External condensation (dew) can occasionally occur on highly insulated glass units in temperate climates. Such occurrences may happen on cloud-free nights when there is little or no wind and usually when a warm front follows a dry spell.
The combination of several factors, namely external air temperature, localised micro climate and thermal transmittance of the glazing itself may contribute to the formation of external condensation. As a consequence of variable temperatures and localised conditions, it is possible to experience a situation whereby both clear and ‘misted’ windows exist at the same time in the same development.
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, Low E reflects heat back into the room and as such the quantity of the 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.