How to Pass an Air Tightness Test First Time
Most failed air tests come down to a few gaps nobody sealed. Here is how to find them before the tester does.
How to Pass an Air Tightness Test First Time
Most failed air tests come down to a few gaps nobody sealed before the boards went on. They are cheap to fix at first fix and expensive after decoration. Here is what we see on site and how to avoid a retest.
Know the figure you have to hit
Under Part L every new home needs an air test, and the measured result goes into the as-built SAP. Approved Document L sets a maximum of 8 m³/(h·m²) at 50 Pa, or 1.57 m³/(h·m²) at 4 Pa when the low-pressure pulse method is used.
That is the legal ceiling, not your target. Your target is the design figure in the SAP, which is usually much lower. If the test comes in under 8 but above the design figure, the as-built SAP is rerun with the real result. It can then fail on emissions, primary energy or fabric efficiency. Get the design figure from your SAP assessor before work starts, and make sure the site team knows it.
Draw the air barrier before you build it
Approved Document L asks for drawings that show where the air barrier runs, how it stays continuous and what it is made of. Mark the line round every junction: wall to floor, wall to roof, round windows and doors, at party walls and at the loft hatch. The designer and the installer should both check it. If nobody can trace the line on the drawing, nobody will build it on site.
The usual leaks and how to stop them
- Service penetrations. Group incoming ducts and cables so there are fewer holes. Core-drill internal penetrations to size and seal them with grommets or collars taped to the barrier. Look hard at pipes boxed in behind baths, kitchen units and soil stacks.
- Masonry walls. Fill and point mortar joints fully, including inside cavities and party walls. Plasterboard is not an air barrier. Dense blockwork may need wet plaster, a parge coat or a liquid membrane on the inside.
- Timber frame. Lap and tape the vapour control layer at every seam and junction, and repair any tears before boarding.
- Windows and doors. Tape the frames to the structural opening so they connect to the air barrier. Sealant or compressible seals can back the tape up, not replace it.
- Joists and steels. Seal round anything that passes through the inner leaf. Joist hangers avoid building joists into the wall in the first place.
- Loft hatches. Where the insulation is at ceiling level, fit a hatch made to be airtight and seal it properly.
- Fixings and later trades. Every fixing, socket or downlight that punctures the barrier needs sealing. A barrier that passed at first fix can be wrecked by the second-fix trades.
Also keep the insulation tight against the air barrier, or fill the gap between them, so air can't move inside the wall or roof.
Too airtight can be a problem too
The current Approved Document F only covers natural ventilation (trickle vents and intermittent fans) in "less airtight" homes: a design figure above 5 m³/(h·m²) at 50 Pa, or an as-built result above 3. If a naturally ventilated home tests below 3, that guidance no longer applies. The options are expert advice or a continuous mechanical extract system. If you are chasing a very low figure, design for continuous extract or MVHR from the start.
Getting ready for test day
- External doors and windows fitted and closing properly.
- Walls, floors and ceilings finished, with skirtings, sockets and switches in.
- Kitchens and bathrooms fitted, with water in the traps.
- All service holes sealed and the loft hatch fitted.
- Power on site.
Walk round the day before. Lift bath panels, open the kitchen units and check the boxing. Open holes there are a common cause of a fail. Don't add temporary tape to chase a number: it won't be there when the family moves in.
If it fails
We go round with smoke to find the leaks, you seal them, and we test again. On a larger site, test the first plots early. Whatever leaks on the first plot will leak on the next twenty, so fix the detail before it is repeated.
Our air testers are registered with the Elmhurst Airtightness Scheme. Building control can accept a certificate from a registered tester as proof the approved procedure was followed. The results have to reach building control within seven days of the final test. You get our report within 5 working days of the visit.
What changes in March 2027
The new Approved Document L (2026 edition) takes effect on 24 March 2027. Projects with their building regulations notice or application in before then, and work started before 24 March 2028, can stay on the current rules. Higher-risk buildings have separate dates. The maximum stays at 8 at 50 Pa and 1.57 at 4 Pa, and the as-built calculation still uses the measured result. The design figure still decides whether you pass.
Questions we get asked
Does every plot need testing? Yes. Every new home is tested. Testing a sample of plots has not been allowed since the 2021 Part L took effect in June 2022.
Is 8 the figure we need to hit? No. It is the maximum. You need to hit the design figure in your SAP, or the as-built SAP may fail.
When should we book? When the envelope is complete and sealed, but before final decoration, so any leaks can still be reached.
Can you test air, sound and ventilation on the same visit? Yes. Booking them together costs less, because we save on travel.
More
More: Air Tightness Testing, Ventilation Testing, Sound Insulation Testing, SAP Calculations, and our guide to ventilation testing for new homes.
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Send us your plot list and the design air permeability figure. We'll price the air test, and sound and ventilation tests on the same visit if you need them, with a free quote.