The window you specify isn't the window that gets installed. BTW models the one that is.
BTW is AFRC Lab's project-specific performance model for glazed façade products — windows, doors and glazing. Set the performance you need, build the actual window from the drawings, and BTW solves the glass to hit it — modelled to ISO 15099, the same method behind the ratings, run on your real window instead of a standard-size sample. A built-in fRsi condensation screen flags potential issues as you go.
ISO 15099 / NFRC 100 · our CFL solver is THERM-validated — 410 runs re-solved, 97.3% on production models · self-serve, in your browser
A rating is a number for a standard product. A model is the full picture of your actual window.
A WERS-style rating was built to compare one generic window against another, on a common basis — a standard catalogue size, the bare window on its own. Useful, but it isn't your project's window. BTW models the window as installed: your real size, system, glass, sub-frames and reveals, to the same ISO 15099 method.
A number, for a standard size
One catalogue size. The bare window, without the sub-frames, accessories and reveals that get fitted on site. A label you search a database for — and that often gets swapped at tender, so the rated value and the installed window drift apart.
Your window, as it's installed
Real size, real system, real glass and sub-frames. And because projects use larger windows than the standard test size — and U-value is area-weighted — the window you install often reads better than its rating. BTW lets you model that, and prove it.
Everything that comes out of a single BTW model.
A rating answers two questions — a U-value and an SHGC. One BTW model answers all of these:
Condensation, embodied carbon and recyclability aren't extras — condensation sits in NCC F8 / H4, and embodied carbon is Green Star / NABERS territory. They're increasingly what you have to show, from the same model that gives you the energy numbers.
You set the targets. BTW solves the glass to hit them.
Pick the performance
Set the Uw / SHGC the project needs — the targets you're trying to meet.
Build the window
Size, splits and system, straight from the drawings — the governing window for each type.
BTW solves the glass
It selects the glazing — and the frame if needed — to hit target. Glass is solved, not fiddled with.
Prove it
Modelled to ISO 15099, exported as audit-ready evidence you can hand to a certifier.
You set the targets; BTW confirms they're achievable and solves the glass to hit them — in one pass, in your browser.
Back your spec with the window that's actually installed.
ESD & thermal consultants
Spec faster than trawling a database, and back the number with the actual window so it survives shop-drawing stage.
Architects & designers
Specify glazing performance you can trust is buildable — and accurate to the real assembly.
Building surveyors & certifiers
Check that the specified values reflect the window that's actually being installed.
Builders & developers
Kill late-stage VE churn — prove the façade spec before it's locked, not after procurement.
Fabricators
Model your own systems, under your own brand — and issue full, un-blurred BTW reports. This is the paid BTW app.
Not sure which?
Send us one window you've already rated. We'll model the real system free, so you can compare the result.
Your systems. Your branding. Your reports.
The paid BTW app puts the model in your hands — load your own frame systems, build any window on demand, and issue full, un-blurred BTW reports under your own brand. Same ISO 15099 method, your name on the output. Talk to us about setting your systems up.
How to read these results. BTW is a project-specific thermal simulation to ISO 15099 / NFRC 100. Modelled results reflect the systems and sizes provided and are indicative at design stage; final NCC compliance is confirmed by your assessor or certifier. "Often reads better than its rating" refers to the area-weighting of larger windows and is not guaranteed for every window — the smallest window of a type is the worst case for Uw. fRsi is an ISO 13788 steady-state condensation screen, not a certification. Embodied-carbon, recyclability and weight are screening estimates (NABERS factors), not a certified EPD. 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. BTW and TIRS™ are trademarks of AFRC Lab Pty Ltd.