Shorts-Blower Door Test Result-Barndominium Project

Those of you that have been following this blog for a while know I have been involved with a local project, I’m calling the Barndominium.  It’s part conditioned storage and part home, nearly 10,000 square feet in total and a true post and frame structure with posts at 6-foot centers and no structural sheathing anywhere.

I recently conducted a blower door test on the entire structure, including both the storage and living spaces.  This is the largest structure; in both square footage and volume I have ever tested.  I also calculated the surface area of the building so that the CFM/square foot of surface area metric could be calculated.  The test results are:

Air changes per hour at 50 Pascals (ACH50).  This is the most common metric used to determine how leaky a structure is.  Codes in new construction in my climate require no more than 3 ACH50 to pass a blower door test.  Most homes I test in my market are around 2 ACH50.  The Passive House standard, which is one of the hardest to pass, requires no more than .6 ACH50.  My goal with this build was around 1 ACH50, we managed to hit .38 ACH50.  The second tightest structure I have ever tested, and my personal best.  (We designed and installed both the air and thermal control layers.)

There are several other ways to look at blower door test results.  For instance, I’m interested in the cubic feet per minute of air moving through the building during the test.  Large buildings, such as this one, have an advantage over small structures when it comes to the ACH50 metric.  A small, tight home might have 250 cubic feet per minute (CFM) of air moving through the home and fail the blower door test using the ACH50 metric.  Most homes I test are around 500 CFM.  I recently tested an older home that had more than 5,000 CFM.  The Barndominium’s CFM number was 945.

One last metric, the CFM per square foot of surface area.  The 2021 International Residential Code, for the first time, allows this metric to be used as an approved test result for homes less than 1500 square feet.  Structures leak air in their surfaces, not in volume.  Most building scientist would like to see this used rather than the ACH50 number.  The CFM/ft² of surface area is a better overall way to judge how leaky a structure is, it levels the playing field between large and small homes.  To pass the 2021 code using this metric, a home must achieve less than 0.30 CFM per ft² of surface area.  The Barndominium hit 0.0389 CFM/ft².  All three ways to show air leakage rates are indicating the Barndominium is very tight.

The test I performed on this structure was a multi-point blower door test.  This test predicts the leakage rate at normal building pressures, not just the higher test pressures of 50 Pascals.  Multi-point tests also estimate the size of the cumulative holes in the structure.  The Barndominium has over 24,000 square feet of surface area.  The software estimates a 7-inch by 7-inch hole over that entire 24,000 square feet at a more normal building pressure of 4 Pascals.

Stay tuned for a couple more blog posts on this build, including the HVAC systems and photos of the finished project. You can view past posts by clicking on the Barndominium Project category to the right of this post.

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