A commercial dehumidifier. Residential units operate on the same refrigerant-cycle principle at smaller capacity. Image: Wikimedia Commons / CC BY-SA.
The Physics of Relative Humidity
Relative humidity (RH) is the ratio of water vapour actually present in air to the maximum amount that air could hold at the same temperature. Warm air can hold more water vapour than cold air. This is why bringing very cold outdoor air (low absolute moisture content) into a warm home produces very low relative humidity — the air's capacity for moisture increases with temperature, but the amount of vapour doesn't change unless it comes from within the home.
The dew point is a more stable indicator of actual moisture content than relative humidity. If indoor air at 20°C has a dew point of 7°C, that air will condense water on any surface cooler than 7°C — such as the inner face of a cold window frame, an exterior wall cavity with insufficient insulation, or a concrete floor slab in contact with grade.
Health Canada recommends maintaining indoor relative humidity between 30% and 55%. The National Building Science Digest and ASHRAE Standard 55 use a slightly narrower comfort range of 30–60%, with 40–50% generally cited as the most comfortable for occupants.
Winter Humidity: The Dry Air Problem
In Prairie provinces and northern regions, outdoor January temperatures regularly reach −20°C to −30°C. At −20°C, even 80% relative humidity outdoor air contains very little absolute moisture — roughly 0.6 grams per kilogram of dry air. When that air enters a home and is heated to 20°C, the relative humidity drops to under 5% if no moisture is added internally.
Normal household activities — cooking, bathing, breathing, and houseplants — add moisture continuously. In an occupied home, this may bring indoor RH to 15–25% without additional humidification. Whether this is adequate depends on occupants' respiratory sensitivity, the type of flooring and furniture, and musical instruments (solid wood instruments can crack below 40% RH).
Central Humidifiers
Central bypass or fan-powered humidifiers install on the furnace plenum and introduce moisture into the forced-air supply stream. They use either a water panel (evaporative drum) or steam generation. Bypass humidifiers are simpler and consume no electricity for the humidification element itself, but require the furnace blower to be running. Fan-powered units operate independently of the furnace and generally humidify more quickly.
The primary maintenance task is replacing the evaporator pad annually, as mineral scale accumulates rapidly. Humidifiers connected to municipal water with high mineral content may need more frequent servicing. An automatic humidistat should be adjusted downward as outdoor temperatures drop — running 45% RH indoors when it's −25°C outside will almost certainly cause condensation on windows and within wall assemblies.
Outdoor-Temperature-Adjusted Targets (Prairie Climate)
- Outdoor −20°C or colder: indoor RH no higher than 25–30%
- Outdoor −10°C to −20°C: indoor RH 30–35%
- Outdoor 0°C to −10°C: indoor RH 35–40%
- Outdoor above 0°C: indoor RH 40–50%
These ranges apply to standard double-pane windows. Homes with triple-pane glazing or interior storm windows can tolerate slightly higher indoor humidity before condensation forms on the glass.
Summer Humidity: The Excess Moisture Problem
Summertime indoor humidity issues are most common in basements, which remain cool year-round. Warm, humid summer air that enters a basement through windows, slab cracks, or foundation walls contacts cold surfaces and can condense. Basements in Ontario and BC coastal areas can sustain mould growth throughout summer months if humidity is not controlled.
Dedicated Outdoor Air System (DOAS) diagram illustrating how ventilation air is conditioned before distribution. Humidity management is integral to modern ventilation design. Image: Wikimedia Commons / CC BY-SA.
Dehumidifiers
Refrigerant-cycle dehumidifiers draw room air over a cold evaporator coil, where water condenses and drains into a reservoir or continuous drain. The dried air passes over the condenser and returns to the room slightly warmer than it entered. This is an important point: dehumidifiers add a small amount of sensible heat to the space, which is a consideration in warm rooms.
Sizing is expressed in litres per day at specific test conditions (typically 27°C / 60% RH under older AHAM standards, or 26.7°C / 60% RH under the newer DOE test method). A unit rated at 30 litres per day will remove less moisture in a cool basement (18°C) than in a warm room, because colder air holds less moisture and the refrigerant cycle is less efficient.
Basement-Specific Considerations
- Close basement windows during humid summer days — outdoor summer air in Ontario can reach 80–90% RH in July, which will raise, not lower, basement humidity when introduced directly
- A dehumidifier target of 50–55% RH in summer is generally sufficient to prevent mould growth on most materials
- Concrete block or poured concrete basement walls will wick moisture from the soil side throughout summer — this is structural moisture and a dehumidifier cannot address it without also addressing the source
- Carpet in basements is generally not recommended in Canadian climates where summer ground contact humidity is significant
Mould Thresholds
Mould growth on organic surfaces (wood framing, drywall paper facing, insulation facing) typically begins when surface relative humidity exceeds 80% for sustained periods. The critical variable is surface RH, not air RH — a wall surface can be at 90% RH while the air a few centimetres away reads 55%, if the surface is cold enough to cause local condensation.
Health Canada's guidance on mould cites the importance of addressing moisture sources rather than simply cleaning visible mould. Recurrence after cleaning indicates the moisture condition that enabled growth has not been corrected.
Measuring Humidity Accurately
Consumer hygrometers vary considerably in accuracy. Capacitive-sensor units (which use an electrically conductive polymer whose capacitance changes with moisture) are generally more accurate than older hair-tension or bimetallic designs, but inexpensive units can still read 5–10% high or low. Calibration against a saturated salt solution (sodium chloride solution equilibrates to approximately 75% RH at room temperature) provides a reference point.
Placement matters: hygrometers near exterior walls, windows, or HVAC supply registers will show readings that don't represent average room conditions. The centre of a room at breathing height (1–1.5 m) is the most representative location for ongoing monitoring.