Condensation in your home: what causes it and how to stop it

Condensation is water from the air inside your home settling on a surface colder than the air: the bedroom window at 7am, the corner of an outside wall, the underside of the loft roof. Left to sit, it feeds mould and damages window frames. We cover the causes, how to stop it on windows and walls, England's extractor fan rules, how to tell it from a leak or damp, and who fixes it in a rented home.

What condensation is

Air always holds some water as invisible vapour. The warmer the air, the more it can hold. When warm indoor air touches a cold surface it cools, and at a temperature called the dew point it can no longer hold all its water, so the surplus turns back into liquid on that surface.

Relative humidity is how full the air is, as a percentage of what it could hold at its temperature. Cool the same air and the percentage climbs, even though no water has been added.

The figures are for a room 4 metres by 3 with a 2.5 metre ceiling, holding about 150 grams of water in its air, roughly a cupful. That is ordinary for a lived-in room. The same air feels fine in the middle of the room and runs down the window.

What causes condensation in a house

Three things cause it together: moisture put into the air, surfaces cold enough to reach the dew point, and too little ventilation to carry the moisture out. Fix any one and condensation drops. Most homes need a little work on all three.

Where the moisture comes from

Everyday living is the source. Cooking, showering, bathing, drying laundry and breathing all put water into the air, and none of it is avoidable. Two adults asleep breathe out up to half a litre of water in a night, which is why bedroom windows run wet in the morning.

Drying laundry indoors is the source most homes can move. Outdoors is best. If it has to dry inside, use one room with the door shut and the window open, so the moisture goes outside rather than round the house.

Why cold surfaces get it first

Condensation forms on the coldest surface in a room, not across the whole room. Glass, window frames, the corners where outside walls meet and low points on walls are the cold spots, so they get wet first. So does the wall behind a wardrobe, where warm air cannot reach.

A room can feel warm in the middle and still have walls cold enough to condense on, especially solid walls with no insulation.

Condensation on windows

Windows are the coldest surface in most rooms, so they show condensation first. Single glazing is the most prone. Double glazing keeps the inner pane warmer, but it still runs wet when the air in the room is humid enough.

For windows that are wet every morning:

• Wipe them first thing. A squeegee or a window vacuum takes the water off before it soaks into the sill, and the water goes down the sink.
• Let air reach the glass at night. Open the bedroom window a little or leave it in the locked vent position, and leave the curtains open.
• Air the house for 20 minutes a day with the windows open, winter included.
• Leave the trickle vents open. These are the slots in the top of the frame, and they let a small, steady flow of air through all day.
• Keep the room steadily warm rather than cold all day and hot for two hours in the evening.

Water you cannot wipe off because it sits between the panes of double glazing is a different fault. The seal around the sealed unit has failed, the unit needs replacing, and ventilating the room will not clear it.

New windows often show more condensation in their first winter. The house is more airtight than it was, so moisture the old draughts carried out now stays in the room until something removes it.

Condensation on walls and in corners

Condensation on walls is harder to spot than on glass, because paint and plaster soak it up. The signs are a cold, clammy feel, peeling wallpaper, a musty smell and dark spots in the corners of outside walls, behind furniture and around window reveals.

A surface that stays damp for more than 24 hours can start to grow mould, and the black mould that follows usually starts in corners and behind wardrobes, where the air moves least. Mould spores can sit in the wall up to a metre around the patch you can see. Treat an area at least a metre wider than the patch, then deal with the moisture, or the mould returns with the next cold spell.

Pulling wardrobes, beds and sofas a few centimetres off outside walls lets warm air reach the plaster. Anti-condensation paint can help on a cold wall, but it takes no moisture out of the air.

Does heating stop condensation?

Heating helps, but it does not stop condensation on its own. Warm air holds more water, so a heated room sits further from its dew point and its walls and windows stay warmer. The moisture is still in the air, though, and it comes out on the first cold surface unless ventilation removes it.

Steady, lower heat works better than cold rooms with short bursts of heat. A house left cold all day lets its walls cool right down, and an hour of heating in the evening warms the air long before it warms the walls. In cold months, run the heating every day, even if not all day.

So condensation can mean a room is too cold, or at least that one surface in it is. An unheated spare bedroom, or a room with a radiator that won't heat up, is where the windows stream and the corners go black first. Radiators on outside walls under windows warm the coldest surfaces, and thermostatic radiator valves let each room hold its own temperature.

Extractor fans and ventilation: the Part F rules

Ventilation takes the moist air out before it reaches a cold surface. In England, Part F of the building regulations sets the minimum, and Approved Document F says kitchens, utility rooms, bathrooms and toilets should each have extract ventilation to the outside, at these minimum rates for a fan that runs when the room is in use.

Fans go as high in the room as practicable and no more than 400mm below the ceiling, where the warm, wet air collects. A cooker hood that recirculates does not take steam out of the kitchen, so without an outside-venting hood the kitchen rate doubles to 60 litres a second.

Run the bathroom fan while you shower and for an hour afterwards. A fan with a humidity sensor, or one wired to the light with an over-run timer, keeps going after you leave the room, so nobody has to remember.

Replacing a worn fan with a new one on the same cabling does not need notifying to building control. Refitting a kitchen or bathroom means keeping or replacing any fan already there. A room that never had one does not have to gain one, unless the work leaves the home less well ventilated than before.

A 2026 edition of Approved Document F takes effect on 24 March 2027 and keeps the same extract rates.

Replacement windows and trickle vents

Replacing the windows is likely to make a house more airtight, so the ventilation rules follow the new frames. If the old windows had trickle vents, the new ones should have them too, no smaller than before and adjustable. If the old windows had none, the home must be no worse ventilated after the work. Trickle vents of at least 8,000mm² in living rooms, bedrooms and the kitchen, and 4,000mm² in the bathroom, meet that.

The same logic applies to insulation and draught-proofing. Energy efficiency work that cuts the draughts an older house relied on can leave it under-ventilated, so plan the fans and vents in the same job.

Wales, Scotland and Northern Ireland

Approved Document F and the rates above apply in England. Wales publishes its own Approved Documents, Scotland works to its own building standards and Northern Ireland has its own building regulations, so a fan or window job in Cardiff, Glasgow or Belfast is checked against that nation's rules. The everyday fixes are the same everywhere.

Dehumidifiers: when one helps

A dehumidifier pulls water out of the air and collects it in a tank. It earns its place where ventilation cannot keep up: a flat with nowhere outside to dry laundry, a bedroom that reads over 60% every morning, or a room drying out after a leak.

Aim for relative humidity of 40% to 60% across the home, and set the dehumidifier to hold the room under 60%. Buy one that removes at least 5 litres of water every 24 hours. A decent machine starts at around £100.

• Refrigerant (compressor) dehumidifiers suit warm, heated rooms, which is most of the house in winter.
• Desiccant dehumidifiers suit cold, unheated rooms, such as a garage, a conservatory or a spare room with the heating off.

A very damp room takes 3 days to a week to dry out with a dehumidifier running. After that, around 2 hours a day usually holds it.

Condensation, a leak or damp: how to tell

Condensation comes and goes with the weather and the day's routine. Damp from outside or from a pipe does not. The government's damp and mould guidance sorts damp in homes into four types, and each leaves a different pattern.

A wet patch that appears after heavy rain, or stays the same size through a dry fortnight, is not condensation. Neither is water coming through the ceiling below a bathroom, which is a leak until proved otherwise, and ventilating the room will not help.

Condensation in the loft

Warm, moist air from the rooms below finds its way into the loft through the hatch, around pipes and through ceiling light fittings. It meets the cold underside of the roof and condenses on the felt, the nails and the timbers. In a cold snap it can drip onto the insulation and stain the ceiling below.

That can look like a roof leak, and the pattern tells them apart. Condensation spreads across the underside of the roof, beads on the nail tips and is worst on cold mornings. A leak is one wet area in the same place, and it follows rain.

The fix for loft condensation is air through the roof space: ventilated soffits at the eaves and roof ventilation tiles let outside air move through. Sealing the loft hatch and the gaps around pipes stops the moist air getting up there in the first place.

When condensation needs a specialist

Most of the fixes above cost little or nothing. Bring in a damp specialist when:

• The damp stays after a few weeks of lower humidity readings and a working fan.
• It sits low on a ground floor wall, or it gets worse after rain.
• Plaster is crumbling, or skirting boards, sills or floorboards feel soft.
• Mould comes back in the same place within weeks of cleaning.
• You are buying the house and the survey flagged damp.

Soft timber matters most. Wood that has stayed wet for months is where dry rot and woodworm damage take hold, and damp and timber specialists treat both.

The survey decides the bill. What damp proofing costs turns on whether the cause is condensation, penetrating damp or rising damp, because a fan and a vent are a different job from a new damp-proof course.

Condensation in a rented home: who fixes it

In England, the landlord has to find and fix the cause of damp and mould. The government's guidance for social and private landlords says it is unreasonable to blame damp and mould on a tenant's lifestyle, because cooking, bathing and drying washing all put moisture into the air.

That guidance sets the landlord's duties alongside the Housing Health and Safety Rating System, under the Housing Act 2004, and the Homes (Fitness for Human Habitation) Act 2018. It expects the landlord to tackle the underlying cause, such as poor ventilation, heating or insulation, rather than only cleaning off the mould.

Social housing tenants in England also have fixed deadlines under Awaab's Law, in force since October 2025. For serious damp and mould, the landlord must:

• investigate within 10 working days
• send a written summary within 3 working days of the investigation finishing
• make the home safe within 5 working days of the investigation
• start any further work within 12 weeks at the latest
• offer somewhere else to stay if the home cannot be made safe in time

Report damp in writing through the landlord's official channels and keep a copy of every message. If the landlord misses the deadlines, use its complaints process, then the Housing Ombudsman. Awaab's Law and the guidance above apply in England only.