Window condensation

Window condensation forms when warm, damp air inside the house meets the cold glass and turns back into water. Where it appears tells you what is wrong. On the room side of the glass, it means the room is too damp or not ventilated enough. Between the panes, it means a failed seal. On the outside, it is usually a sign the window is working well.

Window condensation on the inside of the glass

Every home makes moisture: cooking, showers, drying clothes and simply breathing. When the air can’t escape, it settles on the coldest surface in the room, and that is usually the glass. So the fix starts with the air, not the window.

• Ventilate while the moisture is made. Use the extractor fan when you cook or shower, and keep it running for a while afterwards.
• Open the trickle vents. Most modern windows have slots in the top of the frame. Leave them open, especially in bedrooms overnight.
• Keep the heating steady. Warm glass stays drier than glass in a room that heats up and cools down sharply.
• Dry washing outside or in a vented room. A rack of wet clothes puts a surprising amount of water into the air.
• Wipe the glass in the morning so water doesn’t soak into the sills and seals.

Building Regulations expect replacement windows to keep a room at least as well ventilated as before, and the Planning Portal’s guidance notes that rooms other than kitchens and bathrooms may manage with window openings and trickle vents. If your old windows had trickle vents, your new ones should too.

Window condensation between the panes

Moisture inside a sealed unit is a different problem. The edge seal has failed and the unit can’t be dried out properly. A glazier can replace the unit without changing the frame; see our guide to misted double glazing.

Condensation on the outside of the window

Dew on the outside pane on a cold, still morning means the glass is staying cold because the heat is staying in the room. It clears as the sun comes up. Modern low-energy glass shows it more than old windows did, and there’s nothing to fix.

When window condensation means it is time for new windows

Old single glazing and early double glazing let the inner pane get very cold, so water streams down it all winter however well you ventilate. In that case, replacement windows with a good energy rating make a real difference, because the inner pane stays closer to room temperature. Our guide to energy efficient windows explains the ratings, and uPVC windows are the usual choice on a budget.

Persistent condensation also causes black mould on the seals and reveals. Clean it off with a mould remover, and treat the cause, or it returns.

A quick check before you call anyone

1. Note which side of the glass the water is on.
2. Check whether the trickle vents are open and the extractor fans work.
3. Look for a pattern: bedrooms first thing, the kitchen after cooking, or every room all day.

If the water is between the panes, or the frames are rotting or letting in draughts, it’s worth getting a price. Then compare a few quotes before deciding.