Condensation on Windows in Winter: Causes and Fixes
Condensation forms when a surface is colder than the dew point of the air touching it. In winter the inner pane is often the coldest surface in the room, so it's where water appears first. EPA calls condensation on windows, walls or pipes a sign of high humidity and says to act quickly: dry the surface and reduce the moisture source[1].
Check your own window
Measure room temperature and RH with a hygrometer and the glass temperature with a contact or IR thermometer, then enter them in the condensation risk calculator. Example: at 20 °C and 60% RH the dew point is about 12.0 °C, so any glass colder than ~12 °C will mist.
Fixes that work (EPA)
Reduce the humidity — vent moisture producers outside, run bath and kitchen fans, use a dehumidifier when needed[1].
Increase ventilation or air movement — open doors/windows when practical, use fans[1].
Increase air temperature[1] — warmer air near the glass raises the glass temperature.
Inside vs. outside condensation
Water on the outside of the glass means the outer pane is below the outdoor dew point — common on well-insulated glazing on clear mornings because the outer pane receives little heat from inside. It is not an indoor-humidity problem. Water between panes of sealed double glazing points to a failed seal.
Persistent condensation leads to mould on frames and reveals; EPA advises drying wet materials within 24–48 hours to prevent mould growth[1].
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