Condensation on Windows: Why It Happens and How to Stop It

Condensation is the number one autumn complaint we hear in Bristol, Exeter and Cardiff. The good news is that it is usually easy to diagnose once you know where the water is. There are three kinds, and only one of them means your windows have failed.

1. Condensation on the inside of the glass

This is the most common kind: droplets or a film of water on the room side of the glass, worst first thing in the morning and in bedrooms, kitchens and bathrooms. It happens when warm, moist indoor air meets a cold surface. The glass is simply the coldest surface in the room, so that is where the moisture lands. A family of four produces around ten litres of water vapour a day from breathing, cooking, showering and drying clothes, and in autumn we close the windows and turn the heating on, so that moisture has nowhere to go.

Old single glazing and early double glazing make this much worse because the inside of the glass gets very cold. Modern A-rated units with low-emissivity glass and argon-filled cavities keep the inner pane close to room temperature, so far less moisture condenses on it.

• Ventilate when you make moisture: extractor fans on while cooking and showering, and for ten minutes afterwards.
• Keep trickle vents open. They are fitted to all new windows for exactly this reason and cost almost nothing in heat.
• Dry clothes outside or in a room with the door shut and a window cracked open, not on radiators in the living room.
• Keep a low, steady background heat rather than blasting rooms hot and cold.
• If the problem persists in one room, a small dehumidifier is cheaper than you think and works quickly.

2. Condensation between the panes

Misting, water or a milky haze trapped between the two panes of a double-glazed unit, which never wipes away, means the sealed unit has failed. The edge seal has let moisture into the cavity, the insulating gas has gone, and the unit is no longer doing its job. It will not clear on its own. The fix is a new sealed unit, or a new window if the frame is also tired. If your windows were installed by us, book a service call and we will look at it under your guarantee; if they were installed by someone else, we can still quote to replace the units or the windows.

3. Condensation on the outside of the glass

Dew on the outside of the window on a cold, clear morning, which clears as the day warms up, is a sign of good glazing, not bad. It means so little heat is escaping through the glass that the outer pane stays cold enough for the morning air to condense on it. It is most noticeable on new A-rated and triple-glazed windows. Nothing needs fixing.

When replacement windows are the answer

If you have already improved ventilation and the inside of the glass still streams every morning, or your units are misted, or your frames are single glazed, the windows themselves are the problem. Our Eco Flush and Eco Casement ranges are A-rated as standard and available up to A++, with trickle vents built in and a 15-year guarantee. For period homes in conservation areas, the flush sash design is usually accepted like for like, and our ECOSlide sash windows suit Victorian and Edwardian properties. Every installation is FENSA registered.