Why Do Exterior Home Windows Fog Up?

Temperature, dewpoint and moisture are the keys to most foggy windows.

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.

Identification

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 mold.

Dew Points

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

Considerations

Moisture condenses on the cold side of exterior windows when trapped in tightly sealed buildings, often leading to formation of mold. 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.

References

• Ask the Builder: Condensation on New Windows
• Vanguard Windows: Wiondows and Water: Condensation Facts