Failed double glazed units, more often referred to as misted double glazing, misted up windows, misted windows, fogged double glazing, fogged up windows, fogged windows, steamed double glazing, steamed up windows and steamed windows are caused by a break in the hot melt sealant which is used to form a seal between the two panes of glass and is applied behind the spacer bar of a double glazed sealed unit.
When there is a break in the hot melt sealant it allows external air from the atmosphere which has a moisture content to enter the double glazed unit, some of which is absorbed by the perforated hollow spacer bar which contains moisture absorbing silica pellets.
Over a period of time there is a slow build up of excessive moisture within the double glazed sealed unit, which on warmer days starts to condense (mist) as it cannot escape quickly enough, you will notice on inspection that the mist is more apparent towards the top of the double glazed sealed unit.
As the warm and moist air rises, and the longer a double glazed sealed unit has failed it will increase the likelihood of the internal surfaces of the glass being stained due to the persistent condensing within the unit (condensation in double glazing.) At this point on a warm or cooler day the double glazed unit will always appear misted.