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Why does condensation occur on exterior surfaces of high performance glazing?

By admin on November 27, 2012 in Info & Tips, Windows with 0 comments

With the increasing use of high performance glazing in homes there have been a number of inquiries regarding formation of condensation on the exterior surfaces of windows.

Condensation will start forming on surfaces when the moisture content in the air reaches the saturation point (100% Relative Humidity). When this occurs, the air contains maximum amount of moisture that it can hold under the present conditions. The amount of moisture air can hold depends mainly on the air temperature. As the air temperature drops, the amount of moisture the air can hold decreases. If the air is already at close to 100% Relative Humidity and the temperature drops, the excess moisture will condense and form liquid water.

If the air is at 100% R.H. at a given temperature, and if it comes in contact with surfaces of lower temperature, the air close to the surfaces will be cooled further and condensation will occur. A common example of this is condensation being present in the grass in the morning. Another example is spandrel panels on buildings, having condensation early in the morning. This usually occurs during the fall and spring on mornings when the R.H. is very high and the temperature is moderate.

Under certain conditions the exterior surface temperatures may be several degrees C. lower than the ambient air temperature. This is the case when ground frost forms during clear conditions, even when the air temperature is +2 to +4 C. the reason that the surfaces can reach temperatures lower than the ambient air is the radiation heat loss to the sky. On clear nights, surfaces such as windows, leaves and grass, will radiate energy to the sky. The sky acts as a surface of very low temperature (may be -10 to -50 degrees C, depending on conditions). Because of this radiation loss, the surface temperature will drop well below the ambient air temperature. If the relative humidity is already at – or near 100%, condensation may form.

In the case of conventional low performing window glass, there is a relatively high heat flow from the interior of the house to the exterior that will keep the exterior glass surface temperature high enough to prevent condensation. With the use of high performance glazing this heat flow is reduced and results in a lower exterior glass surface temperature with much higher risk of condensation forming. THE BETTER THE INSULATING VALUE OF THE GLAZING, THE LOWER THE EXTERIOR GLASS SURFACE TEMPERATURE AND THE HIGHER THE RISK OF CONDENSATION. Spandrel panels are better insulated than window glazing (typically around R 15) and external condensation is relatively common.

External condensation on high performance window glass can be a nuisance. It is, however, quite normal and is an indication that the window has a high insulation value. External condensation on window glass, with the use of more high performing glazing, is becoming more common, but should not be a cause of concern.

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