Condensation on the outside of windows
External window condensation (dew) can occasionally occur on highly insulating, energy-efficient double glazing units in temperate climates.
Such occurrences will normally happen on cloud-free nights with little or no wind, usually when a warm front follows a dry spell.
What causes condensation on the outside of windows?
The combination of several factors, namely external air temperature, localised micro-climate and the thermal efficiency of the glazing itself may all contribute to the formation of external water droplets on window panes.
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 on the pane of glass.
This phenomenon is influenced by the thermal insulation of window glass.
Is condensation on the outside of new windows normal?
Yes - it is. And the presence of it is generally an indication of how energy efficient your windows are.
Single glazed windows offer poor thermal insulation. Therefore heat loss from inside the 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 (and energy efficiency) is improved. However, 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.
Why are high-performance windows affected by external condensation?
Low emissivity glass (Low E) reflects the heat back into the room, and as such, the quantity of heat passing through the glazing to the outside is reduced.
Consequently, the outer pane of low emissivity double glazing is not warmed by escaping heat (which instead is retained within the room - a good thing!) and therefore presents a colder surface to the outside environment.
In such cases, and in situations where the external glass temperature is lower than the 'dew-point' of the air (and when weather conditions are comparable to those mentioned previously), exterior condensation can form on the outer pane surface.
However, the combination of these contributing factors is largely unpredictable, and therefore it is not possible to quantify the number of occasions when external condensation occurs.
What should I do if my windows have external condensation?
External condensation on energy-efficient windows is normal, and you do not need to worry about it or do anything.
Instances of external condensation on windows 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, much in the same way as morning dew.