Secondary glazing for cold bathroom windows and louvres
Lots of older Melbourne and Tasmanian houses still have the original bathroom window: a small aluminium or timber frame with obscure glass, or a set of glass louvres. They are the coldest glass in the house. You step out of the shower and the window is streaming, the sill is wet, and there's black mould in the corners of the frame.
Replacing the window means a glazier and often a builder. A frosted acrylic panel on magnetic tape goes on in a few minutes, keeps the privacy, and lifts off when you want to open the window.
Louvre windows
Louvre blades overlap but never seal tightly, so cold air comes in all winter even when they're closed. A panel over the whole louvre frame stops that. The CSIRO puts air leaks at 15–25% of a home's winter heat loss.
Small fixed or awning windows
Single 3mm glass in an aluminium frame has a U-value of about 6.9, the worst in the YourHome table. With a panel added we estimate about 3.0, which roughly halves the heat going through.
What happens to the glass on a Melbourne winter night
| 600 × 900 bathroom window, 18°C inside, 6°C outside | Single glass | With a panel |
|---|---|---|
| Heat lost through the window | about 45 W | about 19 W |
| Inside glass temperature | about 9°C | about 14°C |
| Condensation at 70% humidity (water forms below 12.5°C) | Yes | No |
Estimates from standard U-values. Try your own window and town on the calculator page.
Tips for bathrooms
- Choose frosted acrylic if the window faces a neighbour or the street.
- Clean the frame with the wipe and let it dry fully before sticking the steel strip. Bathroom frames often have soap film on them.
- Keep using the exhaust fan. The panel stops the glass going cold, but the steam still needs somewhere to go.
- For louvres, stick the steel strip on the outer frame, not the blades, so you can still open them with the panel off.
Can you put secondary glazing over louvre windows?
Yes. The panel sits on the flat frame around the louvres and covers all the blades, which stops the draught coming through the gaps between them. Lift the panel off when you want the louvres open.
Will secondary glazing stop bathroom window condensation?
It cuts it a lot. The inner surface of the panel stays several degrees warmer than bare single glass, so on most nights it stays above the temperature where water forms. Run the exhaust fan during and after showers.
Can I get frosted panels for privacy?
Yes. Choose 3mm frosted acrylic in the calculator. It lets light through but you can't see in.