Condensation on windows in winter: what's normal and what isn't
A little mist on a bedroom window after a cold night is normal and nothing to worry about. Water running down the glass every morning, pools on the sill and black mould on the seals is a sign there's more moisture in the air than the room can get rid of, and that tends to turn up as mould somewhere else too.
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Why windows get it first
The air in every home holds water vapour from breathing, cooking, showers, kettles and drying clothes. When that air touches something colder than it, some of the water comes out as droplets. Glass is usually the coldest surface in the room, so it gets wet first, before walls and corners do.
Gov.uk's guidance on damp and mould calls condensation the most common form of damp. It also says, plainly, that it's unreasonable to blame damp and mould on how people live. The real causes it lists are the building: poor ventilation, poor heating and poor insulation.
Usually fine
- A light film on the inside of the glass on cold mornings that clears within an hour or two of the heating coming on.
- Steam on the bathroom or kitchen window while you shower or cook, which goes once the fan's been on for a while.
- Water on the outside of double glazing on a frosty morning. That means the glazing is doing its job.
Worth doing something about
- Glass streaming with water every morning, enough to wet the sill or drip onto the floor.
- Black spots on the rubber seals, the frame or the plaster round the window.
- Paint peeling or wood going soft on the sill or the frame.
- The same thing happening in several rooms, not just the bathroom.
- Curtains that feel damp or have mould along the bottom.
- Mould starting in the corners of the ceiling or behind furniture on outside walls.
At that point the air's too wet for the room. Meaco, which makes dehumidifiers, says mould grows once relative humidity reaches 68%, and suggests keeping homes at about 55%. A cheap hygrometer (a humidity reader) will tell you where your rooms sit.
What helps
- Open the trickle vents at the top of the frame and leave them open. If the window has a locked vent setting, use it at night. Gov.uk's guidance suggests the same thing.
- Wipe the glass and sill first thing with a cloth or a window vacuum and wring it out down the sink, not onto a radiator.
- Keep the bathroom door shut while you shower and run the fan for a good while afterwards.
- Try to keep a little heat in every room rather than one warm room and the rest cold. Cold rooms are where the water settles.
- Pull curtains back during the day so the glass and frame can dry, and leave a gap behind furniture on outside walls.
- Dry clothes outside when you can. Indoors, put them in one room with the door shut and the window on the vent, or the fan on.
A dehumidifier is the quickest way to bring the humidity down, especially if you have to dry washing inside. Our dehumidifier size calculator suggests a size for your home and works out what it costs to run per hour.
When it's more than condensation
If a patch of wall round the window is wet even when the glass is dry, or gets worse after heavy rain, water may be getting in round the frame or through the wall itself. A failed seal, cracked render or a missing drip under the sill can all do it. That's penetrating damp, and a dehumidifier won't stop it. The guide to damp or condensation explains how to tell them apart.
If you've done all of the above for a few weeks and the windows are still streaming, or mould keeps coming back, it's worth having the ventilation and heating looked at. Often it's an extractor fan that barely moves any air, or windows fitted with no trickle vents. Ask for a local specialist and we'll pass your enquiry to one who covers your area.
Renting? Condensation caused by the building is your landlord's to sort out. The tenants' guide explains how to report it.
Sources
Gov.uk, Understanding and addressing the health risks of damp and mould in the home (updated 1 April 2026). Meaco, making the most of your dehumidifier (updated 27 February 2024), for the humidity figures.
Facts, dates and prices on this page were last checked on 2 November 2026.
Common questions
Is condensation on the outside of the glass a problem?
No. On a cold, clear morning, water on the outside of a modern double-glazed window means the outer pane is staying cold because the glazing is keeping the heat in. It burns off once the sun's on it.
Why is there water between the two panes?
That's not room condensation. It means the seal on the double-glazed unit has failed and damp air has got into the gap. Wiping and ventilating won't help. The sealed unit needs replacing, usually without changing the whole frame.
Do I need new windows?
Usually not. Old single glazing does run with water more easily, but the same amount of moisture in the air will just find another cold spot, often a wall corner. Dealing with the moisture comes first.
Should I leave the window open all night?
In winter that cools the room and the walls, which can make condensation elsewhere worse. Opening the trickle vent, or the window on its locked vent setting, gives a steady trickle of air without losing much heat.
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