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PRODUCT DOCUMENTATION
Envirosealed Windows ™
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ADDITIONAL INFORMATION
About internal condensation
The HRV solution
About external condensation
About Low-E Glass

External Condensation

External condensation (dew) can occasionally occur on insulating glass units in temperate climates. Such occurrences will only happen on cloud-free nights when there is little or no wind and usually when a warm front follows a dry spell.

When It Happens
The combination of several factors namely external air temperature, localized microclimate and the thermal transmittance of the glazing itself may all contribute to the formation of external condensation. As a consequence of variable temperatures and localized conditions, it is possible to experience a situation whereby both clear and 'misted' windows exist at the same time in the same development.

Why It Happens
This phenomenon is influenced by the thermal insulation of the glazing. Single glazing offers poor thermal insulation therefore heat escaping from inside a room readily passes through the glass to the outside environment. Consequently, the external surface temperature of single glazing is generally higher than the dew point temperature of the outside air, thus prohibiting the formation of condensation on that surface.

With conventional double glazing the thermal insulation is improved, but sufficient heat still escapes through the glass so as to warm the external surface of the outermost glass, thereby precluding the formation of condensation in most circumstances.

In common with other low emissivity glasses, Pilkington K Glass reflects heat back into the room and as such the quantity of heat passing through the glazing is reduced. Consequently the external pane of low emissivity double glazing is not warmed by escaping heat (which instead is retained within the room) and therefore presents a colder surface to the outside environment.

In such cases, and in situations where the external glass surface temperature is lower than the "dew-point" of the air, (and when weather conditions are comparable to those mentioned previously) condensation can form on the external glass surface.

However, the combination of these contributing factors is largely unpredictable and therefore is not possible to quantify the number of occasions when external condensation will occur.

Instances of external condensation are relatively rare and in all cases it will be a transient effect. Upon any one of the climatological variables changing, the condensation on the glazing will usually dissipate within a short period of time in much the same way as morning dew.

The Bottom Line
In summary, exterior condensation, which forms on the outside pane of the window, typically occurs in the summer. This type of condensation can occur for several reasons: the glass temperature drops below the dew point temperature of the outside air, the air is still, there is a high relative humidity, there is a clear night sky, or there are plants located near your window.

While unsightly, exterior condensation should not concern you since it usually evaporates as the day wears on and will not effect the interior of your home. Since you cannot control the relative humidity outside your home, the only step you can take to combat exterior condensation is to warm the inside surface of the window, as this is a way to warm the outside surface. Seeing exterior condensation on those rare days should be reassurance that your windows are doing their job: keeping your heating and cooling in your home where it belongs and saving you money.

Please also see www.pilkington.com