Waking up to water droplets on your windows is annoying — and concerning. Is something wrong with your windows? Are they failing? Not necessarily. Understanding where condensation comes from helps you solve it.
Let's sort out interior condensation, exterior condensation, and between-the-glass condensation — they're different problems with different solutions.
Interior Condensation (Most Common)
Water on the inside surface of your glass:
Why it happens: - Warm, humid indoor air meets cold glass surface - Water vapor condenses into liquid droplets - Same reason a cold drink "sweats" in summer
This is NOT window failure: - Your windows might be working fine - It's a humidity and/or temperature issue
When it's most common: - New homes or recently renovated (materials release moisture) - Homes with poor ventilation - When outdoor temperature drops suddenly - After cooking, showering, or doing laundry - In bedrooms overnight (breathing releases moisture)
The physics: Condensation forms when glass surface temperature drops below the dew point of indoor air. Colder glass + more humid air = more condensation.
Solving Interior Condensation
Reduce humidity and/or warm the glass:
Reduce indoor humidity: - Use exhaust fans when cooking and showering (run 15+ min after) - Vent dryers outdoors (never indoors) - Don't hang-dry laundry inside - Use a dehumidifier in winter - Open windows briefly for fresh air exchange - Check for water leaks or damp basement issues
Target humidity levels for Ottawa: - When it's -20°C outside: 15-20% indoor humidity - When it's -10°C outside: 25-30% indoor humidity - When it's 0°C outside: 35-40% indoor humidity
Warm the glass surface: - Improve window insulation (triple-pane stays warmer inside) - Open blinds/curtains (lets room heat reach glass) - Direct heat register toward windows - Consider interior storm windows for single-pane
Circulation helps: Moving air prevents moisture from settling. Ceiling fans or small fans near problem windows help.
Exterior Condensation
Water on the OUTSIDE of your windows:
Why it happens: - On cool, humid mornings with clear skies - The glass cooled overnight by radiation to the sky - Humid outdoor air condenses on the cold glass
This is actually GOOD news: - It means your windows are well-insulated - The glass isn't being warmed by heat escaping from inside - More common with efficient triple-pane windows
It goes away: - Once sun warms the glass - When outdoor air warms up - Typically gone by mid-morning
No action needed: Exterior condensation is normal and not a problem.
Condensation Between the Panes
Fog or moisture BETWEEN the glass layers:
What it means: - The seal around the insulated glass unit has failed - Humid air has infiltrated the space between panes - You can't wipe it away from inside or outside
This IS a problem: - The insulating gas (argon) has escaped - Window efficiency is reduced - Will likely get progressively worse - Indicates window is approaching end of life
Solutions: - Glass-only replacement if frame is good - Full window replacement if windows are old - Defogging services exist but are temporary - Check warranty — may be covered
More detail: See our article on foggy windows and seal failure.
When Condensation Indicates a Problem
Watch for these warning signs:
Condensation that pools and causes damage: - Water running down and pooling on sill - Mold or mildew around window - Paint peeling or wood rotting - This indicates either excessive humidity or poor window performance
Condensation on NEW windows: - Some is normal initially (house adjusting) - If excessive, windows may not be performing as expected - Could indicate installation issues (poor air sealing)
Frost forming on interior glass: - Single-pane or very poor double-pane - Indicates window upgrade would help significantly
Condensation in only one room: - Check that room's humidity sources - Verify exhaust fans work - May indicate window in that room is underperforming
The Bottom Line
Interior condensation usually means high indoor humidity, not window failure. Exterior condensation on efficient windows is normal. Condensation between the panes indicates seal failure and requires glass or window replacement.
Unsure what's causing your condensation? Get a free assessment and we'll diagnose the issue and recommend appropriate solutions.
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