Air Tightness Testing for New Builds: A Builder's Guide (2026)
Air tightness testing is now one of the unavoidable steps between a finished build and a signed-off completion certificate. If you're a builder or developer putting up new dwellings in England, the test is no longer something you can sidestep with a default figure and a penalty — the rules tightened under the 2021/2022 Part L uplift, and the test result feeds directly into whether the home passes Building Regulations. This guide explains what air tightness testing is, why it matters, who carries it out, what makes builds fail, and the practical steps that get you through first time.
What Is Air Tightness Testing?
Air tightness testing — also called an air permeability test or air leakage test — measures how much uncontrolled air leaks in and out of a building through gaps, cracks and unsealed junctions in the fabric. It is carried out with a blower door: a temporary frame fitted into an external doorway with a calibrated fan that pressurises (or depressurises) the building to 50 Pascals, then measures the volume of air the fan must move to hold that pressure.
The result is the air permeability figure, expressed in m³/(h·m²) at 50 Pa — cubic metres of air per hour, per square metre of building envelope. A lower number means a tighter building. A leaky shed-quality build might come in at 10 or more; a well-detailed modern home should sit comfortably below 5, and high-performance builds target 3 or under. The figure is purely about uncontrolled leakage — it has nothing to do with intentional ventilation through windows or a mechanical system, which is a separate issue covered by Part F.
Why Is the Test Required?
Air tightness testing exists to support compliance with Building Regulations Approved Document L (conservation of fuel and power). Uncontrolled air leakage is one of the biggest sources of heat loss in a home — warm air escaping through gaps means the heating system works harder, energy bills climb and carbon emissions rise. Part L sets a target for the home's overall energy performance, and air permeability is a major input into that calculation.
For dwellings, the energy assessment runs through SAP (the Standard Assessment Procedure); for non-domestic buildings it runs through SBEM. Both calculations rely on the as-built air leakage figure. At the design stage the assessor enters a target permeability into the model. If the as-built test comes back worse than the figure the design relied on, the SAP/SBEM calculation may no longer pass — and the build cannot be signed off until it does. In short: the test is the moment your real-world workmanship gets measured against the numbers your energy calculation promised.
Do All New Builds Need a Test?
Effectively, yes. All new dwellings in England now require an air tightness test. The old route — where a builder could decline to test and instead accept a punitive default air permeability value in the SAP calculation — has effectively closed under the 2021/2022 Part L uplift. The default figure is now so unfavourable that almost no build can pass its energy target while carrying it, so in practice every new home is tested.
On larger developments the rules also require a sample of each dwelling type to be tested rather than every single plot in some circumstances, but the safer assumption for any builder is that the home you're working on will be tested and must pass. Building it as though it will be tested is the only sensible approach — you cannot retrofit air tightness easily once the plasterboard and finishes are on.
Who Carries Out the Test?
The test must be performed by an accredited air tightness tester — typically someone registered with a scheme such as ATTMA (the Air Tightness Testing and Measurement Association) or an equivalent UKAS-backed accreditation body. Accreditation matters because the test certificate has to be accepted by Building Control and fed into the SAP/SBEM assessment. A figure from an unaccredited tester won't be recognised, and you'll have paid for a test that doesn't count.
Book the tester well ahead of your completion date. Good testers get booked up, and if you fail you'll need them back for a retest — so leaving a buffer in the programme before handover is sensible.
Target Figures and Tightening Standards
The air permeability target depends on what the SAP calculation for the specific plot relies on, not on a single national number. That said, there are practical bands worth knowing. Under the current Part L standards, designers are typically aiming for figures well below the old norms, and the direction of travel is firmly downward as the Future Homes Standard approaches.
| Air permeability m³/(h·m²) at 50 Pa | What it means |
|---|---|
| 10+ | Leaky — old-standard build, would fail a modern Part L target |
| 5–8 | Average older detailing, often marginal under current Part L |
| 3–5 | Good modern build with careful sealing — common Part L target zone |
| 1–3 | High performance, approaching low-energy / Passivhaus-influenced detailing |
| Under 1 | Passivhaus territory (measured differently, but exceptionally tight) |
The key point: don't design to scrape past a high figure. The tighter the figure your SAP calculation assumes, the better your overall energy rating — but you have to actually achieve it on site, or you fail. Many builders deliberately design to a figure with a margin of safety so a small slip in workmanship doesn't tip them into a fail.
How the Test Works on Site
On test day the tester fits the blower door into an external doorway and seals the temporary frame. The fan then drives the building to 50 Pa and records the airflow. For the result to be valid and fair, the building has to be presented correctly — the test measures uncontrolled leakage, so all intentional openings must be sealed or closed in the prescribed way.
Before the tester arrives, the dwelling needs to be:
- Fully weather-tight, with all external doors and windows fitted and closed
- Mechanically complete — trickle vents closed, extract fan and MVHR terminals temporarily sealed per the test method
- Plumbing traps filled with water so they don't register as open leakage paths
- Loft hatch fitted and closed
- All internal doors open so the whole volume is connected
- Free of ongoing wet trades and reasonably clean (the fan moves a lot of air)
Get the test sequencing right in your programme. Testing too early — before the building is genuinely sealed — wastes a test fee and gives a misleading result. Testing too late, after finishes and handover dates are locked in, leaves no room to fix a fail.
Common Causes of a Failed Test
Most failures come down to a handful of repeat offenders. The air barrier is only as good as its weakest junction, and a few unsealed penetrations can ruin an otherwise well-built home. Here are the usual culprits and how to deal with them.
| Common cause of failure | The fix |
|---|---|
| Service penetrations (pipes, cables, waste, soil stacks) | Seal every penetration through the air barrier with grommets, tape or flexible mastic; coordinate first fix to minimise holes |
| Loft hatch leaking | Fit an insulated, draught-sealed loft hatch with compression seals |
| Gaps around windows and doors | Tape and seal the reveal junction between frame and structure; don't rely on expanding foam alone |
| Wet plaster not dry | Let wet plaster fully cure before testing — damp finishes can mask leaks and skew the result |
| MVHR / extract terminals not sealed | Temporarily seal terminals per the test method; ensure ductwork penetrations through the barrier are airtight |
| Floor / wall and wall / ceiling junctions | Maintain a continuous air barrier; parge coat blockwork or seal behind dry lining at perimeters |
| Electrical back boxes and recessed downlights | Use airtight back boxes and sealed / fire-rated airtight downlight covers |
What a Test — and a Failure — Costs
The test itself is one of the cheaper line items on a build. A single dwelling test typically lands in the region of £150–£350 depending on location, size and how many plots the tester is doing in one visit. On a development, testing a sample of plots in a single trip brings the per-unit cost down.
The real cost is failing. If a home fails, you have to find the leaks, open up and reseal the offending junctions, and pay for the tester to return for a retest. If finishes are already on — skirtings, decoration, flooring — chasing a leak behind them can mean ripping out and reinstating work you've already been paid to do once. A fail can quietly turn a £200 test into thousands in remediation and a delayed handover. That economics is exactly why getting it right first time pays for itself many times over.
The Link to MVHR and Ventilation
There is a flip side to building tight: a tight house needs proper ventilation. The whole point of air tightness is to stop uncontrolled leakage so that ventilation can be controlled instead. If you seal a home up well and then under-ventilate it, you get condensation, mould and poor indoor air quality.
This is where Part F (ventilation) comes in alongside Part L. As builds get tighter, more of them use mechanical ventilation with heat recovery (MVHR) — a system that extracts stale, moist air and recovers heat from it to temper incoming fresh air. Below a certain air permeability figure, MVHR effectively becomes the appropriate ventilation strategy because natural background ventilation through the fabric is no longer enough. Plan the ventilation strategy and the air tightness target together, not separately — they are two halves of the same compliance picture.
Practical Steps to Pass First Time
Air tightness is won on site, junction by junction, long before the tester arrives. The builds that pass comfortably are the ones where someone took ownership of the air barrier from day one. Practical steps:
- Identify the air barrier line on the drawings and make sure every trade knows where it is and not to puncture it carelessly.
- Design to a target with margin. If your SAP calculation needs 5, build to achieve 3 or 4 so a minor slip doesn't fail you.
- Coordinate first fix. Every pipe and cable through the barrier is a leak path — minimise penetrations and seal the ones you do make as you go.
- Seal as you build, not at the end. Junctions you can reach during construction become buried and inaccessible later.
- Use the right products: airtight tapes, grommets, parge coats, airtight back boxes, sealed downlight hoods and a draught-sealed loft hatch.
- Consider a pre-test (smoke or pressurisation) before finishes go on, so you can find and fix leaks while junctions are still open.
- Let wet trades dry out and present the building correctly on test day — sealed terminals, filled traps, closed windows, open internal doors.
FAQ
Is air tightness testing a legal requirement for new homes?
In practice, yes. It supports compliance with Building Regulations Approved Document L, and since the 2021/2022 Part L uplift the option of skipping the test and accepting a default permeability value has effectively become unworkable — the default figure is too poor to pass the energy target. Almost every new dwelling in England is now tested.
What is a good air permeability figure?
Lower is better. A figure of 3–5 m³/(h·m²) at 50 Pa is a common, achievable target for a well-detailed modern home; under 3 is high performance. The exact figure you need depends on what your SAP calculation for that plot relies on.
Who can carry out the test?
An accredited tester — for example one registered with ATTMA or another UKAS-backed scheme. An unaccredited result won't be accepted by Building Control or counted in the SAP/SBEM assessment.
What happens if my build fails?
You find and seal the leaks, then pay for a retest. If finishes are already on, remediation can mean opening up completed work, so the cost and delay of a fail far exceed the test fee. This is why building tight from the start matters.
Does a tight house cause condensation problems?
Only if it's under-ventilated. A tight building must be paired with proper Part F ventilation — often MVHR on tighter builds — so that fresh air is supplied in a controlled way rather than leaking through the fabric.
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