How to stop condensation on windows (and stop it coming back)

To stop condensation on windows, you need to do three things: put less moisture into the air, let the damp air out, and keep the glass and walls warmer. Condensation happens when warm, moist air inside your home touches a surface colder than the air's dew point, the temperature at which water vapour turns back into water. Glass is usually the coldest surface in the room, so that's where it shows first.

The cheapest fixes are habits and ventilation: wipe the glass, keep trickle vents open, use the extractor fan and keep internal doors shut when you cook, shower or dry washing. After that come steady background heating, a dehumidifier if you dry clothes indoors, and, last and most expensive, better glazing and insulation.

An air-to-air heat pump can help, but not in the way most people assume. In heating mode it removes no moisture at all. Its job is cheap, steady heat that keeps your walls and windows above the dew point.

Why you get condensation on the inside of your windows

Warm air holds far more water than cold air. At 25°C a cubic metre of air can hold around 20g of water vapour, and that capacity roughly halves for every 10°C drop, according to RICS. When moist room air meets cold glass, it cools, can't hold its water any more, and leaves it on the pane.

The number that matters is the dew point. Here's roughly where it sits for typical indoor air:

Any surface colder than that figure gets wet. On a 0°C night with the room at 20°C, the inside of single glazing sits at about 5°C, older double glazing at about 13°C, and modern A-rated double glazing at about 16 to 17°C. These are our own rough calculations, but the pattern is clear: single glazing will run wet at almost any indoor humidity, and older double glazing starts streaming once the room passes about 60%. Frames and the bottom edge of the glass run colder than the centre, which is why the water collects there first.

Dew point and glass figures are Aircon Adviser calculations and are approximate.

Where all the water comes from

Council guidance puts the moisture an average household adds to the air at around 12 litres a day, and over 20 litres a day where washing is dried indoors. Breathing, showers, baths, cooking with the lids off and kettles all add to it. A wet bath towel alone can hold about a litre.

That's why condensation is the most common form of damp in UK homes, and why the government's damp and mould guidance says poor ventilation, inadequate heating and low energy efficiency are what make it a problem. It also says it is "totally unreasonable" to blame damp and mould on lifestyle choices. Everyday living produces moisture; a home should be able to cope with it.

How to stop condensation on windows overnight

These cost nothing or next to nothing, and they work best together:

• Wipe the glass first thing and wring the cloth into the sink, not back into the room. A window vac makes this quicker.

• Open the trickle vents on your window frames and leave them open. The Energy Saving Trust says they should normally be open all the time.

• Run the extractor fan while you cook or shower and for a while after, and keep the kitchen or bathroom door shut so the moisture doesn't spread.

• Put lids on pans and dry washing in one room with the door closed and a window or vent open, rather than over radiators around the house.

• Keep furniture a few centimetres off outside walls so air can move behind it. Cold corners behind wardrobes are where mould starts.

• Don't block airbricks or seal up every gap. Draught-proofing an already airtight home can make air quality worse, the Energy Saving Trust warns.

Longer-term fixes, from cheapest to most expensive

If the quick wins aren't enough, work up this list. Most homes find the answer in the first three or four.

Prices and rates correct as of October 2026. Running costs use the Ofgem price cap for 1 October to 31 December 2026: 26.32p per kWh for electricity, which carries 0% VAT until 31 March 2027. The cap changes quarterly.

Ventilation: trickle vents, extractor fans and PIV

Controlled ventilation is the backbone of any fix. An extractor fan with a built-in humidistat switches itself on when the air gets humid, so it works even when you forget. Fans such as the Manrose CQF100HT draw about 7W, which is about 0.2p an hour.

Positive input ventilation is a loft-mounted fan that pushes filtered air into the house at a gentle pressure, forcing damp air out through vents and gaps. Units such as the Nuaire Drimaster Eco draw 1.6 to 15.3W, so running one all year costs up to about £35. The catch: it needs a loft, it draws in cold loft air in winter (heated versions exist but cost more to run), and it doesn't replace your bathroom and kitchen fans.

If you're replacing windows, Building Regulations Part F applies. LABC's guidance is that new windows should keep trickle vents if the old ones had them, at least the same size, and that where the old windows had none, ventilation mustn't get worse.

Heating: steady and low beats short and hot

How you heat matters as much as how much. Council guidance recommends low background heat through the day, even when you're out, because "short bursts of heat will only warm up the air, leaving surfaces cold". Some suggest a minimum background temperature of around 15°C. The NHS advises heating the rooms you use most, such as the living room and bedroom, to at least 18°C if you can.

Heat pumps help here. Warmer walls and glass sit further above the dew point, so less water forms on them. Here's what 10kWh of heat costs from different sources at the current cap:

The Energy Saving Trust notes there's little UK data yet on how air-to-air systems perform in real homes, so treat the heat pump figures as a range rather than a promise. Our guide to the cheapest way to heat a room runs the full comparison.

There is a £2,500 Boiler Upgrade Scheme grant for air-to-air heat pumps in England and Wales, but as of mid-October 2026 no installers are certified to claim it yet. Our air-to-air heat pump grant guide tracks where it stands, and the rest of our heating guides cover costs, tariffs and cold-weather performance.

Can air conditioning stop condensation?

Partly, and only if you use the right mode at the right time.

Heating mode removes no moisture. A reverse-cycle air conditioner (an air-to-air heat pump) warms the air, which lowers its relative humidity and warms the room's surfaces, but the amount of water in the air stays the same. Consumer NZ puts it simply: a heat pump "can't heat and dehumidify at the same time". The moisture from cooking, showers and breathing keeps building unless you ventilate or extract it.

Dry mode removes moisture, but it cools the room. It runs the compressor with a low fan so the indoor coil stays cold and water condenses out of the air and drains away. That works well on a mild, muggy day. In winter it's limited: Daikin's manual for its Comfora range rates dry operation for indoor temperatures of 18 to 32°C only, and you can't set a temperature or fan speed in that mode. In a cold January bedroom it won't do much, and other models may differ, so check your own manual.

Air-to-air units recirculate room air. The Energy Saving Trust is clear that you still need to ventilate. A heat pump replaces your heater, not your extractor fan.

In practice, a split's winter contribution is cheap, steady background heat. Dry mode is a useful tool in autumn and spring. For drying washing indoors in winter, a dehumidifier is still the right tool. Our guide on heating with air conditioning covers the running side in more detail.

Dehumidifier or air con dry mode: which costs less?

A dehumidifier wins on cost and on flexibility, because you can move it to the room with the problem. Air con dry mode only treats the room its indoor unit is in, and no manufacturer publishes its power draw in dry mode, so any figure is an estimate. We compare the two in more detail in our guide to whether a dehumidifier can cool a room.

The models named are examples to show running costs, not recommendations. Dry mode figures are our estimate from a 3.5kW split's rated input; manufacturers don't publish them.

Compressor or desiccant for winter?

Compressor dehumidifiers are cheaper to run, but they collect much less water in a cold room. Meaco's 12L Low Energy model removes about 4.9 litres a day at 20°C but only about 1.6 litres at 10°C. Desiccant models keep working in the cold and give off warmth as a bonus.

Worked out per litre removed, at 20°C the compressor costs about 20p a litre against about 52p for the desiccant. At 10°C the gap closes to about 61p against 63p, and the desiccant is warming the room too. So: compressor for heated rooms, desiccant for cold bedrooms, box rooms and conservatories.

Condensation outside the window or between the panes

On the outside of the glass: this isn't a problem. It means your glazing is insulating well, so the outer pane stays colder than the outdoor dew point. The Energy Saving Trust is clear it isn't a fault.

Between the panes of double glazing: the sealed unit has failed and is letting air in. Wiping won't help; the glazed unit needs replacing, though often not the whole frame.

When condensation turns into damp and mould

Left alone, regular condensation soaks into window reveals, plaster and furniture, and mould follows. Mould growth is a sign that humidity has been above about 70%. Aim to keep rooms between 40% and 60% most of the time; our guide to the ideal humidity in a house explains how to measure and manage it.

The government's damp and mould guidance says the main health effects are on the airways and lungs: coughs, wheezing, breathlessness, infections and worse asthma, rhinitis and COPD. It can also irritate eyes and skin, and affect mental health. Those most at risk include children and older people, people with asthma, COPD or heart conditions, people with weakened immunity, pregnant women and new mothers, and people who are housebound.

Clean small patches of mould promptly, find and fix the moisture source, and get help if it keeps coming back. If someone in the household has health problems that may be linked, speak to your GP.