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Tackling local issues for airtightness compliance


As Building Regulations have imposed ever-tougher energy performance criteria on the building envelope, the significance of localised areas of reduced insulation or thermal bridging leading to air leakage has become even more crucial. A. Proctor Group reports


T


he energy consumed by buildings accounts for a significant propor- tion of the UK’s total energy


consumption. Around 45 per cent of UK


CO2 emissions come from the built environment, (27 per cent from domestic dwellings and 18 per cent from non-domes- tic), and therefore Building Regulations relating to energy performance continue to play a major role in helping to achieve our targets for improvement.


Controlling air leakage


Air leakage through cracks, gaps, holes and improperly sealed elements such as doors and windows can cause a significant reduction in the performance of even thermally insulated envelopes. As thermal insulation requirements increase, industry consensus suggests that discrepancies between ‘as built’ and ‘as designed’ performance can be largely attributable to uncontrolled air leakage. Architects are increasingly turning to air barrier membranes as an essential part of the design process for achieving the most effective means of controlling and reducing air leaks.


A misconception when it comes to airtightness is that well-sealed buildings mean uncomfortable, ‘stuffy’ indoor environments; this is largely an effect of poor ventilation rather than airtightness. Buildings with very low rates of air leakage require correspondingly higher levels of ventilation as part of a balanced, holistic design approach. A common misunderstanding is that this increased ventilation will undermine efforts to reduce air leakage and hamper overall efficiency. It’s important to bear in mind that ventilation is controllable, and there- fore can be accounted for within the overall design, whereas uncontrolled air leakage is not.


Managing air flow


Unmanaged or uncontrolled air flow will act as a carrier for moist air, drawing it from outside to in, or pulling it from inside to out, into walls, ceilings, and roofs. The impact of uncontrolled moist air movement can have a long-term detrimen- tal effect on the durability and life of the building.


In terms of the energy efficiency of a building, uncontrolled air flow will almost certainly have a major impact. Initial heat load calculations for heating and cooling equipment will usually make an allowance for a level of natural infiltration or uncontrolled air flow. The higher the infiltration rate, the lower the energy efficiency of the building. Efficiency levels can be affected by both natural and mechanical air movements. The forces of wind and stack effects will lead to a level of air filtration and subsequent efficiency loss. Sealing the shell of the building and any un-designed holes can reduce the impact of wind and stack effects.


Building Regulations compliance


Key guidance related to airtightness compliance is outlined in the Building Regulations Approved Document Part L1A Conservation of fuel and power in new dwellings and Part L2A Conservation of fuel and power in new buildings other than dwellings. A typical approach might aim at reducing the rate of air leakage and increasing the thermal insulation, both of which will contribute to lowering the


building’s CO2 emission rate, however, the implications of each approach can be substantially different. Air leakage is measured in m3


/m2 /hr – the


), over a given time period (hr). The measurement method commonly used is


ADF FEBRUARY 2018


quantity of air moving through the building fabric (m3 (m2


), for a given building floor area


Architects are increasingly turning to air barrier membranes as an essential part of the design process


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