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Dew point and condensation risk in Canadian buildings
Dew point is the temperature at which air becomes fully saturated with moisture. When any surface — a window, an exterior wall, a cold water pipe, or a supply air duct — drops below the dew point of the surrounding air, water vapour condenses on that surface. In Canada, this creates real problems across all seasons: window condensation in winter, pipe sweating in summer, and mould growth in wall cavities year-round where vapour barriers are missing or misplaced.
Indoor RH control is the primary lever for managing condensation risk. In a Toronto home at 21°C indoor temperature with typical double-pane windows (surface temperature around 10°C at -15°C outdoor), condensation starts at about 48% indoor RH. During extreme cold snaps at -30°C on the prairies, even triple-pane windows can fog at indoor RH above 35%. ASHRAE 160 and the National Building Code of Canada both address moisture control in building envelopes, and dew point analysis is central to both.
Rule of thumb: the 1°C per 5% RH approximation
For quick field estimates, dew point drops approximately 1°C for every 5% decrease in relative humidity below 100%. At 20°C and 60% RH: dew point is roughly 20 minus (100 minus 60) divided by 5, which equals 12°C. At 20°C and 30% RH: dew point is roughly 20 minus 70/5 = 6°C. This rule works reasonably well between 50% and 100% RH. For precise calculations — especially near freezing where frost rather than liquid condensation forms — use the full Magnus formula as applied in this calculator.
Ductwork condensation in Canadian summers
Supply air ducts in unconditioned attics and crawlspaces face the opposite problem in summer: cold duct surfaces sweat when warm, humid outdoor air contacts them. A supply duct delivering 55°F air in a 90°F, 65% RH attic will condense moisture on its exterior if the duct surface temperature drops below the outdoor dew point of approximately 77°F. Proper duct insulation with a vapour barrier on the warm side prevents this. Use this calculator to find the outdoor air dew point and compare it to your supply air temperature when diagnosing sweating ductwork. See the psychrometric calculator for full state point analysis of both the duct supply air and the surrounding air.
Frequently Asked Questions
The Magnus formula gives accurate results: dew point = (243.04 × (ln(RH/100) + 17.625×T/(243.04+T))) / (17.625 − (ln(RH/100) + 17.625×T/(243.04+T))), where T is in °C and RH is a percentage. A quick approximation: dew point drops about 1°C per 5% decrease in RH below 100%. So at 22°C and 55% RH, dew point is roughly 22 minus (45/5) = 13°C. For field diagnostics, use this calculator rather than mental math — small errors in dew point lead to incorrect condensation risk assessments. See the full psychrometric calculator for all moist air properties.
It depends on the window surface temperature and indoor air temperature. At 21°C indoor and -15°C outdoor: double-pane windows (surface ~10°C) condense at about 48% RH; triple-pane windows (surface ~15°C) condense at about 70% RH. During -30°C prairie cold snaps, condensation can form on double-pane windows at just 35% indoor RH. Keep indoor RH at or below the threshold for your window type during cold weather. Use the condensation risk calculator for surface-specific thresholds at any indoor and outdoor condition.