Airtightness and Part L: UK Air Permeability Targets Explained

Under the current Part L of the Building Regulations in England (Approved Document L 2021, in force since June 2022), a new dwelling must achieve an air permeability no worse than 8 m³/(h·m²) at 50Pa — and every single new dwelling must now be pressure tested, not just a sample. The notional dwelling that sets the compliance benchmark assumes 5 m³/(h·m²), so in practice most designs need to land at or below 5 to make the SAP calculation work. Here is what those numbers mean, why designing to the limit is a mistake, and how to hit better figures on site.

What Part L says about airtightness

Part L (Conservation of fuel and power) sets the energy performance rules for new buildings. For dwellings in England, the 2021 edition made three changes that matter to anyone responsible for airtightness:

• A limiting air permeability of 8 m³/(h·m²) @50Pa. This is a backstop, not a target — no new dwelling may test worse than 8, regardless of how good its insulation or heating system is.
• A notional dwelling value of 5 m³/(h·m²) @50Pa. Compliance is judged by comparing your dwelling's calculated carbon emissions and energy use against a "notional" version of the same dwelling built to a reference specification — and that reference assumes an air permeability of 5. Test worse than 5 and you must claw back the difference elsewhere in the SAP calculation.
• Universal testing. Previously only a sample of plots on larger schemes needed testing. Now every new dwelling gets a pressure test, so there is nowhere for a leaky plot to hide.

Air permeability vs air changes: know your units

Building Regulations use air permeability: cubic metres of air leaking per hour, per square metre of envelope area, at a 50Pa test pressure. Passivhaus uses air changes per hour: the whole building volume leaking per hour at 50Pa, with a certification target of 0.6 ACH. The two are related but not interchangeable — envelope area and volume differ from house to house. For context, a dwelling meeting the Passivhaus 0.6 ACH target will typically be several times tighter than one scraping through Part L, which shows how much headroom exists above the regulatory minimum.

Why designing to 8 — or even 5 — is risky

Your SAP assessor enters a design air permeability at the start of the job. If the test result comes in worse, the SAP calculation is redone with the real number, and the dwelling can fail compliance even though it passed the "test" in isolation. Chasing the limiting value of 8 leaves no margin at all. Aiming for 5 leaves you hostage to workmanship on the day. The pragmatic approach on site is to design and build for a comfortably better figure — say 3 or below — so that normal site variation still leaves you compliant, and the SAP assessor gains headroom to spend elsewhere in the specification. There is a comfort and running-cost argument too: uncontrolled leakage is heated air you paid for escaping through the fabric. Our heat loss calculator shows what that leakage costs over a heating season.

How to build in a good air permeability score

Airtightness is a design decision first and a workmanship issue second. The projects that test well share three habits:

• One continuous air barrier line, drawn before work starts. Trace it on the sections: walls, floor zones, ceilings, reveals. Every junction where the pen lifts is a future leak. If you want a second pair of eyes, our free drawing markup service will mark the airtight line and flag the risky details on your own drawings.
• A robust primary air barrier. On masonry builds, a liquid applied membrane such as Passive Purple forms the airtight and vapour control layer in one: a two-coat system, spray or brush applied, that bridges blockwork, junctions and joist zones as a single seamless film. It is Passivhaus certified and BBA approved, with tested air leakage of 0.02 L/s·m² at 75Pa (ASTM E2178) — performance far beyond anything Part L asks for. On timber frame, sheet VCL membranes do the same job, with taped laps.
• Details sealed as you go, not at the end. Service penetrations, window reveals and wall-floor junctions cause most test failures. Sealing them while they are accessible — with membrane, tape and sealant — is quick; chasing them after plasterboard is miserable.

For an overview of how the pieces fit together on a real build, see our airtightness solutions page, or browse the full airtight paint range.

Scotland, Wales and Northern Ireland

The figures above are for England's Approved Document L. Scotland (Section 6), Wales (its own Part L) and Northern Ireland (Technical Booklet F) set their own standards and timetables, and they are not identical. If you are building outside England, confirm the current values with your building control body or energy assessor before fixing your design target rather than relying on England's numbers.

Frequently asked questions

What is the maximum air permeability allowed for a new home in England?

8 m³/(h·m²) at 50Pa is the limiting value under Approved Document L 2021. But this is a backstop: because the notional dwelling assumes 5, most designs need a result at or below 5 for the overall SAP calculation to pass.

Does every new house need an airtightness test?

Yes. Since the 2021 edition of Approved Document L took effect in England, every new dwelling must be pressure tested — sample testing on larger developments is no longer accepted.

What air permeability should I actually aim for?

Agree it with your SAP assessor, but building to around 3 m³/(h·m²) or better gives real margin over both the notional 5 and the limiting 8, and is readily achievable with a continuous liquid applied air barrier and disciplined detailing. Passivhaus projects go much further, to 0.6 ACH @50Pa.

Is airtightness bad for ventilation?

No — the principle is "build tight, ventilate right". Part L works alongside Part F, which sets ventilation requirements. A tight envelope with designed ventilation (extract fans or MVHR) gives controlled fresh air; a leaky envelope gives uncontrolled draughts and wasted heat.