Welcome to week 4 of PBA's blog series on the coming updates to the Michigan Energy Code! We're covering essential updates in the new Michigan Commercial Energy Code (based on ASHRAE 90.1-2019) before the adoption date on April 22, 2025. Click here if you've missed our previous posts!
Last week, we started discussing Envelopes - the architectural assemblies that keep the conditioned air inside and keep the rest of the world outside. The excellent article "The Perfect Wall" (link) by building scientist Dr. Joseph Lstiburek describes the four major roles of building Envelopes:
ASHRAE 90.1-2019 includes requirements for air leakage and thermal energy management. Last week we discussed the thermal side. This week, we continue the Envelope discussion with Air Barriers. Depending on your building's geometry, air leakage can be as much or more a source of energy usage as the thermal losses. The need to provide an air barrier isn't new - the outgoing version of the Michigan Energy Code requires air barriers, but doesn't define performance criteria. The new Code retains the requirement, but adds performance criteria and testing requirements.
If air leakage is so bad, why haven't we addressed this before? One of the biggest factors is mold. Lots of materials are fantastic air barriers: polyethylene, aluminum foil, bitumen, etc… Unfortunately, these materials also tend to be exceptional vapor barriers (aka: they block humidity). Although blocking the movement of air is excellent for improving building performance, blocking the movement of humidity can be a building death sentence. We'll skip over those details, but suffice to say: the wrong application of a vapor barrier can cause water to precipitate out of thin air inside your walls, which can lead to mold growth and structural failure. To summarize with a Lstiburek quote, "although air barriers are a good idea everywhere, vapor barriers are not."
Luckily, science has made great strides in building materials, and there are now a variety of air barrier solutions that have minimal vapor retardance. Again, the details are not massively important for an MEP-focused blog, except to say that the technology is mature and available on the market. Implementation details are similarly left as an exercise to our architectural partners.
So what does the Energy Code require? A continuous air barrier. Walls, Roof, Basement Floor, Windows, Doors, the whole shebang. All of the envelope assemblies must work together to create a building that can somewhat be blown up like a balloon. As mentioned before, this isn't new - the outgoing code requires this. What's new is the numbers: 0.4 cfm per square foot of exterior envelope area, when tested at 0.3 inwg. In other words: take a fan, pressurize your building up to 0.3 inwg (about 0.01 psi), and then measure the cubic feet per minute (cfm) of air it takes to maintain that pressure. Normalize that airflow by dividing by the total envelope area of the building in square feet. Normalizing by building size allows us to compare leakage apples-to-apples between projects: a leakage of 1000 cfm might be an excellent result for a large building, but an abject failure for a small building.
Those who have worked with air leakage before, perhaps in venues like the Passivhaus standard, may be more accustomed to seeing air leakage described in a format like "3 ACH 50." This is the same approach as what 90.1-2019 requires, but using different normalization and metric units. "3 ACH" is 3 air changes per hour - instead of dividing the pressurization airflow by the building envelope area in square feet, you divide by the building volume in cubic feet. Instead of 0.3 inwg, you're using 50 Pascals (roughly 0.2 inwg). The two units of measurement are not directly convertible, but 0.4 cfm/ft2 at 0.3 inwg is roughly 3 ACH 50.
Installing the air barrier is great, but if there are gaps then it's as useless as a balloon with holes. The new code requires validation of the air barrier, either through inspection or blower door testing. The 90.1-2019 standard makes it pretty clear they prefer the blower door - a test where a building door is replaced with a calibrated fan, and the required pressurization airflow is measured directly. Larger building? Just use more doors. This test must be done after the air barrier is complete, and should be done before finishes are started. This gives you the chance to correct any issues discovered before doing so would be a major pain. For added flexibility, if you fail the test by less than 50% (i.e. your leakage rate is 0.6 cfm/ft2), you're allowed to just patch the leaks and not re-test. Fail the test with a leakage rate higher than 0.6 cfm/ft2? You'll need to make repairs and then re-test.
In lieu of blower door testing, the Code does allow you to substitute a program of design review and periodic inspections. A third party envelope commissioning authority must verify that your design drawings meet the Code requirements for air barriers, and then make field visits during the installation of the air barrier elements to ensure that they're being installed correctly. Personally, I'm advocating for the blower door testing wherever possible - getting actual performance data on a design is one of the best ways to find out if a new design is working, and to find out where and when improvements are necessary. Learning to design better air barriers through data feedback will help us to build more energy efficient buildings.
A final caveat to the engineer: 0.4 cfm/ft2 is the leakage at the test pressure. We raise the building pressure to stress the envelope into revealing its secrets, but our buildings do not usually operate at 0.3 inwg positive pressure. Appendix G offers a conversion rate between air leakage at test pressure and air leakage at more normal operating conditions. It's roughly 10% of the tested condition - i.e., a building designed to leak at 0.4 cfm/ft2 under test will actually leak about 0.04 cfm/ft2 under operation. But, as mentioned at the start of this blog, that value can still result in heating loads as great as the direct thermal losses through the envelope.
Next week, we'll talk about new requirements for making your roofs ready for solar panels.
