Condensation on Window Glass Exterior
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Condensation may also form on the outside of your windows..?


Condensation On Exterior Surfaces Of Sealed Unit

The sealed units used in windows for homes have become better over time. They are more efficient. In recent years there have been many inquires about condensation forming on the outside surface of the glass on these windows.

Once the moisture content in the air reaches the saturation point (100% relative humidity) condensation will start to develop on surfaces. This means that under the current conditions, the air is holding the maximum amount of moisture that it can. Temperature is the main factor that determines how much moisture air can hold. When the temperature of air drops, the amount of moisture it can hold decreases as well. If the air is already at or near 100% R.H., and then the temperature drops, the excess moisture will condense which will result in the forming of liquid water.

If air is already at 100% R.H., and it comes into contact with a surface that is a lower temperature, condensation will form. This is because the air temperature near the surface will be cooled forcing it to release some of its moisture. Dew on the grass in the morning is a good illustration of this process. A spandrel section on buildings is another example of where condensation will form in the mornings. The fall and spring, when the humidity is high with moderate temperatures, are typical times when this will happen.

At times the outside surface temps might be several degrees C less than the ambient air temperature inside the home. This can happen when ground frost forms as a result of clear conditions, even though the outside air temperatures are + 2 to + 4 degrees C. Surfaces can achieve temps less than the ambient air because of the radiation warmth loss towards the sky. Surfaces like home windows, grass and leaves will radiate energy towards the sky on a clear night. The sky functions like a surface with a really low temperature (might be – 10 to – 50 degrees C). Due to this radiation loss, the surface temperature can drop well underneath the ambient air temperature when the relative humidity is near 100%, and condensation may form.

If the windows have low performing glass, there is flow of heat from inside the house to the outside. This will keep the exterior piece of glass warm enough to prevent condensation. However, when using a high performance sealed unit in the window, this transfer of heat does not happen as readily. This means the outside pane of glass has a much cooler surface temperature and a higher risk of condensation forming. Thus THE BETTER THE PERFORMANCE OF THE WINDOW, THE LOWER THE TEMPERATURE OF THE EXTERIOR GLASS, THUS AN INCREASED LIKELYHOOD OF CONDENSATION. Spandrel panels commonly have condensation because they are even better insulated that sealed units (somewhere around R15).

Exterior condensation on high end window glass may be a nuisance. It’s however really common and it is a sign the window carries a high insulation value. Exterior condensation on window glass, because of a higher performing double glazed unit, has become more prevalent, but should not be any reason for concern.

Another kind of exterior condensation that is very common is in tropical environments high in humidity. Once the interior is cooled with air-conditioning, the high temperature from the exterior surfaces from the glass may also be decreased. When the surface temperatures are decreased sufficiently through the air-conditioning to achieve the dew-point, condensation will form on the outside of surfaces of the glass. As pointed out, this really is normal with double glazed units, but also occurs with sealed IG models when the humidity is sufficiently high. The lower the u-value of the double glazed, the lower the chance of condensation. This kind of condensation can happen throughout all hrs during the day and evening. When the double glazed is subjected to sunshine, the glass temperature increases and condensation won’t normally occur.

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