How Air Tightness Affects Part L Compliance

Airtightness is one of the least visible parts of a build, yet it has one of the biggest effects on SAP results. You can have good insulation, decent glazing and sensible heating, but if the air barrier is weak the numbers move fast. Most builders know about the final air test, but fewer realise how closely the SAP model depends on the airtightness assumption made at design stage. When those two don’t match, compliance becomes harder than it needs to be.

This guide explains why airtightness matters, what the Part L rules require and how to avoid the usual problems at completion.

Why airtightness matters more than people think

Heat escapes from a home in only a few ways. Some of that loss goes through insulation and windows, but a surprising amount goes straight out through gaps and cracks in the building envelope. SAP treats this as uncontrolled infiltration. The leakier the home, the harder the heating system has to work and the more energy is wasted.

A small shift in the air test number can make a larger difference than people expect. Moving from an assumed 5 to an actual 7 does not sound dramatic, but in SAP terms it is enough to push a dwelling close to its limit, especially if other parts of the spec have drifted during construction. This is why airtightness is treated as a major performance driver rather than a finishing detail.

What Part L requires

Under Part L, every new dwelling must be pressure tested unless it falls under one of the narrow exemptions. The test must be carried out by an accredited tester and the measured value must replace the design assumption in the as built SAP. Building Control will not sign off the dwelling without the test certificate, and the EPC cannot be lodged until the final airtightness is entered into the model.

Part L sets an absolute maximum of 8 m³/m²/hr @ 50 Pa. Anything above that fails the minimum fabric standard. In practice, most SAP designs assume 5–6, because building right up against the limit leaves no margin for compliance.

Why design assumptions matter

At design stage, airtightness is only an assumption. Architects often use round numbers like 5 because it keeps the calculation tidy and appears realistic. The problem is that many homes don’t test at 5 unless they have a deliberate air barrier plan. Most fall between 6 and 7 unless extra care has been taken on taping, service penetrations and junctions.

If the SAP model is built around an optimistic value, the design may pass on paper while the as built version is left with no room for the higher test result. Builders who aren’t aware of the assumed value can end up carrying an invisible target without realising it.

If you’re unsure what the plans are assuming, it’s worth checking with your assessor. Our design stage SAP support makes it easier to confirm this early so the target matches the way the home will actually be built.

The link between airtightness and ventilation

Ventilation strategy is tied directly to airtightness in SAP. A leakier home relies more on natural infiltration and intermittent extract fans. A tighter home relies more on designed ventilation, whether that’s continuous mechanical extract or MVHR.

If the home ends up tighter or leakier than expected, the ventilation efficiency in the model can shift. This is another reason airtightness affects the final numbers. A home built tighter than expected can benefit from higher ventilation efficiency in SAP, while a home built leakier than expected may lose the advantage the design relied on.

Where airtightness is lost on site

Most of the leakage in a new dwelling isn’t through the big obvious gaps. It usually comes from smaller, easy-to-overlook points:

• service penetrations that were never sealed
• gaps at first-fix stage hidden by boards
• the junction between plasterboard and window frames
• unsealed areas behind baths and boxing
• slotted brick vents, meter boxes and loft hatches

By the time the plasterboard is on, many of these leakage points are difficult to access. This is why airtightness needs to be planned earlier in the build, not the week before the test.

How to keep airtightness under control

There’s no single trick that guarantees a good result. Instead, it’s a combination of attention to detail and timing. Builders who test early tend to have far fewer problems. A pre-test during second fix is a useful way to catch major leakage points before carpets, skirting and kitchen units make them inaccessible.

The air barrier line should be deliberate rather than assumed. Whether you’re relying on plasterboard, a membrane or a combination of both, the continuity of that layer matters. Wherever the air barrier passes behind a service zone, the sealing should be done before the area is closed up.

Homes that achieve very low results usually do so because the airtightness was planned from the start rather than patched up at the end.

What happens if the test result is higher than expected

A result slightly above the design value does not automatically mean failure. The question is whether the SAP model still meets the Part L targets. Sometimes the impact is small. Other times it exposes earlier optimistic assumptions.

If the result is too high to pass, the options are limited. You can either improve the envelope and retest, add additional insulation where accessible, or adjust other elements of the build to regain the margin. None of these are ideal late in the project, which is why keeping the model updated throughout the build saves time and avoids stress.

If you are struggling with a late-stage fail, our as built SAP guidance can help you understand what is causing the shortfall and what the simplest fixes are.

Final thoughts

Air tightness is one of the most influential parts of a SAP calculation, but it is also one of the easiest to underestimate. The measured value replaces the design assumption, and that shift alone can be enough to change the result. Builders who plan the air barrier early and keep the SAP model updated rarely run into problems. Those who only think about the test at completion often do.

If you want a clearer view of how airtightness affects your project or need help aligning the design target with what is achievable on site, I can guide you through it.