BUILDING ENVELOPE | THERMAL BYPASS RISKS TOWARDS A COMMON LANGUAGE: Closed loop: Isolated from forced convection, which is incurred by the wind or mechanical systems, closed loop bypass occurs within a cavity space. Though a familiar example would be air movement within double- or tripleglazed windows, this also happens in walls and roofs. This natural convection arises within insulation when the temperature difference between the cold and warm faces causes air to change density and generate sufficient force as to enable air movement to take place. The warmer air layer rises and consequently the colder, denser air descends. The net effect of convection upon energy demand is a function of the area affected, the temperature difference, and the duration of the cold weather conditions. How do we prevent thermal bypass? At a conceptual level, the ultimate goal is to completely encapsulate insulation between Figure 1: Insulation between the air and wind barrier Before getting into the detail, it is important that we develop common terms of reference and to do this we need to get our language straight. Air gaps occur on the warm side Cavities are on the cold side Joints are between abutting interfaces Rainscreen Cavity Wind barrier Warm side / inside we need to achieve. The standard of windtightness required can be correlated to the U-value being sought and is discussed in greater detail within the Passivhaus Trusts thermal bypass paper. Joint/interface Cold side /outside Insulation Air barrier Substrate Cavity: Capillary break, drainage and ventilation Air gap the air barrier (red) and wind barrier (blue), and to achieve a suitable standard of workmanship in the process (Figure 1). Implementing successful strategies that manage thermal bypass is somewhat nuanced, which is why the Passivhaus Trusts paper sets out a pragmatic framework that can be applied to building projects. informed, intelligent decisions that will: n Improve the health, wellbeing and energy security of building occupants and owners n Reduce carbon emissions and fuel poverty n Close performance gaps n Avoid costly mistakes n Protect the building fabric from moisture damage. How can the paper help you with your project? The paper isnt just about the problems caused by thermal bypass, rather it gives you all the tips and tricks youll need for preventing thermal bypass risks, so that you can make informed, intelligent decisions about your projects. If you would like to learn more about thermal bypass and how you can address it, download the paper Thermal Bypass Risks: A Technical Review from the Passivhaus Trust website bit.ly/CJNov22PT1. CJ Whether its a new build, a conversion or a renovation, the paper draws upon a sound evidence base that reveals simple, practical steps that policymakers, developers, certifiers, designers and constructors can take to create successful, high-quality, highperformance and robust buildings at speed and scale to meet climate targets. The paper provides new insights that build confidence and help you to make MARK SIDDALL is a director of architecture and research at LovinglyEngineeredArchitecture.com. 44 November 2022 www.cibsejournal.com CIBSE Nov 22 pp42-44 Thermal bypass risks.indd 44 21/10/2022 16:23