The window you specify isn't the window that gets installed.

Spend any time where paper meets site and you learn a quiet truth: there are always two windows. There's the one you specify — a line on a schedule, backed by a rating you searched out of a database. And there's the one that gets installed — a real size, in a real system, with real glass, sub-frames and reveals, fixed into a real opening. Everyone treats them as the same window. They almost never are.

The rating describes the first one. It's a number for a standard catalogue size of the bare window — the product on its own, without the sub-frames, accessories and reveals that get fitted on the actual job. It was built to let you compare one product against another on a common basis, which is a useful thing. It was not built to describe your installed window. And then, more often than not, the specified product gets swapped at tender or fabrication for something close-enough — so even that number drifts away from what turns up on site.

You're not pricing the window you're installing. You're pricing a number for a different one.

Why the gap is the builder's problem

Here's the part that stings. If the installed window underperforms the rating it was specified on, that isn't the supplier's problem — the cost was never in their price. It comes out of the builder's margin, as a redesign, a variation, or worse once it's built. And the margin for "near enough" is shrinking: compliance is being actually scrutinised now, not waved through on a label. A number on paper that nobody has tied to the real window is exactly the kind of thing that unravels at the wrong moment.

The twist: the real window usually reads better

Now the good news, and it's genuinely counter-intuitive. Modelling the actual installed window isn't only risk-cover — it often hands you a better number you can legitimately claim. The reason is simple geometry. Projects use larger windows than the small standard size WERS tests, and whole-window U-value is area-weighted: on a bigger window the frame is a smaller fraction of the whole, so the Uw comes out lower. The installed window the rating treated as a liability is frequently the better performer — you just have to model it to see it. (The flip side, worth knowing: for a Uw target the smallest window of a type is the worst case, so that's the one you prove.)

How you close it

You close the gap by building the window that's actually going in and modelling that — to the same ISO 15099 method the rating uses, just run on your real assembly. That's what BTW does: you set the performance the project needs, build the real configuration from the drawings, and it solves the glass to hit the target. Out comes project-specific evidence — the actual Uw, SHGC and T_vis, plus a condensation screen — that reflects the installed window and supports a Performance Solution, rather than a generic label you're hoping holds.

None of that is a knock on the rating. A rating does the job it was designed for: comparing products. It just isn't a description of your building's window, and on a project where the margin is thin and the compliance is real, the difference between "a number for something like it" and "the number for the one we're installing" is the whole game.

About these claims. "Often reads better" 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 U_w. Modelling is project-specific thermal simulation to ISO 15099 / NFRC 100 — the same method behind the ratings — and produces design-stage evidence; results are indicative to the systems and sizes provided, and final NCC compliance is confirmed by your assessor or certifier. BTW models glazed façade products (windows, doors, glazing). 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.