New thermal bridge requirements in US will influence NZ

20 June 2023 by Jason Quinn

There are new thermal bridging requirements being rolled out in the US, of which ASHRAE has just summarised. ASHRAE standard 90.1 is for all new buildings excepting low rise housing and the 20220 version introducees many changes to improve energy efficiency. 

Fun (?) fact: there are many thousands of different Building Codes across the US. However, 90.1-2022 is a core part of them. MBIE often cites portions of this standard in New Zealand’s Building Code, sometimes without realising the implications of only taking excerpts.

Do remember that the US uses imperial R-value units and these are different from the metric units used to express R-values elsewhere. The numbers are very different! Metric R1 = Imperial R5.678.

In an update to Section 5.5, Building Envelope thermal bridging impacts become tradeable rather than mandatory. Section 5.5.5 outlines linear and point thermal bridges that need to be addressed. As ASHRAE 90.1 is for bigger buildings, significant resources were spent on studies in order to  develop a prescriptive path for increasing R-values to compensate for not-perfect-but-reasonable junctions. If the prescriptive path can’t be taken, then a full calculation method with thermal bridge calcs similar to Passive House modelling is required.

There is an exemption for wood penetration, similar to how we often model Passive House buildings. It is written for materials that have a thermal conductivity below 0.43 W/(mK), which means all wood and lots of fibreglass-based thermal break materials.

I’d love to work through this at length and adapt the lessons for Aotearoa New Zealand but that will have to wait a while given my workload. If someone else gets to this first, please let me know.

Image is from the article and is from ASHRAE 90.1-2002 Appendix K

“Thermal Bridging Requirements in Standard 90.1-2022,” ASHRAE Journal June 2023, pg 42-46.

By Len Sciarra, Aia, Member ASHRAE; Rahul Athalye; Jonathan Humble, Faia, Member ASHRAE; Chris Mathis, Fellow/Life Member ASHRAE; Ben Meyer; Mike Tillou, P.E., Member ASHRAE; Robert Zabcik, P.E., Associate Member ASHRAE

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