UK Building Regulations & Compliance Guide
Airtightness in UK Building Regulations
Airtightness has been part of UK Building Regulations since 2006 and the requirements have tightened with each revision. Under Part L 2021 (which came into force in June 2022), new dwellings must achieve a tested air permeability of ≤8 m³/h·m² @ 50 Pa, with a design target of ≤5 m³/h·m² @ 50 Pa. For non-domestic buildings, the requirements are set out in Part L (Volume 2) and the NCM methodology.
The Future Homes Standard, expected to take effect in 2025/2026, will raise the bar further — with anticipated airtightness requirements tightening to ≤3 m³/h·m² for new dwellings, to support a 75–80% improvement in energy performance over 2013 Part L levels.
Key Airtightness Standards — Quick Reference
UK Building Regulations Part L 2021 — New Dwellings (Volume 1)
Mandatory Design target: ≤5 m³/h·m² @ 50 Pa | Maximum permitted: ≤8 m³/h·m²All new dwellings must have an air permeability test carried out by a UKAS or equivalent accredited tester. If the measured result exceeds 8 m³/h·m², the building fails to comply and remediation is required before sign-off.
UK Building Regulations Part L 2021 — Non-Domestic Buildings (Volume 2)
Mandatory Design target: ≤3 m³/h·m² @ 50 Pa (CIBSE TM23 basis)Non-domestic buildings are subject to air pressure testing under Part L Volume 2. The design target for most building types is ≤3 m³/h·m², with testing carried out in accordance with ATTMA TS2 or CIBSE TM23.
Future Homes Standard (anticipated 2025/2026)
Upcoming Anticipated: ≤3 m³/h·m² @ 50 Pa for new dwellingsThe Future Homes Standard will require new homes to produce 75–80% less carbon than under 2013 Part L. A tighter airtightness standard (approximately ≤3 m³/h·m²) is anticipated. Specifying liquid-applied membranes now future-proofs your projects against this upcoming change.
Passivhaus Classic
Voluntary ≤0.6 ACH @ 50 Pa (≈ ≤1.0 m³/h·m²)The Passivhaus standard requires significantly higher airtightness than UK Building Regulations. Passive Purple is the world's first Passivhaus-certified liquid-applied membrane, with verified test results on completed Passivhaus buildings across the UK and Europe.
EnerPHit — Passivhaus Retrofit Standard
Voluntary ≤1.0 ACH (component method) or ≤0.6 ACH (energy demand method)EnerPHit is the Passivhaus standard for refurbished existing buildings. Passive Purple Retrofit is suitable for EnerPHit projects on existing masonry, concrete and mixed substrates.
How Passive Purple Helps You Comply
| Compliance Requirement | Challenge | Passive Purple Solution |
|---|---|---|
| Part L 2021 — ≤5 m³/h·m² design target | Sheet membranes and tapes leave weak points at laps, junctions and penetrations | Seamless liquid-applied layer eliminates overlaps; fibre-reinforced brush grade seals all junctions |
| Future Homes Standard — ≤3 m³/h·m² | Higher target requires more reliable detailing across the whole envelope | Spray application achieves consistent coverage; liquid membrane follows contours of complex geometry |
| Passivhaus ≤0.6 ACH | Passivhaus requires near-perfect airtightness continuity | Passivhaus Institut certified — used on completed Passivhaus projects achieving 0.3–0.5 ACH |
| Remediation after a failed test | Failed airtightness tests are expensive to remedy, especially if plasterboard is already fixed | Passive Purple Brush can be applied from the inside without removing finishes in many cases |
| Non-domestic Part L — ≤3 m³/h·m² | Commercial buildings have complex service penetrations and structural interfaces | Passive Purple spray application on commercial scale; Passive Purple X for fire-rated commercial applications |
Air Pressure Testing — What You Need to Know
Who Carries Out the Test?
Air pressure tests must be carried out by a UKAS-accredited or BINDT (ATTMA scheme) registered tester for the results to be accepted under Building Regulations. A list of registered testers is available from ATTMA.
When is Testing Required?
Testing is required for all new dwellings and new non-domestic buildings. For dwellings, testing applies unless approved alternative procedures are used. The test must be completed before occupation.
What Does the Test Measure?
A blower door fan is fitted to an external opening and pressurises (or depressurises) the building. The air flow required to maintain 50 Pascals of pressure differential is measured and converted to air permeability (m³/h·m²) or ACH.
What Happens if You Fail?
If the test result exceeds the maximum permitted value, the building cannot be signed off. Remediation — sealing identified leakage paths — is required, followed by a re-test. Passive Purple Brush can often be used for remediation without disrupting finished surfaces.
Related Resources
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