What is air tightness in buildings?

Air tightness (or airtightness) is a measure of how much air leaks uncontrollably through gaps and cracks in a building's fabric - through junction between wall and floor, around window frames, through electrical socket backs, at pipe penetrations, through loft hatches, and around any other opening in the building envelope. Air leakage is a significant source of heat loss in buildings and also causes draughts, condensation in wall cavities, and degraded acoustic performance. An airtight building with controlled ventilation (MVHR) uses far less energy to heat than a leaky building relying on uncontrolled air infiltration.

How is air tightness measured?

Air tightness is measured by a pressure test (also called a blower door test or fan pressurisation test) carried out in accordance with BS EN ISO 9972. A powerful fan is temporarily installed in an external door or window opening and used to pressurise the building to 50 Pa above atmospheric pressure. The airflow required to maintain this pressure is measured and expressed as air permeability in m3 per hour per m2 of building envelope area (m3/h.m2 @ 50 Pa). Lower values indicate a more airtight building. The test also identifies the location of air leakage paths using smoke pencils or thermal imaging.

What air tightness target is required by Building Regulations?

Approved Document L (Conservation of fuel and power) requires all new dwellings to undergo a pressure test and achieve a design air permeability that is used in the SAP energy calculation. The maximum permitted air permeability for new dwellings is 10 m3/(h.m2) @ 50 Pa - but SAP assessors typically design for a target of 5-7 m3/(h.m2) to allow a margin. PassivHaus standard requires 0.6 air changes per hour at 50 Pa (approximately 0.6 ACH@50, equivalent to roughly 0.8-1.0 m3/h.m2 @ 50 Pa). Modern well-built timber frame or masonry houses typically achieve 3-5 m3/h.m2 with careful detailing.

What is the air barrier in a building?

The air barrier is the continuous layer within the building envelope that resists uncontrolled air movement. In a masonry cavity wall, the plaster or plasterboard lining of the inner leaf typically forms the air barrier. In a timber-frame building, a membrane (breather membrane or airtight membrane) is fixed over the structural frame. The critical principle is continuity - any gap in the air barrier becomes a path for air leakage. Junctions (floor to wall, wall to roof, around windows) are the most vulnerable points and require careful detailing with tapes, gaskets, or wet-applied seals to maintain the air barrier through the junction without gaps.

Thermal & Energy noun

Air Tightness

/ ɛː ˈtaɪtnəs /

Also known as: airtightness, air permeability, air leakage, infiltration, blower door test

Definition

Air tightness is a measure of uncontrolled air leakage through the fabric of a building, expressed as air permeability in m3/(h.m2) at 50 Pa. Lower values mean a more airtight building with less heat loss through infiltration and fewer draughts. Tested by pressurising the building to 50 Pa with a fan (blower door test, BS EN ISO 9972). UK Building Regulations require new dwellings to be tested; the maximum permitted air permeability is 10 m3/(h.m2) @ 50 Pa, though designs typically target 5 or less.

In practice

Good air tightness is achieved through continuous detailing of the air barrier - the layer of construction that resists air movement. In masonry construction this is typically the plaster finish on the inner leaf. In timber frame it is a dedicated airtight membrane. The critical details are at junctions: wall-to-floor (DPM and screed continuity), wall-to-roof (the junction between the top of the inner leaf and the ceiling), around window and door frames (proprietary expanding foam or compression tapes), at penetrations (pipe and cable entries sealed with mastic or grommets), and at the loft hatch (a draught-proofed, insulated hatch cover).

Pressure testing is carried out when the building is weather-tight and all major services are in, but before final finishes are applied - this allows leakage paths identified during the test to be accessed and sealed. A smoke pencil or theatrical fog machine is used during the test to visualise air movement at suspected leakage points. In Passivhaus projects, intermediate airtightness tests are carried out at frame stage and at plasterboard stage to identify and fix problems before they are hidden.

Building Regulations

Requirement (Part L)New dwellings must be pressure tested; max 10 m3/(h.m2) @ 50 Pa

Test standardBS EN ISO 9972 - fan pressurisation method

PassivHaus0.6 ACH @ 50 Pa - approximately 0.8-1.0 m3/(h.m2) @ 50 Pa

Ventilation linkAirtight buildings require controlled ventilation (Part F)

Approved Document L (Conservation of fuel and power) requires pressure testing of all new dwellings to be carried out by an accredited tester and results submitted to Building Control before a completion certificate is issued. The tested air permeability is also input into the SAP energy calculation - a building that is more airtight than the design assumption improves the SAP score; a leakier result worsens it and may require compensating improvements. Critically, as buildings become more airtight, controlled ventilation becomes essential - an airtight building without adequate ventilation will have poor indoor air quality and condensation problems. Approved Document F (Ventilation) must be satisfied alongside Part L.

Full Building Regulations guidance

Air Tightness

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