Research Points to Importance of Mitigating Thermal Bridging
Originally published by: Sourceable — October 2, 2014
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Research and monitoring of buildings is increasingly showing the importance of reducing thermal bridging in new construction and mitigating the impact in existing buildings
Thermal bridges are localized areas of high heat flow through walls, roofs and other insulated building envelope components. Thermal bridging is caused by highly conductive elements that penetrate the separation between the interior and exterior planes of a building, allowing heat flow to bypass the insulating layer and reducing its insulation effectiveness.
The thermal bridge may go through a wall and its insulation, through the space between the glass and steel of a curtainwall or simply through a leaky roof.
The impact, however, can be significant to whole building energy use, the risk of condensation on cold surfaces, and occupant comfort.
Green codes such as the USGBC’s LEED standard award high points for buildings that utlise natural light rather than artificial light and enable sunlight penetration through curtainwall, clerestories and large windows. But more glass can often lead to more opportunities for thermal bridging and other penetrations of a building envelope.
The traditional approach of building codes to reducing space heating loads in buildings was to introduce progressively higher levels of thermal insulation and more stringent glazing performance requirements. This was because, in the past, the effects were assumed to be negligible.
However, we now know that the additional heat flow due to major thermal bridges can add up to be a significant portion of the heat flow through opaque envelope assemblies. Studies have shown that the contribution of details that are typically disregarded can result in the underestimation of 20 per cent to 70 per cent of the total heat flow through walls.
The cost of adding extra insulation, not just additional materials but also potentially reduced useable floor space, is not justified if no substantial energy savings are realized in practice.
The primary aim of the guide is to raise awareness of the impact of and methods to mitigate thermal bridging. It provides four key outcomes:
- Catalogues the thermal performance of common building envelope assemblies and interface details
- Provides data driven guidance to enable engineers to more easily and comprehensively consider thermal bridging in building codes and bylaws, design and whole building energy simulations
- Examines the costs associated with improving the thermal performance of opaque building envelope assemblies and interface details, and forecasts the energy impact for several building types and climates
- Evaluates the cost effectiveness of improving the building envelope through more thermally efficient assemblies, interface details and varying insulation levels
The report was produced by international engineering firm Morrison Hershfield developed in collaboration with co-sponsors and industry partners BC Hydro Power Smart, the Canadian Wood Council, Fortis BC, FPInnovations, and the Homeowner Protection Office.