Condensation on the inside of your windows is warm, damp indoor air meeting cold glass and turning to water. A UK household puts several litres of moisture into the air every day, and if that air cannot get out, it settles on the coldest surface in the room. Usually the windows.
Most of the fix costs nothing: move the moisture out and let fresh air in. Here is how, and when heavy condensation is telling you something bigger.
Points to take away
- Condensation on the glass is a humidity problem, not a broken window.
- The main sources are drying washing indoors, showers, cooking and unvented tumble dryers.
- The fix is ventilation plus reducing moisture at source. A dehumidifier helps if it is stubborn.
- New double glazing can make it worse until you use the trickle vents.
- Water trapped between the panes is a failed sealed unit and needs reglazing.
Reading time: about 6 minutes.
Which condensation you have
On the room-facing surface of the glass. You can wipe it off. Warm humid air is hitting the cold pane. This is the common one, and it is about the air in the house, not the window. The rest of this guide is about this.
Between the two panes. You cannot wipe it because it is sealed inside. The unit seal has failed. This needs a new sealed unit fitted into the frame. Our guide to misted double glazing covers the repair.
On the outside of the glass on a clear morning. Nothing wrong. The outer pane is staying cold because your windows are keeping heat inside.
Why it happens
Warm air holds more water vapour than cold air. When it touches a surface below the dew point, the air can no longer hold the moisture and it condenses as droplets. Single glazing and old metal frames are the coldest surfaces in most rooms, which is why they run first. Good double glazing keeps the inner pane warmer, so it condenses less, but it does not remove the moisture from the air.
A typical household adds roughly:
| Activity | Water added to the air per day |
|---|---|
| Breathing and perspiration (2 adults, asleep) | around 1.5 litres |
| Cooking and boiling a kettle | 1 to 2 litres |
| A bath or shower each | 0.5 to 1 litre |
| Drying one load of washing indoors | 2 to 3 litres |
| Unvented tumble dryer | 2 litres plus |
How to stop it, cheapest first
- Stop drying washing on radiators or airers in closed rooms. This is the single biggest cause of morning condensation in UK bedrooms. Dry outside, or in a vented or condenser dryer, or in one room with the door shut and the window ajar.
- Use extractor fans properly. Run the bathroom fan during a shower and for 15 minutes after. Use the cooker hood or open a kitchen window when cooking, and keep lids on pans.
- Open the trickle vents. The small slots in the top of modern window frames are there to let stale, humid air out slowly. Closing them to stop a draught traps the moisture.
- Air each room for 5 to 10 minutes in the morning. Open the windows wide, let the air swap over, close them. Cheaper and faster than heating damp air all day.
- Keep temperatures steadier. Big swings, letting a room go cold then blasting it warm, drive condensation. A lower, more constant setting produces less.
- Run a dehumidifier if the above is not enough. Set it to hold 45 to 55 percent relative humidity. It is the only step that actively removes water rather than relying on the outside air being drier.
Why new windows can make it worse
An old house with draughty windows and gaps around doors ventilates itself, badly and expensively, but it does move moist air out. Fit new sealed windows and that leakage stops. The moisture you produce now stays in the house and finds the next coldest spot: a north-facing wall, a cold corner, behind a wardrobe.
This is normal and it is fixable. Use the trickle vents, run the extractors, and consider a dehumidifier for the first winter while the house dries out.
When condensation is a warning
Surface condensation is not rising or penetrating damp. But if you have it heavily every day, in several rooms, with black mould appearing on frames, sills and walls, the moisture load is high enough to cause damage. That is worth acting on: check for a leaking gutter or overflow, a blocked air brick, an unvented bathroom, or a broken extractor, and get a damp survey if it persists after you have improved ventilation.
If your windows are old, single-glazed or have failing seals, replacing them raises the inner pane temperature and cuts surface condensation. Our guide to the signs you need new windows covers when that is the right call.
Get a free quote and we will match you with vetted local installers who cover your area.