Condensation on the Exterior of Window or Patio Door Glass
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Condensation (moisture) can form on the exterior glass surface of a window or patio door. This collection of moisture is most likely to occur during the spring and fall or when cool nights follow a warm day.
Exterior condensation indicates that the glass in the unit has created an effective barrier between the air circulating within the home and the more humid air on the outside. This is much like a closed bathroom door creates a humidity barrier during a hot shower, which causes the mirror to fog. Exterior condensation may also appear as a haze on the glass.
Exterior Condensation is the Result of Two (2) Key Factors:
Higher humidity levels.
The temperature of the exterior glass surface falls below the dew point.
Understanding Condensation
The condensation seen on the exterior glass surfaces is similar to the dew that can form on car windshields or on the grass some mornings.
Prior to the formation of exterior condensation, you may observe one (1) or more of the following conditions:
An interior temperature that is cooler than the exterior temperature (air conditioning)
A high relative exterior humidity level
Clear night sky
Warm day followed by a cool night
Still air
Vents near windows and/or patio doors
Plants near windows and/or patio doors
The hazy appearance on the glass occurs only at certain times of the day
Windows and patio doors with energy-efficient Low-E4® insulating glass restrict the flow of heat across the space between the panes of glass. Due to this attribute, the exterior glass surface may be a few degrees cooler than the interior glass surface. In fact, the glass temperature may be lower than the adjacent air temperature. While unsightly, exterior condensation usually evaporates throughout the day and will not affect the interior of the home.
A good analogy is when having an iced drink on a warm summer day, the glass has moisture or haze on the outside of it. The warmer air meeting the cooler surface of the glass causes condensation to form.
It may be that only one sash in a bank of windows will have condensation on the pane. This is because the temperature on that glass pane is slightly different from that of other windows around it which could be caused by shadows or some other factor.
