More Solar Heat Gain Than You Can Shake A Stick At
Guest Author, Jacob Zoske of Third Pig Construction in Atlanta, GA
jacob@thirdpigconstruction.com
www.ThirdPigConstruction.com
During the April 2018 Building Science Philosophical Society meeting there was much discussion about the performance of different types of roof systems and how well they keep radiant heat out of our attics. While the group had a variety of ideas and opinions, no one was able to point to any hard data. Leaving the meeting with more questions than answers, I set out to do a small empirical study... in my backyard.
The objective was to build a few residential ”cool” roof systems and see how they performed during an Austin summer. To do this, I built two fully enclosed 4’x4’x2’ “attics” out of ZIP sheathing and suspended a SensorPush temperature/humidity sensor more or less in the middle of the box. See the photo below.
While it would have been useful to measure the actual heat transfer through the roof system, it wasn’t practical for this experiment. Instead I compared the daily temperature inside each “attic” to a control variable to better understand the performance improvements of each “cool” roof system. At the same time, I was also collecting similar data to compare conditioned and vented roof assemblies which will be discussed at the end.
Please keep in mind that the test “attics” were NOT vented and the two sunny sides of the structure were shaded by Phifer SunTex 90
For those interested in more details and temperature data please see the full slide deck.
Roof Types Tested
Galvalume roof with a ¾” air gap
Union Corrugating ribbed steel roof panel
1” x 4” battens
ZIP sheathing
Galvalume roof with a ¾” air gap and 1” foam insulation
Union Corrugating ribbed steel roof panel
1” x 4” battens
1” foam insulation
ZIP sheathing
Asphalt shingle roof with Solarbord OSB
Owens Corning Supreme onyx black 3-tab asphalt shingles
Single layer of #30 felt paper
Solarbord OSB sheathing
“Cool” asphalt shingle roof with Solarbord OSB
Owens Corning Duration Premium Cool Shingles
Single layer of #30 felt paper
Solarbord OSB sheathing
Control Variable
Standard black asphalt shingle roof
Owens Corning Supreme Onyx Black 3-tab asphalt roofing shingles
Single layer of #30 felt paper
ZIP sheathing
Results
Each test was performed over 2-3 weeks. Here are the results from the galvalume roof with a ¾” air gap (red) compared to the standard asphalt shingle roof (blue). While the metal roof didn't exactly keep the attic cool, the peak temperature was consistently 10 to 15°F cooler.
The daily temperature difference(°F) for that test is shown below. Positive numbers indicate that the “attic” under the asphalt shingle roof was warmer than the galvalume roof.
Since the environmental conditions were different during each test period, one way to normalize the data and compare the performance is to use the average temperature difference measured inside the box during each 2-3 week test period. The summary chart below shows the results of all four test systems. Using this metric, the galvalume roof with the insulation performed the best, followed closely by the galvalume roof without insulation. The “cool” shingles and the Solarbord were also pretty effective, but not quite as good.
Conditioned vs Vented Attics
I also collected data from a couple of single story office buildings in North Austin to see how a conditioned attic with spray foam insulation performed. Both buildings have similar construction except for the different attic insulation. Both have basic black asphalt shingles. One is a standard vented attic with code minimum blown-in insulation at the floor. The other has code minimum open cell spray foam under the roof sheathing. Neither would be confused with high performance construction.
The same SensorPush data loggers were placed ~6’ above the attic floor. Each attic space contains two 3-ton air handlers, with standard leaky flex ducts, but no supply or return in the attic space. The results are as follows:
Summary
The galvalume roof with an air gap and 1” of exterior insulation seemed to perform the best, but all of the roof systems tested made a meaningful difference in reducing the solar heat gain.
The best individual way to keep heat out of an attic is... to encapsulate it. Of course an air tight, insulated attic keeps the most heat out! The results are pretty dramatic. Over a few weeks in August the average temperature in the spray foamed attic was 78°F while the vented attic was 95°F. The maximum temperature was 88°F and 129°F respectively.
In order to collect more precise data, a more extensive test is needed using more realistic test structures and testing each system at the same time. Using thermocouples to measure the heat transfer through the different layers could also provide a better understanding of how the individual layers perform.
Join us this Thursday afternoon at the Positive Energy office as our guest author, Jacob, leads us in an in depth discussion on his experiments.