Condensation on the external surfaces of a double-glazing unit can form in a wide variety of circumstances and on either the inside or the outside surface of a building. Where it occurs largely depends on the internal humidity and external temperature levels.
This can occur in certain climatic conditions, particularly cold weather. This is a positive indication that the enhanced thermally insulated units (incorporation Low e glass) are working correctly and reducing heat loss through the windows.
The Low E glass (inner pane) is reflection the heat back inside the room, therefore the external pane is likely to be colder; this is where condensation could form.
Examples of external condensation are rare but occasionally it can manifest itself at certain times of the year. Evidence has shown it is temporary and tends to disappear once the external temperature rises.
Given that the thermal performance of the glass is not affected, and the sealed unit guarantee remains, there is no reason for the glass to be rejected or replaced.
If condensation is evident on the inside surface (room side) then it is likely there is too much humidity and more ventilation may be required.
If there is condensation between the two panes of glass then it is likely the sealed units have failed and must be replaced.