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Tuesday29 July 2014

Tackling Thermal Bypass

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Devereux Architects’ Mark Siddall explains thermal bypass, one of the topics up for discussion at this year’s free Sustainability Now virtual conference.

In developing the design for a residential development that incorporates 25 homes that meet the PassivHaus Standard, concerns relating to the subject of “thermal bypass” have had a significant influence upon the project and lead to the development of new quality assurance systems

Thermal bypass is heat transfer that bypasses the conductive or conductive-radiative heat transfer between two regions. Defined in this manner thermal bypass includes convective loops, air infiltration and wind washing. In this context that 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 architect, and builder, thinks that a design has addressed the performance requirement it is very likely that it has not.

In principal there are two forms of convective loop bypass that occur predominantly through natural convection. “Closed loop” convection may be observed where the air mass remains largely unchanged but temperature differences exist at the boundaries causing re-circulatory air flow whereby the air moves in a loop. A good example of this could be an unventilated cavity wall. “Open loop” convection allows an air mass to be replaced by other air and therefore includes air gaps that permit air flow, and thus heat transfer, between two regions. This form of heat loss is the result of failures in airtigthness and wind-tightness.

“Closed loop” convection can result in significant failures in thermal performance. In principle this is driven by stack effect. A range of studies have shown that even narrow air gaps between the (internal) air barrier and the insulation and small gaps in the joints between insulation have been shown to result in significant heat loss; the proportionate impact being more than a 160% increase compared to the calculated U-value.

In principle “Open loop” convection this is driven by the wind penetrating the thermal envelope. This form of heat loss can be addressed by air tigthness and wind-tightness. Air tightness may be defined as “the property of preventing air from penetrating through the shell” and wind-tightness as “the property preventing air from penetrating into the shell so that the thermal insulation property of the insulation material is not reduced.”

In conditions where there is poor air tightness air flow through 300mm of insulation, with a velocity of 2.5m/s, would result in a 35% reduction in thermal performance. For this reason airtight construction is a perquisite and all joints, cracks and services penetrations through the air barrier should be sealed accordingly.

Wind washing can affect the thermal performance insulation, short-circuit the performance of insulating sheathing, and cool down an air barrier system located towards the outside of the wall assembly (potentially below the dew point temperature). Poor windtightness has been shown to result in catastrophic failures; one study examining as-built defects reports that the heat loss was increase by up to 660% beyond the U-value. Areas that are at particular risk to wind washing are eaves, gables, ridge and corners.

In the context of thermal bypass party walls deserve special attention. The convention in England and Wales, supported by the Accredited Construction details, is to apply the air barrier internally. This has meant that air movement within the party walls between properties has remained unaddressed. Part L 2010 now includes more detailed consideration for this condition and reflects research by Leeds Metropolitan University (LMU). Based upon studies by LMU a number of strategies are now considered within Part L. The first two approaches are considered to achieve a U-value of zero. The first method is to have a solid a party wall however careful consideration should be given to acoustics. The alternative is to fully fill the cavity with insulation. The final option is to provide a continuous seal within the cavity in line with the surrounding insulation – this is achieved by placing an insulated sock wrapped in polythene, but this less successful method achieves a U-value of 0.2 W/m2K. An unfilled and unsealed cavity will have a U-value of 0.5 W/m2K.

If thermal bypass is to be addressed, and carbon reduction objectives are to be met, industry wide training needs to be provided at a national level. Until then building failures will continue and a cost to clients (both private and public), government and the environment. For buildings to perform as expected the ability for designers and non-designers to recognise and avoid thermal bypass mechanisms is critical.

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