Managing indoor humidity through the seasons
Reading relative humidity
Relative humidity (RH) is the amount of water vapour in the air compared with the most the air could hold at that temperature. Warm air can hold more moisture than cold air, so the same amount of water vapour reads as a higher RH when the air cools. That single fact explains most winter humidity problems.
A basic hygrometer — the kind that costs a few dollars — lets you stop guessing. Place one away from direct sun, vents and exterior doors, and read it over a few days rather than once.
Why windows fog in winter
Window glass is one of the coldest indoor surfaces in a Canadian winter. When humid room air touches that cold surface, it cools below its dew point and the vapour condenses into water — or frost if the glass is cold enough. Persistent condensation is a signal that indoor humidity is too high for the current outdoor temperature.
Condensation is a relationship, not a fixed number
The colder it is outside, the lower indoor humidity has to be to keep windows clear. A level that is comfortable in mild weather can fog the glass during a cold snap.
Sensible seasonal ranges
Comfort, condensation and air quality pull in slightly different directions, so the goal is a workable band rather than a single target. As a general approach:
| Season | Typical approach | Watch for |
|---|---|---|
| Winter | Keep humidity moderate and reduce it during deep cold | Window condensation and frost |
| Shoulder seasons | Let ventilation do most of the work | Stuffiness after closed-up days |
| Summer | Limit incoming moisture; dehumidify damp basements | Musty smells, clammy air |
The levers you actually control
- Source control. Run bathroom and kitchen exhaust during and after showers and cooking, where most indoor moisture is produced.
- Ventilation rate. A balanced recovery ventilator dilutes excess moisture with drier outdoor air in winter. See the HRV and ERV guide for how the two differ.
- Dedicated equipment. A humidifier adds moisture to very dry air; a dehumidifier removes it from damp spaces such as basements. Each addresses a different problem.
- Surface temperature. Improving how warm interior glass and wall surfaces stay reduces where condensation can form.
Health Canada publishes general guidance on indoor air and moisture for homeowners who want an authoritative reference.