How to Stop Condensation on Windows (UK Guide)

Air holds water as invisible vapour, and warmer air holds more of it. When moist indoor air touches a surface cold enough, it crosses what physicists call the dew point and the vapour turns back into liquid. In a UK winter the coldest surface in most rooms is the window glass, which is why the water shows up there first, and why it is worst in the morning: overnight the heating is off, the glass is at its coldest, and several hours of breathing have been adding moisture to a closed room.

The quantities involved surprise people. UK council housing guidance uses a standard figure of about 12 litres of moisture released into an average home every day, from cooking, showers, kettles, washing up and simply breathing. Dry your laundry indoors and that can pass 20 litres. None of that water disappears. It either leaves through ventilation or it ends up in your air, on your glass and eventually in your walls.

That is why condensation is a moisture balance problem. The window is just the messenger, and treating the glass (wiping, window vacuums, anti-mist sprays) without changing the balance is a daily chore that never ends. Change what you produce, what you vent and how cold the surfaces get, and the streaming stops on its own.

These cost nothing and fix mild condensation outright. Do all of them before spending a penny.

• Lids on pans, every time. An uncovered pan of pasta is a moisture machine. Lids also bring water to the boil faster, so this one saves energy too.
• Shut the bathroom door while you shower, then vent. Keep the door closed so the steam cannot wander into bedrooms, run the extractor if you have one, and leave it running (or the window cracked) for 15 minutes afterwards.
• Stop drying washing on radiators. Every wet load pushes litres of water straight into your air, and the evaporation cools the radiator so the boiler works harder. Use an airer in one closed room with a window cracked, or better, an airer next to a dehumidifier.
• Wipe the glass in the morning. Not a cure, but a wet window evaporates back into your air as the room warms, so towelling it off and wringing the towel out in the sink genuinely removes water from the house.
• Leave a gap between curtains and glass. Heavy curtains shut tight against a window trap a pocket of cold air and make the glass colder still. A small gap, or blinds raised a few centimetres, lets air circulate.
• Pull furniture off cold external walls. A wardrobe tight against a north-facing wall creates a cold, still air pocket, which is exactly where black mould likes to start.
• Air the house in short bursts. Ten minutes with windows open wide swaps humid air for dry air without cooling the fabric of the building. It feels counterintuitive in January; it works.

Moisture you cannot avoid making has to leave the building, and hoping it seeps out of a modern draught-proofed home is not a plan. Three things matter.

Extractor fans, actually used. The kitchen and bathroom fans are the whole game, because that is where most of the 12 litres is produced. Run the kitchen extractor whenever you cook, not just when something smokes, and check it vents outside rather than recirculating through a grease filter. If a bathroom fan is so loud you avoid using it, that is a real problem worth fixing; quiet replacement fans exist and a spark can fit one in an hour.

Trickle vents, open. Those little slotted vents at the top of window frames are background ventilation, and they are there to be left open through winter. Closing them to keep heat in is a false economy that traps moisture instead. They matter enough that building rules now protect them: under Approved Document F, in force in England since June 2022, replacement windows must not make background ventilation worse than before, so if your old frames had trickle vents, the new ones must have equivalent ventilation too.

Do not seal the building up. Blocked airbricks, taped-over wall vents and sealed chimneys all show up later as condensation and mould. If a room has no extractor, no trickle vent and a draught-proofed door, the moisture from one sleeping adult has nowhere to go, and the window will tell you about it every morning.

Condensation loves a house heated in hot little islands. Warm air from the heated living room carries moisture into the cold unheated bedroom, hits the cold glass and walls there, and dumps its water. This is why the worst mould so often appears in the rooms people are trying to save money by not heating.

A steadier pattern beats bursts: keeping the whole house at a modest background temperature through the coldest weeks keeps wall and window surfaces above the dew point, which stops condensation forming at all. That does not mean tropical; it means avoiding rooms that sit at fridge temperatures for days while the rest of the house steams.

Two practical notes. First, if some rooms are always cold because radiators there barely work, balancing or bleeding them is a free afternoon job that changes the whole pattern. Second, if heating the whole home evenly simply is not affordable, concentrate on ventilation and moisture removal instead: a dehumidifier in the coldest, dampest room costs pennies an hour and protects the room you cannot afford to heat. The UK Health Security Agency's 2023 damp and mould guidance is blunt that living with mould is a health issue, with effects on airways and lungs, so "just put up with it" is the one option we would rule out.

If you have done the free habits and the windows still stream, stop fighting the symptom. A dehumidifier attacks the cause directly by pulling water out of the air, and it is the one fix on this page that works regardless of your landlord, your building or your budget for building work.

It earns its keep in four situations:

• You dry washing indoors. An airer plus a dehumidifier in a closed room dries clothes faster than radiators, without the 20-litre moisture bill.
• You rent. You cannot fit extractor fans or change windows, but you can plug a machine in. It moves house with you too.
• Your home has a cold, damp room nothing else fixes. North-facing bedrooms, basements and rooms over garages are structural cold spots; drying the air is cheaper than rebuilding the wall.
• Someone in the house has asthma or a dust mite allergy. Keeping humidity in the healthy band suppresses both mould and dust mites, which NHS advice links to respiratory flare-ups.

Running cost is the pleasant surprise: an efficient compressor machine draws around 200W, which works out near 5p an hour at the current Ofgem price cap rate of 26.32p per kWh, and it cycles off automatically once the air hits its target. Aim for 50 to 55% relative humidity. One caveat: compressor machines fade badly in unheated spaces below about 15C, so for a garage, conservatory or boat you want a desiccant model instead. Our dehumidifier guide covers sizing, the one we would buy and the desiccant pick for cold rooms.

Condensation season produces a reliable crop of products that treat the symptom, not the balance.

• Window vacuums as a cure. Handy for wiping glass without drips, but the water went back into your air yesterday and will be back tomorrow. Buy one for cleaning, not as a damp strategy.
• Moisture trap crystals for a whole room. Those little tubs of calcium chloride collect an egg cup of water a week. Against a household producing litres a day they are a rounding error. Fine inside a wardrobe or a parked caravan; pointless on a windowsill.
• Anti-condensation paint as a first move. It slightly warms the wall surface and can help as a finishing touch, but painting over a moisture problem without fixing ventilation is how you get mould under the paint.
• Sealing up draughts everywhere. Draught-proofing saves energy and we are for it, but if you seal a home tight and change nothing else, the moisture that used to leak out now stays in. Pair any serious draught-proofing with deliberate ventilation.
• "Self-cleaning" or anti-mist glass sprays. Marginal at best on interior condensation, and another recurring cost that removes no water from the house.

If black mould has already appeared around frames or in corners, deal with it in the right order. Kill and clean the existing growth first with a dedicated mould remover, wearing gloves and with the window open. Then fix the moisture balance using everything above, because cleaned mould returns within weeks if the humidity stays over 60%. A cheap hygrometer tells you whether you are winning.

Renters have more leverage than most realise, and damp is taken more seriously than it used to be. Social landlords now face fixed timescales to investigate and fix damp and mould hazards under Awaab's Law, introduced through the Social Housing (Regulation) Act 2023. Private tenants are covered by the Homes (Fitness for Human Habitation) Act 2018, which lets you hold a landlord to account for a home that is not fit to live in, and your council's environmental health team can inspect and order repairs. Report damp in writing with photos, keep copies, and do not accept "just open a window" as a complete answer where there is no working extractor or background ventilation.

One honest distinction, though: if the moisture is yours (showers, cooking, indoor drying), the landlord fixing the building will not fix the balance. The habits and the dehumidifier above are still your side of the deal.

Everything in this guide treats condensation, which is the most common damp in UK homes but not the only kind. Spend two minutes ruling out the others, because no amount of ventilation fixes a broken gutter.

The morning-pattern test is the giveaway: condensation follows cold nights and busy bathrooms, not rainfall. If your damp tracks the weather or sits in one spreading patch, stop here and get the building looked at first.