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Condensation & mould 15 Oct 2026 5 min read

Condensation stopped being a callback. It's now a compliance failure.

Under NCC 2025, condensation and mould aren't a maintenance afterthought you argue about after handover — they're something the design has to demonstrate up front. Here's what changed, why an energy rating won't answer it, and where the risk actually sits.

DM By Dave Morehouse, founder, AFRC Lab

For most of my time in this industry, condensation was a callback. Someone rings up in July, there's water pooling on the frame and black spots creeping into the reveal, and the conversation that follows is about whose fault it is. Lifestyle? Drying washing indoors? A fan that doesn't get used? It almost never landed on the window, and it almost never landed at design stage. It was a maintenance problem, argued about after the building was occupied.

That era is closing. Condensation management is now written into the National Construction Code — Part F8 for apartments and commercial, Part H4 for houses — and the shift that matters isn't that the rules exist. It's where the burden now sits. Increasingly it's not enough to say a build won't grow mould. You have to be able to show it, and the way you show it is a Mould Index — the same measure the verification pathways and AIRAH's DA07 guidance lean on. A number. Produced before it's built. Owned by whoever signed the design off.

Condensation didn't get more common. It got harder to argue your way out of.

Why the rating on your window won't answer it

Here's the trap I see most. A window carries an energy rating, so people assume compliance is handled. But an energy rating answers an energy question — how much heat flows through, how much sun gets in. U-value and SHGC. It was built to compare one product against another on a common basis. It was never built to tell you whether the installed window will stay above the temperature where moisture condenses and mould takes hold.

Those are different physics. Condensation is about surface temperature — the coldest spot on the assembly on a cold night. And the coldest spot is almost never the glass in the middle of the pane. It's the frame, the edge of the glazing, and above all the junction where the window meets the wall — the interface, where a thermal bridge can pull the inside surface cold enough to sweat even when the headline rating looks fine. A rating of the bare window, at a standard size, on a test stand, simply doesn't see that.

Where the risk actually sits

So the question isn't "what does the window rate?" It's "how cold does the actual installed assembly get, at its worst point, in this climate — and does that clear the Mould Index the Code now wants demonstrated?" You can screen the window itself with a surface-temperature factor (fRsi) — a quick, steady-state indicator of whether a junction is heading for trouble. But a screen is a screen. The place it bites, the window-to-wall interface, is its own assessment: that's what a TIRS assessment is for — taking the governing junction and establishing the Mould Index, the number with the science behind it, that you hand to a certifier.

The reason to do this before procurement is money, not theory. Caught as a redline at design stage, a condensation risk is a line-item change — reselect a glass, a spacer, a sub-frame, a detail. Let it through to a variation once the package is procured and it costs more and drags the program. Let it reach a handed-over, occupied building and you're into rectification — re-glazing, remediating mould — which runs to the order of $25,000 a defect by the time you're there (Equity Economics, 2019). The cheapest version of this problem is the one you find on a drawing.

What proving it looks like — three rungs

Screen it — fRsi. Straight out of the BTW model: a fast surface-temperature factor that flags which junctions are heading for trouble, so you know where to look.
Prove it — TIRS MI-6. The quick, sharp Mould Index assessment on the part that matters — a window junction, a wall, any element — giving the MI number with the supporting science behind it. That's all the NCC condensation provisions actually need: certifier-ready evidence, minimum fuss.
Understand it — Full TIRS. The full-spectrum assessment — solar loads, ambient conditions, summer and winter — for complex or high-risk façades, where you want to see how the system performs across the whole season, not just clear the line.

None of this is exotic. It's just doing the condensation check where the condensation happens — on the installed assembly, at the interface — and doing it early enough that the answer is still cheap to change. The Code moved the goalposts from "we'll deal with it if someone complains" to "show us it won't happen." That's a good thing. It just means the proof has to exist before the building does.

Not sure your junctions clear? Send us one.

Pick the window and junction you're least sure about. We'll model the real assembly and run the fRsi screen to tell you plainly whether it's heading for trouble — and when you need the compliance evidence, TIRS MI-6 delivers the Mould Index number, with the science behind it, that the NCC asks for. All while it's still a redline, not a rectification.

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About these figures. Surface-temperature-factor (fRsi) results are an ISO 13788 steady-state screening indicator only; they flag risk, they do not certify condensation compliance. Under NCC 2025, Parts F8 and H4 determine compliance from a Mould Index, which a TIRS interface assessment establishes. Modelled results are indicative to the systems, junctions and sizes provided; final NCC compliance is confirmed by your assessor or certifier. The ~$25k rectification figure is an industry estimate (Equity Economics, 2019) and varies by project. AFRC Lab (Advanced Fenestration Research & Certification) is an independent consultancy; it is not the AFRC and is not affiliated with WERS or the Australian Fenestration Rating Council. TIRS™ is a trademark of AFRC Lab Pty Ltd.