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Walls

Airtightness

Airtightness describes how little uncontrolled air leaks through a building envelope, measured with a blower door test.

Also known as air tightness, air leakage, blower door test, air barrier, air permeability

Air leaking through gaps in walls and roofs carries heat out and moisture into the construction. A tight envelope saves energy, prevents condensation and lets ventilation work as designed. It depends on a continuous air barrier, usually the vapour barrier or sealed boards, with every joint and penetration taped.

It is tested by pressurising the building with a fan (a blower door) and measuring the leakage, for example as air changes per hour at 50 Pa or litres per second per square metre of envelope.

Related terms
Vapour barrierA vapour barrier is a membrane on the warm side of the insulation that limits how much moisture from indoor air can migrate into the wall or roof. Building envelopeThe building envelope is the physical barrier between inside and outside: the external walls, roof, lowest floor, windows and doors. Element jointAn element joint is the connection between two prefabricated elements on site, where structure, insulation, air barrier and cladding must be made continuous.
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