Rated small.
Built big.
WERS deemed-to-satisfy rates one standard-size window. Nobody builds the standard size. And because a window’s U-value is area-weighted, the real window you install usually performs better than its rating — you’re just not allowed to claim it under DTS. Drag the sliders and watch the gap open up.
The rating isn’t wrong. It’s the wrong window.
A WERS number is modelled on a single standard test size. Your project almost never uses that size — and the gap between the two is performance you’ve paid for but can’t claim.
One standard size
WERS rates a fixed catalogue size — a fixed light at 1.5×1.2 m (H×W), an awning just 0.6 m high. That number travels onto every project, whatever you actually build.
U-value is area-weighted
The frame is the thermal weak point. A bigger window is mostly glass — the frame fraction shrinks, so the whole-of-window U-value drops. Bigger window, better number.
You claim the small one
Deemed-to-satisfy locks you to the standard-size rating. A Performance Solution models the window as installed — the real size — and lets you claim what it actually does.
Fixed window.
Left: the WERS standard-size rating, locked. Right: the same system, modelled at your size. Drag the width and height and watch the real U-value pull ahead.
Awning window.
The awning is the clearest case of all — WERS tests a tiny 0.6 m-high sash that is over half frame. Build a real one and the number transforms.
2-panel sliding door.
Big glass, big frames, a chunky interlock down the middle. Exactly where modelling the real size — not the standard one — pays off most.
The window DTS can’t rate as one.
Real architecture isn’t a catalogue rectangle. Here’s a fixed light with an awning down the right and a lowlight fixed below — one composite window. There’s no single deemed-to-satisfy number for it…
WERS has no number for the whole window. So the certifier has to split it — pull the awning out, rate it separately from the fixed, and work with two separate values for one window — or just nominate a target and hope the build meets it. Neither is the window as installed.
Whole-of-window result, area-weighted across the three elements and their shared junctions — the actual window, to ISO 15099.
Composite result is the area-weighted combination of the three generic elements at the sizes shown, using the same engine as the comparisons above. A Performance Solution models your actual composite as one window; deemed-to-satisfy can only split it into separate ratings or chase a nominated target.
What you get on the page.
A WERS certificate hands the certifier two numbers for a standard window. A BTW report models the window you’re installing and returns the whole set — energy, condensation, carbon and weight.
Illustrative WERS-style label — representative format only, not any real company’s certificate.
Stop certifying the small window.
Send us one window you’ve rated to WERS — or a whole project — and we’ll model the real system, at the real size, as a Performance Solution. See exactly how much you’ve been leaving on the table.