WINDOW CONDENSATION
The Learning Center
It is caused by the same phenomenon that produces dew on grass or on your car if you leave it outside at night. What causes this is
nighttime heat radiance. On nights when the sky is clear, a surface facing the sky sends and receives radiant heat to and from the sky. The
radiant temperature of a clear sky is usually well below zero, particularly at night. Because of this, most objects give up heat to the sky at
night, causing the surface temperature of many objects, including windows, to descend below the temperature of the air outdoors. When
this happens, water vapor in the atmosphere will condense on those surfaces. This occurs when a cool night follows a warmer day, most
typically during the spring and fall seasons.
However, this can also happen during humid days in the summer. Typically it will clear as the day warms. You will also probably notice
condensation more on those surfaces that face west because the sun, which eventually evaporates the condensation, does not shine on
them in the morning. Since it is not harmful, and it happens only under certain atmospheric conditions that occur only a few days during the
year, it is nothing to worry about. The only way of reducing exterior condensation is to increase the temperature of the outer glass surface
above the dew point of the air. To do this, you can try increasing the temperature inside your home or by opening blinds, shades, or drapes,
but you may not be able to eliminate it entirely. Exterior condensation has no detrimental effect on materials that are impervious to
moisture, such as glass and vinyl.
In the summer, houses with air-conditioning can also have condensation form on the exterior window surface when it is cooler than the dew
point temperature of the outside air. On very humid summer days, dew point temperatures (the temperatures at which condensation will
start to form) range from about 65oF to 75oF. When temperatures inside the building are within this range, summer condensation problems
can occur.
5. Why is there moisture on the exterior glass of my new windows?
< back to list of questions about window condensation
3245 Miracle Drive
Murrysville, PA 15668
724-387-2991
PA Contractor ID
PA001856
The Learning Center
It is caused by the same phenomenon that produces dew on grass or on your car if you leave it outside at night. What causes this is
nighttime heat radiance. On nights when the sky is clear, a surface facing the sky sends and receives radiant heat to and from the sky. The
radiant temperature of a clear sky is usually well below zero, particularly at night. Because of this, most objects give up heat to the sky at
night, causing the surface temperature of many objects, including windows, to descend below the temperature of the air outdoors. When
this happens, water vapor in the atmosphere will condense on those surfaces. This occurs when a cool night follows a warmer day, most
typically during the spring and fall seasons.
However, this can also happen during humid days in the summer. Typically it will clear as the day warms. You will also probably notice
condensation more on those surfaces that face west because the sun, which eventually evaporates the condensation, does not shine on
them in the morning. Since it is not harmful, and it happens only under certain atmospheric conditions that occur only a few days during the
year, it is nothing to worry about. The only way of reducing exterior condensation is to increase the temperature of the outer glass surface
above the dew point of the air. To do this, you can try increasing the temperature inside your home or by opening blinds, shades, or drapes,
but you may not be able to eliminate it entirely. Exterior condensation has no detrimental effect on materials that are impervious to
moisture, such as glass and vinyl.
In the summer, houses with air-conditioning can also have condensation form on the exterior window surface when it is cooler than the dew
point temperature of the outside air. On very humid summer days, dew point temperatures (the temperatures at which condensation will
start to form) range from about 65oF to 75oF. When temperatures inside the building are within this range, summer condensation problems
can occur.
5. Why is there moisture on the exterior glass of my new windows?
< back to list of questions about window condensation
3245 Miracle Drive
Murrysville, PA 15668
724-387-2991
PA Contractor ID
PA001856
