For years, mould had a convenient home: the resident's lifestyle. Too many showers, not enough ventilation, washing dried indoors. The window maker was clear, the builder was clear, and the person living with the black spots wore it. That story is wearing thin — because the building code, the insurers and the tribunals have all started treating mould as something that should have been designed out, not cleaned off.
From lifestyle to liability
Two shifts did it. First, the NCC: under the 2025 code, condensation and mould management sit squarely in Parts F8 and H4, and compliance is judged against a Mould Index. Mould is now a thing the design has to demonstrate it won't grow — a box on the compliance path, not an afterthought. Second, the disputes: persistent mould is increasingly argued as a building defect and a health issue, and when it is, the conversation stops being about open windows and starts being about who warranted the build would perform.
Mould used to follow the occupant home. Now it follows the signature.
That's the part worth sitting with. When mould is a defect, the risk tends to land on the party who signed off the design or the compliance — the certifier, the designer, the builder, depending on the job — not the person who happens to live there. "We told them to ventilate" is a much weaker position than it used to be. (That's a general observation about where the exposure sits, not legal advice — the specifics turn on the contract and the facts.)
Why it's so often the window
Mould needs a cold surface and moist air, and in a modern, well-sealed building the coldest surface in the room is almost always around the glazing — the frame, the edge of the glass, and the junction where the window meets the wall. Air that's perfectly comfortable in the middle of the room hits that cold line, gives up its moisture, and the mould has what it needs. It's not bad housekeeping; it's physics meeting a thermal weak point. Which means it's foreseeable — and foreseeable is exactly what turns "bad luck" into "should have been caught."
Getting in front of it
The good news is that this is the most provable risk on the job, and the proof is cheap compared with the claim. You establish the Mould Index on the governing junction before it's built — a TIRS MI-6 assessment delivers that number with the supporting science, which is exactly what the NCC condensation provisions ask for. Do it at design stage and the mould question is settled on paper: here's the evidence it stays under the line. Skip it, and the first time anyone runs the number may be after the spots appear, in a room you no longer control, in front of people deciding who pays.
Mould stopped being a housekeeping problem the moment the code asked you to prove it wouldn't happen. The smart move is to treat it like any other defect you'd rather catch early — find it on a drawing, not in a handover inspection.
Worried about a junction? Prove it before it's built.
Tell us the window, the junction and the climate zone. A TIRS MI-6 assessment puts the Mould Index — the evidence the NCC wants — on the governing detail, so the mould question is answered at design stage, not in a dispute.
Get the evidence → Try BTW yourself →General information, not legal advice. Where liability for mould ultimately falls depends on the contract, the facts and professional advice. Under NCC 2025, condensation and mould management sit in Parts F8 and H4, with compliance determined from a Mould Index; a TIRS MI-6 assessment establishes that Mould Index with its supporting science on the relevant element. Results are indicative to the systems, elements and conditions provided; final NCC compliance is confirmed by your assessor or certifier. AFRC Lab (Advanced Fenestration Research & Certification) is an independent consultancy; it is not the AFRC. TIRS™ is a trademark of AFRC Lab Pty Ltd.