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Window Condensation in Winter: Normal Humidity or a Failed Seal?
Where the water appears tells you whether it's humidity or a broken glass unit.
Wet glass and puddles on the sill are a familiar winter sight in Canada. Sometimes it's harmless and easy to fix. Sometimes it means a window is no longer doing its job.
Where the water shows up tells you which one you're dealing with.
Moisture on the inside surface
When warm, humid indoor air touches cold glass, it condenses. The colder it gets outside, the lower your indoor humidity needs to be to avoid it. Bedrooms with closed doors, bathrooms and kitchens are the usual problem spots.
Cooking, showers, drying laundry indoors, aquariums and lots of plants all raise humidity. So does a humidifier set too high.
How to reduce it
- Use a hygrometer and lower humidity on the coldest days
- Run bathroom fans during and after showers
- Use the range hood when cooking
- Open blinds and curtains so warm air reaches the glass
- Have your HRV or ventilation checked if you have one
Fog between the panes
If you see haze, droplets or a milky film that you can't wipe off from either side, the seal around the insulated glass unit has failed. The gas fill has leaked and moist air has got in.
The window still keeps out weather, but it insulates worse and the view won't clear up. A window contractor can usually measure and order a replacement glass unit for the existing frame.
Frost or water on the frame
Frost on the frame or around the edges, or drafts you can feel, point to worn weatherstripping or air leaks around the window. Those are often fixable with new seals or exterior caulking.
Common questions
Occasional condensation isn't, but water sitting on wooden sills and frames all winter can cause rot and mould.
Defogging services exist, but replacing the sealed glass unit is the more common fix.
Not necessarily. Many issues are fixed with humidity control or new glass units. Old, drafty or rotting frames are when full replacement makes sense.