Condensation on Windows: The Real Cause
Every autumn the same ritual: you wake up, the windows are fogged, and you blame the windows. But the glass is innocent. Condensation is your indoor air confessing that it’s carrying more water than the coldest surface in the room can tolerate.
Your window is a dew point detector
Air holds water as invisible vapour — up to a limit that depends on temperature. Cool the air down and that limit drops. The temperature at which the water in your air would start turning back into liquid is called the dew point.
Any surface colder than the dew point of the air touching it gets wet. In most homes, the coldest surface is the window pane, because it’s the thinnest barrier between inside and outside. So the glass fogs up first — not because it’s faulty, but because it’s the messenger.
Some rough numbers for indoor air at 20 °C:
| Indoor RH at 20 °C | Dew point (approx.) |
|---|---|
| 50% | ~9 °C |
| 60% | ~12 °C |
| 70% | ~14 °C |
At 70% relative humidity, any surface below roughly 14 °C starts collecting water. On a cold night, a single-glazed pane — or the aluminium edge of a double-glazed one — gets there easily.
Where the water comes from
The water on your window was in your air, and before that, it came from you. Breathing, cooking, showers, drying laundry indoors: a household releases roughly several litres of water vapour per day. In summer you ventilate constantly without thinking about it. In autumn and winter, windows stay shut, and every litre you release stays in the air until it finds a cold surface to land on.
That’s why condensation season starts when the heating season does. The outdoor air gets cold, your walls and glass get cold, and your indoor moisture — produced at the same rate as always — suddenly has somewhere to condense. It’s the same mechanism that makes your whole home feel damp in autumn.
Why wiping the glass doesn’t fix anything
Wiping the window removes the symptom for an hour. The water you wipe off is a tiny fraction of what’s dissolved in your air, and the air will happily redeposit it overnight.
The other classic mistake is trusting the % on a cheap hygrometer to decide when to ventilate. Relative humidity is a percentage of what air could hold at its current temperature — a cold pane can be dripping while the room reads a reassuring number. What matters is the actual water content of the air, its absolute humidity, and whether the air outside carries less of it than the air inside.
What actually stops condensation
Two levers, in order of effectiveness:
- Get the water out. In cold weather, outdoor air is almost always drier in absolute terms than indoor air, even when the forecast says 90% RH — cold air simply cannot hold much water. Short, wide-open airing several times a day (Stoßlüften) swaps your humid air for dry air without chilling the walls.
- Keep surfaces warmer. Heat the rooms you use, don’t let bedrooms drop to cellar temperatures, and keep curtains from trapping a pocket of cold air against the glass overnight.
Reduce the input too: lids on pots, bathroom door shut and aired after showers, laundry dried with a plan rather than on a radiator.
Condensation isn’t a cleaning problem or a glazing problem. It’s a water-balance problem — litres in, litres out — and ventilation at the right moments is how you balance the books.
Let the app watch for you
Knowing that outdoor air is drier in absolute terms is exactly the calculation Open/Shut does for you, using real-time weather for your exact location. It tells you when opening the windows will genuinely dry your home — and when it won’t. Free, private, no account.