Based on our theory and from information obtained from double glazing manufacturers, Double glazed units have a cavity bar or frame between the two panes of glass. This bar has been filled with tiny silicone gel crystals. These crystals absorb most of the moisture that can buiild up inside the double glazed unit but unfortunatly can only absorb so much. Therefore once they have obsorbed as much as they can hold, the window will then condensate!
You will generally find this happening on the windows which are south facing or get plenty of sunlight on them.
One thing we must stress, no double glazed units will last forever. All units will fail in the end!
We can replace your misted double glazed units for a fraction of the cost of having new windows fitted!