Is wet plaster really airtight?

A continuous coat of wet plaster (or a parge coat) is genuinely airtight — the problem is continuity, not the material. Plaster typically stops at floor and ceiling level and behind dry-lining, leaving gaps at exactly the junctions that leak. As a strategy it works only if the plaster (or a dedicated parge coat) is deliberately carried continuous behind skirtings, behind dry-lining and connected to the floor and ceiling air barriers.

Membrane or board — which airtight layer is better?

Both work; the right choice depends on the construction and the trades. Taped racking boards (OSB/sheathing) are robust and hard to damage but need every joint and penetration taped; internal membranes (AVCLs) are flexible and continuous but more vulnerable to puncture, which is why a service void to keep them intact is valuable. The best layer is the one your team can install continuously and protect.

Why do I need an 'airtightness champion'?

Because airtightness is destroyed incrementally by follow-on trades who each make 'just one small hole'. A named person who owns the air-barrier strategy, briefs every trade, and inspects and reseals after each penetration is the single most reliable way to translate a good design into a good test result. Without one, responsibility diffuses and the layer leaks.

Airtightness Strategy: The Continuous Air Barrier & Detailing

The single biggest mistake in airtightness is treating it as a snagging exercise — chasing leaks with mastic after the building is finished. That approach plateaus quickly and never reaches good numbers. Genuine airtightness comes from designing a continuous air barrier on paper, choosing materials that can deliver it, detailing every junction, and appointing someone to protect it on site. Sealing then becomes verification, not the strategy.

The continuous air barrier — and the 'red pen' rule

The air barrier is the continuous layer that stops air passing through the envelope. The defining test is the 'red pen' (or 'pencil') rule: on every section and plan drawing, you must be able to trace the air barrier around the entire building — through walls, roof, floor, around every window and door and through every junction — without lifting your pen off the page. Wherever you have to lift the pen, you have a designed-in air leak. Do this exercise on the drawings, before anyone builds anything.

Choosing the airtight layer

Different constructions use different primary airtight layers. The key is to choose one continuous element per assembly and detail everything else to connect to it:

Common primary airtight layers

Approach	Airtight layer	Notes
Masonry, wet finish	Continuous wet (parge/plaster) coat	The plaster is the air barrier — but only if it's continuous behind dry-lining and down to floor/ceiling
Timber frame / I-joist	Internal membrane (AVCL) or racking board (OSB/sheathing) with taped joints	Membrane or board taped at every lap, junction and penetration
Service zone build-up	Air barrier behind a battened service void	Keeps cables/pipes out of the air barrier — fewer penetrations to seal
Roof / ceiling	Continuous membrane or sealed board	Must connect to the wall air barrier and around every downlight, pipe and hatch

Whichever layer you choose, the products that connect it — airtight tapes, grommets, flexible junction membranes, pre-formed corners, airtight back boxes — must be compatible and designed for durability. Cheap mastic that cracks in a year is not an air-barrier strategy.

Detailing the hard bits — junctions and penetrations

Leakage concentrates at junctions and penetrations, so that's where the detailing effort goes:

Window and door openings — the frame's airtight seal must bond continuously to the wall air barrier (airtight inside, weather-tight outside).
Wall-to-floor and wall-to-roof junctions — the wall air barrier must physically connect to the floor and ceiling air barriers, not just stop near them.
Service penetrations — minimise them, group them, and pass each through a purpose-made airtight grommet or collar rather than a hopeful blob of sealant.
Intermediate floor / wall junctions — design the air barrier to run continuously past the floor zone, not be interrupted by it.
Electrical boxes on external walls — use airtight back boxes, or keep electrics in a service void inboard of the air barrier so they never penetrate it.

The service void — design out the penetrations

One of the most effective strategies is to position the air barrier and then build a battened service void (typically ~25–50 mm) on the warm side of it, for running cables, sockets, pipes and switches. Because all those services sit inboard of the air barrier, they never penetrate it — eliminating dozens of small leaks at source and making the airtight layer far easier to keep continuous. It also protects the air barrier from being punctured by later occupant DIY.

Sequencing and the airtightness champion

A good design fails if the build doesn't protect it. Two organisational measures make the difference:

Sequencing — the air barrier must be installed, inspected and (ideally) test-verified while it's still accessible, before it's covered by insulation, plasterboard, screeds and finishes. An early-stage blower door test catches problems when they cost minutes to fix, not days.
An 'airtightness champion' — one named person on site responsible for understanding the air-barrier strategy, briefing trades, protecting the layer from the inevitable follow-on punctures (the plumber's new pipe, the electrician's extra cable), and signing off the detailing. Airtightness is a team sport; without an owner, every trade assumes someone else is handling it, and the layer dies by a thousand cuts.

Airtightness strategy and detailing — the continuous air barrier.