Air Tightness Testing Requirements

What This Requirement Covers

Air permeability testing (also called air tightness testing or pressure testing) measures the rate of uncontrolled air leakage through the building fabric. Uncontrolled air leakage wastes energy and can cause draughts, condensation, and noise transmission. Approved Document L requires air permeability testing for all new dwellings and most new non-domestic buildings.

Key Requirements

Testing Standard

1. Air permeability testing must be carried out to CIBSE TM23 (Testing buildings for air leakage) or the ATTMA (Air Tightness Testing and Measurement Association) TSL1 standard
2. The test measures the volume of air passing through the building envelope per hour at a pressure differential of 50 Pascals, expressed as m³/(h.m²) of envelope area

Maximum Air Permeability

1. New dwellings: Maximum 8 m³/(h.m²) at 50 Pa (limiting value)
2. The design air permeability used in the SAP calculation may be lower than the maximum; the tested result must be equal to or better than the design value
3. Non-domestic buildings: The limiting value depends on the building type and the BRUKL calculation
4. Buildings achieving Passivhaus certification target 0.6 ACH at 50 Pa (approximately 1-2 m³/(h.m²))

When Testing Is Required

1. All new dwellings must be tested, with certain sampling rules:

- On developments of up to 2 dwellings: all units tested - On larger developments: a sample of units may be tested (minimum 3 units or 50%, whichever is greater, per dwelling type) - If any tested unit fails, all units of that type must be tested

1. The test must be carried out before the EPC is issued and before occupation

Common Air Leakage Paths

1. Around window and door frames
2. At the junction of walls and floors (particularly timber floors)
3. Through service penetrations (pipes, cables, flues, extract ducts)
4. At loft hatches and access panels
5. Through suspended timber floors
6. At masonry joints and mortar joints
7. Around electrical sockets and switches on external walls

Practical Compliance Tips

1. Design for air tightness from the outset; identify the air barrier line on the drawings and ensure continuity at all junctions
2. Use a dedicated air barrier membrane in timber frame construction, taped and sealed at all joints
3. Seal all service penetrations through the air barrier with appropriate sealants or grommets
4. Plaster is the primary air barrier in masonry construction; ensure continuous parge coat or plaster on internal wall surfaces
5. Carry out a preliminary (indicative) air test before finishes are complete to identify and fix leakage paths
6. Seal around loft hatches, access panels, and any penetrations through the ceiling
7. Brief all trades on the importance of air tightness; a single unsealed penetration can significantly increase the air leakage rate
8. Book the final air test early; ATTMA-registered testers can be busy during peak handover periods