Thermal bypass a basic step to improving building performance

11 April 2024 by Jason Quinn

I get asked periodically how to increase building performance without spending extra money. Well, better-than-Code performance is going to cost something so I focus on low-hanging fruit: what’s the greatest benefit for the least cost? Let’s assume continuous mechanical ventilation is already included, because that’s the single most important thing. After that, I recommend preventing thermal bypass via a dedicated air control layer. Thermal bypassing is bad and fixing it offers a lot of bang for your buck. 

For those who don’t want to spend the money to take this advice, then at least consider the approach recommended in the US EPA’s document, Thermal Bypass Checklist. You’ll note it dates back to 2008 (the design of the document looks like something out of the 1970s) so this is not cutting edge work. But it’s a starting point if the project isn’t going with a dedicated ACL. 

Cover of Mark Siddall's report Thermal bypass risks

Thermal bypassing is bad. It’s the air movement through and around insulation and it has a very big impact on building performance. In the High-Performance Construction Details Handbook, I pointed out that thermal bypass can reduce the thermal performance of wall assemblies by 30% or more. We’ve discussed thermal bypass previously and recommended a brilliant research paper (illustrated above). However I’m sharing the EPA Checklist because it was prepared by and written for builders—and airtightness expert Sean Maxwell recommended it. So if you want hands-on advice, this is it.

The authors describe the document as “a comprehensive list of building details where thermal bypass, or the movement of heat around or through insulation, frequently occurs due to missing air barriers or gaps between the air barrier and insulation.”

Big caveat: this is an US document written for American conditions. The text is applicable for us in New Zealand but some photos illustrate practices unsuitable for our climates, such as a polythene vapour barrier layer. That’s only OK in some very cold climates.

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