What happens if an air test fails

How common it is

An analysis of six years of the ATTMA lodgement database, published in 2021, found about 3,500 air tests recorded each week, of which around 15% were retests. A fail doesn't mean the tester got it wrong, or that the house is badly built throughout. It means some air paths need sealing before the home can be signed off.

Two ways to fail

Above the limiting value. Approved Document L sets a backstop of 8.0 m³/(h·m²) at 50 Pa (or 1.57 at 4 Pa). A home above it fails whatever else is in the calculation.
The as-built calculation misses its targets. The home's energy and emission rates are recalculated with the measured air permeability, and must not be worse than the targets. If the design assumed, say, 4 and the test measured 6, the as-built SAP may no longer meet the targets even though 6 is under the backstop.

A result worse than the design value doesn't automatically mean a fail: what counts is whether the as-built calculation, using the measured value, still meets the targets. Your SAP assessor can tell you.

What the regulations say happens next

The home's air permeability should be improved, and new tests carried out until it meets the criteria (Approved Document L 2021, paragraph 7.6; 2026 edition, paragraph 6.7).
The results of all pressure tests, including failures, should be reported to building control (paragraph 7.7; 2026 edition, 6.8).
Notice of the test results must be given to building control no later than seven days after the final test (Regulation 43).

Finding the leaks

With the fan pressurisation ("blower door") method the home is held under pressure, so the tester can look for leaks during the test, for example with a smoke pen. Ask the tester to show you or note where the air was getting in. Places often listed by building control include:

extractor fans, cooker hoods and dryer vents;
pipes through walls and ceilings, including the top of the soil stack;
cracks and holes in the inner walls and linings, behind coving, and at the top and bottom of skirting boards;
sockets, switches, fused spurs, heating controls, ceiling roses and recessed spotlights;
eaves, and the ends of floor joists where they go into walls;
windows and their frames, beneath inner window sills, around door frames, and letter boxes and keyholes.

Fix it for good, not for the test

Researchers at UCL, working with ATTMA on results from over 100,000 new homes, found a disproportionate number of results just inside the targets, and evidence that some homes were being sealed in the short term to pass. Short-term sealing tends to fail quickly and leaves a leakier home than intended. Fix the air path in the construction, at the detail that caused it, and record what was done.

Don't ask a tester to tape over vents or openings that should be left as built, or to adjust a result. Recording a result honestly is part of the tester's professional duty and of our code of conduct.

The retest

Agree the retest fee and date with the tester. Jobs on TestConnect can set the retest fee in advance.
Use the readiness check honestly: only confirm the plot is ready once the remedial work is finished.
Look at the other plots of the same house type. If the cause was a detail, the next plots will probably have it too.

How TestConnect helps

The tester records the fail with the measured value, the target, defects found, photos and recommendations. The retest is booked as a new attempt linked to the first, and both stay on the plot's record, so you can see which house types and details keep failing. Read more on air tightness testing.

This guide summarises published guidance; it isn't compliance advice. Check anything you're unsure about with your building control body, the tester's scheme, or your SAP assessor.

Sources

Last checked: October 2026.