How to Stop Condensation on Windows
Condensation appears when warm, moist indoor air touches cold glass. Cutting the moisture and keeping the glass a little warmer stops most of it; the rest is about habits, fans and fresh air.
4 min
Condensation on the inside of windows comes from two things meeting: moist indoor air and a cold pane of glass. To stop it, reduce how much water vapour gets into the air, remove the vapour that does build up through ventilation and extraction, and keep room and glass temperatures from dropping too low. Wiping the glass deals with the symptom. Moisture control deals with the cause.
What causes condensation on windows
Air can hold only a certain amount of water vapour, and warm air holds more than cold air. When room air touches a surface colder than its dew point, the air right next to that surface cools, can no longer carry all its moisture and drops the excess as tiny droplets. Windows are usually the coldest surfaces in a room, especially single glazing, older double glazing and metal frames, so they mist up first.
A household produces a surprising amount of moisture every day without anyone noticing. The main sources are:
- Cooking, particularly boiling pans without lids.
- Showers and baths.
- Drying laundry indoors on airers or radiators.
- Tumble dryers that are not vented to the outside.
- Breathing, which is why bedroom windows are often wettest in the morning.
- Flueless gas or paraffin heaters, which release water vapour as they burn.
- Large numbers of houseplants, aquariums and kettles left steaming.
Inside, outside or between the panes?
Where the water sits tells you a lot.
| Where it forms | What it usually means | What to do |
|---|---|---|
| Inside the room | Too much indoor moisture, cold glass or both | Work through the steps below |
| Outside the glass | Cool, humid morning air meeting a well-insulated pane that stays cold | Nothing; it tends to be a sign the glazing is keeping heat in |
| Between the two panes | The seal of a double-glazed unit has failed | The sealed unit normally needs replacing; the frame may be fine |
How to stop condensation on windows: daily habits
- Put lids on pans and use the cooker hood whenever you cook, ideally one that vents outside rather than recirculating.
- Run the bathroom fan during and for a while after a shower, and keep the bathroom door shut so steam does not drift through the house.
- Dry clothes outside when you can. If they must dry indoors, use a small room with the door closed and a window slightly open or a dehumidifier running.
- Open windows briefly each morning. Ten minutes of cross-ventilation swaps damp overnight air for drier air without chilling the walls much.
- Leave trickle vents open. These small slots at the top of many modern frames let air change slowly all day.
- Wipe the glass first thing with a squeegee or absorbent cloth and wring it out into the sink, not back onto the sill.
- Keep furniture slightly away from external walls so air can move behind it.
Warmer glass, drier air
Keep the heating steady
Rooms that swing between warm and very cold are more prone to condensation than rooms kept at a modest, even temperature. Cold bedrooms that are heated only briefly before sleep are a classic example. If some radiators are slow to warm, it may simply be trapped air; the guide on how to bleed a radiator covers the fix.
Use a hygrometer and dehumidifier
A cheap digital hygrometer shows relative humidity, which makes it easier to tell whether changes are working. Where readings stay high, a dehumidifier in the worst room can pull litres of water from the air. Empty and clean it regularly.
Think about curtains and blinds
Heavy curtains closed tight against the glass trap a pocket of cold air, and the window can stream behind them. Open them during the day and leave a small gap at the bottom at night if streaming is bad.
Improve the glazing
Secondary glazing, replacement sealed units or better frames keep the inner pane warmer. These are larger jobs and work best once moisture sources are under control.
Draughts versus ventilation
Sealing draughts and providing ventilation are not opposites. Uncontrolled gaps around doors waste heat; controlled ventilation, such as trickle vents and extractor fans, removes moisture where it is produced. The guide on weatherstripping a door explains how to seal the first without blocking the second.
When condensation turns into mould
Water that sits on sills and frames day after day can lead to black spots of mould on seals, plaster and curtains. Mould can affect breathing, particularly for people with asthma, allergies or weakened immune systems, so anyone with symptoms should speak to a doctor. Persistent damp that does not respond to better ventilation, such as wet patches on walls or a musty smell that will not clear, may have a structural cause like a leak or failed damp-proofing and is worth a professional inspection.
Tenants should report ongoing damp to the landlord in writing and keep dated photos. Recording the condition of windows and walls on day one, as set out in the move-in inspection checklist for renters, makes later conversations far simpler.
Desk habits
Small routines that save time later
Picked up while writing the notes on this site.
Photograph the meter
A dated photo settles most arguments about estimated readings.
Name files by date
Year, month, day at the start of a filename keeps invoices and letters in order.
Sketch before you measure
A rough floor plan stops you missing an alcove or counting a strip twice.
Update the router
Check for firmware updates whenever you change your Wi-Fi password.
Put terms in writing
A short email confirming price and due date beats any verbal promise.
