Every window and door system manufactured in Australia carries a Window Energy Rating Scheme (WERS) certificate. WERS is the independent body that tests the thermal performance of a window assembly — the frame, the glass, and the spacer between them — and publishes the numbers under a standard methodology. If the system on your quote does not have a WERS rating, the quote is incomplete.
Two of those numbers do most of the work. The first is U-value. U-value measures how much heat the window assembly lets through — measured in watts per square metre per degree, lower is better. A single-glazed aluminium frame typically reads U≈6.0. A double-glazed thermally-broken system reads U≈2.0 or lower. The difference is roughly threefold; the difference in your winter heating bill is more than that, because windows are the weakest link in a wall.
The second is SHGC — Solar Heat Gain Coefficient. SHGC measures the fraction of solar energy that passes through the glass into the room, expressed as a number between 0 and 1. SHGC 0.7 means seventy per cent of the sun's heat makes it through; SHGC 0.3 means thirty. Unlike U-value, you do not want SHGC universally low. On a north-facing window in Melbourne, a higher SHGC pulls in free winter heat. On a west-facing one in February, the same number bakes the room.
NCC 2025 has made these numbers the specification. Since May 2024, new Victorian homes have had to reach a NatHERS 7-star rating. The rating is calculated from the whole envelope — walls, roof, slab, and windows — but the windows are usually the largest single lever. You hit 7-star by specifying the right U-value and the right SHGC on every elevation, not by averaging across the house.
A good glazing specification reads every window separately. North wants a higher SHGC because you want the winter sun. South wants a low U-value because you lose heat there year-round. East and west — but particularly west — want a low SHGC because the afternoon sun is the main overheating risk. External shading on west elevations is often cheaper than upgrading the glass.
What this looks like in practice: on a Toorak renovation last year we specified four different glass build-ups across one house — clear low-E IGU on north for solar gain, neutral low-E IGU on south for the U-value, a low-SHGC tinted IGU on west for summer protection, and a slimline IGU on the heritage-overlay frontage that could not be replaced. Same building. Different numbers. One certificate.
The framing matters as much as the glass. A standard aluminium frame conducts heat straight through; a thermally-broken frame puts a polyamide bar between the inside and the outside, so the cold does not bridge the metal. On winter mornings this is the difference between a frame that runs at room temperature and one that sits at the outside temperature and pulls condensation out of the air. NCC 2025 has tightened condensation provisions specifically because aluminium frames have been the weak point in too many recent builds.
Where it gets expensive is the small numbers. The cost difference between U-value 2.4 and U-value 2.0 — both within the thermally-broken double-glazed range — is significant, and it does not always earn its keep. We bring the WERS data into the design conversation early so the spec can be optimised, not maximised, and the cost lands where it produces real performance.
The short version: every system on your quote should name its U-value and its SHGC, and the choice should be made per elevation. A house full of the same window is a house spending more than it needs to on the elevations that did not need the upgrade — and missing performance on the ones that did.
Written by Jason Ah-Young, Director of Maison Glass & Aluminium. For advice on a specific project, book a consultation.
