Waterproof, Watertight and Vapour-Intelligent: How Passive Purple Manages Water and Vapour

One coating, three jobs 💧

Airtight. Watertight. Vapour-intelligent. On paper those sound like they might pull against each other — how can a layer keep water and air out while still letting the structure breathe and dry? That balance is precisely what a good vapour control layer is designed to strike, and it's what Passive Purple is built to do as a single liquid-applied coating.

Here's how it manages water and vapour, and why that combination matters for durable, low-energy buildings.

Watertight: EN 14891

Passive Purple was tested for watertightness to EN 14891 — the standard for liquid-applied waterproofing — and returned a PASS (per the EPD summary). In plain terms: as a continuous, seamless film with no joints or laps, it keeps liquid water out. That's what lets one product serve as both the airtight and the waterproofing layer, rather than stacking up separate systems that each need their own detailing.

Airtight, yet breathing: intelligent vapour control

This is the part that surprises people. Being airtight and watertight does not mean being a vapour barrier that traps moisture in the build-up. Passive Purple is a vapour control layer, and it's designed to let the structure dry.

Tested for vapour resistance to EN ISO 12572, it records:

• A µ-value of 76,584.
• An Sd of 22.9 m (dry) at 0.3 mm thickness.
• An Sd of 34.4 m (dry) at 0.45 mm thickness.

The important idea is that it behaves as intelligent vapour control across an Sd range of roughly 0.8 to 40 m. Rather than a fixed, single-value barrier, it adapts its vapour resistance to conditions — tighter when it needs to hold vapour back, more open when the structure needs to release moisture. That's the mechanism that lets a build-up stay airtight and watertight while still being able to dry out, which is what keeps constructions healthy over their lifetime.

Flexible enough to move with the building

Buildings are not static. They expand, contract, settle and flex, and substrates crack. A rigid airtight layer bridged across a moving junction is a failure waiting to happen. A liquid-applied coating only earns its keep if it can stretch and bridge that movement without splitting.

Passive Purple's elongation was tested to EN 12311-1 at BBRI:

• ~219% elongation before ageing.
• ~230% elongation after ageing — so it doesn't grow brittle with time; if anything it held up.
• The EPD cites elongation after ageing of 350.5% (to BS EN ISO 527-3).

Stretching to more than twice its length — and staying that flexible after ageing — is what gives the coating its crack-bridging ability. A hairline crack opening up in the substrate beneath it doesn't tear the membrane; the film simply stretches across the gap and keeps the airtight, watertight line intact.

That resilience under repeated movement is confirmed separately: fatigue and movement testing to EOTA TR008 returned a PASS. It's not just a one-off stretch in a lab — it's designed to keep bridging movement cycle after cycle.

A note on Passive Purple External 🧱

It's worth distinguishing the grades briefly. The vapour-intelligent behaviour above describes the internal airtight and vapour-control layer. Passive Purple External does a different job on the outside of the building: it's a raintight but vapour-breathable façade membrane, designed to shed driving rain while letting the wall breathe outward. Same liquid-applied, seamless approach — tuned for its position in the wall.

Why it matters

Specifying one coating that is airtight, watertight and vapour-intelligent removes a whole category of coordination risk. You're not lapping an airtight membrane into a separate waterproofing system into a separate VCL, hoping every transition is detailed and installed perfectly. It's one continuous layer doing all three, flexible enough to move with the building rather than fight it.

• Watertight to EN 14891 (PASS).
• Intelligent vapour control across an Sd range of ~0.8–40 m — airtight, yet able to dry.
• High elongation (~219% pre-ageing, ~230% post) for crack-bridging and movement.
• Fatigue-tested to EOTA TR008 (PASS).

All of the figures above come from independent, accredited testing and the product's EPD. To see the full data set and the wider certification package in one place:

Explore the full picture: view the full test data & certifications.