Air-tightness

The air-tightness of a building envelope has a direct effect upon the energy performance of the building. High air leakage levels will account for a substantial proportion of energy losses for the occupier. Conversely, good air-tightness in a building reduces capital spend on heating and cooling systems, and reduced running costs. Air leakage typically accounts for 25-50% of the heat loss from a building.

Air-tightness can vary enormously depending upon construction method, site practice and build quality. For a typical shed, with conservative joint leakage figures for built-up or composite cladding, air leakage through joints accounts for less than 20% of the overall leakage. Other factors such as poor interface detailing and leakage through masonry walls contribute significantly to air leakage. Tests performed by Oxford Brookes University Colorcoat® Centre for the Building Envelope show that it is easily possible to build air-tight steel clad buildings with air permeability well within Building Regulation requirements, and that:

• Pre-finished steel cladding is inherently air-tight unlike blockwork which is air permeable. Air leakage will mainly occur at joints and through gaps in the construction, particularly at interfaces with other cladding materials and unplanned penetrations through the building envelope.
• The liner sheet must be identified as the air-tightness barrier, except when an acoustic liner is specified, in which case a membrane will be required, which should be positioned between the dense acoustic slab and the thermal insulation quilt
• Experience has shown that with close attention to detail, steel-clad buildings should have no difficulty in achieving air-leakage rates of no more than 5 m3/m2/h, and often even less than 3 m3/m2/h
• Based on Part L back stop values, air infiltration accounts for ~30% of heat energy loss through the envelope (dependant on building geometry)

Reducing energy usage through efficient air-tightness of the building envelope is clearly a major step towards cutting CO2 emissions. With ever-rising energy prices, this also makes economic sense, since attention to detail in construction of the building will continue to pay off for the full operational lifetime.