This is the quick-answers companion to the complete thermal bridging guide and the series behind it. Each question below is answered in a line or two, then links straight to the piece that works it through in full.
Thermal bridging, in plain terms
What is thermal bridging?
A thermal bridge is a more conductive path — a steel stud, a slab edge, a metal window frame — that lets heat bypass the insulation around it. It loses more heat than the insulated R-value suggests, and leaves a cold spot on the inside surface, which is where condensation and mould begin.
The complete guide →If a bridge only loses a little heat, why does it matter?
Because the damage isn't on the energy report — it's on the surface. The bridge is the coldest continuous line in the room, and cold air saturates sooner, so it's the first place surface humidity climbs, mould takes hold and condensation forms. Under the NCC's condensation provisions that makes it a health and compliance question, not just an energy one.
Why it matters: the cold line →Can a 1D R-value capture a thermal bridge?
Not fully. A bridge is a two- and three-dimensional heat-flow problem — heat spreads sideways and concentrates at a surface — while an R-value is one-dimensional resistance straight through a layer. Resolving the worst surface temperature needs 2D/3D modelling to ISO 10211 for construction details and ISO 15099 for windows, the method behind LBNL THERM.
How it's actually assessed →What the NCC counts — and doesn't
Does the NCC regulate thermal bridging in Australia?
Yes, but narrowly. Under NCC 2022, NatHERS accounts for thermal bridging in repeating steel framing (studs and joists) to NZS 4214:2006, with timber exempt, and steel walls clad directly to the frame need a thermal break of at least R0.2. The window frame isn't counted as a bridge — it's folded into the whole-window U-value.
What the NCC & NatHERS require →What is the minimum thermal break for a steel frame?
At least R0.2 where cladding fixes directly to the same steel member. That inverts to a U-value of 5 W/m²K — about the insulating value of a single pane of glass — and it's a through-thickness test, so a thin strip over the framing faces satisfies it, while indirect fixing over a batten cavity removes the requirement entirely.
The thermal break and its limits →Does the NCC count the window frame as a thermal bridge?
No. NatHERS counts repeating steel wall framing (to NZS 4214:2006) and exempts timber, but the window frame is not treated as a wall bridge. Its conductivity is folded into the area-weighted whole-window U-value instead — even though it's the most conductive metal in the wall.
The bridge nobody counts →The window frame nobody counts
How many lineal metres of window frame are in a typical house?
As a rule of thumb, roughly 80 lineal metres for about 25m² of glazing — around three lineal metres of frame per square metre of glass. In aluminium that's a continuous run of highly conductive metal, fully exposed outside to inside, around every opening.
The thermal bridge nobody counts →Why does a whole-window U-value hide the frame?
The whole-window U-value (Uw) is an area-weighted average of glass and frame, and glass is usually 75–90% of the area. So it dominates the number and the frame's much higher conductivity is smoothed into a respectable average — hiding the coldest continuous surface, where condensation and mould begin.
The thermal bridge nobody counts →Is a window's U-value an average?
Yes. Uw is an area-weighted average of the glazing and the frame sections — a sliding door is solved as about seven sections, then averaged into one number. It describes the average performance, not the single coldest point.
U-value, averaged — the sliding door →“Thermally broken” — does it fix the bridge?
Does a thermally broken window remove the thermal bridge?
Not necessarily. A break interrupts the main metal path through the frame — a real improvement — but the bridge can survive through the sill or threshold, an incomplete break where only one track is broken, the interlock, or metal hardware. “Thermally broken” describes a component, not the coldest line of the installed frame.
The thermal break that isn't →Can a thermally broken sliding door still get cold or condense?
Yes. The residual cold paths are usually the interlock or meeting stile, the bottom track and its drainage channel, an incomplete break that leaves one sash solid aluminium, or steel roller hardware linking the sash to the cold outer sill. Any of these can run below the dew point on a “broken” system.
The thermal break that isn't →A thermally broken slider grows mould on one side of the sill but not the other — why?
Because the break often stops one door short. Modelling one slider's sill under both doors, the frame U-value barely moved (5.80 vs 5.75 W/m²K) — but the unbroken inner sill ran about 3°C colder and reached mould risk at 41% room humidity, where the broken outer side held to 50%. The average looks identical; the surface that actually condenses does not.
We modelled it: one slider, two doors →Why doesn't the U-value show the residual bridge?
Because the frame's conductivity is area-weighted into the whole-window U-value, and that figure is often averaged again across a project. The residual bridge on one part of the frame is smoothed into one respectable number, so the coldest line — where condensation forms — never appears as a bridge.
The thermal break that isn't →U-value vs condensation: the coldest line
Can a window have a good U-value and still get condensation?
Yes. Condensation forms at the single coldest surface, not the average. A bridged break or a conductive sill can run cold enough to condense and grow mould while the area-weighted Uw still meets target — especially on a glass-dominated slider, where good glazing flatters the average.
A good U-value, a wet sill →What is the difference between fRsi and the Mould Index?
fRsi is a surface-temperature factor — a quick steady-state screen that flags condensation risk at a junction. The Mould Index is the measure the NCC uses to determine condensation compliance, estimating mould growth over time. fRsi flags risk; the Mould Index is the yardstick compliance is judged against.
fRsi, the Mould Index and TIRS →Does fRsi prove NCC condensation compliance?
No. fRsi is a screening indicator. Under NCC 2025 Parts F8 and H4 compliance is determined from a Mould Index, which a TIRS MI-6 assessment delivers with its supporting science — that's the evidence the NCC needs.
fRsi, the Mould Index and TIRS →The rating vs the window that gets installed
Why doesn't a window's rating match the window that gets installed?
A WERS-style rating is measured on a standard catalogue size of the bare window — without the sub-frames, accessories and reveals fitted on site — and the specified product is often swapped at tender. So the rated value and the installed window drift apart.
Specified vs installed →What size is a window rating based on?
A generic rating is produced at a standard test size set by the scheme — for a sliding door, a two-panel door much smaller and simpler than most real installations. Real project doors are typically larger, more glazed and multi-panel, which changes the glass-to-frame ratio and the resulting number.
What size a rating assumes →Is a project-specific model better or worse than a WERS rating?
It's more accurate — it models the real installed window to the same ISO 15099 method. And because projects use larger windows than the test size and Uw is area-weighted, the installed window often reads better than its rating: a smaller frame fraction gives a lower Uw.
Specified vs installed →Can a modelled result be used for NCC compliance?
Project-specific modelling to ISO 15099 produces design-stage evidence that supports a Performance Solution. Results are indicative to the systems and sizes provided; final NCC compliance is confirmed by the project's assessor or certifier.
Specified vs installed →Model the coldest line, not the average.
Send us one window or junction you’ve already got a rating for. We’ll model the actual section in 2D to ISO 15099 — the frame, the glass and the surface temperature on the coldest line — so you can see the thermal bridge the whole-window number averages away. The first one costs you nothing.
Start with one window →Plain-language answers, not project advice. These answers summarise AFRC Lab’s published analysis of thermal bridging in Australian windows and buildings; the linked pieces carry the full detail, assumptions and sources. The ~80-lineal-metre and ~25m² figures are representative estimates for a typical house, not published statistics. Project-specific thermal simulation to ISO 15099 / NFRC 100; modelled results are indicative to the systems and details provided, and final NCC compliance is determined by your assessor or certifier. AFRC Lab (Advanced Fenestration Research & Certification) is an independent company; it is not the AFRC and is not affiliated with WERS. TIRS™ is a trademark of AFRC Lab Pty Ltd (patent pending).