Mould and Fungal Resistance: How Passive Purple Protects Airtight Interiors

Airtight buildings and the mould question 🦠

Make a building airtight and you change how moisture behaves inside it. That's the point — you stop uncontrolled draughts, cut energy loss and take charge of ventilation. But it also means the materials lining your walls have a bigger job to do. Get moisture management wrong and warm, humid indoor air can find cool surfaces, condense, and give mould exactly the damp foothold it needs.

So for any layer that sits on the warm side of the structure, mould and fungal resistance isn't a nice-to-have. It's part of delivering a healthy building. We had Passive Purple tested for both, independently, at MicroStar Lab in the USA.

Mould growth: ASTM D3273-21

ASTM D3273-21 is the standard test for mould growth on the surface of interior coatings. Samples sit in a sealed, high-humidity chamber — a deliberately hostile environment designed to encourage mould — and are scored each week over 28 days on a 0 to 10 scale, where 10 means no mould defacement at all and 0 means the surface is overrun.

The results:

• Passive Purple: 10/10, every week for four weeks. No defacement.
• Passive Purple Brush: 10/10, every week for four weeks. No defacement.
• Untreated controls: dropped to 0/0. Comprehensively colonised.

Side by side, in the same chamber, under the same humidity: the untreated surfaces were taken over while the Passive Purple surfaces stayed perfectly clean for the full 28 days. Not a marginal improvement — a perfect score against a control that failed completely.

Fungal resistance: ASTM G21-15

ASTM G21-15 takes a slightly different angle. Rather than mould defacement in humid air, it measures a material's resistance to fungi directly, exposing samples to a mix of fungal species and rating growth from 0 (no growth) upward.

• Passive Purple (interior): rated 0 — no growth.
• Passive Purple Brush: rated 0 to 1.
• Passive Purple External grades: rated 1 to 2 — light growth.

We want to be precise here, because accuracy matters more than a tidy headline. The very best mould and fungal results are for the interior grades — which is exactly where they count, since that's the warm-side layer living inside the conditioned, occupied space. The External grades, designed for a different job on the outside of the building, showed light growth of grades 1 to 2. That's the honest picture, and it maps sensibly onto where each product is actually used.

Why a VCL helps in the first place 💧

Mould needs moisture. Starve it of moisture and it can't establish, however hospitable the surface. This is where Passive Purple does more than simply resist growth on its own face.

As a continuous vapour control layer, it helps govern how much moisture-laden air reaches the structure in the first place. By managing that vapour flow — and by being airtight, so humid indoor air isn't leaking into cold constructions where it can condense — it works to control the very moisture that feeds mould. The coating's own resistance is the last line of defence; the vapour control is the first.

Put together, that's a layer doing two complementary things at once:

• Managing humidity and condensation risk by controlling vapour and stopping air leakage.
• Resisting mould and fungi on its own surface, verified to ASTM D3273-21 and ASTM G21-15.

What this means for occupants

Healthy indoor air is increasingly what clients ask for, and mould is one of the clearest ways a building can fail its occupants — visibly, and for their health. For anyone specifying airtight and low-energy construction, being able to point to a warm-side layer with a perfect 28-day mould score and independent fungal ratings is a genuinely useful piece of evidence.

It also reflects the wider philosophy behind Passive Purple: build tight, manage moisture deliberately, and don't leave indoor air quality to chance.

These results are from independent, accredited testing. To see the full mould and fungal data alongside the rest of the certification package:

Read the evidence: view the full test data & certifications.