This post first appeared on the Rockwool R-Class Website
The first time I visited one of Kyle Stumpenhorse’s (RR Buildings) jobsites was in December of 2022. I made the trip to Illinois to blower door test one of Kyle’s Barndominium projects, the project he calls “Building a Barndominium”. The barndo we tested was post and frame construction using closed cell spray foam (CCSF) as both the insulation and air sealing for the project. The home tested very well, 0.41 ACH50.
I got to know Kyle a little on that trip, I was very impressed by his attention to detail and willingness to learn. He pushes himself to do better on every new build. I wasn’t surprised to find out his next project he was planning on using Rockwool insulation. He also had a goal, beat the blower door score of the project using closed cell spray foam as the air barrier. Could it be done?
Before we get into the air control layer products Kyle used and the blower door test results of his most recent project, which he is calling “Building the BEST House”, let’s chat about how a barndominium is constructed. The biggest difference is how the structure is framed compared to standard light framing methods. Post and frame structures are constructed by having posts that act the wall frame. Posts can be set in a drilled hole that extends below the frost line which is common for non-barndominium builds. Posts can also be secured to poured concrete footings or as part of a monolithic slab. Once the posts are set, horizontal 2x wall girts are fastened directly to the posts. These wall girts are installed both inside and outside the posts and become the “wall sheathing”. Spacing between the wall girts will depend on engineering specifications, but 2-foot centers are common. Roof trusses are set next, usually fastened to the posts. Much like the walls, 2x boards are nailed on top of the trusses, these purlins become the “roof sheathing”, 2-foot centers are again common. Once the framing is complete, the frame is often covered, both on the walls and roof, by a corrugated steel panel, 3 feet wide by usually whatever length is needed.
On Kyle’s latest project, he chose to use Rockwool instead of CCSF. Seeing as the structure is framed with 8-foot on center posts, (Kyle coined the term advanced, advanced framing), the choice to use Rockwool Comfort Batt in 24” wide widths work perfectly to completely fill the cavity. The density of Rockwool’s insulations helps to reduce the amount of air that can move through the product, but it’s still a fibrous insulation, it won’t completely stop air. Closed cell spray foam can do a better job at reducing air movement, but CCSF is also a vapor retarder, slowing the movement of moisture. Being vapor open, Rockwool has an advantage at allowing assemblies to dry, but you will need to use additional products to reduce air infiltration and exfiltration. The wall assembly on Kyle’s latest project uses two products for air tightness, on the exterior, Kyle used LP’s Weatherlogic sheathing product (a coated oriented strand board (OSB) with integrated weather resistive barrier (WRB) using tapes and sealants to create an airtight assembly). On the interior, a membrane was used. Siga’s Majrex, a membrane designed to act as both an air barrier and vapor retarder. Both the OSB sheeting, and membrane greatly reduce air movements through the wall, but still allow for some drying potential in the inward and outward directions.
So, how did Kyle do on the blower door test for his second high performance barndominium? I brought in some help for this test, Jake McAlpine from The Energy Conservatory and Minneapolis Blower Door made the trip to Illinois with me for the testing. Kyle left a few open holes around the building to test how they can affect the overall blower door numbers. The first blower door test had this barndo at right around 1 ACH50. A good score seeing as the air tightness code requirement for this part of Illinois is 4 ACH50. I could tell right away that Kyle was very disappointed seeing as his previous barndominium tested at .41 ACH50. We began looking for the holes that required sealing and found an unexpected large one, a 20-foot seam in the interior Siga membrane that connected to the attached garage was missing tape. After sealing this opening, a second test was conducted. The new blower door score came in at .43 ACH50. 170 CFM50 was the actual air leakage rate, Kyles’ previous barndominium (with the closed cell spray foam) had a leakage rate of 377 CFM50, more than twice his latest build.
This is where it gets confusing. What number matters more, the air exchange per hour metric (ACH), or the air leakage rate of the building envelope (CFM per square foot of surface area)? The ACH compares the cubic feet per minute of air leaks to the home’s volume, but buildings don’t leak in volume – they leak at the surfaces, the shell of the building.
Considering that the first barndominium is 4,800 sq. ft., and the second, 2,600 sq. ft., it’s hard to compare “apples to apples” on the ACH. A better comparison is the CFM per square foot of surface area number, which is simply a better indication of tightness. The surface area of Kyles first barndominium was 13,450 square feet. The leakage rate was 377 CFM. If we divide the CFM of air leakage at 50 Pascals by square feet of surface area (377/13450=.028) we end up with .028 CFM/square foot of surface area. The Best House project had a leakage rate of 170 CFM and a surface area of 7730 square feet. By using the same formula, the Best House project has a leakage rate of .022 CFM/square foot of surface area. (The square foot of surface area is calculated by adding the area of all six sides of a home’s conditioned space, this includes conditioned basements and attics.)
Comparing the blower door test results on Kyle’s 2022 “Building a Barndominium” project and his 2023 “Building the BEST House” project to see which was more airtight. The Best House project is the tighter build.
Back to my original question, can you build an airtight barndominium without using closed cell spray foam. Absolutely! Choosing the right materials and taking the time to properly install those materials can work to make a post and frame structure airtight, Kyle’s latest build is proof.
Here’s Kyle’s video on the process of testing his Building the Best House project. (31) Can a House be too Air Tight? – YouTube