Why do windows fog and drip in winter?
Windows fog in winter because they are the coldest surface in the room. Warm indoor air carries moisture; when that air touches a single-pane window cooled to outdoor temperature, the moisture condenses out as water on the glass. It is the same physics as a cold drink sweating in summer. The condensation is just your home's water budget made visible.
Is window condensation a sign of a problem?
Light morning condensation that clears within an hour is normal in older Australian homes. Heavy condensation that drips onto sills, pools around frames or leaves persistent wet patches is a sign the indoor humidity is running too high — and that the wet sills are about to grow mould if they haven't already. It is the gateway condition for window-frame mould.
What causes high indoor humidity in winter?
- Drying clothes inside without ventilation
- Unflued gas heaters (every litre of gas burned produces about a litre of water vapour)
- Showering with the bathroom door open
- Closed-up rooms where breathing alone raises humidity overnight
- Cooking without using the rangehood
- Aquariums, indoor plants and large pets all adding moisture you might not have counted
How to reduce window condensation without renovating
- Open windows for ten minutes a morning, even in winter. The cold dry air outside replaces the warm wet air inside.
- Use the rangehood when cooking and the bathroom fan during and after showers.
- Move drying clothes out of bedrooms — bedrooms are where most winter condensation happens.
- Keep curtains and blinds open during the day so air moves across the glass.
- Add passive moisture absorbers to closed bedrooms, especially the bedroom of anyone who sleeps with the door shut.
When to consider double glazing or secondary glazing
If you are doing major renovations, double glazing is the long-term fix for condensation. If you rent or can't justify the spend, secondary glazing kits (an acrylic panel that fits inside the existing frame) cost a small fraction of double glazing and reduce condensation by 70–90%. Both work because they keep the inside glass surface warmer.
What about the mould on the bottom of the frame?
That patch of black around the bottom of the window frame is the most common winter mould in Australian homes. Treat it early with a vinegar solution and a soft brush. Dry the area thoroughly. Then fix the cause — usually a combination of high bedroom humidity and single-pane glass. Without the cause fix, the mould returns within weeks.
