Why Do Exterior Home Windows Fog Up?

Written by laura reynolds
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Why Do Exterior Home Windows Fog Up?
Temperature, dewpoint and moisture are the keys to most foggy windows. (window image by Marek Kosmal from Fotolia.com)

Windows fog for the same reason that cold glass of lemonade frosts in the summer time: condensation forms on the cool glass surface. Exterior window condensation stems from one of several environmental or climatological causes.

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That fog on your window is actually gaseous water. As it hits the cooler surface of the window, it condenses like clouds in a cooling atmosphere. Condensation on exterior windows can be an occasional nuisance, or evidence of a more serious problem, such as mould.

Dew Points

Warm air holds water as vapour. As air cools, the vapour condenses until it forms liquid water. The dew point is the temperature at which water vapour condenses into liquid water.

Seasonal Condensation

When air on one side of a piece of glass is warmer than the other side, condensation may occur where moisture meets cool glass or cool air. Fog and frost form on the inside of windows when winter temperatures drop suddenly. Condensation also forms when outside temperatures heat the glass, and interior air is cooled below the dew point.


Moisture condenses on the cold side of exterior windows when trapped in tightly sealed buildings, often leading to formation of mould. Excess moisture also condenses on metal window frames.

Insulated Windows

Condensation appears inside sealed thermopane windows when the protective seal fails and the vacuum is compromised. Moisture interacts with the gas between the panes to form a milky fog.

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