Introducing The Spring Street Passive House: A Healthy Home Project By Kristof & Diane Irwin

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 Electrification of Domestic Hot Water: Heat Pump Water Heater Adoption in U.S. Residential Construction

The residential construction market in the United States is undergoing a fundamental transformation, driven by the dual imperatives of grid modernization and enhanced indoor air quality. Central to this shift is the increasing adoption of Heat Pump Water Heaters (HPWHs). These highly efficient, all-electric systems represent a critical technology for decarbonizing buildings and fostering a more resilient energy infrastructure. While current national adoption rates remain modest, market dynamics indicate a significant acceleration, propelled by robust governmental policies, escalating consumer interest in new construction, and continuous technological advancements.

HPWHs function by moving heat rather than generating it, offering substantial energy savings and eliminating on-site combustion byproducts that compromise indoor air quality. The evolution of HPWH technology, including integrated, split, and emerging 120V plug-in models, directly addresses historical installation complexities and upfront costs. However, widespread adoption faces persistent barriers, notably the high initial investment and the challenge of emergency replacements, which often favor conventional, less efficient alternatives. Addressing these challenges requires a multi-faceted approach, emphasizing streamlined incentives, comprehensive workforce development, and enhanced consumer education to fully realize the environmental, economic, and health benefits of residential electrification.

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The Electrification Imperative in Residential Construction

The transition to all-electric homes, particularly through the integration of technologies like Heat Pump Water Heaters (HPWHs), is emerging as a strategic imperative across the United States. This profound shift is driven by a two-fold objective: adapting to a rapidly evolving energy grid and significantly improving indoor air quality by eliminating combustion from residential spaces. HPWHs are increasingly recognized as a vital technology for the clean energy transition and for substantially lowering building emissions, primarily due to their ability to efficiently provide heating by replacing the use of onsite fossil fuels.[1] They are progressively acknowledged as a critical technology for heat decarbonization efforts.[2]

The broader transformation of the electric grid, which HPWH adoption directly supports, is propelled by several interconnected factors. These include a rising demand for electricity, the increasing economic and technical viability of diverse energy generation sources, the rapid growth of distributed energy resources (DERs), and ambitious state-level clean energy and decarbonization policy goals.[3] This context positions HPWH adoption as a fundamental component of a larger national energy strategy. The widespread adoption of HPWHs signifies more than just a technological upgrade; it represents a fundamental societal shift in how homes interact with the energy ecosystem. This transformation is deeply rooted in a collective commitment to decarbonization and grid modernization, driven by both environmental imperatives and significant economic opportunities. Architects designing for HPWHs are not merely specifying an appliance but are actively contributing to a national infrastructure and public health transformation.

At their core, Heat Pump Water Heaters operate on a principle distinct from conventional water heating methods. Unlike traditional water heaters that generate heat directly through the combustion of fossil fuels (e.g., natural gas) or through electric resistance, HPWHs utilize electricity to move existing thermal energy from one location to another. This process involves extracting heat from the surrounding air and transferring it to the water within a storage tank.[4] This "refrigerator in reverse" mechanism makes them remarkably energy efficient, typically two to three times more efficient than conventional electric resistance water heaters.[4] This superior efficiency directly translates into significant annual energy bill savings for homeowners, making them an economically attractive option over the appliance's lifespan.[4]

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Current State of Heat Pump Water Heater Adoption in the U.S.

Market Dynamics and Growth Trajectory

The U.S. residential heat pump water heater market, while still maturing, exhibits a clear growth trajectory. In 2022, the market size was valued at USD 468.22 million and is projected to grow at a Compound Annual Growth Rate (CAGR) of 5.90% during the forecast period.2 Globally, the HPWH market reached $1.7 billion in 2024 and is expected to expand to $2.22 billion by 2033, reflecting a steady growth rate of 3%.[16] Historical data indicates a significant acceleration, with U.S. sales of HPWHs doubling from 2016 to 2020.[2] More recently, 2023 saw over 190,000 HPWHs shipped in the U.S., marking a substantial 35% increase over 2022 and representing the largest annual increase ever recorded for this technology.[17]

Despite these impressive growth rates, the overall national adoption rate of HPWHs remains relatively low, estimated at approximately 3% of all households.[18] In 2023, HPWHs constituted about 4% of residential electric water heater sales.1 Further data suggests that currently, only 1% of homes in the U.S. utilize electric heat pump water heaters for their hot water needs.[20] This presents a critical distinction between the low overall national adoption rate of HPWHs and the higher reported figures for consumer preference and integration in new construction. While the installed base is small, there are strong signals of growing consumer interest and integration in new construction. More than 40% of residential consumers are now reportedly opting for HPWHs over conventional systems, a choice driven by their energy-saving capabilities and reduced carbon emissions.[16] Furthermore, a significant trend in new residential construction indicates that over 45% of new builds are integrating heat pump systems.16 North America, particularly eco-conscious states, accounts for over 45% of residential units adopting heat pump technologies, with the U.S. and Canada experiencing over 38% growth in residential installations.[16] The higher figures for "consumers opting for HPWHs" and "new builds integrating heat pump systems" likely refer to new purchases or intent for water heaters, or the broader category of heat pump systems (including space heating) in new construction, rather than representing the total installed base of HPWHs. This nuance is crucial for understanding the true pace and potential of market transformation, indicating that while the momentum is strong, the existing housing stock still presents a vast opportunity for retrofits.

The American water heater market is largely dominated by three key manufacturers: Rheem, A. O. Smith, and Bradford White.[21] Rheem currently holds the largest HPWH market share in the U.S., benefiting from strategic partnerships with major retailers and homebuilders.[21] Bradford White ranks as the third-largest HPWH market player, with manufacturing operations located in Middleville, Michigan.2 Other notable U.S. manufacturers contributing to the residential HPWH market include Vaughn and Nyle Systems.[2]

Looking ahead, ambitious sales targets underscore the projected market shift. Rewiring America sets a target for HPWHs to comprise 100% of water heater sales by 2040, which would lead to a complete turnover of fossil fuel-based water heating stock by 2050.[20] To achieve this aggressive goal, HPWH sales need to increase more than tenfold over the business-as-usual scenario by 2032.[20] The U.S. Department of Energy (DOE) supports this trajectory, projecting that its 2024 efficiency standards, with compliance starting in 2029, will result in over 50% of newly manufactured electric storage water heaters utilizing heat pump technology, a substantial leap from the current 3%.[13] These ambitious sales targets and projected rapid market shifts for HPWHs are not organic growth projections alone; they are directly linked to, and in many cases, mandated by recent and upcoming policy changes. The DOE's efficiency standards and the Inflation Reduction Act are creating a powerful regulatory and financial tailwind that will fundamentally transform the HPWH market, pushing it towards dominance.

Policy and Incentives Catalyzing Adoption

Governmental policies and financial incentives are playing a pivotal role in accelerating HPWH adoption. The U.S. Department of Energy (DOE) finalized new energy-efficiency standards for residential water heaters on April 30, 2024. These standards are projected to save American households approximately $7.6 billion per year on energy and water bills and reduce 332 million metric tons of carbon dioxide emissions over 30 years of shipments.[13] This initiative represents the largest energy savings action by the Appliance Standards Program in history.13 Compliance with these new standards will be required starting in 2029, and they are expected to result in over 50% of newly manufactured electric storage water heaters utilizing heat pump technology, a substantial increase from the current 3%.[13] These standards are designed to more than double the efficiency of electric storage water heaters.[13]

Further catalyzing adoption is the Inflation Reduction Act (IRA), which significantly expands the accessibility and affordability of heat pump water heaters through various tax credits and rebates.[13] Homeowners can claim a federal tax credit valued at up to 30% of the HPWH project cost, capped at $2,000 per year.[12] This credit has no lifetime limit, enabling homeowners to claim it annually for eligible improvements until 2033.[23] To qualify for these tax credits, HPWHs must be ENERGY STAR certified.[24] In addition to tax credits, the Home Electrification and Appliance Rebate program, also under the IRA, offers up to $1,750 for ENERGY STAR-certified electric HPWHs.22 For low- to moderate-income (LMI) households, these rebates can be even more substantial, covering 50-100% of the HPWH costs, up to $1,750.[26] Eligibility for these rebates typically includes new construction, replacement of a non-electric water heater, or a first-time purchase of a HPWH for an existing home.[27]

Beyond federal initiatives, state and local programs, along with utilities, are actively managing their own energy efficiency and appliance upgrade rebate programs.[27] Examples include instant rebates offered in Massachusetts ($750-$1,500) and California ($500-$900).26 Utilities like TVA EnergyRight also provide residential rebates for qualifying HPWH systems.[28] Many programs are actively exploring time-of-use pricing structures to further incentivize HPWH adoption and maximize the benefits of off-peak energy consumption.[29] The comprehensive suite of government policies and incentives for HPWHs extends beyond purely environmental objectives; it acts as a significant economic stimulus for the burgeoning HPWH market. This stimulus drives manufacturing investment, fosters job creation across the supply chain [3], and accelerates consumer adoption. Furthermore, the tiered structure of IRA rebates, especially for low- and moderate-income households, directly addresses energy equity, ensuring that the benefits of clean energy technologies are accessible across all socioeconomic strata. The simultaneous implementation of stringent efficiency standards (a "push" from the supply side) and generous consumer incentives (a "pull" from the demand side) reveals a sophisticated and comprehensive market transformation strategy. This dual approach is designed to overcome the inherent inertia and initial cost barriers associated with new technology adoption, accelerating the shift away from conventional water heaters towards HPWHs across the entire market.

Table 1: U.S. Residential HPWH Market Overview (2022-2033)

This table provides a concise, quantitative overview of the HPWH market's current scale and its projected trajectory. For architects, this context is essential to understand the industry's momentum and the increasing relevance of HPWHs in residential design, moving beyond anecdotal evidence to present a data-driven overview. By outlining current adoption rates alongside future projections and policy impacts, the table helps architects make informed decisions about specifying HPWHs in their projects, highlighting that while current penetration is low, the market is on a steep growth curve, indicating that HPWHs are becoming a standard rather than a niche technology. The inclusion of CAGR and specific targets from authoritative sources visually reinforces the strong trend towards electrification in water heating, which can be invaluable for architects when advocating for HPWH integration to clients, demonstrating alignment with broader industry and policy directions.

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Dual Benefits of HPWH Electrification: Grid Resilience and Indoor Air Quality

The widespread adoption of Heat Pump Water Heaters offers profound benefits that extend beyond individual household energy savings, directly addressing critical challenges in energy infrastructure and public health.

Playing A Role In Grid Stability and Efficiency

Heat pump water heaters are uniquely positioned to act as flexible loads within the electrical grid due to their inherent thermal storage capabilities.[31] The large storage tank allows them to optimize the timing of electricity consumption without compromising hot water delivery service to occupants.31 This ability to store thermal energy enables HPWHs to reduce strain on the electric grid during peak electricity demand periods.[8] The widespread adoption of grid-interactive HPWHs represents a significant, decentralized infrastructure investment that directly enhances overall grid reliability and resilience. For architects, understanding this benefit is paramount, as it positions their projects not merely as individual energy-efficient structures, but as active contributors to broader national energy security and sustainability goals. By integrating HPWHs, buildings become dynamic participants in grid management, offering a scalable solution for managing increasing electricity demands and integrating renewables.

HPWHs can actively participate in utility demand management programs.[8] This allows for strategic load shifting, where electricity consumption is moved from high-price or peak demand periods to low-price or off-peak times.[31] Strategies employed include pre-heating water when electricity is abundant and cheap, adjusting temperature setpoints, or temporarily preventing the use of less efficient electric resistance heating elements during peak events.[8] HPWHs can start or stop heating quickly, making them highly responsive to variable grid signals.[31] This demand flexibility is crucial for integrating intermittent renewable energy sources, such as solar and wind power, into the grid. By shifting demand to match periods of high renewable generation, HPWHs help balance supply and demand, improving grid stability and maximizing the utilization of clean energy.[31] They can effectively absorb excess renewable generation, preventing curtailment and enhancing grid efficiency.[48]

HPWHs are a key component of Grid-interactive Efficient Buildings (GEBs), which integrate energy efficiency, demand flexibility, and smart technologies to serve the grid as distributed energy resources (DERs).[47] National adoption of GEBs is projected to yield $100-200 billion in U.S. electric power system cost savings and contribute to a 6% annual reduction in CO2 emissions by 2030.[51] The concept of "transactive energy" further refines this, envisioning a system where DERs like HPWHs are coordinated with smart loads through dynamic, automated transactions. This approach has the potential to reduce daily load swings by 20-44% and generate billions in annual economic benefits by optimizing grid operations.[49] The transformation positions HPWHs as not just energy-efficient appliances, but as integral parts of a future-proof energy infrastructure, contributing to both local building performance and national energy security.

Improving Indoor Air Quality and Home Health

A direct and immediate benefit of electrifying water heating with HPWHs is the complete elimination of on-site combustion within the home.[9] This removes a major source of toxic combustion exhaust gases and associated pollutants that are typically generated by natural gas, propane, or oil-fired water heaters.9 Furthermore, by removing a fuel-fired appliance, HPWHs also eliminate the inherent risk of fire or explosion that can be caused by gas leaks or combustion malfunctions.[15]

Traditional fossil fuel-burning appliances, including water heaters, furnaces, and stoves, produce a range of harmful byproducts when fuel is incompletely burned.[56] It’s a proper panoply These include Carbon Monoxide (CO), an odorless, colorless, and highly toxic gas that reduces the blood's ability to carry oxygen. Acute exposure can cause fatigue, headaches, nausea, dizziness, and impaired vision, and at high levels, it can lead to loss of consciousness and death.[56] Another significant byproduct is Nitrogen Dioxide (NO2), a respiratory irritant that can cause airway inflammation, coughing, wheezing, and increased asthma attacks.[56] Scientific studies have consistently shown higher NO2 concentrations in homes with gas stoves, and exposure is linked to increased risk of asthma in children and more severe symptoms for those with respiratory illnesses.[59] Particulate Matter (PM, PM2.5), microscopic solids and liquids, can irritate eyes, nose, and throat, lodge in the lungs causing irritation or damage, lead to inflammation, heart problems, and increase the risk of premature death. Some particles may contain cancer-causing substances.[56] Other pollutants include carbon dioxide (CO2), sulfur dioxide (SO2), various hydrocarbons (e.g., benzene), and aldehydes.[56]

While furnaces and water heaters are typically vented to the outside, their emissions still contribute to outdoor air pollution.[57] Unvented combustion devices, such as gas stoves or unvented heaters, pose even higher risks by releasing pollutants directly into the living space.[59] ASHRAE's position emphasizes source control and adequate ventilation as key means to dilute indoor contaminants and improve indoor air quality.[62] By eliminating the combustion source entirely, HPWHs offer a proactive approach to mitigating these indoor air quality concerns. Electrifying water heating with HPWHs directly removes a significant and consistent source of harmful indoor air pollutants, leading to tangible and measurable health benefits for building occupants. This is particularly impactful for vulnerable populations such as children, older adults, and individuals with pre-existing respiratory conditions. This shifts the conversation from abstract "environmental benefits" to concrete "health and safety" improvements directly within the home, a powerful consideration for architects designing healthy living spaces.

Accelerating Broad Scale Adoption By Identifying Opportunities and Challenges

Key Advantages and Drivers

The momentum behind Heat Pump Water Heater adoption is driven by a confluence of compelling advantages and supportive market forces. Foremost among these are the significant energy and cost savings. HPWHs are remarkably energy-efficient, typically 3 to 4 times more efficient than conventional electric resistance water heaters.[10] This efficiency translates into substantial annual energy bill savings for homeowners, ranging from $80 to $550 per year, and over $5,600 in savings over the product's lifetime.[10]

Beyond economic benefits, HPWHs offer profound environmental advantages and a reduced carbon footprint. By consuming significantly less energy and operating on electricity (which is increasingly decarbonized through renewable sources), HPWHs dramatically reduce greenhouse gas emissions.[10] Replacing a single gas water heater with a HPWH can save over 2,000 lbs of CO2 emissions annually, an amount equivalent to growing more than 17 trees for 10 years.[64]

The technology itself is maturing rapidly. While HPWHs have existed since the 1970s, their mainstream adoption has primarily occurred in the past decade, indicating a shift from niche to proven technology.[38] They are now considered a reliable solution [10] and benefit from continuous innovation in efficiency, sound reduction, and installer-friendly features, such as top water connections and duct-ready designs.[7]

Finally, increasing governmental and utility support acts as a powerful accelerant. Strong policy drivers, including the DOE's finalized efficiency standards [13] and the comprehensive incentives provided by the Inflation Reduction Act [12], are significantly accelerating market growth. Utilities are also actively developing and implementing programs, including rebates and online platforms, to streamline HPWH adoption and educate consumers.[29]

Persistent Barriers and Areas for Improvement

Despite the clear advantages, several persistent barriers impede broad-scale HPWH adoption in the U.S. residential market.

The most significant barrier remains the high upfront and installation costs.[18] HPWHs frequently retail for at least $2,000, which is substantially higher than low-to-medium efficiency gas or electric resistance water heaters, often priced at $600 or less.[43] The installation cost often exceeds the equipment price itself; for contractor installations, the average cost was roughly $2,700, contributing to an overall average project cost of $3,200-$4,700.[43] This high upfront cost is critically exacerbated by the fact that approximately 85-90% of water heater replacements occur during emergency situations.[19] In these urgent, unplanned scenarios, homeowners are highly inclined to opt for quick, familiar, and seemingly cheaper conventional solutions, bypassing HPWHs despite their long-term energy and cost savings. This creates a cycle where the immediate need for replacement, driven by appliance failure, actively impedes the adoption of more efficient and environmentally beneficial technology.

Installation complexities also pose a significant hurdle. HPWHs are generally taller and heavier than conventional units [36], requiring significant air space (450-1000 cubic feet) for efficient operation.6 Replacing a gas water heater with a HPWH often necessitates a new 240V circuit or an electrical panel upgrade, adding to the cost and complexity.[14] Furthermore, HPWHs produce condensate that requires proper drainage, which may involve installing a new drain line or a condensate pump if a gravity drain is not readily available.[9] The cool, dehumidified air exhausted by HPWHs can lower the ambient temperature of the installation space, potentially causing discomfort or increasing heating loads in conditioned areas. If not properly vented or managed, this can lead to moisture damage and mold growth on cold surfaces.[4]

A critical bottleneck in the market transformation is workforce development and availability. A significant barrier is the skilled labor shortage in the HVAC and plumbing trades.[71] Workforce challenges, exacerbated by factors like the COVID-19 pandemic, have led to retention issues and staffing problems, complicating HPWH installations.[70] The insufficient supply of adequately trained and experienced HPWH installers directly translates into higher installation costs, slower project completion times, and a greater risk of improper installations that can undermine system performance and consumer satisfaction.[43] This workforce gap limits the ability to scale HPWH adoption despite growing demand and policy support. There is a clear need for clearer guidance for installers on the post-installation startup process, including diagnostic run times and electric element behavior.[70]

Finally, consumer awareness, while growing, remains low in many areas, with only 29% of households in some regions familiar with heat pump technology.[16] This lack of understanding of the long-term cost savings and environmental benefits contributes to a general installer and consumer bias towards conventional models.[33]

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What Needs To Happen Next

The U.S. residential construction market is at a pivotal juncture, with Heat Pump Water Heaters emerging as a cornerstone of the electrification movement. The transition to HPWHs is not merely an appliance upgrade; it represents a fundamental societal shift towards a more resilient, decarbonized energy grid and healthier indoor environments. The technology is rapidly advancing, with innovations addressing efficiency, sound, cold-climate performance, and installation ease, including the critical development of 120V plug-in models that simplify retrofits. Furthermore, comprehensive policy support from the DOE and the Inflation Reduction Act is creating a powerful market transformation strategy, utilizing both regulatory mandates and financial incentives to accelerate adoption.

However, significant barriers persist, primarily the high upfront and installation costs, which are exacerbated by the prevalence of emergency replacements. The current shortage of skilled installers further compounds these cost and complexity issues, creating a bottleneck that hinders widespread deployment. To fully realize the profound environmental, economic, and health benefits of HPWHs, a concerted effort is required across all stakeholders.

For architects, the implications are clear: designing with HPWHs is no longer a niche consideration but a strategic imperative that contributes to a building's holistic performance and broader societal goals. To accelerate broad-scale adoption, the following recommendations are critical, even if not all are in each of our sphere of influence.

1. Streamline and Publicize Incentives: While federal incentives exist, their complexity and the emergency nature of most water heater replacements often prevent homeowners from leveraging them. Utilities and government agencies should collaborate to offer more point-of-sale rebates and direct-to-contractor incentives, simplifying the financial process at the moment of purchase. Clear, accessible communication about available tax credits and rebates is paramount.

2. Invest in Workforce Development: Addressing the skilled labor shortage is crucial. This requires increased funding and support for training programs specifically focused on HPWH installation, maintenance, and troubleshooting for plumbers and HVAC technicians. Programs should include practical, hands-on training to build installer confidence and efficiency, ultimately reducing labor costs and installation times. Exploring alternative licensing pathways for HPWH installers, separate from full plumbing licenses, could also expand the workforce, particularly in rural areas.

3. Enhance Consumer and Contractor Education: Despite growing interest, a significant portion of the population remains unaware of HPWH benefits or misinformed about installation requirements. Targeted educational campaigns, leveraging trusted sources like building science organizations and MEP firms, should highlight the long-term energy savings, improved indoor air quality, and grid benefits. For contractors, clearer guidance on installation best practices, particularly regarding air volume, venting, and condensate management, is essential to prevent performance issues and ensure customer satisfaction.

4. Promote "Retrofit-Ready" Solutions: The emergence of 120V plug-in HPWHs is a game-changer for the existing housing stock. Policy and incentive programs should specifically promote these "drop-in" solutions to address the electrical panel constraints common in older homes, making the transition from fossil fuels more accessible and affordable during emergency replacements.

5. Integrate HPWHs into Holistic Building Design: Architects should approach HPWH specification not as an isolated component, but as an integral part of a building's overall energy and environmental strategy. This includes designing spaces with adequate air volume and proper ventilation for optimal HPWH performance, considering the unit's sound profile relative to living areas, and planning for grid-interactive capabilities to maximize demand response benefits. Collaboration with MEP engineers and building science consultants from the earliest design phases can ensure seamless integration and optimized performance.

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The Damp Deception: How a Well-Intentioned Code Change is Fostering Mold in New Homes

By Positive Energy staff

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Introduction: The Air We Breathe – A Tale of Good Intentions and Unforeseen Consequences

The promise of a new home often includes visions of a healthier, more energy-efficient living space. However, a subtle yet significant regulatory shift in U.S. building codes, particularly affecting hot-humid climate zones, may be inadvertently undermining this very promise. Before 2021, residential ventilation requirements were often loosely enforced; homes were typically required to have a ventilator, but the actual volume of air exchanged was not mandated to be measured. This frequently led to systems being ineffectively installed or even "sabotaged" by HVAC contractors, rendering them inoperable or improperly configured from the outset.[1] Consequently, many homes, even in that period, did not achieve consistent fresh air exchange. Compounding this, most residential HVAC systems lacked any form of supplemental or dedicated dehumidification, a feature that building science experts have increasingly recognized as crucial, especially for high-performance homes in moisture-laden environments.[3]

The 2021 International Energy Conservation Code (IECC) sought to address ventilation deficiencies by introducing a pivotal change: a mandate for measured outside ventilation air, ostensibly in the name of improving indoor air quality (IAQ). Specifically, section R403.6.3 of the 2021 IECC added a new requirement for flow rate testing on mechanical ventilation systems, ensuring a prescribed amount of outdoor air is introduced into the home.[4] The intentions were sound; the 2021 IECC aimed to enhance both energy efficiency and IAQ, with proponents suggesting that homes built to this standard would be less prone to issues like mold and moisture.[5]

However, this well-intentioned advancement carried a critical oversight: the lack of a corresponding regulatory requirement for supplemental or dedicated dehumidification in these hot-humid climates. This omission has set the stage for an emerging crisis. By mandating a consistent intake of hot, humid outdoor air without ensuring a means to adequately remove the excess moisture, the code has inadvertently created conditions ripe for widespread problems. The historical ineffectiveness or "sabotage" of older ventilation systems, while detrimental in its own way, may have unintentionally masked the full impact of introducing large volumes of unconditioned humid air because, in many cases, these systems were not delivering significant ventilation. The 2021 code, by ensuring ventilation systems do operate as measured, has unmasked and amplified the underlying physics challenge of managing moisture in humid climates. The code addressed a symptom—inconsistent or non-existent airflow—but failed to holistically address the root challenge in humid regions: the quality and moisture content of that mandated incoming air.

The Science of Humidity – Why Standard AC Isn't a Silver Bullet in Hot-Humid Climates

Understanding the challenge requires a grasp of how buildings, particularly in hot-humid climates, manage heat and moisture. HVAC systems contend with two types of heat loads: sensible load (temperature) and latent load (moisture in the air). Standard residential air conditioners are primarily designed to tackle sensible loads. While they do remove some moisture as a byproduct of cooling, their capacity to do so is often limited and less efficient, especially during "shoulder seasons" (spring and fall) or under part-load conditions when outdoor temperatures are mild, but humidity remains high.[7] During these periods, the AC runs less frequently to meet the lower temperature demand, thereby performing less incidental dehumidification. Research indicates that optimizing dehumidification by central air-conditioning systems, particularly during part-load conditions, often requires modified control settings and specific airflow strategies, implying standard operation is insufficient.[7]

The drive towards greater energy efficiency, a cornerstone of modern building codes like the IECC 5, has led to tighter building envelopes and better insulation. These improvements reduce the sensible cooling load, meaning HVAC systems run less often. Paradoxically, this reduced runtime for cooling further diminishes the system's ability to remove moisture.[3] Building Science Corporation has explicitly noted that "most building efficiency improvements...are directed at lowering sensible gains while latent (moisture) gains remain mostly unchanged" and that "supplemental dehumidification was needed in high performance, low sensible heat gain homes in order to maintain indoor relative humidity below 60% year-round".[8]

Into this scenario, the 2021 IECC introduces the requirement for measured mechanical ventilation, forcing a specific volume (Cubic Feet per Minute, or CFM) of outdoor air into the home.4 In hot-humid climates, this outdoor air is inherently laden with moisture, directly increasing the latent load that the HVAC system must manage. Even before the 2021 mandate for measured ventilation, studies had identified that high-performance homes in hot-humid climates could experience elevated indoor humidity levels when ventilating to the rates prescribed by standards like ASHRAE 62.2.3 The 2021 IECC, by ensuring these ventilation rates are consistently met, likely exacerbates this pre-existing vulnerability. While ASHRAE 62.2 itself provides ventilation rate calculations and mentions potential exceptions for "extreme humidity" [10], the IECC's adoption of these rates without concurrently mandating a robust humidity control solution for these specific climates is the crux of the problem.

This reveals a significant regulatory blind spot. While the 2021 IECC stringently mandates and verifies ventilation airflow [4], it does not impose a corresponding requirement for supplemental or dedicated dehumidification systems in residential buildings in hot-humid climates.11 This is despite the scientifically established need for such systems to maintain healthy and durable indoor environments under these conditions.[3] This omission is particularly glaring when contrasted with specific commercial or specialized applications where dehumidification is considered essential and sometimes mandated, such as for controlled environment horticulture or swimming pool areas.[12] The regulatory framework appears to operate in silos: the energy code focuses on ventilation rates and energy metrics, but the crucial synergistic understanding of how ventilation interacts with humidity in specific climates—and the need for integrated solutions—seems to be lost. The responsibility for ensuring the entire system (house-as-a-system) functions correctly to manage both air exchange and moisture falls through the cracks of the primary energy code that drives widespread construction practices.

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A Breeding Ground – How Unconditioned Ventilation Air Turns HVAC Systems into Mold Incubators

The consequences of introducing a continuous stream of hot, humid outdoor air into a home without adequate dehumidification are particularly acute within the HVAC system itself. As described by the user, this moisture-laden ventilation air is often "dumped directly into the return plenum of a standard HVAC system". Return plenums and associated ductwork, especially if constructed from porous materials like fiberboard-based duct board, become prime locations for condensation. When this warm, moist air encounters cooler surfaces within the HVAC system—such as the evaporator coil, or even the cooler conditioned air already in the return—its temperature can drop below the dew point, causing water vapor to condense into liquid.[14] Building science principles confirm that the highest relative humidity, and thus the first point of condensation, will occur next to the coldest surfaces.[15] The HVAC evaporator coil and the ductwork immediately surrounding it are classic examples of such surfaces.

These damp conditions create an ideal breeding ground for mold. Mold requires three primary ingredients to thrive: moisture, a food source (which includes organic materials like the paper facing on duct board, dust, and cellulose particles commonly found in HVAC systems), and suitable temperatures, which are typically the same temperatures humans find comfortable.[15] Introducing a constant supply of humid ventilation air directly threatens the ability to keep susceptible building materials below the moisture content thresholds that inhibit mold growth (e.g., below 20% moisture content for wood and wood-based products).[15] Faulty HVAC installations have long been associated with moisture and mold growth due to issues like condensation from improperly insulated ductwork.[1] The current code scenario effectively institutionalizes a system flaw that mimics such faulty installations by design. While HVAC systems themselves, with their metallic surfaces, are not typically initial generators of mold, they can readily support and distribute mold if organic debris accumulates and moisture is persistently present [16]—conditions which the new ventilation mandate can unfortunately create.

The choice of duct material, particularly porous duct board, exacerbates this vulnerability. Duct board can absorb and retain moisture, providing a sustained damp environment conducive to mold proliferation. Its fibrous nature can also trap dust and organic particulates, which serve as a nutrient source for mold. While specific research on "duct board mold" resulting directly from the 2021 code is nascent, the principles of building science and observations of mold growth in humid conditions strongly support this concern.[14] A material choice that might have been marginally acceptable before 2021 becomes a significant design flaw when combined with the new ventilation requirements that deliver a consistent moisture load directly into these materials. This points to a lack of holistic, systems-thinking in material specification guidelines relative to evolving code mandates. The code-mandated measured ventilation, intended to ensure fresh air distribution, ironically transforms the HVAC system into a highly efficient moisture distribution system when dehumidification is absent, delivering humidity precisely to the components most susceptible to mold growth.

Table 1: Common Mold Hotspots in Newer Homes (Hot-Humid Climates) due to Code Imbalance

This table synthesizes information from the user query and building science principles discussed in the cited sources to highlight areas particularly at risk.

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The Fallout – IAQ in Decline and Reputations Tarnished

The proliferation of mold within the HVAC system inevitably leads to a significant decline in indoor air quality, directly contradicting the primary intention behind the 2021 IECC's enhanced ventilation requirements. As mold colonies mature, they release spores, mycotoxins (toxic compounds produced by some molds), and microbial volatile organic compounds (MVOCs) into the airstream.[18] The HVAC system, designed to distribute conditioned air, then becomes an efficient distributor of these harmful bioaerosols throughout the entire home.[18] Even if an HVAC system is designed to filter incoming outdoor air, if the system components themselves become contaminated, it transforms from a solution for IAQ into a source of indoor pollution.[20] This creates a scenario where the air intended to be "fresh" becomes foul and potentially hazardous.

This situation is compounded by the codified trend towards increased air tightness in modern homes, a crucial strategy for energy efficiency heavily promoted by codes like the IECC.[4] However, we need to caveat that we absolutely are in favor of air tight homes. While air tightness is beneficial for reducing energy consumption, it also means that homes don’t dry out like they used to when they were built to be leaky, making effective mechanical ventilation and, critically, humidity control even more important.[19] Tighter envelopes reduce the outdated poor strategy of uncontrolled exchange of indoor and outdoor air, meaning that internally generated pollutants or moisture can become trapped and concentrated if not actively managed. The American Society of Civil Engineers has noted that "energy-efficient buildings are so airtight that they can no longer breathe," and that "the main culprit to blame for mold problems in energy-efficient buildings...is insufficient ventilation".[21] The current predicament is not insufficient ventilation volume, but rather ventilation that is improperly conditioned for the climate.

A damaging consequence of this emerging problem is the potential for the air tightness standards themselves to be unfairly blamed for the resulting mold and IAQ issues. When homeowners in new, tight, and purportedly "efficient" homes experience musty odors, visible mold, and health complaints, they may erroneously conclude that air tightness is the problem. This can lead to a terrible reputation for even the basic air tightness stringencies of code minimum homes, fostering resistance to these beneficial energy-saving measures in the future. This misattribution occurs because the root cause—the imbalance between mandated ventilation and absent dehumidification—is less obvious than the visible symptom of mold in a tightly sealed home. Thus, compliance with one aspect of the energy code (measured ventilation for IAQ) can inadvertently undermine the goals and reputation of other vital aspects (energy efficiency through air tightness).

The focus within the 2021 IECC on quantifying ventilation (i.e., ensuring a certain CFM of air is delivered and tested for [4]) without equally robust requirements for qualifying that air (i.e., ensuring it is appropriately dry for hot-humid climates) represents a fundamental oversight in the regulatory approach to IAQ. The code prioritizes the delivery mechanism over the quality of the delivered product, which, in these specific climatic conditions, can lead to outcomes directly opposed to the stated goal of healthier indoor environments.

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The Broad Ripple Effect – Public Health, Economic, and Environmental Tolls

The regulatory omission of mandatory dehumidification in conjunction with measured ventilation in hot-humid climates is not merely a technical misstep; it is sowing the seeds for significant public health consequences, substantial economic losses, and avoidable environmental damage.

Public Health Crisis in the Making:

Exposure to damp and moldy environments is unequivocally linked to a range of adverse health effects. Authoritative bodies like the U.S. Centers for Disease Control and Prevention (CDC) warn that such exposure can cause stuffy noses, sore throats, coughing or wheezing, burning eyes, and skin rashes. For individuals with asthma or mold allergies, reactions can be severe, and those with compromised immune systems or chronic lung disease may develop serious lung infections.[22] The National Institute for Occupational Safety and Health (NIOSH), part of the CDC, further associates damp buildings with respiratory symptoms, infections, the development or worsening of asthma, hypersensitivity pneumonitis, allergic rhinitis, and eczema.[23] An ASHRAE position document on limiting indoor mold underscores that "persistent dampness in buildings contributes to negative health outcomes" and that "public health authorities have documented consistent associations between damp buildings and increased risks of adverse health effects".[24] The document explicitly recommends humidity control to prevent such health-relevant dampness. This building code oversight, therefore, has direct negative public health externalities that extend beyond individual discomfort, potentially burdening healthcare systems and reducing productivity, with a disproportionate impact on vulnerable populations such as children, the elderly, and those with pre-existing respiratory conditions.

Economic Burdens on Families and Businesses:

The financial toll of addressing mold infestations is considerable. Homeowners face significant costs for mold remediation, repair of damaged building components like drywall and insulation, and replacement of contaminated HVAC ductwork. Professional mold remediation can average $2,365 to $3,500, with costs easily escalating to $9,000 or more depending on the extent and location of the infestation.[25] Remediation of mold within HVAC systems can range from $3,000 to $10,000, and whole-house remediation, which might become necessary in severe cases, can cost between $10,000 and $30,000.[25] Beyond direct remediation, there's the cost of repairing or replacing materials damaged by moisture and mold; for instance, extensive drywall replacement can run into many thousands of dollars.[26] These unexpected expenses represent a severe financial blow to families. For builders, this situation can lead to increased warranty claims, costly litigation, and significant reputational damage. The economic burden extends further, potentially affecting insurers through increased claims (if mold damage is covered) and even local governments, as widespread mold issues could lead to devalued properties and impact the tax base.

Table 2: Estimated Economic Impact of Mold Remediation and Repair per Household (Hot-Humid Climate, Post-2021 Construction)

This table illustrates potential cumulative costs based on data from cited sources and general construction cost knowledge. Actual costs will vary significantly based on the severity and specifics of each case.

The Carbon Footprint of Failure: Environmental Repercussions:

The cycle of damage and repair also carries a significant, often overlooked, environmental cost. The premature replacement of mold-damaged building materials—such as drywall, insulation, and ductwork—necessitates the manufacturing of new materials and the disposal of the old, both of which have associated embodied carbon emissions. Embodied energy, or embodied carbon, refers to the total energy consumed (and greenhouse gases emitted) during a material's lifecycle, from raw material extraction, manufacturing, and transportation to installation.[27] Studies indicate that it can take many years, even decades, for an energy-efficient new building to offset the negative climate change impacts stemming from the embodied energy of its initial construction.[27] When building components fail prematurely due to issues like mold, this payback period is effectively nullified for those components, and new embodied carbon is incurred with their replacement. For example, common materials like plasterboard have an embodied energy of around 15.1 MJ/kg, glasswool insulation around 57.5 MJ/kg, and various steel components used in HVAC or structures range from 38.8 to 79.6 MJ/kg.28 Repeated replacements amplify this environmental burden. This hidden environmental cost directly conflicts with the overarching energy conservation and carbon reduction goals of the IECC. The code, in its current iteration for these climates, may inadvertently reduce operational carbon at the expense of increased embodied carbon due to recurrent, avoidable repairs.

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Rectifying the Oversight – A Call for Healthier, More Resilient, and Genuinely Efficient Homes

The issues stemming from the 2021 IECC's ventilation mandate in hot-humid climates are not an indictment of ventilation itself, nor of the pursuit of air tightness. Both are crucial components of modern, high-performance buildings. Instead, this situation highlights the urgent need for a more holistic, systems-based approach within our building codes—one that recognizes the intricate interplay between ventilation, air tightness, and moisture management, especially in challenging climates.

The most direct path to rectifying this oversight is through code reform. There is a compelling case for integrating mandatory supplemental or dedicated dehumidification requirements into the IECC and adopted state-level energy codes for all new residential construction in hot-humid climate zones (typically ASHRAE Climate Zones 1A, 2A, 3A, and potentially moisture-prone areas of 4A [11]). Building science organizations have already developed technical guidance and capacity recommendations for such systems, demonstrating that viable solutions exist and are well understood.[3] Mandating appropriate dehumidification is not an "additional burden" but rather a crucial correction to ensure that the primary IAQ and energy performance goals of the code are actually met, preventing the code from inadvertently causing harm. It is about making the entire building system work as intended in these specific, challenging environments.

Concerns about the upfront cost of installing dehumidifiers must be weighed against the far greater costs of inaction. While a supplemental dehumidification system might add $400 to $2,000 to the initial construction cost 8, this pales in comparison to the thousands, or even tens of thousands, of dollars required for mold remediation, structural repairs, and health-related expenses.[25] A life-cycle cost (LCC) analysis, which considers all costs and benefits over the lifespan of the building or equipment, would almost certainly demonstrate that the initial investment in dehumidification is highly cost-effective when the avoided downstream costs are factored in.[29] The Department of Energy already has established methodologies for evaluating the cost-effectiveness of code changes, providing a framework for assessing such a requirement.[30]

The benefits of a corrected approach are manifold:

• Genuinely Protected IAQ: Homes will have consistently managed humidity levels, drastically reducing the risk of mold growth and the circulation of bioaerosols.

• Enhanced Occupant Health and Comfort: Reduced exposure to mold and dampness will lead to fewer respiratory problems and allergic reactions, and greater thermal comfort.

• Preservation of Building Durability and Value: Preventing moisture damage will protect the structural integrity of homes and maintain their market value.

• Reduced Economic Losses: Families will be spared the financial burden of remediation and health costs, and builders will face fewer warranty issues and reputational risks.

• Lowered Life-Cycle Carbon Emissions: Avoiding the premature replacement of building materials will reduce the overall embodied carbon footprint of these homes.

• Restored Faith in High-Performance Building Standards: Demonstrating that air tightness and ventilation can be successfully implemented without adverse side effects will bolster confidence in modern building science.

The "vapor management declaration" discussed in proposed changes to the IECC, while a positive step toward documenting passive moisture control strategies like vapor retarders [31], is insufficient on its own. Passive measures primarily address moisture movement via diffusion and incidental air leakage; they cannot adequately manage the substantial bulk moisture loads actively introduced by mechanical ventilation systems in humid climates. A comprehensive solution requires both robust passive design and appropriate active mechanical moisture control.

Furthermore, addressing this regulatory gap could spur beneficial industry innovation. A clear code requirement for effective, integrated dehumidification and ventilation solutions would create market demand, encouraging manufacturers to develop more sophisticated systems and prompting better training for HVAC designers and installers.[2] This aligns with the IECC's stated intent to "provide flexibility to permit the use of innovative approaches and techniques".[32]

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Conclusion and Call to Action:

The 2021 IECC's mandate for measured ventilation air was a step towards improving indoor air quality in new homes. However, its failure to concurrently require supplemental/dedicated dehumidification in hot-humid U.S. climate zones represents a critical oversight with escalating negative consequences. This regulatory gap is leading to widespread moisture issues, fostering mold growth within HVAC systems and living spaces, degrading IAQ, tarnishing the reputation of air-tight construction, and imposing significant public health burdens, economic losses, and environmental impacts from avoidable repairs and material replacements.

It is imperative that stakeholders—including building code officials at national and state levels, policymakers, the building industry, HVAC designers and contractors, and public health advocates—recognize the severity of this unintended consequence and act decisively. The path forward involves amending building energy codes to require effective mechanical dehumidification strategies as an integral part of the ventilation system in new homes constructed in hot-humid climates. Such a change is not merely about adding another piece of equipment; it is about ensuring that our pursuit of energy efficiency and fresh air does not inadvertently create unhealthy and unsustainable living environments. By adopting a truly holistic, systems-based approach to building design and regulation, we can ensure that new homes are genuinely healthy, comfortable, durable, and efficient for decades to come.

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What Have We Learned About Air Conditioning & The Coronavirus

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:

1. 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. 

2. 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. 

From Dr. Taylor’s IAQ Radio interview: 

“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. 

1. Better sensors and analytic tools - NGS next generation sequencing equipment.

2. New data streams (IAQ data) - PTR-ToF-MS (proton transfer reaction, time of flight, mass spectrometers).

3. Rapid IT development - we can handle big data sets and find the needles in the haystacks.

4. Goal to personalize Healthcare - human genome unlocks microbial genomes as well.

   1. 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.

   2. 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.

   3. 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.

   4. Proteomics is the large-scale study of proteomes. A proteome is a set of proteins produced in an organism, system, or biological context.

   5. 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.