Air permeability of partitions is the ability of building elements (walls, roofs, windows, doors) to allow air to pass through. The movement of air through partitions is influenced by the pressure difference between the inside and outside environment. Air permeability determines the volume of air that passes through the partition at a given time and pressure. It is most often expressed in m³/(m²-h) or in terms of the n₅₀ factor in a building airtightness test (the so-called Blower Door test).
In modern energy-efficient construction, the air permeability of the envelope is strived for as low as possible. Uncontrolled infiltration leads to heat loss, draughts and compromises the effectiveness of mechanical ventilation. The airtightness of the envelope is achieved through the use of vapour and windproofing membranes, sealing tapes, assembly foams and careful execution of the connections between the construction elements.
Too much airtightness is also not beneficial if the building does not have efficient ventilation. It is important to maintain a balance between reducing unwanted leaks and ensuring controlled air exchange.
