Not just a U‑value. The whole window, one build.
Build The Window models the window you actually make — your systems, your sizes — to ISO 15099. One build returns the U-value, SHGC, condensation, embodied carbon and weight, and reverse-engineers the glass to hit your target. Self-serve, as many as you like, under your own branding.
Watch it build a window.
This is the real BTW app. Pick the system, glass and climate zone — it models the window to ISO 15099 and solves the glass to hit your target, live.
A rating compares products. BTW models yours.
A WERS number rates a standard catalogue size on a common basis — built to compare one product against another. BTW models the window you’re actually installing, to the same ISO 15099 method, and hands back far more than a single figure.
The window as installed
Real size, system, glass and sub-frames — the assembly that goes on the building, not a standard test sample. The sub-frame is the real thermal lever, and BTW models it.
More than a U-value
One build returns Uw, SHGC and Tvis, plus a condensation & Mould Index scan, embodied carbon, recyclability and weight. The whole picture, not one number.
Reverse-engineer the glass
Set the Uw / SHGC target and BTW selects the glass — and the frame if needed — to hit it. You set the goal; it solves the make-up.
Your systems, your branding
Model your own suite as many times as you like, self-serve, and export clean, un-blurred reports under your own name — ready to hand to a client or certifier.
One build, the whole output set.
This is the “not just a U-value” part — every one of these comes out of the same single model of your window.
3.48
W/m²K · whole-window
0.67
solar heat gain
0.72
daylight
254.6
kg · system mass
Example build — the window in the demo above. Every build returns the full set, condensation and carbon included.
Condensation, embodied carbon and recyclability aren’t extras — condensation sits in NCC F8 / H4, and carbon is increasingly Green Star and NABERS territory. One build keeps you ahead of all of it.
Every number is modelled to ISO 15099 by our CFL solver — cross-validated head-to-head against LBNL THERM: 222 results at 98.7% average accuracy.
How it works
Modelling that used to mean THERM/WINDOW and a consultant engagement — now self-serve, in one pass.
Pick the performance
Set the Uw / SHGC target the project needs — or the band you want to market your suite at.
Your targetBuild the configuration
Size, splits and system, straight from the drawings. Build the governing window per type — the smallest for Uw, the largest where an SHGC cap governs.
Your systemBTW solves the glass
It selects the glass — and the frame if needed — to hit the target, modelled to ISO 15099. Glass is solved, not fiddled with.
ISO 15099Export the evidence
Audit-ready output under your branding, with the source drawing attached for provenance — ready to pass to a client or certifier.
Audit-readyThe window you specify isn’t always the window that’s installed.
WERS was built to compare the relative performance of window products on a common basis — a standard test size, the bare window, a database you search. It was never built to certify the specific window on your project. The specified product is routinely swapped at tender or fabrication, so the rated value and the installed window drift apart. BTW models the real thing — your project’s actual size, system and glass — to the same standard the ratings are built on, and because projects use larger windows than the standard test size and Uw is area-weighted, the modelled result is often a better, less over-specified number too.
“A handy service in an ever-changing industry. AFRC Lab’s knowledge, experience and professional approach ensure accurate compliance for projects and systems alike.”
Frame choice decides more than the U‑value.
This interactive study puts eight sill systems on the same glass and shows which keep the mould out — and which don’t — across three climate zones. It’s the clearest proof of why modelling the whole window beats a single rating — and exactly the kind of story your own systems could tell.
Which window keeps the mould out?
Eight fixed-sill systems, one glass, three climate zones — modelled for Mould Index, minimum surface temperature and holistic comfort. Pick a zone, compare any two, and read it off yourself.
Build one, free. Compare the result.
Try the live BTW demo, or send us one window you’ve rated to WERS and we’ll model the actual system free — so you can compare the result for yourself.
Try BTW — live demoHave us assess one for youIndependent & product-neutral. AFRC Lab is a third party. We provide the modelling method, the app and the verification data — we don’t supply, sell or endorse any product or brand. Project-specific thermal simulation to ISO 15099 / NFRC 100; modelled results reflect the systems and sizes provided, and final NCC compliance is determined by your assessor / certifier.