Due to the enhanced thermal properties of a double-glazed unit comprising a low-E glass, the outer pane of glass does not get warmed by heat escaping from inside the building through the glass. This results in the outer pane being kept cooler in comparison to a less thermally efficient insulating unit. Certain climatic conditions or high humidity levels, without rain can lead to the formation of condensation on the external surface of the outer pane. It is also possible that due to shelter from nearby trees or buildings, external condensation will appear on some windows but not others. This phenomenon is due to localised atmospheric conditions and is in no way an indication of a defective unit. Indeed this can be seen as a positive indication of performance.