Thermal bypassing publication a must-read Mark Siddall’s work is outstanding

16 October 2022 by Jason Quinn

If you design building assemblies you need to make the time to read this new publication.

Thermal bypass—the air movement through and around insulation—has a very big impact on building performance. In the High-Performance Construction Details Handbook, I noted that thermal bypass can reduce the thermal performance of wall assemblies by 30% or more.

The Passivhaus Trust in the UK has published an incredibly valuable resource written by Mark Siddall on thermal bypass and its impacts on buildings. It’s very well referenced with the useful information included in the text (where else do you find airflow rates inside of insulated cavities for physics simulations?).

Mark wrote an excellent short technical paper in 2009* which I summarised down to the three most important rules to follow in order to prevent thermal bypassing.

  1. The thermal control layer is contiguous (eg at the roof wall junction, and around the windows to the insulation gap between the glass) and that it lines up.
  2. There is no windwashing through the insulation.
  3. The insulation is fitted tightly without gaps.

Mark spoke at the very first Australasian Passive House Conference in Auckland and I’ve been referencing his excellent work on real world measurements of Passive House for years, for instance Reality check: PHPP accuracy in the real world and Supermodels are good for business

* “Thermal Bypass – The impact of natural and forces convection upon building performance,” Green Building Magazine, (2009)

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