Passive Purple External: Fire Class B, Watertight and Breathable
Passive Purple External: Fire Class B, Watertight and Breathable
Most façade build-ups need three or four separate products to do what one liquid membrane can do here. Passive Purple External is a liquid-applied membrane that is fire-rated, waterproof, airtight and vapour-open in a single seamless coat, applied behind cladding and rainscreens, over timber frame, or directly onto concrete for protection.
This is a distinct product from the internal Passive Purple. The internal membrane is a high-resistance vapour control layer; External is deliberately the opposite. It lets the wall breathe outward while shedding rain from the outside. More on that below.
Fire performance 🔥
Reaction to fire is where liquid membranes usually fall down. Passive Purple External is classified Euroclass B-s1,d0 to EN 13501-1, tested standalone by PAVUS in the Czech Republic. That classification is valid on non-combustible A1 substrates with a density of at least 653 kg/m³ — an important condition to specify correctly, because the substrate is part of the tested system.
For projects following US standards, it also achieves ASTM E84 Class A — more on the US certification below.
Independently certified in the UK and the USA 🛡️
Passive Purple External isn't certified to just one rulebook. It is independently tested and certified on both sides of the Atlantic — to European/UK and US standards — and it doesn't simply scrape a pass on either. On both, it clears the bar comfortably.
In the UK and Europe, it holds a Kiwa BDA Agrément (certificate BAW-21-214-P-A-UK), which independently assesses it for use across all four UK nations with an assessed durability equivalent to the life of the building — around 60 years — sitting alongside its EN fire, watertightness and breathability classifications.
In the USA, it achieves ASTM E84 Class A, the top interior surface-burning class — and it earns it with room to spare: a Flame Spread Index of 0 and a Smoke Developed Index of 15, against limits of 25 and 450. It has also passed the full ICC-ES AC212 water-resistive barrier test battery (bond, freeze/thaw, water penetration, racking and UV weathering).
Two standards, two continents — both met, and both comfortably beaten. For a practice working across markets, that means one façade membrane with the paperwork already in place wherever the project lands.
Watertight, yet breathable 💧
The combination that makes this membrane unusual is its ability to keep liquid water out while letting water vapour pass through.
- Watertightness Class W1 to EN 13859-2 — the top watertightness class for wall underlays.
- Water permeability Class W3 to EN 1062-3.
- Near-zero water absorption to EN 16302.
- Vapour permeability Sd ≈ 0.6 m to EN ISO 12572 — genuinely vapour-open.
That Sd value is the headline. At roughly 0.6 m, the membrane is highly breathable, allowing moisture to escape outward through the wall rather than becoming trapped in the construction. This is the crucial distinction from the internal Passive Purple VCL: External breathes out while it sheds rain, which is exactly what you want on the cold, outer side of an insulated wall.
Movement and durability
Buildings move, and coatings that cannot move with them crack. Passive Purple External is crack-bridging Class A3 at 23°C and Class A4 at -10°C to EN 1062-7 — retaining crack-bridging performance even in cold conditions.
On weathering, it passes 400–500 hours of UV exposure; notably its tensile strength actually increases after 500 hours of UV, rather than degrading. It is also airtight to EN 12114, carries a low-emission EMICODE EC2 rating, and achieves adhesion of up to 1.23 N/mm² on CLT to EN ISO 4624.
Protecting concrete ☀️
For concrete structures, Passive Purple External acts as a CO₂ and carbonation barrier, Class C1 to the relevant test. By limiting carbon dioxide ingress it helps slow carbonation, which protects embedded reinforcement and extends the service life of the concrete — while still allowing the substrate to breathe.
Passive House component
Passive Purple External is certified by the Passive House Institute as a component in Class phB. Note the distinction: External is phB, not phA. It is a strong performer for airtight, low-energy façades, but the classification should be quoted accurately in Passive House documentation.
Why one membrane matters
Specifying separately for fire, water, air and vapour means more products, more interfaces and more ways for a detail to fail. Passive Purple External brings those functions into a single, seamless, liquid-applied layer:
- Fire-rated — Euroclass B-s1,d0 standalone (on qualifying A1 substrates).
- Waterproof — Class W1 watertightness, near-zero absorption.
- Airtight — tested to EN 12114.
- Breathable — Sd ≈ 0.6 m, so the wall dries outward.
That makes it well suited to rainscreen and ETICS façades, timber-frame walls, and concrete protection — one product doing the work of several, seamlessly detailed around penetrations and junctions.
For the complete test data and certification references, see the full test data & certifications, or view the product page for Passive Purple External.