With U.S. building codes now starting to address air tightness with respect to energy efficiency, many builders find themselves struggling to meet the defined ACH50 values (air changes per hour at 50 Pascal) required by their local codes. This measurement is an assessment of air leaks as monitored via the pressurization of the whole building in a Blower Door Test.

That, however, is not the case at Beartooth Passive House, a young building company located in Red Lodge, Montana. In their first building ever constructed, they have achieved staggering ACH50 average values of 0.16-0.19 (1/h) for depressurization and pressurization.

ACH50 average values of 0.16-0.19 (1/h)!!

“This is proof that anyone can build an airtight building envelope as long as the details are thought out in advance and executed with care,”

says Tully Gallagher, founder of Beartooth Passive House, and a Certified Passive House Tradesperson.

“I’m definitely a detail oriented person, but I’ve never built a house before. If I can do it, there should be no excuse for anyone else that it’s too hard to build air tight.”

Those ACH50 results are remarkably better than the maximum value allowed to achieve Passive House building certification, which is set at 0.6 (1/h).

Compared to the typical national building code requirement of 3.0 ACH50, Beartooth’s first building is about 15-20 times more airtight. It’s also worth noting that the state of Montana actually amended their state building codes to leniently allow for up to 4.0 ACH50.

The third party crew that performed the Blower Door Test was stunned by the values they recorded. Said their spokesperson,

“I’m not saying this is impossible. I’ve just never seen anything quite like this before”.


Passive House is internationally recognized as the most stringent energy efficiency standard in the world. With the primary focus being placed on delivering high quality building thermal envelopes, the resulting structures are very healthy and comfortable buildings. Focusing on the durability and resiliency of the indoor environment incidentally leads to a 75% reduction in energy demand for heating and cooling compared to code-compliant new builds (and 90% less energy compared to existing buildings). Implementing passive solar strategies within a holistic thermal envelope approach and best practice building science, Passive House is a streamlined pathway for any building (including retrofits) to reach Net Zero.

Beartooth Passive House was the first company to join Emu Systems’ North American Pilot Program. Emu has been developing pre-designed construction systems to enable American builders to affordably meet the Passive House standard in different climate zones of continental North America. The Beartooth Passive House project, located in Climate Zone 6, is currently going through Passive House building certification, with the process scheduled to be completed once the house is occupied by the end of summer.


Tully’s take away after testing his first building?

“The amazing part of all this is that the relatively small amount of time spent on proper detailing results in decades upon decades of realized comfort, health, and cost savings.

It will never be easier to properly detail and seal an air barrier than upon initial construction of the building. Imagine if someone told you that if you just spend an additional 30 minutes a day exercising over the next month, you would be able to stay healthy and in good shape for the next 50 years. How many people would pass up that opportunity?”


Stay tuned for a follow up post about the moisture readings we took on site after the Blower Door Test, as part of our Pilot Program. To stay updated, sign up for our newsletter!

Learn more:
Emu’s North American Pilot Program (accepting applications)
Advanced CPHT Builder Training (registration for May class open)
Why is airtightness important?
The potential benefit of an early ‘Blower Door Test’

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