How to pass an air permeability test (and what to do if you fail) - UK guide
The air permeability test is the moment of truth for airtightness — a pass is needed to sign off the build, and a fail means delays, remedial work and a re-test. The good news: passing is almost entirely about preparation and continuity, decided long before the tester arrives. This guide covers what the test measures, how to pass first time, and what to do if you don't.
What it measures: a blower door fan pressurises the building and measures how much air leaks out, giving an air permeability figure (m³/h·m² at 50 Pa). The lower the number, the tighter the building. You're given a target to meet — standard Building Regs, or a much tighter figure for low-energy and Passivhaus work.
Passing starts at design and first fix, not test day
Airtightness is a line you draw continuously around the heated envelope — walls, floor, roof — with no breaks. The single biggest reason buildings pass is that someone identified that line early and protected it at every junction and penetration as the build went up. Trying to chase leaks at the end is far harder than building them out from the start.
Where buildings leak
Air finds the smallest gaps. The usual offenders:
- Service penetrations — pipes, cables, soil stacks and ducts passing through the airtight layer.
- Window and door perimeters — the gap between frame and structure.
- Wall-to-floor and wall-to-roof junctions — where one element meets another.
- Loft hatches, downlighters and sockets — small, numerous and easy to miss.
- Unsealed laps and tears in a membrane airtight layer.
How to prepare so you pass
Make the airtight layer continuous and seal every interruption:
- Use a robust, continuous airtight layer. A liquid-applied layer such as Passive Purple Internal Airtight & Vapour Control Paint is seamless and self-details around junctions, removing the laps and tapes that a sheet membrane relies on.
- Seal service penetrations as they're installed — grommets for cables and pipes, and Passivhaus airtight sealant or The GOAT airtight flexible sealant at gaps and junctions.
- Tape membrane laps and frame perimeters with quality airtight tapes.
- Don't forget the small stuff: loft hatches, downlights, socket boxes on external walls.
A smoke pencil or a trial depressurisation before the formal test will reveal leaks while you can still fix them cheaply.
What to do if you fail
A fail isn't a disaster — it's a to-do list. The test can usually be run in smoke or leak-detection mode to find exactly where air is getting through. Then:
- Find the leaks with smoke, a thermal camera or simply a hand on a windy day near suspect junctions.
- Seal them in priority order — big, obvious paths first (service penetrations, perimeters, junctions).
- Re-test once the sealing has cured.
Most fails come down to a handful of missed penetrations or junctions — targeted sealing usually turns a fail into a comfortable pass.
Related reading
- How to make a building airtight
- Airtightness detailing around windows and doors
- Which side of the insulation does a vapour control layer go?
Frequently asked questions
What's a good air permeability result?
Standard Building Regulations set a maximum you must stay under; low-energy and Passivhaus projects aim far lower (Passivhaus is measured differently again, as air changes per hour). Your target is set by the project — build to beat it, not just meet it.
Can I air-test before plastering?
A preliminary (first-fix) test is a great idea — it finds leaks while the airtight layer is still accessible and cheap to fix, before finishes cover everything.
Is airtightness the same as having no ventilation?
No — “build tight, ventilate right.” A tight envelope is paired with controlled ventilation so the building stays healthy. Airtightness controls uncontrolled leakage, not deliberate ventilation.
Preparing for an air test? Our technical team can help you specify the airtight layer, sealants and tapes to pass first time.