This means that the perimeter edge seal has lost its integrity and that it is letting in moisture somewhere.

The moisture itself is hard to spot at first, sometimes becoming visible only as the Sun or another heat source 'draws' the moisture up. Typically, the moisture will be drawn in as the atmospheric pressure changes, and it will build up, eventually condensing into water inside the sealed unit.

Although there many reasons why a double-glazed sealed unit may mist up inside when it has 'broken down' because of premature failure of the perimeter seal, the essential cause will lie in how the sealed unit was made, how the framework it is fitted into was made or how the double-glazed sealed unit was fitted into the framework.

On top of this, environmental conditions may play a part, such as wind or sonic loads and possible exposure of the perimeter seal to aggressive chemicals such as those found in washing-up liquid. This may all sound a bit complicated, but the cause of a premature failure of a double-glazed sealed unit can usually be fairly easily traced once it has been removed from its frame.