The short answer
On figures published by a Melbourne uPVC maker, double glazed uPVC has a whole-window U-value of 1.6 to 1.7↗, thermally broken aluminium 2.0 to 2.4↗, and single glazed aluminium 5.8↗. Lower means less heat through. Timber double glazed sash windows are priced at $2,800 to $5,800 per window↗, against $2,300 to $3,500 per window↗ for uPVC.
What is a U-value in plain terms?
It is the rate heat passes through the window. Lower keeps more heat where you want it, in winter and in summer. The number worth comparing is the whole-window one, because the frame leaks too: WERS reports Uw and SHGCw↗, which count the frame, the edge of the glass and the centre of the glass rather than the glass alone.
How far apart are the frames?
Reported whole-window U-values
Show the numbers
| Item | Range |
|---|---|
| Single glazed aluminium | 5.8 W/m²K, lower is better |
| Thermally broken aluminium | 2.0 to 2.4 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 |
The interesting part of that chart is where the distance is. Going from an old single glazed aluminium window to any double glazed frame is a jump of several units. Choosing between a thermally broken aluminium frame and a uPVC one moves the number by a few tenths. If your house still has single glazing, the glass decision matters more than the 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 honest 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?
| Frame | Reported U-value | Reported installed price |
|---|---|---|
| Aluminium, single glazed | 5.8↗ | Not published by our sources |
| Aluminium, thermally broken | 2.0 to 2.4↗ | Not published by our sources |
| uPVC, double glazed | 1.6 to 1.7↗ | $2,300 to $3,500 per window↗ |
| Timber sash, double glazed | Not published by our sources | $2,800 to $5,800 per window↗ |
| uPVC, triple glazed | 1.2 to 1.4↗ | Not published by our sources |
We could not find an installed price for double glazed aluminium windows from a source we can link to, so the cell is empty rather than filled with 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.
- Check your climate zone. The building code sets conductance limits in climate zones 2 to 8↗ and solar heat gain limits 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.