How do I stop condensation?

A household of four produces somewhere in the region of ten litres of water vapour a day just by breathing, cooking, showering and drying clothes. That water has to go somewhere. In a well-ventilated house it leaves with the air; in a sealed, under-heated one it condenses on the coldest surface available — usually a window, an external wall, or the corner behind a wardrobe. Every fix below is a version of one of three moves: make less moisture, get rid of it faster, or make the cold surface less cold.

What you'll need

• Hygrometer to measure humidity (optional)
• Window vac or absorbent cloth (optional)

Step by step

1. Put lids on pans while cooking and use the extractor fan on its highest setting.An uncovered pot of boiling water is one of the largest single moisture sources in a home.
2. Run the bathroom extractor during showers and for 15–20 minutes afterwards.Most of the moisture leaves the room after you do. Fans that stop with the light switch are doing half a job.
3. Stop drying laundry on radiators or indoor airers where possible.A single wash load releases roughly two litres of water into the air as it dries.
4. If you must dry indoors, do it in one room with the door shut and a window open a crack.Containing it beats spreading it through the house.
5. Open windows for 5–10 minutes twice a day, ideally on opposite sides of the house.Short and wide beats a window on the latch all day — you exchange the air without cooling the walls.
6. Keep heating low and steady rather than short hot bursts.Surfaces stay above the dew point. Letting a house go cold and then blasting it is what produces streaming windows.
7. Pull furniture 5cm away from external walls and leave trickle vents open.Airflow across a cold surface stops moisture settling on it.
8. Wipe up standing condensation each morning rather than letting it soak into frames and sills.This is damage limitation, not a fix — but wet timber and plaster is how condensation turns into rot and mould.

Tips

• Buy a hygrometer before buying a dehumidifier. If you're consistently above 60% relative humidity you have a real problem worth spending money on; if you're at 45% the streaming window is a cold-glass problem, not a humidity one.
• Condensation on the *inside* of double glazing means the glazing unit's seal has failed and the unit needs replacing. Condensation on the room-side surface is a ventilation issue.
• A dehumidifier is a reasonable purchase for a genuinely damp property, but it costs electricity to run continuously. Fix ventilation first and you may not need one.

Common mistakes

• Blocking air bricks and trickle vents to stop draughts — They're the designed ventilation path. Sealing them is a common cause of a sudden new damp problem.
• Turning the heating off entirely when out all day — Cold surfaces at the point you come home and start cooking is exactly when condensation forms.
• Treating recurring window condensation as normal — Sustained moisture on frames and reveals leads to mould and, in timber, to rot.

If it doesn't work

Questions people ask