Air Tightness Solutions

Air Testing Glossary and Symbols

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.


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