Protecting Windows in Winter
Cold weather puts windows under a particular kind of stress: warm humid air on the inside meets very cold glass, and moisture has to go somewhere. Understanding where it goes explains most winter window problems.
Condensation, explained simply
Air holds a certain amount of water vapour, and warmer air holds more. When indoor air touches a cold surface, it cools, and the water it can no longer hold settles out as droplets. On a single-pane window in January, the inner surface can be cold enough for this to happen across the whole pane. The result is the familiar fogging, and at lower temperatures, frost.
Condensation responds to two things you can influence indoors: how much moisture the air carries, and how cold the glass surface is. Lowering indoor humidity and raising the inner glass temperature both reduce it.
What glazing layers actually do
The number of panes changes how cold the inner surface becomes. A sealed gap between panes slows heat loss, so the surface you touch stays warmer and is less likely to reach the point where moisture condenses.
| Assembly | Inner-surface behaviour | Common context |
|---|---|---|
| Single pane | Inner surface near outdoor temperature | Older homes, unheated spaces |
| Double pane | Warmer inner surface, sealed gap | Most current Canadian housing |
| Triple pane | Warmest inner surface, two gaps | Cold-climate and high-efficiency builds |
Reducing indoor humidity
- Run kitchen and bathroom exhaust fans during and after moisture-producing activities.
- Avoid drying laundry indoors without ventilation.
- Keep furniture and heavy curtains from trapping cold air against the glass, which keeps the surface colder.
- Open blinds during the day so warm room air can reach the pane.
Frost, ice, and the frame
Persistent frost on the inner pane is a signal that either humidity is high or the glass is unusually cold, often around the edges where the seal sits. Water that repeatedly forms and refreezes can work into frames and sills over time, so wiping down condensation and improving airflow protects more than the glass itself.
Air sealing around the opening
Drafts are separate from condensation but related to comfort. Weatherstripping that has gone hard or compressed lets cold air bypass the glazing entirely. Checking the seal where the sash meets the frame, and confirming that interior trim is not hiding gaps, is a low-cost seasonal task.
For an overview of the broader subject, see the general reference on condensation, and Natural Resources Canada's guidance on energy-efficient windows.