Liquid Membranes for Timber Frame Construction

Why does timber frame leak air?

Not because the panels are badly made — a factory-built panel is more dimensionally accurate than almost anything built in situ. The leakage comes from what happens after the panels leave the factory, and from the number of interfaces a timber wall contains once it is a building rather than a kit.

Count one plot. Panel-to-panel joints down every stud line. Sole plate to slab, head binder to the floor above. Every board joint across the elevation. Joist ends — dozens per floor, each a timber passing through the plane the airtight line is meant to occupy. Party wall junctions. Every window and door opening, with reveals, cill, head and cut corners. Service penetrations for soil pipes, ducts, cables and vents, most installed by trades who arrive after the membrane is nominally complete. And behind all of it, timber moves: it shrinks as it dries and the building settles.

A tape-and-sheet strategy addresses each individually: every board joint a strip, every reveal a cut piece, every penetration a collar, every lap a bonded seam whose performance depends on the board being clean, dry and warm enough for the adhesive when it was rolled. Across one plot that is hundreds of separate operations. None are inspectable once insulation and lining go on, and each is a single point of failure.

A liquid membrane changes the arithmetic. Passive Purple is sprayed onto the structure and cures into one continuous film. Board joints are not sealed one at a time — they are part of the surface being coated, along with fixings, screw heads, corners and small penetrations. There are no laps because there are no sheets. What still needs deliberate attention — joist zones, larger penetrations, window and door perimeters — is detailed with the brush grade of the same chemistry, so detail and field are one material rather than two systems relying on an adhesive between them.

Where should the air barrier line go in a timber frame wall?

Approved Document L asks you to answer this on paper first. Paragraph 4.21(a) asks for drawings that "clearly identify the position, continuity and extent of the air barrier". Continuity and extent are what most schemes design for; position gets skipped, and it decides the other two — which layer carries the seal: sheathing board, OSB, or plasterboard.

The guidance is explicit about one of those. Paragraph 4.21(e) states: "Internal plasterboard linings are not appropriate for use as an air barrier solution." That sits in the 2021 edition incorporating the 2023 amendments, Volume 1, for new dwellings in England — check the document that applies to your project.

The numbers are well known: a limiting air permeability of 8.0 m³/(h·m²) at 50 Pa, notional dwelling 5.0. The quieter change matters more — every new dwelling is now pressure tested, which removes the averaging a sampled regime allowed. The strategy has to be repeatable by whoever is on site that week, on every plot.

On timber frame that argues for the line on the structure: the OSB or sheathing face, coated as one film, with the service void kept clear of the barrier. Services then run in front of the airtight layer instead of through it. Note that Passivhaus 0.6 ACH at 50 Pa and Part L air permeability in m³/(h·m²) measure different things and do not convert into each other.

We do not carry out condensation risk analysis — insulation manufacturers do, several free of charge for a specified build-up; see condensation risk analysis. Further reading: where to draw the Part L air barrier line and Part L air permeability targets explained.

Will it stick to OSB?

Yes — and the honest answer includes the substrates where it does not, because those are the ones worth discussing before the specification is fixed. Pull-off adhesion, BS EN ISO 4624:

OSB at 0.42 N/mm² is a sound bond for the field of a timber frame wall. Wood fibre at 0.04 N/mm² is not, and publishing that figure is the point: where the membrane runs onto wood fibre or similar low-adhesion faces, prime it. Supergrip Primer covers 0.2–0.3 kg/m²; Lime Prime and IM Primer 0.15–0.25 kg/m². More in what does Passive Purple stick to.

Which products go on a timber frame?

Alongside them: FRED, IM Fleece Tape, the Joist End Bundle, and primers — Supergrip, IM Primer, Lime Prime.

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What about fire, on a combustible frame?

Timber frame and spray foam both bring real performance, and both are combustible. Under BS EN 13501-1 untreated polyurethane spray foam typically classifies only to Euroclass E, and exposed structural timber needs protecting where it forms part of the fire strategy — a tested thermal barrier is normally required so the assembly satisfies Approved Document B, building control and insurers.

FRED is a liquid intumescent coating that swells in a fire into an insulating char, delaying ignition and slowing flame spread. It is Class A2-s1,d0 to EN 13501-1 and carries a 25-minute Warringtonfire rating, tested on a load-bearing timber frame wall at 8 kN per linear metre to BS EN 1365-1:2012. Over cured spray foam it goes on a Supergrip FR fire-rated primer; foam plus primer plus FRED achieves a Class B reaction to fire.

Used with Passive Purple on 9 mm OSB, the build-up achieves a Class B rating with up to 30 minutes of fire protection — airtightness, vapour control and certified fire performance in one system rather than three. On the external face, Passive Purple External is itself Class B-s1,d0 to EN 13501-1 under PAVUS report PK1-01-20-065-E-1. EN 13501-1 and the ASTM schemes used in North America are separate systems and must never be merged. See fire protection for timber frame and spray foam.

What about joist ends, party walls and window perimeters?

These three details decide a timber frame air test, and all three are hidden by the time anyone would think to check them. Joist ends leave a gap around every single timber — trivial individually, dozens per floor, and unreachable once the deck and ceiling are on. Seal them before the floor build-up closes them off: flexible airtight sealant around the timber-to-structure gap, tied into the wall's airtight layer. The joist ends guide covers the sequence.

Party and separating walls break the internal line by definition, which is why position matters on terraces and semis: a line on the external sheathing face carries past a separating wall in a way a line on the internal lining cannot. Window and door perimeters are the third — the gap between frame and structure moves, and in a different direction from the board either side. Use Passive Purple Brush at the perimeter, or IM Fleece Tape across the frame-to-structure gap.

Has it been proved on timber frame?

A Croydon timber frame house used Passive Purple External Brush on joints, junctions and structural connections before the external coat, with a continuous internal Passive Purple layer inside. It tested at 0.17 ACH.

At Arrington outside Cambridge, a White-Haus timber frame home took the full system: Waterproof Blue liquid DPM at the kicker blocks, Passive Purple External on the erected frame, Passive Purple Brush on joints and fixings, and a sprayed internal layer over 50 mm PIR. The View in Ampthill, a hybrid timber frame barn conversion by the same manufacturer, ran a 326 mm build-up with Passive Purple External on the OSB outside and a continuous spray coat of Passive Purple inside.

Our best result on a school is 0.078 ACH at Wester Hailes High School in Edinburgh — a whole-building pre-test after our works, measured by EDP Building Sciences to ATTMA TSL4 in November 2025, against a 0.6 design target, on McLaughlin & Harvey's £44m Passivhaus school.

Croydon — 0.17 ACH Cambridge MMC timber frame Ampthill barn conversion Wester Hailes — 0.078 ACH

See it on site

Inspect a completed application before specifying. The 360° tours cover coated panels and junctions; the videos show the spray and detailing sequence.

360° site tours Application videos

Related: light gauge steel · masonry · retrofit and social housing · schools · commercial and high-rise.

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