Moisture
Managing window condensation and winter frost
A film of fog on the inside of a window during a cold snap is one of the most common things Canadian households notice each winter. It is rarely a problem with the window itself — more often it is the glass reporting on the air in the room.
Why it happens
Air holds a limited amount of water vapour, and that limit drops sharply as temperature falls. When warm, humid indoor air meets the cold surface of a windowpane, the air right at the glass cools below the point where it can hold all its moisture, and the excess condenses into droplets. The colder the glass and the more humid the room, the more readily it forms.
This is why condensation tends to appear first on the coldest windows — older single-glazed units, or panes in rooms with little air movement — and why it is worst on the most frigid mornings.
What it tells you
Occasional, light condensation that clears as the day warms is normal in cold weather. Persistent heavy condensation, or frost building on the interior glass, usually points to indoor humidity that is high for the outdoor temperature.
General guidance from Canadian public-health and energy sources is to lower indoor relative humidity in winter as outdoor temperatures drop, because the colder the outside air, the more readily moisture condenses on cold surfaces inside.
Where the moisture comes from
- Cooking, showering, and dishwashing release large amounts of vapour in short bursts.
- Drying laundry indoors adds moisture steadily over hours.
- A tightly sealed, well-insulated home traps that moisture rather than leaking it outdoors.
Measured steps to reduce it
- Ventilate the wet rooms. Run kitchen and bathroom exhaust fans during and after cooking or bathing to move humid air outside at the source.
- Let air reach the glass. Opening blinds during the day and keeping furniture slightly off cold exterior walls helps air circulate across the pane.
- Watch humidifier settings. If a humidifier runs in winter, lowering it during the coldest stretches often clears persistent fogging.
- Wipe standing water. Drying the sill prevents pooled condensation from sitting against frames and finishes.
Condensation between the panes
Fog trapped between two layers of a sealed glazing unit is different. It indicates the seal has failed and the insulating gas has been replaced by humid air, and it cannot be wiped away from either side. That is a window-unit issue rather than a humidity-management one, and it is the point at which a qualified installer is the right contact.