| Parameter | Value | Note |
|---|
Maximum Safe Indoor RH by Window Type and Outdoor Temperature
| Outdoor Temp | Single Pane (U=5.7) | Double Pane (U=2.8) | Triple Pane Low-E (U=1.2) |
|---|
Condensation risk in Canadian buildings: the dew point comparison
Condensation occurs whenever a surface's temperature drops below the dew point temperature of the surrounding air. It's a simple comparison: calculate indoor dew point from indoor temperature and RH, then compare it to the coldest surface in the space. If the surface is colder than the dew point, water vapour condenses onto it. This is the single most useful diagnostic for predicting window condensation, wall moisture problems, and mould risk in Canadian winter conditions.
Window surface temperature depends heavily on glazing technology. A single-pane window can have an interior surface temperature within a few degrees of outdoor temperature on a cold Canadian winter day. A modern triple-pane, low-E, argon-filled window maintains an interior surface temperature much closer to room temperature, dramatically reducing condensation risk even at the same indoor humidity level.
Calculating surface temperature from U-value
If you don't have a direct surface temperature measurement, you can estimate it from the window or wall U-value using the temperature ratio method: T_surface = T_indoor − (U-value × R_interior_film) × (T_indoor − T_outdoor), where R_interior_film is approximately 0.13 m²K/W (still air film resistance) for vertical surfaces. This calculator performs this estimate automatically when you select the U-value input method, letting you assess condensation risk for proposed window specifications before installation.
Safe indoor humidity targets for Canadian winters
The maximum safe indoor RH depends on your coldest surface and your local outdoor design temperature. As a general guide for homes with double-pane windows: keep indoor RH below 30% when outdoor temperature drops below -20°C, below 40% between -10°C and -20°C, and below 50% above -10°C. Homes with high-performance triple-pane windows can typically support meaningfully higher indoor RH without condensation risk, since their surface temperature stays much closer to room temperature even in extreme cold.
Beyond visible window condensation, sustained surface moisture above 70% RH at the surface promotes mould growth even without visible liquid water. Use the relative humidity calculator to verify your target indoor RH, and the dew point calculator to double-check dew point against any cold surface in your building.
Frequently Asked Questions
Condensation occurs when a surface temperature drops below the dew point of the surrounding air. Calculate indoor dew point from indoor temperature and RH, then compare to your window's interior surface temperature. At 40% RH and 21°C, dew point is about 7°C. Any window surface colder than 7°C will condense. Single-pane windows are far more prone to this than triple-pane low-E windows. Use the dew point calculator to find your exact dew point, then compare to a measured or rated surface temperature.
It depends on your coldest surface temperature and local outdoor design temperature. For homes with double-pane windows: keep indoor RH below 30% when outdoor temp is below -20°C, below 40% between -10°C and -20°C, and below 50% above -10°C. Homes with triple-pane low-E windows can support higher RH safely. Always verify with this calculator using your actual window surface temperature or U-value rather than relying on generic guidelines alone, since performance varies significantly by window type and age.