It has long been recognised that buildings, even when all doors, windows and other openings are closed, are not fully airtight. Wind forces create pressure differences which give rise to an unquantifiable and often unwanted air flow through a building, usually called air infiltration to distinguish it from planned ventilation. In cold weather, air infiltration leads to an additional heat load and may also cause discomfort due to cold draughts. In hot weather, it can disturb the temperature control and air distribution performance of ACMV equipment. Both of these effects make the correct sizing of heating and temperature control equipment difficult, and in the past have often led to deliberate oversizing to compensate. Furthermore, the relative importance of air leakage has increased as insulation and energy conservation standards have risen. This is because the heat loads due to air infiltration are proportionally more significant.
For example, consider a typical semi-detached house built prior to the current standards, and with an air leakage typical of much older dwellings. The heat loss due to air infiltration would be about one third or less of the total heat loss through the fabric. If the fabric insulation of the house were raised to meet the current LI standard without changes to its airtightness, the infiltration losses and the fabric losses would be approximately equal. By sealing to the new airtightness standard required by Part L, the air infiltration loss would fall and would again be about one third of the fabric loss. In practice, it is probable that careful attention to design detail and proper supervision of on-site construction will produce dwellings with an airtightness better than the Part L requirement, thus reducing the relative importance of air leakage even further.
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