Air permeability targets and as-built test failures
An air permeability figure in a Part L calculation is a promise about site workmanship. Here is where the leakage happens, how the test works and what to do when it fails.
An air permeability test failure means the as-built figure came back worse than the value used in the compliance calculation, and the consequence is that the calculation no longer demonstrates compliance. Approved Document L sets a limiting value of 8.0 m³/(h·m²) at 50 Pa for a new dwelling, and most designs commit to something better than that so as to hit the primary energy and carbon targets. Beat the limit and miss your own design figure, and you still have a problem, because the calculation has to be re-run with the tested number.
What the design target commits you to
The figure in the calculation is not an aspiration. It is an input, and the whole compliance case rests on it alongside the U-values, the ventilation strategy and the heating system efficiency.
Two things follow. A dwelling designed at 3.0 with a heat pump sized for that fabric will underperform if it tests at 6.0, and the calculation may fail even though the limiting value is met. And because building control signs off against the as-built calculation rather than the design one, the sign-off is what the test result feeds into.
Where a planning condition or a local plan policy has required a commitment to a stated air permeability figure, missing it also becomes a planning compliance question. Say in the Energy Statement what the design value is and how it will be achieved on site, rather than quoting a number with no strategy behind it.
Where the air actually leaks
Leakage happens at junctions and penetrations, not through the middle of a wall. The recurring locations:
- Joist ends built into external masonry, and the perimeter of an intermediate floor
- The wall to floor junction at ground level, behind the skirting
- Service penetrations: soil vent pipes, extract ducts, incoming water, gas and electricity, and the services intake enclosure
- Window and door perimeters, particularly where the frame is fixed before the reveal is sealed
- Loft hatches, and the ceiling line where a sloping ceiling meets a wall
- Recessed downlights, ceiling roses and sockets on external walls
- The party wall cavity in a semi-detached or terraced dwelling, which acts as a bypass unless it is sealed at the perimeter
- Wet room extract terminals and cooker hood ducts with no backdraught closure
A dwelling that tests at 7.5 is rarely leaking in one dramatic place. It is leaking a small amount at forty junctions, which is why remedial work after a failure costs more than getting it right during the build.
Drawing the airtightness line and keeping it
The single most useful drawing on a self-build is a set of sections with the airtightness line marked in one continuous colour, never broken. Every point where that line changes material is a detail someone has to build.
Nominate the barrier per element and write it down. Wet plaster or a parge coat on masonry inner leaves. A taped membrane in a timber frame or roof. A proprietary tape at every sheet joint, a grommet at every cable, and a flexible collar at every pipe. Then work out who owns each junction, because the plasterer, the carpenter, the electrician and the plumber all cross the line and none of them will treat it as their responsibility unless told.
Thermal bridging sits alongside this. Using default junction values in the calculation penalises the result, while calculated psi values for your actual details improve it, and the details you calculate have to be the ones built. That is a separate check from the pressure test and it is worth budgeting for.
How the test is done and who does it
The test measures air flow through the envelope at a reference pressure difference, using either a fan pressurisation method or a low-pressure pulse method, carried out by a competent tester whose result building control will accept.
Preparation decides the outcome as much as workmanship:
- The building has to be complete, with all penetrations sealed and wet trades dry
- Trickle vents and purge openings are closed and sealed, and mechanical ventilation terminals are temporarily sealed
- Traps are filled so that drainage does not provide a direct path
- Internal doors are open, and loft hatches closed
- The envelope area is measured to the convention in the test standard, because an error in the area changes the result
Book the test early in the programme and book a pre-test. A preliminary test once the barrier is complete but before plasterboard and finishes go on is the difference between a two-hour fix and a fortnight of remedial work.
When a test fails
Nothing dramatic happens at the moment of failure. The tester issues a result, and you then have three routes.
Re-run the calculation with the actual figure. If the dwelling still complies on primary energy, carbon and fabric energy efficiency, and the limiting value is met, you may be finished, though the heating system may now be oversized or the predicted bills higher than the buyer was told.
Find and fix the leaks. A smoke pencil at negative pressure, a thermal camera on a cold morning with the fan running, and a methodical walk of the junction list will locate most of them. Common fixes cost very little: mastic behind skirtings, foam and tape at the services intake enclosure, a collar on the soil vent pipe, sealing the loft hatch, and closing the party wall cavity at eaves and perimeter.
Change something else in the calculation. Better glazing, a different heating control strategy or additional renewables can recover a shortfall, though each change has a cost and some need planning permission if they are visible.
Illustrative example: a two-storey timber frame house designed at 3.0 tests at 5.8, and thermal imaging finds leakage at the intermediate floor perimeter, four unsealed cable penetrations and an open loft hatch surround. After sealing, a retest returns 3.4 and the recalculated compliance passes. Figures here are illustrative only.
Remedial work and retesting
Retesting is routine and testers expect it. Fix in one campaign rather than in stages, because each test costs money and access.
Two cautions. Sealing work carried out after decoration damages finishes, so a failure at that point is expensive; and there is a temptation to seal the ventilation provision itself, which improves the number and breaches Approved Document F. A tester who finds trickle vents permanently sealed will say so.
Record what was done. Photographs of each remediated junction, keyed to the section drawings, form the evidence for building control and for any warranty provider.
Ventilation, and the risk of building too tight
A very airtight dwelling cannot rely on background ventilators and infiltration. Approved Document F expects a continuous mechanical ventilation strategy where the design air permeability is very low, in the region of 3.0 m³/(h·m²) or below, so check the current text before selecting a natural ventilation strategy for a highly sealed build.
The interaction with overheating matters too. A sealed, well insulated dwelling with large glazed areas overheats readily, and the assessment under Approved Document O will drive shading, glazing area and purge ventilation. Read the Overheating Risk Assessment requirements alongside the airtightness strategy rather than after it, and look at the roof early where plant, panels or a biosolar build-up will puncture the barrier. Where a heat pump is planned, the fabric result feeds directly into its sizing, as our note on ground source heat pumps and borehole permitting touches on.
Browse the PlanScope report library to see which energy and sustainability documents a scheme is likely to need.
This is general guidance rather than legal advice. Check the position with your local planning authority, your building control body or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is the limiting air permeability for a new dwelling?
Approved Document L sets a limiting value of 8.0 m³/(h·m²) at 50 Pa for a new dwelling. Most designs commit to a better figure because the primary energy and carbon targets are hard to meet at the limiting value, and that design figure is the one the as-built calculation has to be re-run against.
What happens if the as-built test is worse than the design value?
The compliance calculation is re-run with the tested figure. If the dwelling still meets the primary energy, carbon and fabric energy efficiency targets, it may still comply. If it does not, you either find and seal the leakage and retest, or improve something else in the calculation such as glazing or heating.
Can I seal trickle vents to pass the test?
Vents are closed and temporarily sealed for the test itself, which is part of the standard method. Leaving them permanently sealed breaches the ventilation requirements in Approved Document F and will be reported. A very airtight dwelling needs a mechanical ventilation strategy rather than reliance on background ventilators.
Where do most dwellings leak?
At junctions and penetrations: joist ends built into masonry, the intermediate floor perimeter, the wall to floor junction behind skirtings, service penetrations and services intake enclosures, window and door perimeters, loft hatches, recessed downlights, and the party wall cavity acting as a bypass. Failures are usually the sum of many small leaks.
Is a pre-test worth the cost?
Almost always. A preliminary test once the airtightness barrier is complete, before plasterboard and finishes, lets leaks be sealed while they are still accessible. Finding the same leaks after decoration turns a two-hour fix into remedial work that damages finishes and delays handover.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.