Condensation on Windows: Why It Happens & How to Stop It

Published 29 October 2026 by Yorkshire Damp & Mould

Mould and condensation damage beside a window

Waking up to water running down the glass, puddles on the sill and black specks on the seals is one of the most familiar sights in a British winter. Window condensation is not a fault with the window as such. It is a sign that the air indoors is holding more moisture than the coldest surfaces in the room can cope with. Left alone, the water soaks into frames and plaster and gives mould a foothold, so it is worth understanding what is going on.

Why it happens on glass first

Air can only hold a certain amount of water vapour at a given temperature. When warm, humid air touches something cold enough, it cools below its dew point and the excess vapour turns back into liquid. Glass and the metal or uPVC around it are usually the coldest surfaces in a room, so they collect the water first. That is why the problem is worst on frosty mornings, after a night with doors and windows shut.

Where the moisture comes from

Every household produces water vapour through ordinary living. The main sources are:

• Breathing, especially in bedrooms with two or more people asleep.

• Cooking without lids or without the cooker hood running.

• Baths and showers, particularly with the bathroom door left open.

• Drying washing on radiators or clothes horses indoors.

• Tumble dryers that are not vented to the outside.

• Bottled gas or paraffin heaters, which release water vapour as they burn.

If you rely on a portable gas heater or notice any problem with a gas appliance, flue or boiler, stop using it and call a qualified professional. Never block vents that a gas appliance needs to work safely.

Inside, outside or between the panes?

Where the water forms tells you what kind of problem you have. Water on the room side of the glass is down to humidity indoors and is the type this guide is about. Droplets on the outside of the window on a cool morning are normal with efficient glazing and clear by themselves. Mist trapped between the two panes of a double-glazed unit means the seal has broken, and the unit needs replacing.

Practical ways to cut it down

Small changes to daily routine make a noticeable difference. Put lids on pans and use the extractor hood. Keep the bathroom door shut while bathing and leave the fan running afterwards. Dry clothes outside when you can, or in a room with the door closed and a window slightly open. Pull curtains and blinds back during the day so the glass can warm up, and avoid packing furniture tight against outside walls.

Steady, moderate heating also helps. A house that is allowed to get very cold and is then heated in short bursts has more cold surfaces for longer, so moisture settles before the room warms through.

Ventilation that keeps working

Habits only go so far if the home cannot exchange its air. Trickle vents in the window frames give constant background ventilation without leaving windows open; keep them open, and if your frames do not have them they can often be added. Extractor fans in kitchens and bathrooms remove steam at the source, and fans with a run-on timer or humidity sensor keep going after you leave the room. Where condensation appears in several rooms at once, a Positive Input Ventilation unit can lower humidity across the whole house.

When it points to something more

If condensation persists despite good habits, or the wall around the window feels wet after rain, there may be another problem such as a failed seal around the frame, a leaking sill or water coming through the wall. Mould on the reveal that keeps returning is another sign to get it checked.

We install trickle vents, Envirovent extractor fans and PIV systems across Leeds, Bradford, Wakefield and Harrogate. If your windows stream every morning, call 07518 705555 or use the enquiry form and we can arrange a visit to look at the cause.

Frequently Asked Questions

Is condensation on the outside of my windows a problem?

Usually not. Moisture on the outside face of modern glazing on a cool, clear morning is a sign the glass is keeping heat in. It clears as the day warms up and does not affect the inside of the home.

Why is there moisture trapped between the panes of my double glazing?

That normally means the seal on the sealed unit has failed and damp air has got into the gap. It cannot be wiped away and does not come from your room. The usual fix is replacing the glass unit rather than the whole window.

Should I wipe my windows dry every morning?

Yes. Wiping glass and sills with a cloth or window vacuum stops water soaking into frames, seals and the plaster around the reveal, where mould tends to start. Wring the cloth out in a sink rather than leaving it on a radiator.

Can a dehumidifier stop window condensation?

It can reduce it by pulling water out of the air, especially in a room where clothes are drying. It treats the symptom rather than the cause, so it works best alongside better ventilation and less moisture being produced.