Room-side droplets: humidity meets a cold surface
Condensation forms when a surface is colder than the dew point of the air touching it. Indoors, that depends on both the moisture in the air and the window’s surface temperature. Cooking, bathing and drying clothes add moisture; cold glass, restricted room-air circulation and local air leaks can contribute to the conditions at the window.
Wipe up standing water and note when it returns. Compare several rooms, use a humidity meter and check whether curtains or furniture restrict warm air reaching the glass. Persistent moisture around one frame, especially after rain, also needs investigation for leakage rather than being labelled ordinary condensation.
Outdoor dew: consider the weather
Exterior glass can cool below the outdoor dew point, particularly on clear nights. Dew may then be visible in the morning and evaporate later. Well-insulated glazing can allow the outside pane to remain cool because less indoor heat reaches it.
That observation does not certify a window’s energy rating, and occasional outdoor dew alone is not a reason to replace the window. Confirm the moisture really is on the outer surface without leaning out or using unsafe access. Record unusual staining or water entering the building separately.
Between panes: investigate the sealed glass
Moisture within a factory-sealed IGU points to a seal problem and warrants a manufacturer or glass-service assessment. The haze may be intermittent. A separate storm-window cavity is a different assembly, so identify what you have before ordering parts.
Ask whether a compatible glass unit or sash can be replaced while retaining a sound frame. If indoor condensation is the issue, review moisture sources and ventilation first; if the frame or wall is wet, address the water path as well. Replacing glass will not resolve every cause of a wet sill.
Technical references
- Andersen: Exterior glass condensation — Exterior dew forms when glass drops below outdoor dew point.
- Andersen: Window condensation explained — Differentiate room-side, outdoor and between-pane moisture.
- NRCan: Upgrading windows and exterior doors — Condensation control and repair assessment.
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