The short answer
On figures a Melbourne uPVC maker publishes, a uPVC frame on its own has a U-value of 1.5 to 2.5, thermally broken aluminium 2.5 to 4.0, and aluminium without a thermal break 5.5 to 7.0. Lower means less heat through. As whole windows, double glazed uPVC comes in at 1.6 to 1.7 against 5.8 for old single glazed aluminium. Timber double glazed sash windows are priced at $2,800 to $5,800 per window.
What is a U-value in plain terms?
It is the rate heat passes through. Lower keeps more heat where you want it, in winter and in summer. There are two kinds worth keeping apart. A frame U-value (Uf) describes the frame alone. A whole-window U-value is the one to compare between products, because it counts everything: WERS reports Uw and SHGCw, which take in the frame, the edge of the glass and the centre of the glass.
How far apart are the frames?
Reported frame-only U-values
Show the numbers
| Item | Range |
|---|---|
| Aluminium, no thermal break | 5.5 to 7.0 W/m²K, lower is better |
| Aluminium, thermally broken | 2.5 to 4.0 W/m²K, lower is better |
| uPVC | 1.5 to 2.5 W/m²K, lower is better |
The big gap is between plain aluminium and everything else. A thermal break, the insulating strip that stops the metal carrying heat straight through, takes aluminium most of the way towards uPVC, and the two ranges meet at their edges. That is why "aluminium" on a quote tells you very little until you know whether the frame is thermally broken.
What happens when you add the glass?
Reported whole-window U-values
Show the numbers
| Item | Range |
|---|---|
| Single glazed aluminium | 5.8 W/m²K, lower is better |
| Double glazed uPVC | 1.6 to 1.7 W/m²K, lower is better |
| Triple glazed uPVC | 1.2 to 1.4 W/m²K, lower is better |
Going from an old single glazed aluminium window to double glazed uPVC is a jump of several units. Going from double to triple glazing moves the number by a few tenths. If your house still has single glazing, the glass decision matters more than any frame argument.
Whose numbers are these?
All of them come from a Melbourne company that makes uPVC windows. That does not make the figures wrong, and they are the clearest published set we could link to, but an aluminium maker would pick different comparisons. The fair way to settle it for your own job is to ask each supplier for the WERS rating of the exact product they are quoting, then compare like with like.
What do the frames cost?
We could not find an installed price for double glazed aluminium windows from a source we can link to and rely on, so the cell stays empty rather than holding a guess. Ask two aluminium suppliers and you will have a better local figure than any national average.
So how should you choose?
- Start with the glazing, not the frame. Single to double is the big step in every figure above.
- If it's aluminium, ask whether it is thermally broken. That one detail changes the frame's figure more than anything.
- Check your climate zone. The building code has a conductance check in climate zones 2 to 8 and a solar heat gain limit in climate zones 1 to 7, so the right glass in Cairns is not the right glass in Ballarat.
- Ask for the WERS heating and cooling stars, which run 0 to 10 stars, and the Uw and SHGCw behind them.
- Compare the same window in the same size. A quote for a smaller opening will always look better.
- Timber is the heritage answer and the dearest of the set on these figures, so price it separately.