Thermal Pane Insulated Glass Unit
Thermal Pane Insulated Glass Units (IGUs) consist of two panes of glass separated by a spacer band and are vacuum sealed with some kind of gas; usually dried air.
Thermal Panes insulate the property by conducting less heat than single pane glass and can improve insulation with gas fillings like Argon and/or a Low-E coating.
When the vacuum seal fails, the glass unit loses insulation and begins to develop condensation between the panes of glass. This leads to the fog and bacterial growths you can see inside a failed unit. Long broken seals will decay and start to delaminate; the panes of glass separate from the spacer band.
Some suggest using a window defogging service to fix a failed seal, but the process is flawed. Defoggers will drill two small holes in one pane, insert a chemical wash and place gaskets in the holes to vacuum out the condensation. The problem is, the seal is broken around the perimeter of the glass. Any condensation that has been removed will return as the moist air flows in from the broken seal. The only way to fix a broken seal is to replace the glass unit.