Condensation on Windows in Winter: Causes and Fixes

Water droplets or frost on the inside of your windows in winter mean the indoor air holds more moisture than the cold glass can handle. When warm, humid air touches glass that is below its dew point, the moisture condenses. A little fog on the coldest mornings is normal in Canada, but condensation that sits on the glass every day will peel paint, rot wooden sills and frames, and let mould grow.

The usual cause is high indoor humidity from cooking, showers, drying laundry indoors, lots of houseplants, or a furnace humidifier set too high. Newer, more airtight homes hold on to that moisture unless they are ventilated. The other half of the problem is cold glass: older single-pane windows, double-pane units whose seals have failed, and heavy curtains or blinds that trap cold air against the glass all make condensation worse.

As it gets colder outside, indoor humidity needs to come down to keep the glass dry. As a general guide, aim for about 35% indoor relative humidity when it is -10°C outside, about 30% at -20°C, and 25% or lower when it drops below -25°C. Windows with better glass can handle a little more. A digital hygrometer (CAD $10–$25) shows your indoor humidity; place it in a main living area, not in the kitchen or bathroom.

1. Measure indoor humidity with a hygrometer for a few days to see where you are.
2. Run the bathroom fan during every shower and for 20 to 30 minutes afterwards.
3. Use a range hood that vents outdoors whenever you cook, especially when boiling water.
4. Turn the furnace humidifier down, or off, as outdoor temperatures drop. Many humidifiers have a dial marked with outdoor temperatures for this.
5. Dry laundry in a dryer that vents outdoors rather than on racks inside.
6. Open curtains and blinds during the day, leave a gap at the bottom at night, and keep heating vents under windows uncovered so warm air reaches the glass.
7. If your home has an HRV (heat recovery ventilator), run it and keep its filters and core clean.
8. Wipe condensation off the glass and sills each morning. Clean any small black spots with a mild detergent solution and dry the area well.

If moisture or fog appears between the two panes of a double- or triple-pane window, the seal on the glass unit has failed. Lowering indoor humidity will not fix it. A glazier or window company can usually replace just the sealed glass unit without replacing the whole window frame.

Call a professional if mould is spreading beyond a few small spots, if window sills or frames feel soft or rotted, or if condensation continues even when indoor humidity is in the recommended range. That usually points to a ventilation problem; an HVAC contractor can check your fans and HRV, or recommend adding one. A window specialist can replace failed sealed units or upgrade windows that are too cold.