Causes:

Condensation forms on the outdoor surface of glass when its temperature drops below the outdoor dew point temperature. 

Insulating units containing an energy efficient, low-emissivity glass such as SGG PLANITHERM TOTAL, have enhanced thermal insulation properties thanks to a high performance transparent coating that reflects heat back into the room. As a result the outer pane of glass does not get warmed by heat escaping from inside the building through the glass and remains cooler in comparison to less thermally efficient windows.

External condensation only occurs in certain climatic conditions with high humidity levels and/or particularly cold weather. It is possible that, external condensation will appear on some windows but not on others. This is due to localised atmospheric conditions such as shelter from nearby trees or buildings, variable air currents and wind speeds and varying levels of nearby vegetation.