Updated 2 November 2026 · By Diego Gómez
Condensation and frost on windows in winter: causes and fixes
Why Canadian windows fog up or frost on the inside in winter, and how to fix it with humidity, air flow and ventilation.
When it's −20 °C outside, the inside of a window pane can be well below freezing. Moist indoor air meets that cold glass and leaves water or frost.
What to do
- Lower indoor humidity on very cold days: turn humidifiers down and aim for around 30–35%.
- Run exhaust fans when showering and cooking, and for a while after.
- Let warm air reach the glass: open blinds and curtains slightly and keep vents clear.
- Check your HRV or ventilation system is running if you have one.
- Wipe up water so it doesn't damage sills and frames.
If the problem is the reverse, a damp, musty basement, see basement dehumidifiers.
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FAQs
Does a dehumidifier help with winter window frost?
Sometimes, but in winter the usual cause is too much added humidity (humidifiers, drying laundry, cooking) combined with very cold glass. Reduce humidity sources first.