Part L and the Air Barrier Line: Where You Draw It Decides Whether You Pass

The headline numbers in the current Part L are well known: a limiting air permeability of 8 m³/(h·m²) at 50Pa, and a notional dwelling at 5. The change that decides whether a scheme passes is quieter than either. Every dwelling is now tested, and Approved Document L is specific about which layer of the wall the airtight line should sit on.

Last updated: August 2026

What actually changed in the 2021 edition?

The Table 4.1 limiting value tightened from 10.0 to 8.0 m³/(h·m²) at 50Pa, and the notional dwelling assumes 5. Those are the numbers everyone quotes. The structural change is at paragraph 7.4: "An air pressure test should be carried out on every dwelling."

Under AD L1A 2013 it was a sample — "three units of each dwelling type or 50 per cent of all instances of that dwelling type, whichever is the less." On a 60-plot scheme of four house types, twelve tests could carry the site, so a method that worked most of the time was good enough. Universal testing removes that averaging: the air barrier strategy now has to be repeatable by whoever is on site that week, not just achievable by the best gang on the best day.

Where does the air barrier line go?

Approved Document L asks you to answer this on paper first, at 4.21(a): "Drawings: all relevant drawings should be provided to clearly identify the position, continuity and extent of the air barrier."

Three words are doing the work. Continuity and extent are what people design for — the unbroken pen line round the section. Position gets skipped, and it decides the other two: which physical layer carries the seal, blockwork, sheathing board, OSB, or lining. Until that is fixed, "continuous" is an aspiration rather than a specification, because you cannot detail a junction between layers you have not named. Our technical details library sets the line on standard junctions.

Paragraph 4.21(c) covers what happens when the line meets reality: "Where membranes are penetrated, careful detailing should be used to achieve a robust and durable seal at these penetrations." Note robust and durable — the guidance is not only asking whether the seal works at handover.

What does the guidance say about plasterboard?

Paragraph 4.21(e) states: "Internal plasterboard linings are not appropriate for use as an air barrier solution." The same paragraph continues: "Where dense aggregate blocks have been used, plaster, parge coat or liquid membranes should be applied internally to reduce air permeability."

Two things need saying plainly. First, the language: Approved Document L states this — it does not prohibit it. AD L is statutory guidance, not the statute; the legal test is the pressure test under regulation 43. You can depart from the guidance and show compliance another way. What you cannot do is depart from it accidentally.

Second, the scope. That sentence sits in Volume 1 (Dwellings), 2021 edition incorporating 2023 amendments, in the guidance for new dwellings with cavity walls, in England. It is not in Volume 2 for non-domestic buildings, nor in the existing-dwellings guidance, and Wales, Scotland and Northern Ireland run their own wording. Outside those bounds, check your own document.

So why is plasterboard still used as the line?

Because it is, and it would be dishonest to pretend otherwise. ATTMA TSL1 Issue 3 observes that internal plasterboard lining is commonly used as the air barrier line where the target sits above 5 m³/(h·m²) at 50Pa.

That is a fair description of site practice, but it is not statutory guidance and not a design recommendation — it is an observation made in a document about how to conduct a test. It also carries its caveat in the threshold: above 5. The notional dwelling is 5, and most schemes now need to land at or below it.

Why does the void behind dab-fixed board leak?

There is direct evidence. Johnston and Lowe, in Building Services Engineering Research and Technology 27(1) in 2006, took dry-lined masonry dwellings testing at 24–26 m³/h·m² at 50Pa and sealed them by injecting foam ribbons into the void behind the plasterboard, cutting permeability "by almost 55%, to a mean of just over 11 m³/h per m² at 50 Pa".

Nothing on the room-side face changed. Over half the leakage was moving through the cavity between board and blockwork — an interconnected void running behind sockets, round reveals, into the ceiling zone and past the joist ends. That is why sealing the perimeter of a dab-fixed lining is difficult: the leak is not at the visible joint, it is in the space behind it.

What does a continuous liquid membrane do differently?

It changes the position, not the diligence. If the guidance says the internal plasterboard lining is not appropriate as the air barrier, then any strategy whose method is to seal the gaps around that lining is sealing around a layer never meant to carry the airtight line. The work may be careful and the result may test well. The line is still on the wrong layer.

Intelligent Membranes puts the barrier on the structure itself — blockwork, sheathing board, OSB — as one continuous liquid film behind the lining. Paragraph 4.21(e) names liquid membranes directly, alongside plaster and parge coat, as what should be applied internally where dense aggregate blocks have been used. The line runs where the guidance says to draw it, on a layer that is monolithic rather than jointed, and the void behind the board stops being part of the strategy. Our masonry approach and internal airtightness system pages show how the film meets junctions, joist zones and penetrations.

Does the seal still work in year three?

The regulations say nothing about sealants degrading, and it would be an overreach to claim they do. The evidence comes from elsewhere.

The Passivhaus Trust's Good Practice Guide to Airtightness (June 2020), §3.1.1.4, lists products "commonly used but do not form robust and reliable air barriers": "duct tape, gaffa tape, aluminium foil tape, expanded foam, silicon sealant, decorators caulk, dot & dab plasterboard, skim finish, OSB, vinyl, plywood, flooring and masking tape". Its permitted list includes "Wet plaster > 5mm depth." Continuous applied films are in; jointed and adhesive-dependent assemblies are out.

The AIVC reaches the same place from field data: "airtightness tends to deteriorate after completion", changes "occur mostly within the first (1 or 2) year(s)", and causes include "poor workmanship" and "unsuitable implementation conditions for adhesives and mastic such as cold and/or dusty conditions." A February first fix in an unheated shell is exactly that condition — and the practical content of robust and durable in 4.21(c).

How does your airtightness figure decide ventilation?

The figure you commit to is not only a Part L number. Approved Document F, Volume 1 permits natural ventilation only for what it calls "less airtight dwellings" — design air permeability above 5, as-built above 3 m³/(h·m²) at 50Pa. Build tighter and you need continuous mechanical ventilation: MVHR or continuous MEV.

So a scheme aiming below 5 to make SAP work has already chosen a mechanical strategy. Its service runs then puncture the air barrier, returning you to 4.21(c) and to which layer they are sealed against. A barrier on the structure is sealed at first fix, while it is still reachable.

What should you do on your next project?

• Name the layer before you draw the line. Write it on the section — "airtight line: blockwork face" — so 4.21(a) is answered with a noun rather than a hope.
• Check what the line is made of. If any part of it is a plasterboard lining, or the seal around one, 4.21(e) has already commented on it.
• Design for margin, not the limit. With every plot tested, built-in tolerance is all that stands between site variation and a failed SAP calculation.
• Fix ventilation at the same meeting. The AD F thresholds of 5 and 3 make airtightness and ventilation one decision.
• Sequence the penetrations. Seal them against the structural layer at first fix, while access exists.

Our project case studies show what this produces on tested schemes.