Double-glazing provides a layer of still air that improves the insulation value of what would otherwise be highly conductive glass. Condensation on the outdoor surface of double glazed windows is in no way a result of a defective unit. In fact, this is a sign that the enhanced thermally insulating double glazed window is actively reducing the heat loss through the glass.

When a unit is defective, the condensation will form on the inner face of the double glazed window. This means the air is no longer completely sealed. In this case, the double glazed window will need to be replaced.