Liquid Membranes for Light Gauge Steel Frame (LGS)

Why do SFS envelopes leak?

Light gauge steel framing behind a rainscreen is fast, light and dimensionally accurate. It is also one of the most junction-dense forms of construction in use. The airtight line sits on the face of the sheathing board — cement particle board, gypsum sheathing or OSB — and that face is not a continuous plane. It is an assembly of boards inside a frame that is itself broken by structure.

Count what one elevation contains. A head track at every floor, usually a deflection head that must stay free to move. A base track at every floor, on the slab edge or a shelf angle. The slab edge itself, with its band of fire stopping and cavity barriers. Every window and door opening, with four reveals, a cill, a head and eight cut corners. Penetrations for helping hand brackets, rainwater outlets, vents, extract terminals and service entries — the bracket count alone runs to several per square metre. And behind all of it, board-to-board joints on 400 mm or 600 mm stud centres across the full elevation.

A tape and sheet system addresses each of those individually. Every board joint takes a strip of tape, every reveal a cut piece and a corner detail, every bracket a collar, every deflection head a movement-tolerant detail. The sheet has to be lapped, and each lap is a bonded seam whose performance depends on the board being clean, dry and dust free when it is rolled. Across a mid-rise elevation that is thousands of separate operations, each an opportunity for a leak, none of them inspectable once insulation and rails go on.

A liquid membrane inverts the problem. Passive Purple is sprayed onto the board and cures into one continuous film. Board joints are not sealed one at a time — they are part of the surface being coated. Reveals, corners, bracket penetrations and the fixings themselves are wetted out in the same pass. There are no laps, because there are no sheets. What still needs deliberate attention — deflection heads, movement joints, the slab edge and larger penetrations — is detailed with the brush grade of the same material, so detail and field are the same product rather than two systems relying on an adhesive between them.

Movement matters too. Steel, board and insulation move at different rates, and a deflection head is designed to move. Passive Purple External is crack-bridging classified A3 at 23 °C and A4 at −10 °C to BS EN 1062-7, so the film accommodates that movement rather than tearing where the frame is meant to flex.

Can it be applied from the outside, on an occupied building?

Yes, and on re-cladding and overcladding that is usually the reason it is chosen. The airtight and vapour control layer goes on the outer face as part of the cladding works — same scaffold, same access, same trade sequence. Nothing has to happen inside the flats.

The commercial consequence is direct: no decant, no appointments with residents, no internal works package, no making good, no restricted working hours in occupied dwellings. On social housing remediation, student accommodation, hotels and hospitals, the cost and programme risk of getting inside every unit is often larger than the cost of the membrane.

Working externally also keeps the barrier continuous past what breaks an internal line — compartment floors, party walls, risers and finishes the fire strategy requires to stay intact. The site sequence: prepare and, where needed, prime; detail deflection heads, movement joints, openings and larger penetrations with the brush grade; spray the open areas to the specified coverage; install insulation, rails and cladding over the cured film.

Fire — the reason it can go where others cannot

Most liquid air barriers are ruled out of external build-ups on fire grounds before their airtightness is ever discussed. Passive Purple External is classified Class B-s1,d0 in accordance with EN 13501-1, under PAVUS classification report PK1-01-20-065-E-1: reaction to fire B, s1 for smoke production, d0 for flaming droplets.

That is what allows the airtight layer to sit on the outer face of the sheathing board within a rainscreen build-up rather than being pushed to the warm side by the fire strategy. Quote it in full: EN 13501-1 Class B-s1,d0, PAVUS PK1-01-20-065-E-1. EN 13501-1 and the ASTM E84 scheme used in North America are separate systems and are not interchangeable — use whichever applies to the project, and never merge the two.

The membrane is also vapour open: sd 0.62 m (3.1 MNs/g) to EN ISO 12572 under Kiwa BDA Agrément BAW-21-214-P-A-UK. The wall dries outwards while the film resists driving rain before cladding goes on — water permeability Class W3 to BS EN 1062-3, capillary water absorption 0.004 ml/cm² to BS EN 16302.

How well does it bond to the boards SFS actually uses?

Pull-off adhesion to BS EN ISO 4624, as reported in the Kiwa BDA Agrément, section 2.5.2. The table includes the substrates where adhesion is low: those are the places to discuss a primer before the specification is fixed, which is why it is published in full.

Which products go on an SFS wall?

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Has it been proved on a steel-framed building?

At the Bedford Academy expansion, a two-storey teaching extension, the airtight line had to cross steel interfaces, OSB board joints and beam ends alongside masonry, timber and plasterboard. Beam ends, floor-to-wall connections, steel interfaces, board joints and service penetrations were detailed with Passive Purple Brush and the associated tapes and grommets, with field areas sprayed. The completed building tested at 0.43 ACH, inside the 0.6 ACH we guarantee.

Our best result on a school is 0.078 ACH at Wester Hailes High School, 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.

Read the Bedford Academy case study

See it on site

Inspect a completed application before specifying. The 360° tours cover coated elevations, junctions and reveals; the video channel shows the spray and detailing sequence on site.

360° site tours Application videos

Drawings and specification text

Steel frame detail drawings — deflection heads, slab edges, openings, penetrations — sit with the rest of our airtightness and vapour control details. Specifiers working to a CSI-based master specification can take the section text directly.

Technical details and drawings CSI 07 27 26.21 specification

Frequently asked questions