External condensation explained.

In temperate climates such as New Zealand, external condensation (dew) can sometimes form on double glazing that has high insulation performance.  Though it may not seem it, this does not indicate a fault in the double glazing, but in fact shows that it is working well.

What is external condensation?

Unlike internal condensation that forms when the temperature inside the home drops so low that the water vapour in the air condenses on the glass, external condensation only happens 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.

External condensation on double glazed window

Single glazing offers poor thermal insulation therefore heat escaping from inside a room readily passes through the glass to the outside environment. This means the temperature on the outside face of your single glazing is generally higher than the ‘dew-point’ temperature (the temperature at which air is cool enough to release moisture) of the outside air, thus preventing condensation from forming on that surface.

Standard float double-glazing may improve the thermal insulation when compared to single glazing, but there is still enough heat escaping through the glass that the outside surface will begin to warm up as it attempts to find equilibrium, thereby precluding the formation of condensation in most circumstances.

Double glazing with Low-E glass.

Low-E, (or low emissivity), glass reflects heat back into the room meaning less heat passes through the glass so the outside face of the double glazing is not warmed (which instead is retained within the room).  This means the external face of the double glazing is cooler and can even be 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.

The combination of these contributing factors is largely unpredictable and therefore it 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 won’t last for long as the conditions change.

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