What Does Passive Purple Stick To? Independent Adhesion Testing Across Substrates

What does Passive Purple actually stick to?

It's a fair question, and one specifiers ask us constantly. An airtight layer is only as good as its bond to the building. If the membrane lets go — at a junction, behind a service penetration, where two materials meet — the airtight line is broken and the detail has failed.

So we had Passive Purple tested. Independently, at accredited European laboratories, across the substrates real UK and EU projects are actually built from: masonry, timber, steel, gypsum, cement boards and insulation. The results below are mean pull-off adhesion values to ISO 4624 / EN ISO 4624 (or EN 1542 where noted), carried out at BBRI in Belgium and the University of Liège.

Why liquid-applied changes the question

Traditional airtightness relies on membranes, tapes and mechanical fixings. Each of those is a potential point of failure — a tape that lifts, a lap that peels, a fixing that works loose over time.

Passive Purple is different because it's liquid-applied. It's rolled or sprayed on wet, so it impregnates the surface and cures into one continuous, seamless layer that is chemically bonded to the substrate. There are no joints to fail and no separate adhesive doing the work. The coating is the bond.

That's why the pull-off numbers matter — and why one finding keeps repeating across every test: when something does let go, it's the substrate that fails, not the membrane.

The results, substrate by substrate 🧱

• Galvanised steel — 2.7 N/mm². The strongest bond in the set. Reassuring for steel frames, structural connections and metal details.
• Gypsum block — 1.6 N/mm². Here the gypsum itself failed before the coating did. The bond was stronger than the block.
• CLT (cross-laminated timber) — 1.83 N/mm². Again, the timber fibres failed first. The coating held; the wood gave way.
• OSB — 0.51 N/mm². A dependable bond to one of the most common sheathing boards on UK sites.
• Red clay brick (Porotherm) — 1.2 to 1.5 N/mm². Crucially, adhesion was maintained or improved after accelerated ageing — the bond doesn't tire over time.
• CEMGOLD cement board — up to 0.82 N/mm². The board failed first in testing, not the coating.
• PUR / PIR / PS / Kingspan Kooltherm insulation — 0.05 to 0.29 N/mm². These figures look modest, but the reason is important: the insulation board always failed internally before the bond did. The coating's grip exceeds the board's own tensile strength — you cannot pull the membrane off without tearing the insulation apart first.

Reading the numbers the right way

It's tempting to rank substrates purely by newtons per square millimetre, but that misses the point. On gypsum, CLT, cement board and insulation, the limiting factor is the material, not Passive Purple. The coating is stronger than the thing it's stuck to. You physically cannot get a cleaner bond, because the substrate surrenders first.

That's exactly what you want from an airtight and vapour-control layer. It means the weak link in your detail isn't the membrane — it's whatever the membrane is protecting, and Passive Purple is holding onto it harder than that material can hold onto itself.

Why it matters for your project

Real buildings are messy. A single airtight line might cross blockwork, then a timber junction, then a steel column, then a run of PIR, then a service penetration through plasterboard — often within a metre or two. A tape-and-membrane approach needs a different product, and a different detail, for each transition.

Passive Purple bonds continuously across all of them with one coating. No changes of material at junctions, no accumulation of tape laps at exactly the points where airtightness is hardest to guarantee. For Passivhaus and low-energy work, where the airtightness test is unforgiving, that continuity is the whole game.

• Works across masonry, timber, steel, gypsum, boards and insulation.
• Continuous bond — no mechanical fixings or tapes to fail.
• Adhesion holds after accelerated ageing.
• Where failure occurs, it's the substrate — the bond beats the material it's stuck to.

These results come from independent, accredited testing — not our own marketing bench. If you'd like to see the full pull-off data, the labs behind it and the wider certification package, we've put everything in one place.

See the numbers for yourself: view the full test data & certifications.