BUILDING ENVELOPE | THERMAL BYPASS RISKS PASS JUDGEMENT Often slipping under the radar in building design, thermal bypass can result in major energy performance gaps. Mark Siddall, director of architecture and research at LEAP, discusses measures to mitigate this problem Wind barrier at Larch Corner Passivhaus T hermal bypass is a largely unregulated, little-known phenomenon with the potential to cause failures in building performance. However, with the correct knowledge and skills, both in practice and onsite, failures in design and construction can be avoided and buildings can perform as predicted. Founded upon sound physics, buildings that are built to the Passivhaus standard have been demonstrated to perform reliably as intended. However, this is far from the norm within the construction industry at large. There is an increasing recognition of the energy performance gap and the consequential development of means to reduce this. One of the major contributors to the energy performance gap is faulty, illconsidered design and construction that can result in a thermal bypass. A thermal bypass is heat transfer that bypasses the conductive or conductive-radiative heat transfer between two regions. Defined in this manner, convective loops, which include both air infiltration and windwashing, constitute a form of thermal bypass. (See graphic for common languages, page 26, when talking about thermal bypass.) Earlier this month, I completed a paper which draws together 68 years of research. Summarising 170 peer-reviewed papers and related documents, the paper establishes that thermal bypass, air movement across, Defective workmanship showing insulation gaps The Passivhaus Goldsmith Street development in Norwich within and behind insulation, can lead to about 10 times more heat loss than predicted leading to larger bills and CO2 emissions plus drafts, discomfort, and mould. But this need not be the case. Workmanship and buildability As all good designers will appreciate, we cant entirely close performance gaps. We have to allow some tolerance. Thats why, following research undertaken in Norway, the paper proposes a standard that limits the effect of air movement to 5% of the calculated U-value. But we first need to recognise that the ability of insulation to fill an air gap is dependent upon the standards of workmanship on site, and that these standards of workmanship are in turn heavily dependent upon the appropriate design, detailing and specification of an assembly. Often overlooked, factors such as the level and flatness of a substrate can have 42 November 2022 www.cibsejournal.com CIBSE Nov 22 pp42-44 Thermal bypass risks.indd 42 21/10/2022 16:22