Misted double glazing is a sign that the double glazing unit has failed or blown. The most common cause of this is holes around the seal which allow moisture to get in between the two panes of glass. This moisture cannot escape so will just build up as more water enters the gap between the panes.
The only real solution to this problem is to replace the failed double glazed unit.
This is less expensive than you think and the lead time to replace the pane is significantly less than ordering a whole new window frame and glazing; this simply isn’t necessary. Contact us to discuss and we may even be able to give you an estimate over the phone.