Air leakage is the uncontrolled movement of air through gaps, cracks, joints etc in the fabric of a building envelope. It commonly occurs at interfaces between construction materials and around service penetrations. It can have a negative impact on comfort, health and energy. The heat loss associated with air leakage can be significant.

During the design process for a building it is important to develop an air tightness strategy for the construction to ensure continuity of the air barrier. Good quality control procedures are required during the construction stage in order to achieve the expected air tightness level.

Air permeability is measured by means of a fan pressure test in units of m3/h.m2 at 50 Pa. This translates as the quantity of air (in m3) which leaks into or out of the dwelling per hour divided by the internal area (M2) of the building fabric at a pressure of 50 Pa. In accordance with the current Technical Guidance Documents (TGD) for the Conservation of Fuel and Energy – Dwellings, Part L 2019, air pressure testing should be carried out on all dwellings and show attainment of a backstop value of 5m3/hr/m2. The design may require a lower air tightness level if mechanical ventilation is installed in the building. The tests should be carried out by a person certified by an independent third party to carry out this work, such as the National Standards Authority of Ireland (NSAI).