Condensation forms on windows because glass is usually the coldest surface in a room, and when its temperature falls below the dew point of the room air, water vapor condenses onto it. On a winter morning a bedroom at 18 °C with 65 percent RH has a dew point of about 11 °C; single glazing at 5 °C and the edges of double glazing at 9 °C are both below it, so they are wet. Either lower the dew point (ventilate, remove moisture at source) or raise the glass temperature (better glazing, heating, curtains open) and the condensation stops.
The two numbers
Every case of condensation is the dew point of the air on one side and the temperature of a surface on the other. See what dew point is. Windows lose because:
- Glass conducts heat far better than an insulated wall, so its inner surface runs closer to the outdoor temperature.
- Spacer bars around double-glazed units are colder than the middle of the pane, which is why condensation starts at the bottom edge.
- Curtains and blinds trap a layer of still, cold air against the glass and make it colder.
Where the moisture comes from
| Source | Typical water added |
|---|---|
| Two adults sleeping, 8 hours | About 0.5 to 0.8 liters |
| One shower | Up to 1.5 liters into the air if not extracted |
| Cooking a meal | 1 to 3 liters |
| Drying a load of laundry indoors | 2 to 5 liters |
| Unvented gas heater or hob | About 0.8 liters per kWh of gas |
A bedroom is the usual site because breathing adds moisture all night into a closed room while the outdoor temperature, and therefore the glass, drops to its minimum. See why humidity changes throughout the day.
Is it a problem?
Light condensation on single glazing in a cold snap is ordinary. Persistent condensation, water pooling on sills, black spotting on frames or reveals, or condensation on double glazing away from the edges are signs the room’s dew point is too high. That same moisture is condensing invisibly on cold wall corners and behind furniture, where mold grows. Condensation between the panes of a sealed unit is a failed seal and a different problem.
Fixes, in order of effectiveness
- Extract moisture at source. Run bathroom and kitchen extract fans during and for 15 minutes after use; vent the tumble dryer outside; do not dry laundry in bedrooms.
- Ventilate bedrooms. Trickle vents open, window cracked or door ajar overnight. This lowers the dew point directly and helps CO₂ too; see why indoor CO₂ rises overnight.
- Keep the glass warmer. Steady low heating rather than off overnight; curtains open a little at the bottom so warm air reaches the glass; secondary glazing or replacement units with warm-edge spacers.
- Dehumidify in rooms that cannot be ventilated, aiming for a dew point a few degrees below the coldest surface.
- Wipe and dry as a stopgap so water does not soak into frames and plaster.
Measuring it
A hygrometer with dew point display in the bedroom and an infrared thermometer on the glass tells you exactly how far apart the two numbers are. The humidity checker computes the dew point; a thermal camera shows every cold surface in the room at once, covered in best thermal cameras for home inspection.
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Frequently Asked Questions
Is condensation on windows a problem?
On the inside of single glazing on a cold morning it is normal, but persistent condensation means the room is holding more moisture than it should and mold on frames and reveals usually follows.
Why do only the bedroom windows get wet?
Two people breathing overnight in a closed room raise the dew point by several degrees, and bedrooms are often cooler than living rooms, so the glass is colder too.
What about condensation between the panes?
That means the sealed unit has failed and moist air is entering the gap. It cannot be fixed by ventilation; the glass unit needs replacing.
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