The short answer: to stop condensation on windows, you need to do three things at once: make less moisture, remove the moisture you do make, and keep cold surfaces warmer. In practice that means using extractor fans and trickle vents every day, drying laundry in one closed and ventilated room rather than on radiators, keeping a steady background temperature instead of short hot blasts, and wiping glass and sills each morning before water soaks into frames and plaster. Do that consistently through October and most homes see streaming windows reduce within days, and mould is far less likely to take hold before winter.

The first cold nights of autumn are when it starts. You open the curtains and the glass is running with water, the sill is wet, and there is a faint musty smell in the corner of the bedroom. It feels like a window problem. In most UK homes it is actually an air problem, and it is fixable without replacing windows or buying expensive kit.

Why do windows get condensation in October?

Condensation forms when moist air touches a surface that is colder than the air itself. The Energy Saving Trust describes it in exactly those terms: moisture in the air becomes droplets of water when it hits a cold surface, such as a window. In summer your glass is relatively warm and your windows are open more often, so the moisture you produce escapes or never settles. In October three things change at once. Outside temperatures fall, so the inner pane of glass gets colder, especially overnight. You close windows for longer to keep heat in, so ventilation drops. And you start drying more laundry indoors, running hotter showers, and cooking with lids off pans in a sealed kitchen. The air holds more moisture, the glass is colder, and the moisture has nowhere to go. It lands on the coldest surface it can find, which is usually your bedroom window at dawn.

The scale of moisture in an ordinary home surprises most people. The Energy Saving Trust notes that a typical household can produce up to 100 litres of moisture a week from cooking, showering, boiling kettles and breathing. If that moisture is not removed through ventilation, it stays in the air or builds up on surfaces. Bedrooms are hit hardest overnight because two people breathing in a closed room for eight hours steadily raise humidity while the window cools. By morning the glass is below the dew point of the room air, and water runs down the pane.

How can you tell if it is condensation or a different damp problem?

Not all damp is condensation, and the fix depends on the cause. Condensation is the most common type in UK homes, and it has a recognisable pattern. The Energy Saving Trust lists excessive condensation around windows, black mould and musty smells as typical signs. Condensation tends to appear on windows, external walls, and in corners or behind furniture where air cannot circulate. It is usually worse in winter and improves when you ventilate.

Rising damp is different. It shows as a tidemark along the bottom of a wall where groundwater moves up through masonry, often because a damp proof course has failed. Penetrating damp is also different. That is rainwater getting in through a leaking roof, blocked guttering, cracked render or poorly sealed windows, and it usually appears as a concentrated damp patch that gets worse after rain. If you have a tidemark, a patch that grows after storms, crumbling plaster low on a wall, or water coming through a ceiling, that is not a ventilation problem and it needs a building fix first. If your main symptom is water on glass in the morning, steamed mirrors that take an hour to clear, and black spots around window frames and in room corners, you are almost certainly dealing with condensation.

One other distinction matters with modern glazing. Condensation on the outside of a window on a cold, still morning is not a fault. It usually means the window is insulating well, so the outer pane stays cold. Condensation between the panes of a double glazed unit is different again: it suggests the sealed unit has failed and air has got inside. That needs a glazier, not a dehumidifier. Condensation on the room side of the glass, the type you can wipe off with your hand, is the type this guide deals with.

What should you do first to stop condensation on windows?

Start with removal, not products. Moist air has to leave the house. The cheapest and most effective change for most homes is to use the ventilation that is already fitted and use it for longer than feels natural.

• Keep trickle vents open. The Energy Saving Trust advises making sure trickle vents are always open and clear of obstructions. They are the small vents at the top of modern window frames. Closing them to stop draughts also traps moisture. If your windows have them, leave them open all day and night through the heating season.
• Run extractor fans for longer. Use the bathroom extractor during a shower or bath and let it run for at least 15 minutes afterwards. Do the same in the kitchen: use the cooker extractor whenever you cook and let it run for at least 15 minutes after you finish. If you do not have fans, open a window in that room during use and leave it open for at least 15 minutes afterwards.
• Open windows in short bursts. The Energy Saving Trust suggests opening windows several times throughout the day for at least five minutes, and longer if you can. A short, wide open window changes the air without cooling the walls as much as a window left on the latch all day. A good routine is five to ten minutes in bedrooms first thing, and a cross draught through the house once a day when you can manage it.
• Close the right doors. When you are cooking or showering, close the kitchen or bathroom door so steam does not drift into colder bedrooms and hallways, where it will condense on cooler surfaces. Open the door again once the fan has cleared the room.

If you only do one thing differently this week, make it the extractor overrun. Most fans are switched off the moment someone leaves the bathroom, while the air is still saturated. Fifteen minutes of overrun clears the moisture that would otherwise settle on tiles, ceilings and the nearest bedroom window.

How do you reduce the moisture you make in the first place?

Ventilation removes moisture. Source control stops it building up so fast. Small habits make a measurable difference because they act on the largest moisture sources in the home.

• Put lids on pans. The Energy Saving Trust lists covering pots and pans with a lid when cooking as a basic prevention step. It reduces steam and it also reduces the energy needed to keep a pan at a simmer.
• Dry clothes carefully. Dry clothes outside when the weather is warm or dry. When you have to dry indoors, use one room, ideally the bathroom, close the door, open a window or run the extractor, and avoid draping washing over radiators throughout the house. Putting wet laundry on a radiator in a bedroom sends moisture straight to the coldest window in that room overnight. A heated airer in a closed, ventilated bathroom, or a dehumidifier in the same closed room, is far better than radiators in every room.
• Wipe down wet surfaces. After a shower, wipe walls and shower screens with a squeegee or towel, and wipe windows and sills in the morning. The Energy Saving Trust recommends wiping down walls after showering and doing the same for windows. Water you remove with a cloth is water that cannot soak into grout, silicone or plaster.
• Avoid unflued gas heaters. Local authority guidance across the UK consistently advises against portable bottled gas and paraffin heaters in homes with condensation, because burning gas produces a large amount of water vapour as well as heat.

A simple hygrometer helps you stop guessing. These small digital gauges cost very little and show relative humidity and temperature at a glance. UK building and health guidance generally treats 40 to 60 percent relative humidity as a comfortable band for occupied rooms. Sustained readings above 60 percent in living rooms and bedrooms suggest you are making more moisture than your ventilation is removing, especially in cold weather when walls and windows run colder than the room air. Use the reading to test your routine: if humidity is still above 60 percent an hour after a shower, your extractor needs longer, a window needs opening, or the door was left open so steam spread through the house.

Does heating affect condensation on windows?

Yes, because warm surfaces condense less than cold ones. A room that is heated in short, fierce bursts and then allowed to go cold overnight gives you the worst combination: warm, moist air in the evening meeting glass and external walls that cool rapidly after the heating goes off. A steadier background temperature keeps surfaces warmer and reduces the temperature gap that causes water to drop out of the air.

The Energy Saving Trust makes two balanced points here. A home that is not heated properly can contribute to damp, and rooms where the heating is turned off or kept very low in colder months are prone to problems. At the same time, leaving heating on all the time just to avoid damp is not a sensible or affordable solution. The practical middle ground for most homes is a consistent, moderate background temperature in occupied rooms, heating that does not swing wildly between hot and cold, and attention to cold spots. If one bedroom is always unheated and always wet, warming that room a little and ventilating it each morning will usually help more than turning the main living room up by several degrees.

Cold spots matter as much as average temperature. Gaps in insulation, an uninsulated section at the top of a wall, or a single glazed window in an otherwise insulated room will attract condensation because that surface is colder than everything around it. The Energy Saving Trust notes that insulation reduces condensation risk when it is suitable, the right type and properly installed, because it keeps surfaces warmer. It also warns that adding insulation reduces natural air leakage, so installers should consider ventilation at the same time. In practical terms: do not fit new draught proofing in a room that already has a condensation problem unless you also improve ventilation. Sealing a wet room tighter makes it wetter.

What is the room by room plan for condensation season?

Condensation is easiest to beat when you treat each room according to how it produces moisture.

Bedrooms. Open the window for five to ten minutes when you get up, wipe the glass and sill straight away, and leave trickle vents open. Avoid drying laundry in bedrooms. Leave a small gap between wardrobes and external walls so air can circulate, and avoid pushing a bed hard against a cold external wall if you can. If black mould appears behind a wardrobe, that is an airflow problem in that corner as much as a humidity problem for the whole house.

Bathrooms. Run the extractor for every shower and bath and for at least 15 minutes afterwards. Keep the door closed while the fan runs. Wipe the shower screen and tiles, and if there is no fan, open the window during use and for at least 15 minutes after. Check that the fan actually extracts: hold a piece of tissue near it and see whether it is drawn towards the grille. A fan that is clogged with dust or venting into a loft rather than outside will not clear steam, however long you run it.

Kitchens. Use the cooker hood or extractor every time you cook, not only for frying. Keep lids on pans, close the kitchen door while you cook if steam is spreading, and let the fan overrun for at least 15 minutes. If you use a tumble dryer that is not a condenser model, it should vent outside. A dryer venting into the house can undo a whole day of good ventilation in one cycle.

Living rooms and hallways. These rooms produce less moisture but often contain the coldest external walls and the furniture that traps damp air behind it. Leave gaps between sofas and wardrobes and external walls. The Energy Saving Trust also suggests leaving internal doors open where possible and ensuring there is a gap under internal doors, so air can move between rooms rather than leaving one cold, still room to collect moisture. Keep trickle vents open even in rooms you use less often.

When does a dehumidifier actually help?

A dehumidifier can help, but it is not a substitute for ventilation. The Energy Saving Trust suggests considering a dehumidifier for dealing with excessive moisture or to help with drying washing inside. Used well, in a closed room where laundry is drying, a dehumidifier pulls moisture from the air faster than a window alone on a wet day, and it adds a small amount of warmth as it runs, which also helps keep surfaces dry. Used badly, left in a hallway with doors open to the whole house and windows closed, it will run all day and make little difference to a bathroom that is never extracted properly.

If you buy one, use it where the moisture is made: in the bathroom or utility room with the laundry, door closed, for the hours the washing is drying. Empty the tank, clean the filter as the manufacturer advises, and still open windows and use extractors for cooking and showers. A dehumidifier removes water from air. It does not bring fresh air in, and it does not fix a leaking gutter, a broken extractor duct or a failed sealed window unit. Treat it as a useful tool for laundry days and persistently humid rooms, not as the whole answer.

How do you stop mould from following condensation?

Mould grows where moisture sits for long periods on a surface that feeds it: paint, wallpaper paste, grout, silicone, timber and dust on cold plaster. The Energy Saving Trust is clear that mould can affect health, causing symptoms such as nasal stuffiness, wheezing, and eye or skin irritation, and that children, older people and people with skin or respiratory conditions are particularly at risk from moisture build up in the home. That is why wiping windows is not cosmetic. Water left on a sill each morning soaks into the frame and the plaster reveal, and black mould follows in the corners where the air is stillest.

Daily drying is prevention. Cleaning is treatment. If mould has already appeared in small patches on window frames, tiles or silicone, the Energy Saving Trust advises using an antifungal cleaning product or a product designed to kill mould, using a disposable cloth and throwing it away after use, wearing gloves and a mask and any other protection the product label recommends, only wiping mould when it is wet so spores are not spread into the air, and keeping the room ventilated while you clean with doors to other rooms closed. Do not dry brush mould, and do not paint over active mould with ordinary paint and expect it to stay away. If mould covers a large area, keeps returning in the same place despite good ventilation, or is linked to a leak or rising damp, get professional advice. In a rented home, report persistent damp and mould to your landlord rather than only cleaning it repeatedly, because the underlying cause may be a building or ventilation fault that cleaning cannot fix.

What should you check before winter gets colder?

October is the easiest month to get ahead of this. A short check now prevents a winter of daily mopping and mould cleaning.

1. Test every extractor fan. Switch it on, check it pulls air, clean dust from the grille, and confirm it vents outside or into a properly ducted system. Set timers or remember the 15 minute overrun.
2. Open every trickle vent. Go room by room and open them. If a vent is painted shut or blocked by a blind or curtain, clear it.
3. Put a hygrometer in the worst bedroom. Watch morning readings for a week. If you are regularly above 60 percent, adjust ventilation and laundry habits before you spend money on products.
4. Move furniture off external walls. Even a small gap behind a wardrobe or sofa lets air circulate and reduces cold, still corners where mould starts.
5. Fix the obvious building issues. Clear blocked gutters, check for leaking pipes under baths and sinks, and look for gaps in insulation at the top of walls or around loft hatches. Condensation advice assumes rain and plumbing water are not also getting in.
6. Set a heating routine that avoids cold rooms. A steady background temperature in occupied rooms, with no bedroom left completely cold for weeks, keeps surfaces warmer and reduces overnight condensation.

None of this requires new windows. In fact, replacing windows without fixing ventilation often moves condensation from the glass to the next coldest surface, usually an external wall corner. Fix the air first, keep surfaces reasonably warm, and use the glass as your daily indicator. When the bedroom window is dry three mornings in a row, your routine is working.

How to stop condensation on windows: frequently asked questions

How do I stop condensation on windows overnight? Reduce moisture during the day, run extractor fans for at least 15 minutes after showers and cooking, keep trickle vents open, avoid drying laundry in bedrooms, keep a steady background temperature, and wipe glass and sills in the morning. Open the bedroom window for five to ten minutes when you get up to clear moist air.

Should trickle vents be open or closed in winter? Open. The Energy Saving Trust advises making sure trickle vents are always open and clear of obstructions. They provide background ventilation that removes moisture without opening a window wide.

How long should I run an extractor fan after a shower? The Energy Saving Trust advises letting extractor fans run for at least 15 minutes after taking a shower or a bath, and at least 15 minutes after finishing cooking in the kitchen.

What humidity should a house be to avoid condensation? UK guidance generally treats 40 to 60 percent relative humidity as a comfortable band for occupied rooms. Sustained readings above 60 percent suggest too much moisture is being produced or too little is being removed. A low cost hygrometer makes the level visible.

Does a dehumidifier stop window condensation? It can help, especially when drying laundry indoors in a closed room, and the Energy Saving Trust suggests considering one for excessive moisture or indoor washing. It does not replace extractor fans, trickle vents or opening windows, and it does not fix leaks or failed window units.

Why is condensation only on my bedroom window? Bedrooms combine overnight breathing in a closed room with the coldest glass in the house before dawn. Moist air from two sleepers meets a window that has cooled all night, so water forms there first. Morning ventilation, trickle vents and wiping the sill usually improve bedrooms fastest.