U-Value and SHGC — Choosing Window Glazing for a Melbourne Energy Report
If you're reading this with an energy report in hand, you're in the right place. This page explains the two numbers on it — U-value and SHGC — what's actually achievable in aluminium windows, and the exact process we use to match certified glazing to your targets.
What is a U-value and what's a good one for Melbourne?
The U-value measures how much heat passes through the whole window — frame and glass together. Lower is better: a lower U-value means less warmth escaping in winter and less heat pushing in during summer.
Across the systems we supply, whole-window U-values run from about 1.4 at the best thermally broken end to 6.6 for basic single glazing. To put that in context, a standard single-glazed clear awning sits around 6.4, plain double glazing brings the same window to about 4.0, and a good Low-E double-glazed unit gets it to around 3.0. There's no single "right" number — your NatHERS or BASIX energy report sets the target for your specific house, and we select glazing to meet it.
What is SHGC?
SHGC stands for Solar Heat Gain Coefficient. It's a number between 0 and 1 describing how much of the sun's heat the window lets through — 0.6 means about 60 per cent gets in, 0.2 means about 20 per cent.
Unlike U-value, lower isn't automatically better. A high SHGC on a north-facing window is free winter heating; the same glass on a west-facing window is an afternoon oven in February. Across our range SHGC runs from 0.13 to 0.78, which is a very wide span, so there's real room to tune it orientation by orientation. In our experience that's where the biggest comfort gains come from on Melbourne homes — not from one glass type everywhere, but from matching each elevation.
What's the difference between the glass U-value and the window U-value?
The glass U-value describes the glazing on its own. The whole-window U-value, written Uw, describes the complete window — glass, frame, spacer and all — and it's always the one that matters for compliance.
The distinction catches people out because the frame is a significant share of the opening, and aluminium conducts heat, so a whole window always performs worse than its glass alone. That's the entire reason thermally broken frames exist: our best thermally broken fixed window reaches a whole-window U-value of 1.4, against about 2.2 for the best standard-framed fixed lite with comparable glass. Your energy report and your compliance certificate both call for the whole-window figure.
What is WERS?
WERS is the Window Energy Rating Scheme — the national system that certifies the energy performance of whole windows. Rather than rating the glass alone, it tests the complete window: frame, glass and spacer together, which is what actually matters for your house.
Each certified combination of frame system and glazing gets a whole-window U-value, an SHGC, a visible transmittance figure and separate heating and cooling star ratings. It's the authority we use when matching windows to an energy report, because those figures are directly comparable with what NatHERS and BASIX assessments call for. The systems we supply have several thousand certified combinations between residential and commercial ranges.
What do the heating and cooling stars mean?
They're WERS's plain-language summary of a window's performance — heating stars for how well it holds warmth in during winter, cooling stars for how well it keeps heat out during summer, each out of ten.
They're useful for comparing options at a glance. In our Comet awning system, single glazed 4mm clear rates 3.8 heating and 1.1 cooling stars; plain double glazing takes that to 5.8 and 2.4; and the best Low-E double glazed options reach about 6.1 heating and 3.7 cooling. The two move together but not identically, which is why we look at the actual U-value and SHGC figures rather than the stars alone when matching an energy report.
My energy report specifies a U-value and SHGC — how do you match it?
We work item by item. For each window we take the required U-value and SHGC from your report, then look up the certified combinations for that frame system and operating type in the WERS database, and select options where both the U-value and the SHGC meet the target.
Where several options qualify we recommend the most economical one that does — typically working up from clear single glazing, to Low-E single, to double glazing, and to thermally broken double glazing for the lowest U-values. If nothing in a standard residential system reaches the target, that's the signal to step the item up to a thermally broken system. The values we use then go onto your compliance certificate alongside your NatHERS or BASIX certificate number.
Should I use the same glass on every window in the house?
Usually not, and it's one of the more common things we talk clients out of. Your north-facing windows benefit from letting solar heat in through winter, which means a higher SHGC; your west-facing windows want the opposite, because that's where Melbourne's summer afternoon heat arrives. Our range spans SHGC from 0.13 to 0.78, so there's genuine scope to specify differently by elevation.
Your energy report may set a single target or different targets per item. Either way we'd rather work through the schedule orientation by orientation than apply one glass type across the whole job.
What's the best U-value you can supply?
The lowest whole-window U-value we can supply is 1.4, on a 50mm thermally broken fixed window with a high-performance Low-E argon-filled double-glazed unit. A thermally broken awning in the same range reaches 1.8, and our entry door reaches 1.8 double glazed.
Within standard non-thermally-broken systems, the best figures are around 2.2 for a Comet fixed lite and 2.6 for a Comet sliding door. Bear in mind these are the best-case combinations in each system, not typical ones — the achievable figure depends on which glazing make-up you choose, and we'd match it to the target in your energy report rather than chase the lowest number for its own sake.
Why do fixed windows perform better than opening ones?
Because there's nothing to seal. A fixed lite has the glass set directly into the frame with no sash, no gap and no moving parts, so there's less frame material conducting heat and no join for air to pass through.
You can see it directly in our numbers. In the Comet range, a double-glazed fixed lite reaches a U-value of 2.2 while the best double-glazed awning in the same range reaches 3.0. That's a meaningful difference across a large glazed area. It's why we often suggest making the large picture window fixed and putting a smaller awning beside it for ventilation, rather than making the whole opening operable.
What about thermally broken windows?
Thermally broken frames are the step beyond everything on this page — an insulating barrier through the frame itself that takes whole-window U-values as low as 1.4. They're also the most oversold product in aluminium windows, which is why we gave them their own page with like-for-like certified comparisons: what the break is genuinely worth on each window type, and when a standard frame gets you there anyway.
Thermally Broken Windows in Melbourne — When They're Worth It