Passivhaus Affiliate

NEW Thermal Bypass paper

Known as thermal bypass, air movement across, within and behind insulation increases heat loss, causes discomfort and enables mould growth. With the correct knowledge and skills, both in practice and onsite, failures in design and construction can be avoided and deliver buildings that perform as predicted.

Whether it’s a newbuild, a conversion or a renovation, this guide draws upon a sound evidence base which reveals simple practical steps policymakers, developers, certifiers, designers and constructors can take to create successful high-quality, high-performance and robust buildings at speed and scale to meet climate targets.
Read this paper to gain new insights, build confidence and discover how you can make informed, intelligent decisions that will:

  • improve the health, well-being and energy security of building occupants and owners

  • reduce carbon emissions and fuel poverty

  • close performance gaps

  • avoid costly mistakes

  • protect the building fabric from moisture damage


Types of thermal bypass (a) closed loop (b) open loop: wind washing (c)open loop: air leakage (infiltration or exfiltration) (d) reality


By contextualising an understanding of thermal bypass, this paper provides a comprehensive and technical demonstration of how Passivhaus reliably closes the performance gap. If thermal performance gaps are to remain within an acceptable tolerance, then continuous insulation should be encapsulated on all sides by uninterrupted, unbroken air and wind barriers.

The paper emphasises the invaluable opportunity of implementing Passivhaus at scale to provide a robust solution that delivers high performance construction, energy savings, carbon emissions, comfort, health and well-being that owners, investors, occupants can rely on both now and in future.

This paper has been authored by PHT technical advisor Mark Siddall, PHT member LEAP.




Thermal Bypass risks

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 You can hear more about the paper from the author, Mark Siddall, in this Passive House Accelerator podcast





This paper sets out the issues created with poorly installed insulation in a clear and precise way and has the potential to change the way the industry thinks about insulation and how we design to avoid thermal bypass in the future.



This is an important and timely publication that provides a comprehensive and systematic review of the literature available on the relatively little known and understood phenomena of thermal bypassing. The publication will be of relevance to both practitioners, policy makers and academics, providing not only an invaluable reference source but also practical guidance on how to minimise thermal bypassing.

Leeds Beckett University


This is a thorough publication which provides technical readers with useful guidance on both the building physics background and practical implications. CIBSE welcome its contribution to the delivery of energy efficient new builds and retrofits, and to reducing risks associated with poor performance.



As national climate strategies continue to emphasise reducing emissions from buildings, this is a useful guide for practitioners in the construction sector who are dealing with the issue of thermal bypass. The guide blends analysis of peer-reviewed research with practical guidance for those involved in building and retrofitting buildings. As a single point of reference on a complex topic, this guide will be a valuable resource for the sector as it delivers the sustainable buildings that our low emissions future demands.



This is a very useful paper which brings all aspects of thermal bypass into one place. The solid evidence base presented makes a case for better practice that helps close the performance gap. Going forward it is a great resource for members’ own research and development.



The ASBP fully endorses the Passivhaus Trust’s technical review of thermal bypass risks.  The document offers a much-needed depth of coverage and provides an invaluable point of reference for an area of thermal performance often overlooked.  This can only help raise the bar in terms of good design and performance expectations.



There is a general lack of awareness of thermal bypass, which has a significant impact on the thermal performance of our homes. The Good Homes Alliance is pleased to endorse this guide that will help address this issue, with clear diagrams highlighting the practical implications of thermal bypass and what we can all do about it.

Good Homes Alliance


We warmly welcome this review of the risks that thermal bypass poses to energy efficiency and thermal performance.

Building Performance Network


Energy efficiency is one of the key pillars of Net Zero carbon buildings, alongside low carbon heat, renewable energy and demand flexibility. This detailed technical paper on thermal bypass risks by the Passivhaus Trust brings current and previous research information into one place and provides clear advice on how to minimise these risks during design and construction. LETI supports this excellent paper and the continued effort by the PHT to share and disseminate this type of knowledge in the building industry.



In optimizing Passive House performance the relationship of insulation and air movement is a particularly tricky one to navigate. So it is with great pleasure that I read The Passivhaus Trust's report on Thermal bypass risks. This report is clear, authoritative, and actionable. I expect it will help practitioners - not only across the UK but around the world - to make more effective high-performance enclosures. Well done!

The Passive House Network


We are pleased to see this important document being published. We hope this publication will help educate and inform the industry, from designers to builders to regulators, so that we can see improved construction durability and higher thermal performance.

Passive House Institute New Zealand


This paper provides both a useful primer and serves as a detailed reference guide for all those involved in high performance buildings to identify and mitigate risks in regards to thermal bypass.

Passive House Canada


This paper, published by the Passivhaus Trust, clearly explains the relevance of thermal bypass for a building's energy performance and highlights the importance of good design and on site implementation to successfully achieve wind-tight construction. The clear recommendations and easy-to-understand guidelines on how to successfully prevent thermal bypass are particularly valuable.

iPHA (international Passive House Association)


Further information

Technical Guidance - Thermal bypass risks

PHT Guidance

Treehugger: What is Thermal Bypass? - 7 October 2022

30th September 2022

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