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Air Tightness & Testing

Airtightness is the control of unwanted heat loss through the building envelope via the adoption of a carefully engineered design, using various products such as vapour control layers (VCL).


An airtight building has a number of positive impacts when used as part of a timber frame construction project; especially when combined with an appropriate natural, mechanical or hybrid ventilation system:


  • Reduced heating bills, thanks to minimal levels of heat loss
  • Reduced need for cooling/air conditioning
  • Allows ventilation systems to work to optimum levels of performance
  • Less chance of damp, mould and rot – since moisture is less likely to enter the property
  • Fewer draughts – therefore increasing thermal comfort.

Building airtightness (also called envelope air tightness) can be defined as the resistance to inward or outward air leakage through unintentional leakage points or areas in the building envelope. This air leakage is driven by differential pressures across the building envelope due to the combined effects of stack, external wind and mechanical ventilation systems.

Airtightness is the fundamental building property that impacts infiltration (the uncontrolled inward leakage of outdoor air through cracks, interstices or other unintentional openings of a building, caused by pressure effects of the wind and/or stack effect).

A number of studies have shown substantial energy savings by tightening building envelopes. The ASIEPI project technical report on building and ductwork airtightness estimates the energy impact of envelope airtightness in the order of 10 kWh per m2 of floor area per year, for the heating needs in a moderately cold region (2500 degree-days).