Will your windows condense? Dew point vs glass.

Short answer

Indoor air at 21 C and 50% relative humidity has a dew point of 10.2 C. A window pane at 10 C is below that, so it will condense; the highest humidity that pane can take at 21 C is about 49%. Lower the humidity, warm the glass, or both.

How it works, with a worked example

Water vapour condenses on any surface colder than the dew point of the air touching it. The tool computes the dew point from the National Weather Service form of the Magnus formula: vapour pressure e = RH x 6.112 x exp[17.67T / (T + 243.5)] hPa, then T_d = 243.5 ln(e/6.112) / (17.67 - ln(e/6.112)). The highest humidity before condensation is the humidity at which the dew point equals the surface temperature: 100 x e_s(surface) / e_s(air).

Worked example. Air at 21 C and 50% has e = 0.50 x 24.9 = 12.4 hPa and a dew point of 10.2 C. A glass surface at 10 C is 0.2 degrees below it, so it condenses. Saturation pressure at 10 C divided by that at 21 C gives 49%, the highest humidity that pane can tolerate. At 40% RH the dew point drops to about 6.9 C and the same 10 C pane stays dry.

Condensation on windows, walls or pipes is the moisture-control signal EPA mentions. Dry the wet surface, lower the humidity or the humidifier setting, and EPA also suggests venting moisture-producing appliances such as dryers and stoves to the outside where possible. Hidden condensation inside walls or attics will not show here.