Airtightness, Fire & Protection for Commercial & High-Rise Buildings

What actually fails on a commercial or high-rise envelope?

Not the field of the wall. Rarely the field of the wall. What fails is the small proportion of the elevation where the barrier has to change direction, cross a structural element, or accommodate something passing through it and on a tall building that proportion is much larger than it looks from the drawing.

Take one storey of a rainscreen elevation. A head track at every floor, usually a deflection head that has to stay free to move. A base track 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, four reveals, a cill, a head and eight cut corners. Helping hand brackets at several per square metre, each a fixing through the barrier. Rainwater outlets, vents, service entries, balcony connections and soffits. All of it repeated by the number of floors, executed at height, often in weather.

Then add the two continuity problems that only appear in commercial and residential-at-height work: compartment floors and party or separating walls. Both are lines the fire strategy requires to stay intact, and both interrupt any airtight layer that has been drawn on the internal face. An internal barrier that stops at a compartment floor and restarts above it is not continuous, whatever the drawing says, and it is precisely at that break that the pressure test finds the leakage.

Why do sheet membranes struggle on this kind of wall?

This is an argument about mechanism, not about brands. A self-adhered sheet performs superbly on what it was designed for: a large, flat, clean, dry, dimensionally stable plane. The further an elevation departs from that description, the more of its performance depends on hand-formed details rather than on the sheet itself.

Four departures matter here. SFS infill is a jointed board face inside a frame broken by structure at every floor, so the plane is not a plane. Complex penetrations, bracket arrays, service entries, irregular openings, each need a collar or patch, and a patch is a bonded seam whose performance depends on the substrate being clean, dry and warm enough for the adhesive when it was rolled. Irregular substrates, aged concrete, patched render, exposed blockwork, made-good repairs, do not offer the uniform surface a pressure-sensitive adhesive needs. And continuity past compartment floors and party walls is exactly where a lapped system has to be interrupted and restarted.

A liquid membrane inverts the arithmetic. Passive Purple is sprayed on and cures into one continuous film. Board joints are not sealed one at a time; they are part of the surface being coated. Brackets and fixings 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, larger penetrations, is detailed with the brush grade of the same chemistry, so detail and field are the same material rather than two systems relying on an adhesive between them. Passive Purple External is crack-bridging classified A3 at 23 °C and A4 at −10 °C, so it accommodates the movement a deflection head is built to allow.

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

Yes, and on re-cladding and over-cladding 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 remediation, student accommodation, hotels and hospitals, the cost and programme risk of getting inside every unit is frequently larger than the cost of the membrane. It also keeps the barrier continuous past the things that break 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. Further reading: airtight and vapour control for high-rise re-cladding, installed from the outside in and spray-applied external airtightness for retrofit and re-cladding.

What does the fire classification allow?

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. Quote it in full, and quote it in the right scheme - EN 13501-1 and the ASTM system used in North America are separate classification schemes and must never be merged.

That classification is what allows the airtight layer to sit on the outer face of the sheathing within a rainscreen build-up, rather than being pushed to the warm side by the fire strategy, which is the whole basis of the outside-in approach above.

The membrane is also vapour open: sd 0.62 m 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. Passivhaus certified phB at 0.16 m³/(hm²), component ID 2380as00. Where fire is the dominant driver, FRED adds an intumescent layer over combustible substrates: Class A2-s1,d0 to EN 13501-1, with a 25-minute Warringtonfire rating to BS EN 1365-1:2012. See Passive Purple External: fire Class B, watertight, breathable and intumescent paint explained.

Which products go on a commercial or high-rise façade?

External range All products Specifier hub

Also: IM Fleece Tape for glazing interfaces, Intelligent Protect AC/FR for exposed steel, and temporary protection for glazing during the works.

Will it bond to the substrates a commercial envelope actually uses?

Pull-off adhesion to BS EN ISO 4624. The low figures are published alongside the high ones, because they are the honest reason to specify a primer rather than discover the problem on site.

Primers: Supergrip 0.2–0.3 kg/m², Lime Prime 0.15–0.25 kg/m², IM Primer 0.15–0.25 kg/m². Full figures on our coverage rates page, with the testing behind them in what Passive Purple sticks to.

We do not carry out condensation risk analysis. Insulation manufacturers do, several free of charge for a specified build-up - see condensation risk analysis for what to send and who to ask.

Has it been proved?

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

At Unity Campus Phase 2, a £100m technology and research campus at Sawston near Cambridge delivered by Morgan Sindall Construction, Passive Purple and Airtight White were used across all three buildings, brush grade on joints, fixings and penetrations first, then the sprayed membrane, with IM Fleece Tape at the glazing interfaces. The buildings averaged 0.43 ACH.

On a modular new-build hospital in East Anglia, Airtight Fleece Tape was applied to joint steels and key junctions and 3,500 m² of Passive Purple internal and brush membranes were installed over two days. The building achieved 0.2 ACH.

Wester Hailes - 0.078 ACH Unity Campus - 0.43 ACH Modular hospital - 0.2 ACH

See it on site

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

360° site tours Application videos

Related: light gauge steel frame · timber frame · masonry · retrofit and social housing · schools and education · asbestos encapsulation. Further guides: airtight, weatherproof, fire-rated facades - spray or roller · the complete external façade kit · the external detailing kit · anti-corrosion and flame-retardant protection for steel.

Frequently asked questions