Condensation has its own small vocabulary, and the words get swapped around in a way that causes real confusion on a job — people think they've proven something when they've only flagged it, or they commission a full study when a short one would have cleared the box. So let's separate the three you'll actually hear: fRsi, the Mould Index, and TIRS. They do different jobs.
fRsi — the screen
fRsi is a surface-temperature factor: a quick, steady-state indicator of how cold the inside surface of a junction gets relative to the air. It's the smoke alarm. Run it — it sits inside the BTW model — and it tells you which junctions are heading toward condensation and deserve a closer look. What it doesn't do is prove compliance. It's a screen, by design: fast, directional, not the final word.
Mould Index — the measure the Code uses
The Mould Index isn't a service you buy; it's the yardstick. It estimates how much mould would actually grow on a surface over time, given its temperature and humidity. Under NCC 2025, the condensation provisions — Parts F8 and H4 — judge compliance against a Mould Index, in line with the approach in AIRAH's DA07 guidance. So when a certifier asks you to demonstrate a build won't grow mould, this is the number they mean. fRsi says "this junction looks risky"; the Mould Index says "and here's whether that actually becomes a problem."
fRsi flags it. The Mould Index settles it. TIRS is how you produce the Mould Index.
TIRS — the assessment that produces the evidence
This is where the two tiers come in, and the distinction matters for what you spend.
The quick, sharp assessment: it delivers the Mould Index number plus the supporting science, on whatever element matters — a window junction, a wall, any part. That is all the NCC condensation provisions actually need. If your job is to clear F8 / H4 cleanly and move on, this is the one.
The full spectrum — solar loads, ambient conditions, summer and winter. Not a bigger compliance tick, but a real understanding of how the system behaves across the whole year. For complex or high-risk façades, or when you want to design with the numbers rather than just satisfy them.
So which do you need?
In order: screen it with fRsi to find the junctions at risk; prove it with TIRS MI-6 to put the Mould Index — the evidence the Code wants — on the governing one; and reach for Full TIRS only when you need to understand performance across the season, not just clear the line. Most jobs stop at MI-6, because that's exactly what the NCC asks for. The mistake is treating the screen as the proof, or paying for the full study when the minimum was all that was required.
Not sure which one your job needs?
Tell us the element and the climate zone. We'll say plainly whether an fRsi screen settles it, whether you need TIRS MI-6 for the compliance evidence, or whether the façade warrants the full-season picture — before you spend on the wrong one.
Ask the lab →About these terms. fRsi is an ISO 13788 steady-state screening indicator only and does not certify compliance. Under NCC 2025, Parts F8 and H4 determine condensation compliance from a Mould Index. TIRS MI-6 delivers that Mould Index with its supporting science on the relevant element (a window junction, a wall, any part); Full TIRS adds solar-load and seasonal (ambient, summer/winter) analysis. 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.