Sealed double - glazed units are designed to last for approximately 20 years. Water vapour passing through the edge seals will eventually saturate the desiccant material contained in the perforated alloy or plastic spacer bars around the perimeter, resulting in internal misting. Properly installed windows in sheltered conditions might last longer than 20 years, but windows in exposed positions, or strong sun light can fail sooner.
We say that the double- glazed sealed unit has 'broken down' or 'failed' , when the perimeter edge seal has lost its integrity and is letting in moisture.
This moisture is hard to spot at first, but over time, and as the temperature rises, it becomes more visible as misting/fogging or in more extreme cases, water in between the panes.
We are often asked if it is possible to split open, clean and reseal the unit. This is not commercially viable. The only recourse is to replace the unit with a new one. This is done with minimal disruption, with a factory guaranteed unit.