How much warmer does secondary glazing make a room?
Plain single glazing is the weakest part of most older Australian houses. Glass is thin, so heat goes straight through it, and on a cold night the inside of the pane gets cold enough to run with condensation. A second clear layer with still air trapped behind it slows that down a lot.
Here is what has actually been measured, then a calculator so you can try your own window.
What the research says
| Finding | Number | Source |
|---|---|---|
| Share of a home's heating energy that can be lost through windows | up to 40% | YourHome, Australian Government [1] |
| Single glazed window, aluminium frame, 3mm glass (U-value) | 6.9 W/m²K | YourHome [1] |
| Single glazed window, timber frame, 3mm glass | 5.5 W/m²K | YourHome [1] |
| Factory double glazing, timber frame, 6mm air gap | 3.0 W/m²K | YourHome [1] |
| Removable acrylic panel on magnetic strips, typical | about 2.7 W/m²K | Building Conservation [3] |
| Heat loss cut by secondary glazing, two homes in the ACT | up to 60.8% | Bulut et al. 2022 [2] |
| Electricity saved in those homes | up to 9.96% | Bulut et al. 2022 [2] |
| Australian buildings still on single glazing (estimate) | about 85% | Bulut et al. 2022 [2] |
| Heat loss cut by secondary glazing on an old sash window, lab test | 63% | Historic Scotland / Glasgow Caledonian Uni [3] |
| Share of winter heat loss caused by air leaks | 15–25% | CSIRO [4] |
A lower U-value means less heat gets through. Going from about 5.5–6.9 down to about 2.7–3.0 roughly halves the heat lost through the glass, which lines up with the 50–60% reductions measured in real homes. The panel also seals the gaps around old sashes and louvres, and that is where much of the draught comes from.
Try your own window
How we work it out: heat flow = U-value × area × (inside temperature − outside temperature). U-values are from YourHome; with a panel we add the resistance of a still air gap and 3mm acrylic, which gives about 3.0 for aluminium frames and 2.7 for timber. Glass surface temperature uses the standard inside surface resistance of 0.13 m²K/W. These are estimates for comparing options, not a guarantee.
Does thicker acrylic keep more heat in?
| Added to a single-glazed window | U-value, estimated | Less heat lost |
|---|---|---|
| No panel | about 5.8 | — |
| 3mm acrylic panel | about 2.78 | about 52% |
| 4mm acrylic panel | about 2.74 | about 53% |
| 6mm acrylic panel | about 2.66 | about 54% |
Hardly at all. Acrylic conducts about 0.19 W/m·K, so a few extra millimetres add very little. The still air layer does most of the work. Three things make a bigger difference than thickness:
- The air gap. A gap of around 6mm gives roughly two thirds of the benefit; from about 15–20mm it's close to the best you'll get for warmth. Wider gaps help with noise rather than heat.
- A sealed edge. A gap anywhere around the panel lets warm air circulate out. Foam-backed magnetic tape in one continuous loop matters more than thickness.
- A flat panel. Large thin panels can bow and lift at the edge, which breaks the seal. That's why we switch to 4mm above 1.4 m².
Thicker panels do cut a little more noise because they're heavier, but for real noise reduction the air gap needs to be 100mm or more, which a frame-mounted magnetic panel usually can't give you.
Calculated with standard surface resistances (inside 0.13, outside 0.04 m²K/W) and about 0.17 m²K/W for a still air layer of 15mm or more.
Being straight about small windows
A small bathroom window loses tens of watts, not hundreds. On its own it won't make a big dent in your power bill. What changes is how the room feels and how wet the glass gets: with a panel in place, the inside surface stays several degrees warmer, so it stops streaming with water every morning and the frame stops growing mould. For a whole house of single-glazed windows, the savings add up.
- YourHome, Australian Government, “Glazing”: yourhome.gov.au/passive-design/glazing
- Bulut, Wilkinson, Khan, Jin, Lee (2022), “Thermal performance of retrofitted secondary glazed windows in residential buildings: two cases from Australia”, Smart and Sustainable Built Environment 11(4), doi:10.1108/SASBE-03-2021-0050
- Building Conservation, “Secondary Glazing”, summarising P. Baker (2008), Improving the Thermal Performance of Traditional Windows, Historic Scotland and Glasgow Caledonian University
- CSIRO (2024), “Cold draughts in your home”