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Iain Fairnington of the
A.Proctor Group, discusses how the performance of the building envelope can be improved through airtight membranes
he need for airtightness is driven by the global challenge to reduce carbon emissions and the drive to create more energy efficient buildings which waste less energy, reduce costs and protect both the building fabric and well being of its occupants.
The Government’s long-term carbon reduction strategy set out in the Climate Change Act means by 2050 the UK must achieve a reduction of 80 per cent against 1990 levels. While changes of government and the scrapping of the Zero Carbon Homes Policy have created some uncer- tainty, the need to follow through with these plans remains a fundamental part of the design and construction of the UK’s building programme.
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-domestic), and space heating is accounted for much of this energy. The Building Regulations’ increasingly stringent performance criteria for building envelope has led to higher standards of insulation being specified for roofs, walls, windows and floors. However, identifying localised areas of reduced insulation or thermal bridging causing air leakage has become even more crucial.
Air leakage through cracks, gaps, holes and improperly sealed elements such as doors and windows can cause a significant reduction in the performance of thermally insulated envelopes, in some cases reducing their effectiveness by up to 70 per cent. Discrepancies between ‘as built’ and ‘as designed’ performance are largely attributa- ble to uncontrolled air leakage, prompting architects and developers to increasingly turn to air barrier membranes as an essen- tial part of the design process in achieving the most effective means of controlling and reducing air leaks.
A common misconception regarding airtightness is that well-sealed buildings mean uncomfortable, ‘stuffy’ indoor
Sealing the deal of airtightness T
environments, which are in fact created by poor ventilation. Buildings with very low rates of air leakage require correspondingly higher levels of ventilation as part of a balanced design approach. It is a myth that increased ventilation hampers overall effi- ciency, because ventilation is controllable and can be accounted for within the design.
Guidance, legislation and compliance
The key guidance relevant 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. However, it is also important to take a holistic approach when considering compliance with Building Regulations. Both reducing the rate of air leakage and increas-
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