The impact of natural and forced convection upon building performance. There is mounting evidence to suggest that buildings that are being designed to achieve thermal performance standards, including the Building Regulations, are in fact consuming in excess of 40% more energy than the predicted values. In some cases the increase in energy consumption can be up to 70% greater than that predicted, says Mark Siddall.
This is an eight page article. First published in June 2009
In developing the design for a residential development that incorporates 25 PassivHaus standard homes at the Racecourse Estate, Sunderland, (see last issue) concerns relating to air movement within and through walls, otherwise known as ‘thermal bypass’, have had a significant influence upon the project. Here Mark Siddall reviews existing literature relating to the subject.
This study serves to develop an understanding of the extent to which thermal bypass mechanisms can impair thermal performance, enable the adoption of appropriate performance targets and, where possible, inform the reader of some of the technical strategies and solutions that are available.
Harrje,  describes thermal bypass as heat transfer that bypasses the conductive or conductive-radiative heat transfer between two regions. Defined in this manner, convective loops, which can include both air infiltration and wind washing, constitute a form of thermal bypass. In this context it should be recognised that the term thermal bypass is being applied to largely unfamiliar, and often unregulated, heat transfer. Furthermore it is an acknowledgement that air movement can lead to a significant increase in the heat loss when compared to predicted values. This means that even when the designer thinks that a design has addressed the performance requirement, it is very likely that it has not.
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