Air tightness / air permeability / air leakage – defined as the resistance of the building envelope to inward or outward air permeation. Air leakage is driven by pressure differentials between inside and outside a building caused by the wind, stack effect and mechanical ventilation systems.

Air barrier or air seal line – the physical components that make up the airtight envelope of the building. The air barrier needs to be continuous around the whole envelope – roof, walls and ground floors – and needs to be durable and maintainable in the long term. The air seal line can be drawn on construction drawings.

Air test or air leakage pressure test – the building is pressure tested by connecting a fan and measuring the airflow rates required to keep the building at various positive pressures.

Air permeability – expressed as the amount of air leakage in cubic metres, per hour, per square metre of envelope at a nominal pressure differential of 50 Pascals, between inside and outside the building envelope.

Q50 – air flow rate required to pressurise the building envelope to 50 Pascals, the measured unit of which is cubic metres per second.