor, where six ceiling air conditioners were distributed… infection was bound to spread quickly.
Surprisingly, the four employees who worked inside Starbucks were fine. [They] went to the second floor from time to time, but they wore KF94 masks throughout the working hours.”
The story was eventually picked up by Bloomberg, who reported that the incident illustrates a lot about both the effectiveness of masks and the role of air conditioning in the spread of the disease.
“The Starbucks case is one of “the most important opportunities to study risk factors among a more or less controlled cohort of people,” said Arnold Bosman, director at Transmissible BV, a Netherlands-based developer of training materials for outbreak control. “This Starbucks event will be a very valuable training exercise for future generations of epidemiologists.”
Indeed, this scenario is an important case study for researchers across the scientific community to examine how pollutants and pathogens can be spread in indoor environments. And as far as Positive Energy is interested in this unfortunate case study, we want to examine the action of the building systems and their contribution to poor health outcomes. Like doctors, professional engineers need to at minimum “do no harm,” although this minimum is not a sufficient standard of care given how easily it can bias expectations toward cost-only-optimized-solutions. When we identify what doesn’t work, it informs and refines our understanding of design strategies to help keep our clients comfortable, safe and healthy indoors.
What Does The Starbucks Case Teach Us?
The Starbucks case seems to affirm a growing body of scientific research on the effectiveness of masks at preventing transmission, but the scenario also begs our core question - how does air conditioning impact transmission? The answer is related to the reasons why masks are beneficial. Both are operating to either move or prevent the movement of air. In the case of air conditioning systems the air they move and mix is a potential vector for spreading SARS-CoV-2 around a building and dispersing it into the volume of indoor air. Masks prevent this potentially virus-mixed air from entering our lungs. Again, air is the common link. A solution of solid or liquid particles suspended in air is an air-solution, or aero-solution, now commonly referred to as an aerosol. The important aspect of particulate or liquid matter in an aerosol is that it is a solution, this means the solid or liquid does not readily fall out, it stays suspended in the air for a long time, hours to weeks. The fact that the virus can be carried via aerosolization shapes how we understand and deal with it.
A recent NYT Opinion Column by Dr. Linsey C. Marr, an engineering professor at Virginia Tech, articulates this well:
“As we cough and sneeze, talk or just breathe, we naturally release droplets (small particles of fluid) and aerosols (smaller particles of fluid) into the air. In a peer-reviewed study published in Scientific Reports on Wednesday, researchers at the University of Nebraska Medical Center found that aerosols collected in the hospital rooms of Covid-19 patients contained the coronavirus. This confirms the results of a study from late May (not peer-reviewed) in which Covid-19 patients were found to release SARS-CoV-2 simply by exhaling — without coughing or even talking. The authors of that study said the finding implied that airborne transmission “plays a major role” in spreading the virus.”
Given that the virus is airborne, it makes sense to employ our knowledge of the behavior and flow of air in indoor spaces (or better yet, use modeling tools to do so), but that is not as simple as it may seem. From a recent study on droplet behavior:
“The dynamics of virus transmission is not well understood, with one challenge being the complicated fluid and flow characteristics involved in the fate and transport of virus, including source dynamics (e.g., exhale velocity and temperature, droplet sizes, virus load, and droplet–virus correlations), ambient conditions (e.g., mean and turbulent flows, temperature, and humidity), and virus dynamics (e.g., virus viability and infectious rate) (e.g., Lindsley et al., 2015; Feng et al., 2020; Dbouk and Drikakis, 2020a; and Mittal et al., 2020). Understanding the fundamental fluid dynamics of expiratory virus-laden droplets is critical to the prediction of the transport and fate of droplets and associated potential threats of infectious disease transmission and will provide quantitative guidance for making a public health policy for disease mitigation, e.g., decisions on social distancing and face covering in various indoor and outdoor environments (Dbouk and Drikakis, 2020b; Verma et al., 2020).”
So, given these facts, just how dangerous can air conditioners really be? As you might expect, it highly depends on how well designed and installed those systems are. Air conditioners are not themselves inherently problematic, but left to the devices of traditional industry practices, they can be disastrous for human health.
We can safely assume that many buildings are not employing robust filtration or ventilation strategies, which are both known to be effective in mitigating airborne particulates on which the SARS-CoV-2 virus is carried. If you have not yet listened to our recent podcast episode on this topic with Dr. Ty Newell, PhD, P.E., it is a true education on the matter. Conditioned spaces create unique hygrothermal conditions and the behavior of pollutants in a given space is largely determined by its conditioning strategies and how well they were implemented. This is important to note primarily because most conditioned spaces have systems that are insufficient to protect human health and do no harm.
In fact, the first COVID-19 patient in Wuhan spread it to others via an air conditioning unit even though they were more than 6 feet away. In a published study of the patient one scenario, a swab sample from the air conditioning system near the patient tested negative, indicating that the virus droplets indeed were not filtered and likely circulating around the restaurant via the air conditioner’s blower. We’re inferring here that the COVID laden particles were being circulated by, not through, the air conditioning system.
Dr. Marr again:
“Consider the case of a restaurant in Guangzhou, southern China, at the beginning of the year, in which one diner infected with SARS-CoV-2 at one table spread the virus to a total of nine people seated at their table and two other tables.
Yuguo Li, a professor of engineering at the University of Hong Kong, and colleagues analyzed video footage from the restaurant and in a preprint (not peer reviewed) published in April found no evidence of close contact between the diners.”
“Droplets can’t account for transmission in this case, at least not among the people at the tables other than the infected person’s: The droplets would have fallen to the floor before reaching those tables.”
But the three tables were in a poorly ventilated section of the restaurant, and an air conditioning unit pushed air across them. Notably, too, no staff member and none of the other diners in the restaurant — including at two tables just beyond the air conditioner’s airstream — became infected.”
All evidence considered, the Starbucks case in South Korea is strikingly similar to the case of patient one in Wuhan. Air conditioned spaces with insufficient strategies employed for human health can and do cause serious health issues.
What Could Have Prevented These Infections?
To state the obvious, staying home or utilizing a curbside pickup system would have certainly prevented this particular infection cluster, but since many people are opting to continue some degree of public life as it was before the pandemic, let’s look at the other strategies available in hopes that more buildings can “bake in” protective measures without relying on occupant behavior.
Profs. Linsey Marr (Virginia Tech), Shelly Miller (CU Boulder), Kimberly Prather (UC-San Diego), Charles Haas (Drexel University), William Bahnfleth (Penn State), Richard Corsi (Portland State), and Jose-Luis Jimenez (CU Boulder) have written a fantastic and exhaustive FAQ document with lots of really great information. We’ve simplified a few salient points for those who aren’t able to dive in to that depth just yet.
Protective Masks
Wearing protective masks is a demonstrably effective strategy as evidenced by the Starbucks employees who did not become infected. Researchers have, for quite some time, known that masks can prevent people from spreading airway germs to others. These findings have driven much of the conversation around masks during the coronavirus pandemic and have been the catalyst for further research. As cases have continued to rise across the world (and especially here in the US), experts are pointing to a growing body of evidence suggesting that masks also protect the people wearing them, lessening the severity of symptoms, or in some instances, staving off infection entirely. This a growing body of research spans disciplines of virology, epidemiology, and ecology and the results so far suggest that universal masking not only protects others from a potentially infected individual, but also protects the mask wearer. The mechanism of protection is the reduction of the “inoculum” or dose of the virus for the mask wearer, leading to more mild and asymptomatic infection manifestations. Ideas about the importance of viral dose in the development of various diseases have been studied since the 1930s and what we have learned has contributed to the development of strategies to protect us against other airborne pollutants as well.
With regard to the SARS-CoV-2 virus, there is a notable new paper out on the effectiveness of mask wearing. Dr. Monica Gandhi, an infectious disease physician at the University of California, San Francisco wrote in a recent article:
“As governments and workplaces began to recommend or mandate mask-wearing, my colleagues and I noticed an interesting trend. In places where most people wore masks, those who did get infected seemed dramatically less likely to get severely ill compared to places with less mask-wearing.
It seems people get less sick if they wear a mask.
When you wear a mask – even a cloth mask – you typically are exposed to a lower dose of the coronavirus than if you didn’t. Both recent experiments in animal models using coronavirus and nearly a hundred years of viral research show that lower viral doses usually means less severe disease.
No mask is perfect, and wearing one might not prevent you from getting infected. But it might be the difference between a case of Covid-19 that sends you to the hospital and a case so mild you don’t even realize you’re infected.”
There you have it. Protective masks are a simple, relatively straightforward and inexpensive strategy to protect yourself and others from viral transmission.
Humidity Control
The impact of humidity on human comfort and health is important to understand and important to include in mechanical system designs. Humans and viruses prefer different indoor temperatures and humidities to thrive. Keeping indoor spaces in the Goldilocks zone of 40-60% relative humidity is an effective way to mitigate the spread of viruses like COVID. Our bodies natural defenses, our cilia and mucous tissues air impaired when the air gets too dry. Too wet, and the resultant microecology of damp buildings creates an ecosystem for a host of microbes, including fungi, bacteria and viruses impact the indoor microbiome in ways that negatively impact our health.
There are new approaches to modeling airborne droplet behaviors that illustrate the expelled droplets that carry the SARS-CoV-2 virus are sensitive to environmental conditions, including temperature, humidity, and ambient flows. Since these droplets play a key role in viral and other pollutant spread, we should have a keen sensitivity to controlling humidity in indoor environments. Further convincing evidence suggests this modeling strategy’s accuracy as noted in another study:
“At a higher humidity, the droplets grow faster, fall to the ground earlier and can be inhaled less by healthy people. A humidity level of at least 40 percent in public buildings and local transport would therefore not only reduce the effects of COVID-19, but also of other viral diseases such as seasonal flu.”
There is, of course, nuance here (this is a tricky set of topics). Take into account Stephanie H. Taylor MD M Architecture, CIC and her work in creating sufficient levels of humidity to support healthy immune function. Generally speaking, viruses thrive in dry conditions because they aerosolize and thus stay in the air longer. It’s also such that when your mucus tissues dry out, the cilia (which protect against viruses and other pollutants) don’t work like they should; the microbiome on the surfaces of your muco-cilia system don’t produce the right recipe to fight viruses.
“When our mucous lining becomes thick effective,particle capture is reduced. Particle capture becomes ineffective at as little as a 6% increase in mucous viscosity. Cystic fibrosis patients experience more infections because infectious particles settle and macrophages and dendritic cells don’t secrete needed proteins.
“Low ambient humidity impairs barrier function and innate resistance against influenza infection.” Akiko Iwasaki study found that the mammalian immune system is impaired at 10%-20% RH
The comfort zone is 40%-60%. Staying within the comfort zone is the goal. Staying within the comfort zone reduces infectivity.”
But in the case of SARS-CoV-2, and to add even more complexity to the topic of humidity and viral spread, its’ worth noting that the virus in question seems to behave a bit differently than its counterparts. This was recently described by Lew Harriman on an episode of IAQ Radio, in which he discussed the new ASHRAE document “Damp Buildings, Human Health and HVAC Design”. Harriman reminded listeners that, while Dr. Taylor’s findings are true, the SARS-CoV-2 virus is actually able to remain in the air for hours at a time at 50%RH. He also noted that the level of humidity control really depends on the building typology; grocery stores have different usage patterns than a home, for example.
So while studies of other viron can and do provide meaningful insights to reduce transmission in general terms, it is important to understand the specifics of the viral behavior in question before recommending or adopting a strategy. And, as with all science, the research body grows and our understanding will change. Remember that nothing is final, but in the applied science profession, we do our best to recommend solutions that help people based on the latest peer-reviewed research.
Ventilation
We’ve mentioned this strategy previously in this article and other articles we’ve written and podcast episodes we’ve recorded and we cannot overstate the importance of sufficient ventilation. Researchers, such as Jeffrey Siegel, are taking this message mainstream. The NPR segment, Marketplace, recently aired an interview with Siegel about the state of ventilation in buildings and how it’s negatively impacting virus transmission indoors.
From that interview:
“Molly Wood: It is my understanding that a lot of existing [heating, ventilation and air conditioning] systems, particularly in commercial buildings, do recirculate a lot of air in order to keep either cooled or heated air in the system. That was for efficiency purposes?
Siegel: That’s absolutely correct.
Wood: So in hindsight, was that a terrible mistake?
Siegel: No, absolutely not. We have a climate crisis. Energy use associated with buildings is a very big part of our energy footprint. Conditioning that air is one of the big users of energy within a building. So it’s important that we do it in an energy-efficient manner. I think that the bigger problem is that we have to be much more cognizant of how we’re managing ventilation. I think COVID-19 adds some variables to how we might manage ventilation. But in general, I think that we have the tools to do it. It might take some investment and so on, [but] we just have to be a little bit more proactive and engaged in how we manage the ventilation in our systems.”
This begs a common question we get from practitioners across the AEC industry - if we’re going through all the effort to design and build energy efficient buildings, but we’re also being told to ventilate, how do we reconcile those outcomes? And the truth is that it takes some careful consideration and calculation, but that is exactly the role of a good mechanical designer who has a sympathetic understanding of enclosures, energy performance, and human health. With the right framework, communication flows, and process, multiple simultaneous positive outcomes (energy efficiency, healthy air, budget sensible approach) are achievable.
If you were wondering about the ventilation of the building in the Starbucks case, Starbucks was specifically asked about their ventilation, and noted that "... the windows were opened for more than 10 minutes twice a day to ventilate," but that most of the windows were fixed glass and the only operable windows opened a narrow width of 30cm. There was no functional or known mechanical ventilation strategy.
When we spoke with Ty Newell recently about the role of ventilation in virus prevention, he walked us through a graph (see below) that was presented in a webinar he’d given (based on his research) in early August that may surprise you. Providing sufficient fresh air in indoor spaces is a clearly effective strategy in preventing virus transmission.
From Dr. Newell’s recent paper, Killing Ourselves With Comfort:
“Reduction of disease transmission within buildings and homes requires increased fresh air flow rates (at least doubling to 40cfm per person) and improved air filtration (to at least MERV11 filtration). Carbon dioxide concentration monitoring of every indoor building space is the key to reducing indoor virus transmission rates. Carbon dioxide concentration is a direct measure of human respiration rates, and therefore, virus concentration in the indoor environment. Maintaining indoor carbon dioxide concentrations below 800ppm, equivalent to doubling today’s inadequate, odor-based ventilation rates, will reduce disease transmission rates below the limit required for decay of Covid-19 transmission.”
But of course, like all subjects, there is complexity to consider in some situations. The unhealthy air from raging fires in California can actually make people more susceptible to COVID-19 as their lungs and immune systems can become overtaxed with the presence of toxic particulates via smoke inhalation. So robust filtration comes into focus as a crucial strategy for good indoor air quality.
Filtration
As we have pointed out in previous articles, ASHRAE suggests using filters with a minimum MERV-13 rating. Condensing significantly, MERV ratings are based on a filter’s performance/ability to filter out particles between 0.3 and 10 microns. SARS-CoV-2 can be found in respiratory droplets or attached to other pollutants in this size range; the higher the MERV number, the higher the probability that the filter will remove these droplets. However, the solution to all problems is not to install a higher MERV rated filter to a building’s central air conditioning system, filters are part of a system and as such the parameters of the rest of the system can aid or impair a filter’s ability to capture pollutants (which are substances that are harmful to human health). The filtration system can’t leak air or let air bypass the filter and find another path to the conditioned space. Filtration efficacy is also dependent on the type of filtration media, it’s electrostatic properties and the velocity (speed and direction) of the particles as the approach the filter.
An interesting quirk of the physics of filtration is the very smallest particles are actually easier to filter out than the 0.3 micron ones. The smallest particles get pushed toward filter fibers because of their collisions with gas molecules in the air. We recorded a fascinating, but relatively slow podcast episode on the wild world of filtration some years ago that is worth your consideration. Even Vox is getting in on the air quality conversation, with a recent article about the effectiveness of air filtration and virus transmission prevention. These ideas are not only on the radar of scientists anymore, but major media outlets.
Remember that simply replacing a non-HEPA with a HEPA filter in existing equipment may worsen the problem. Make sure your system can accommodate the air flow needs of a HEPA filter. If your system can’t, you can explore a more decentralized approach through portable room air cleaners instead. Take a look at The Wirecutter’s recent review of portable room air cleaners for a pretty comprehensive list of consumer grade pieces of equipment you can buy online today.
In summary, we were already in the midst of a revolution of understanding in the field of IAQ when the SARS-CoV-2 virus abruptly entered our lives and brought the field more sharply into focus. As is evidenced by the Starbucks case, studying the impact of HVAC systems on human health, especially during a pandemic, is crucial to protect us against future outbreaks. With the data gathered from the diligent research currently taking place, we will continue to understand a more complete picture of how we can use indoor air quality as a public health tool that’s “baked in” to our society’s buildings. We have a lot of work to do, a lot to learn and understand, but we have the tools, the data and the motivation like never before. Pandemics don’t just work themselves out - they end when smart people take good science, communicate it effectively to the public, and we work together to take care of one another.
On The Horizon - Emergent Knowledge
The following are some examples of topics in the emerging research field of indoor air quality.
Better sensors and analytic tools - NGS next generation sequencing equipment.
New data streams (IAQ data) - PTR-ToF-MS (proton transfer reaction, time of flight, mass spectrometers).
Rapid IT development - we can handle big data sets and find the needles in the haystacks.
Goal to personalize Healthcare - human genome unlocks microbial genomes as well.
Metagenomics is the study of genetic material recovered directly from environmental samples. The broad field may also be referred to as environmental genomics, ecogenomics or community genomics.
Epigenetics focuses on processes that regulate how and when certain genes are turned on and turned off, while epigenomics pertains to analysis of epigenetic changes across many genes in a cell or entire organism. ... The epigenome can mark DNA in two ways, both of which play a role in turning genes off or on.
Metabolomics is the large-scale study of small molecules , commonly known as metabolites, within cells, biofluids, tissues or organisms. Collectively, these small molecules and their interactions within a biological system are known as the metabolome.
Proteomics is the large-scale study of proteomes. A proteome is a set of proteins produced in an organism, system, or biological context.
Glycomics is the comprehensive study of glycomes (the entire complement of sugars, whether free or present in more complex molecules of an organism), including genetic, physiologic, pathologic, and other aspects.
State of the Art HVAC: Five keys to flawless space conditioning.
By Kristof Irwin, originally published in The Journal of Light Construction, July 10, 2019
In an ideal building science based world, the Perfect Wall concept would be matched and complemented by a Flawless HVAC concept.
Just as there are fundamental physics based principles supporting the Perfect Wall, the same applies to Flawless HVAC, but, alas, they are far less widely known, understood, or put into practice. Many know to “keep the outside out, and the inside in” but what exactly is the “inside” we keep “in”? The “inside” is a volume of air that we immerse ourselves and our loved ones in. We live most of our lives immersed in fishbowl of air of our own making. The qualities of this air are readily controllable and impact our health, comfort and well-being. Alas, being invisible, air gets less attention but is no less important to understand or do well.
The Perfect Wall has Rain, Air, Vapor Thermal control functions. Flawless HVAC has Capacity, Distribution, Ventilation, Filtration and Dehumidification (in humid climates) control functions. More specifically, the basics are: (1) variable capacity heat pump equipment (also known as Variable Refrigerant Flow, or VRF), (2) rigid metal ductwork, (3) continuous balanced tempered ventilation air, (4) effective particulate capture, and (5) dedicated dehumidification.
I’m the principal of Positive Energy, a full-service building science consulting firm based in Austin, Texas. We have an amazing team and we know that both our technical skills and our ability to evolve the systems and processes that deliver buildings to society are important. Among the services we provide to architects and builders are heating and cooling system design, building pressure testing, duct pressure testing, and duct flow balancing and verification. We have the good fortune to serve a high-end custom home market where clients are willing and able to spend the money to get an hvac system the quality of which matches the quality of the rest of the building. Working in the residential space we avoid much of the split incentive situation that plagues the developer world, where the person making the decisions is viewing primarily through an economic lens. Accordingly, we don’t cut corners with low quality equipment or ductwork. In an industry where the lowest common denominator often controls, we are focused on providing top quality solutions for our clients. In this story, I’ll talk about the principles and practices that guide our designs for state-of-the-art hvac systems.
Five Rules for a Healthy Building
As building scientists, we recognize that the house is a system. What do these buildings do? They take inputs of electricity, water, gas, and data, and they output human beings: healthy, functional members of society. Our philosophy is that we design to optimize that human output. Our motto is, “Design Around People, a Good Building Follows.”
There are five principles to creating a healthy indoor environment for the space where we spend 90% of our lives:
start with a good enclosure
minimize indoor emissions
keep it dry
ventilate
filter.
The first two items on the list aren’t part of the hvac system (although technically, the enclosure provides the connection between the supply and the return air, and so in a functional sense could be considered part of the mechanical system). The good enclosure is the builder’s responsibility, and minimizing indoor emissions is in large part the responsibility of the homeowners and building occupants. But keeping the building dry, ventilating, and filtering the air are part of hvac system design.
You’ll notice I didn’t mention heating and cooling. Those are important for comfort, but they’re not related to the top priority: the health of the humans living in the space. (Yes, of course, there are climates and weather events where temperature control is a life-safety matter. My point is simply that much of the time, heating and cooling is not a health matter.) But while every hvac system does heating and cooling, it’s shocking how many systems don’t address the vital health priority of supplying dry, fresh, filtered air.
Heating and Cooling Equipment
These days, we prefer to specify VRF equipment, which represents the future of the hvac industry. VRF stands for Variable Refrigerant Flow, and modern VRF equipment offers advantages in at least three areas: efficiency, occupant comfort, and zoning capability.
In the old days, air conditioner or heat pump compressors had two modes of delivering power. Either full on, or off. More recently, dual-stage and unloading compressors have come into the market that add a second option, at either 50% or 65% of full capacity. VRF is a generation ahead of that dual-stage equipment. What VRF supplies is the ability to continuously vary the capacity of the machine. Quick reminder that capacity (power) is a rate, not an amount (energy). The goal is to vary the rate of heating or cooling to match the rate of heat leaking out or in through the enclosure.
I use a car analogy to explain the difference. Suppose you hop into your truck to go somewhere, and the rules are, you have to floor the accelerator all the time, and you control the speed of the truck by turning the ignition key on and off. That’s standard single-stage equipment. With VRF, you now have a gas pedal: You can smoothly vary the power output of the engine depending on how fast you need the vehicle to go. A four-ton VRF compressor like the Mitsubishi City Multi can smoothly vary its power all the way from 48,000 Btu/hr down to 15% of that, or anywhere in between.
This capability in the VRF equipment provides the ability to efficiently manage “part-load” conditions, when standard equipment suffers from the problem of over-sizing. ACCA Manual J is the industry standard manual for sizing hvac equipment. Manual J is often referred to as a load calculation. A word is actually missing there: It’s a peak load calculation. The Manual J load is representative of the peak heating and cooling loads you’re going to see in your climate zone for 1% of the hours throughout the year. Designers size their equipment to handle the peak load. But the vast majority of the time, your building will not see loads that high. It will see loads at what we call “part-load” conditions, when one-stage equipment runs in less efficient stop-and-start mode, that also causes more wear and tear on components. VRF equipment with its ability to give variable capacity is able to meet part-load conditions more efficiently.
Matching power to the load is not the only reason that a VRF compressor such as the Mitsubishi City Multi is more efficient than a standard compressor. The other reason is the design of the compressor motor. The electronically commutated motors in these units are driven by an inverter, and the inverter has the capability of adjusting not just the frequency of the current being delivered to that motor, but also the voltage. By playing with those two parameters in concert with one another, the motor achieves the highest power factor possible at any given speed and any given load that the motor is under. This improves the Energy Efficiency Ratio (EER) of the equipment (which expresses how many Btus of heat are moved for every watt of energy that you have to purchase). Simply put, you’re getting more heating or cooling per watt out of the VRF equipment at any speed. We’re getting more mechanical work than we were with the previous generation of equipment, for the same amount of power. So even at peak load, a 4-ton VRF system with inverter drive runs much more efficiently than a 4-ton single-stage or dual-stage system sitting next to it.
You can think of this in terms of the amps required to start and run the compressor motor. A standard single-stage four-ton unit will take about 100 amps of power to get started, and then will run at about 40 amps continuously once it gets going. A four-ton Mitsubishi City Multi will start out at about 2 amps, then it will ramp up slowly if necessary to meet the demand, up to about 24 or 26 amps. When the temperature in the space approaches the set point, the VRF unit will slowly reduce power and creep up to the set point, and, guided by its software, will then run just hard enough to maintain the temperature at exactly that set point. The traditional equipment will overshoot the set point, shut off, and then wait until the temperature rises above the set point again before it starts up again.
In practice, the lower amp draw combined with the the precise control of the VRF unit adds up to a savings of 20% to 40% in energy consumption. And because with a properly functioning controller the unit maintains a rock-steady set point, it also provides better comfort, without swings in temperature.
One last automotive metaphor that fits here. Remember carburators? They went away. They’re no longer used not because they did not work, but rather because fuel injection systems performed the same functional role more of distributing fuel to the engine efficiently and reliably. If you take nothing more from this article, please reset your view of VRF. VRF (which has been around since the 1980s) is not new, it’s not a fad that will die out. In fact, it could be that not to switch to VRF is the risky decision. Consider this, in 10-15 years when you need replacement parts, what will be occupying the shelf-space in distributors’ warehouses? Beyond the availability of parts, as someone who used to rebuild carburetors, both the parts and the installer expertise are needed to make things work. Will future generations of installers resist or appreciate the ability to connect a computer?
Air Handlers and Zoning
The outdoor compressor is linked to indoor units by refrigerant lines. Depending on the size and model of the compressor, a VRF compressor can handle anywhere from several indoor units up to dozens of units (in the case of big commercial equipment running on three-phase power). The homes we’re designing for typically have single-phase power, so we’re restricted to the equipment that can run on single-phase. We typically call for one or more Mitsubishi City Multi S-Series compressors, rated at 3, 4, or 5 tons, each of which can serve 8 independently controllable indoor units.
The indoor units could be anything from wall-mounted units or ceiling cassettes to variable-speed vertical or horizontal air handlers (commonly known as “multi-position”) much like the form-factor of air handlers for a traditional system. Our clientele has not embraced the visible wall-mounted units, so we typically specify one or more Mitsubishi multi-position air handlers and conventional ductwork. This form factor also leverages our ability to impact architectural decisions early in the design process. Again we benefit from non-split-incentive decision making: most homeowners understand that impairing access to their AHUs impairs the ability to provide quality installation and maintenance.
When it comes to zoning and duct design, there have to be conversations with the owners and the architect. Many in the industry, particularly residential, have grown accustomed to a process based only on an installation and not on any planning during the design stage. Architects don’t always consider the ductwork when they’re drawing house plans, but they should. I want architects to be thinking about the ductwork early enough in the process that the ductwork can be allowed for. Not leaving room for the “lungs of the home” or building is not really a full design. Perhaps calling it “ductwork” conceals that fact that we are talking about the distribution system that delivers thermal comfort and indoor air quailty. By “leaving room” I mean two things: Both room in the design process, and room within the architectural and framing designs. The simple concept of an integrated process, one that aligns architectural, structural and mechanical designs, is catching on strongly because it’s simple, effective and improves outcomes.
As for zoning, that requires a conversation with the customer on how they plan to live in the space as well as an analysis of the building. We zone the building by load profile and use profile. Load profile means, for example: “This room is facing east. That room is facing west. Those are different load profiles. This room’s on the first floor with very little exterior load and glazing. This room’s on the third floor. So those are different load profiles.”
You can also zone based on use profile: “This is the bedroom, it’s not occupied during the day. This is the central core. It’s rarely occupied at night. Those will be different zones. This is the man’s office. He wants to have it at a certain temperature. Or this is the woman’s sanctuary inside the house. She wants to keep it in her comfort zone. These two rooms are occupied by a teenage daughter and an 8-year-old son. They’re not going to want things the same, so give them each their own control.”
In the case of the east and west sides of the building, we may choose to give each zone its own outdoor compressor. That way, during a season with chilly nights and warm days, if the sun starts to overheat the east side in the morning while the west side is still cool enough to need heating, we can handle both needs at once.
But most zones aren’t going to have opposite needs, so multiple zones can usually be run off the same compressor using refrigerant lines and controls. In that case we give each zone a dedicated air handler and air distribution system that serves that area. Because we can have multiple air handlers served by the same outdoor system, VRF gives us the flexibility to do that and keep the initial cost down. This also minimizes the footprint necessary for all the equipment.
Sometimes, we get into a situation where the zones are too small even for the smallest air handler. In that case, we do “air-side zoning” — we zone the areas using dampers and controls in the duct system served by a single air handler. And occasionally, there’s a point load that is best handled by a wall-mounted unit, such as a laundry room or a garage.
Duct Systems
Duct board box plenums and flex-duct supply lines are typical in the industry in our market. We don’t do things that way: We specify metal duct for all our designs. In our view, flex-duct and duct board air distribution systems need to go away. Why? Well, think about it. People put a lot of effort into constructing a durable, functional enclosure. You have one chance to get it right, and then it’s inconvenient to fix it forever. The ductwork is the same way: It’s a durable, functional, passive assembly; you have one good chance to get it right, and then it’s inconvenient to fix it forever. And together with the enclosure, the duct system defines the breathing zone of the occupied space. The air distribution system is a permanent, durable part of the home that serves you well forever, or serves you poorly forever. Metal duct is appropriate for that situation.
Metal is a durable material. It will last the life of the home, if attached well and done well. And it’s a recyclable material, so at the end of its life cycle there is something we can do with it.
Metal has a natural galvanic action that retards the growth of indoor micro-biological organisms. That includes mold and bacteria, and even viruses and protozoa and all kinds of little living creatures. With air quality in mind, we always aim for fiber-free air distribution systems. The nooks and crannies of ductboard and turbulence created by flex duct spiral pressure liners do not help keep distribution systems clean.
Clean is another way of saying free of food, or substrates on which to grow unhealthy indoor microbiomes. If you think flex duct and ductboard is “fine” please keep in mind that your assessment is not an immutable physical law. It’s an assessment based on comparative metrics. Be clear on what your comparing to and what outcomes are priorities. You “eat” air all the time, is poor IAQ “fine”? Perhaps in the way that a greasy burger and fries is a “fine” diet compared to starving in sub-Saharan Africa. What really makes flex duct and ductboard the norm is the fact that it supports a beneficial economic outcome. Our industry is based on both interchangeable parts and exploitable and interchangeable labor. But that’s a topic for another day.
You get one good chance to get it right. This is perhaps the key consideration: an air distribution system moves tens of thousands of pounds of air every day. It will do so with either a lot of friction, very little friction, or somewhere in between. Using low-friction metal distribution systems based on the principles of fluid mechanics is analogous to having the right amount of air in your tires. Rolling resistance resists motion. So does friction in duct systems.
If you buy an efficient car, but then you drive on tires that are nearly flat, you’re going to lose a lot of the efficiency in that vehicle to rolling resistance. Of course you can always inflate your tires. You’re not going to roll around in your Prius with your tires half flat. But if you have ductwork with high friction resistance — like most duct board and flex duct the way it is typically installed today — you’re stuck with it forever. Just because you and your clients don’t see or value the ducts does not mean they don’t matter. Air distribution systems matter for the life of the home. It only makes sense to do it right when you’ve got the chance.
Filtration
Why filter the air in a home? It’s just dust, right? Oh, if only it were “just dust” — bits of leaves or soil, or even gross things like skin flakes. But dust is like a candy-coated M&M, and the candy coating is things like chemical pollutants and biotoxins. You breathe those things in with the dust, and if the particles are small enough, they can lodge in your lungs. The best way to keep from being exposed to those toxins is to filter the air, with at least a MERV-13 filter.
The MERV-8 filters that a lot of installers put in are touted as being 99% effective at catching dust. But all they catch is larger dust that your bronchial cilia are capable of catching and expelling from your system. MERV-8 filters are there only to keep the air conditioner coil from fouling. They’re not there to protect the health of the people in the building. Based on our expertise in IAQ and also per ASHRAE standards, we specify MERV-13 filtration at a minimum; these capture most of the smaller particles that your bronchial tubes won’t catch and clear. If the clients are sensitive, we may go up to MERV-16 or even to a whole-house HEPA filter.
The state of residential filtration provides a simple but powerful illustration of how far from human health principles our industry mainstream has drifted. We know that capturing particulate pollutants is important for health and should be happening whenever the home is occupied. Do we do that? Not so much. Our industry has somehow decided that the right time to filter the air is either when the temperature is too hot and we need cooling, or too cold and we need heating. The impacts of our societal and industry lens of home as a visual-spatial and an economic asset has a powerful distorting effect on our decisions and actions.
Fresh Air and Dehumidification
Humidity control is important for occupant comfort, and also for building health. If you maintain the air relative humidity (RH) in an acceptable range of 35-55% or 50-55% in hot humid climates, the occupant’s thermal comfort will be satisfied over an expanded range of sensible temperatures. That can make up for situations like an overheated sunroom: If I keep it dry, I am able to evaporate moisture off the occupants’ skin, which is part of cooling.
Controlling moisture helps maintain the stability of trim, or of musical instruments in the house.
But most importantly, dry air is critical for the health of the human occupants of the building, because humid air supports the growth of all the organisms in the “microbiome” of the home. Fungi, bacteria, and other organisms battle for supremacy in a humid environment, and they release biotoxins that cause human health problems. If we keep the air dry, we take away a major factor in that health threat.
And here’s the thing: As the energy code evolves, it’s increasing the need to independently manage humidity. Tighter enclosures, more insulation, and better windows are reducing the sensible load in the house. That means air conditioners — which are the only dehumidification equipment in most houses — are running less often. In essence, the code says, “Thou shalt run thy air conditioner less.” And if the air conditioner is not running, you’re not removing humidity. Meanwhile, required fresh air ventilation is bringing moisture into the home during much of the season.
So for our clients, we always specify a dedicated dehumidifier with its own controls. Typically that is an Ultra Aire unit, because we have a good relationship with Ultra Aire, have the ability to access their technical teams, and we have a solid track record with their product. We pull air from the conditioned space into the dehumidifier, and send it to the supply air distribution system. We also use a dedicated damper-controlled ventilation port on the dehumidifier unit to draw in fresh air and distribute it also using the heating and cooling air distribution system. Note that this system needs to be designed to account for the additional volume of dehumidified air.
The dehumidifier runs in response to relative humidity in the house. It doesn’t run only when the air conditioner or heat is running. But it doesn’t require the air handler to be running — the fan in the dehumidifier unit is sufficient to get the dry air where it needs to go.
Summary
The term air conditioning is so familiar that perhaps we don’t hear it. Conditioning does not mean cooling. Conditioning means that we are creating an indoor environment that is conditioned to be suitable for human occupancy.