Why do windows fog up in winter?
←→to navigate
Slide 1 of 6
tl;dr
Warm indoor air holds invisible moisture; the winter windowpane is the coldest thing it touches, so the air cools past its dew point right at the glass and sheds its water there as fog.
Show the written answerThe written answer
Windows fog in winter because warm indoor air carries invisible moisture, and the glass is the coldest surface it touches. Cool air can't hold as much water vapor as warm air — so when your cozy room air meets the chilled pane, it drops below its dew point and sheds water right there, as fog.
The dew point line
Every batch of air has a dew point: the temperature where its water vapor starts condensing into liquid. Warm air holds a lot of vapor; chill it and the ceiling drops. A winter windowpane is often the one indoor surface below that line — so the air unloads on it.
Why your house makes it worse
Showers, cooking, drying laundry, even breathing — a household pumps liters of moisture into the air daily. The more humid the room and the colder the glass, the faster the fog forms. That's why the kitchen and bathroom windows go first.
What actually helps
Crack a window to vent moist air, run extractor fans, keep lids on pots, and let air circulate near the glass — closed curtains trap a cold pocket that fogs faster. Persistent fog between double-glazing panes is different: that seal has failed.
cited
Sources
- [01]Window Condensation 101 — Andersen Windows
“Condensation occurs when warm air comes into contact with a cool window pane — as the air cools, it releases its moisture onto the glass.”
- [02]Condensation on Windows Explained — Home Air Quality Lab
“Condensation happens when the temperature of the glass is at or below the dew point of the air in your house.”
people also ask
Keep asking
Why does fog form inside, not outside, in winter?
The moisture source is indoors — cooking, showers, breathing — and the glass is the coldest indoor surface. In humid summers the pattern flips: cooled indoor air chills the outside of the pane.
Is window fog a sign of a bad window?
On the room-side surface it just means humid air met cold glass. Fog trapped BETWEEN the panes of double glazing is the real fault — the sealed unit has leaked and needs replacing.
What's the fastest fix?
Lower the humidity or warm the glass: vent while cooking and showering, run fans, open curtains so room air keeps the pane warmer, and air the house briefly even in winter.
same shelf
