What Is Airtightness? — Building Envelope Testing

Definition

Airtightness refers to the resistance of a building's envelope to uncontrolled air leakage. It is measured as air permeability in m³/(h·m²) at 50 Pascals pressure difference, representing the volume of air leaking through each square metre of the building envelope per hour. A lower number means a tighter building. Airtightness is a key factor in energy efficiency and is regulated under Part L of the Building Regulations.

When Is It Used?

Airtightness testing is required for all new dwellings under Part L 2021 and for new non-domestic buildings over 500 m². It is also relevant for retrofit projects targeting EPC improvements, Passivhaus certification, and any project seeking to minimise energy consumption. The test is typically carried out before the final Building Control inspection, after all services are installed but ideally before decoration.

Key Facts

• Part L 2021 backstop: 8 m³/(h·m²) at 50 Pa; practical target: 3-5 m³/(h·m²)
• Passivhaus target: 0.6 ACH at 50 Pa (approximately 1 m³/(h·m²))
• Testing must be done by a UKAS-accredited or ATTMA-registered tester
• A blower door test typically costs £200-£400 for a domestic property
• The air barrier line must be continuous and identifiable on drawings
• In masonry construction, the wet plaster coat acts as the main air barrier
• In timber frame construction, the vapour control layer (VCL) is the air barrier
• Smoke pencils or theatrical smoke machines help locate leaks during testing

Related Calculators

Use the U-Value Calculator for fabric performance, or the Heat Loss Calculator to see the impact of airtightness on heating demand. See also the MVHR glossary entry for ventilation in airtight buildings.