Condensation will form on any surface as soon as the surface temperature falls below the dew point of the air. External condensation will only occur on cloud free nights when there is little or no wind and usually when a warm front follows a cold dry spell.
As gardeners know, the air temperature in their garden can vary on any day or night from one part of the garden to another. A hedge, a shrub, an open flowerbed or a projecting wall or garage can all affect the air temperature in their close proximity.
It is a combination of weather conditions and local microclimate, which can contribute to the formation of external condensation. On occasions and as a result of the local conditions it is possible to see clear and condensed windows in the same home.
However, as would be expected the formation of external condensation on glazing is also affected by the thermal insulation performance of the glass.
The thermal insulation provided by single glazing is very poor and heat from the home passes through the glass and escapes to the outside world.
As a result of this the external surface of a single glazed window is warmer than the dew point of the outside air and this prohibits the formation of condensation on the glass.
With ordinary double-glazing the level of insulation is improved. However, sufficient heat still escapes through the glass to warm the external pane sufficiently to prohibit the formation of condensation.
Low emissivity glass works differently to ordinary glass. The low emissivity coating reflects heat back into the room and as a result the amount of heat passing through the glazing is greatly reduced.
The external pane of a double glazing unit containing low emissivity glass is not warmed up by escaping heat and therefore presents a colder surface to the outside environment.
When the glass surface temperature is lower than the dew point of air and conditions are comparable to those mentioned earlier, condensation could form on the external glass surface.
Unfortunately, it is not possible to quantify the number of occasions when external condensation will occur because nobody can predict the coincidence of still air and clear night skies. However, it will be a relatively rare and transient occurrence.