Misty double glazing: what's wrong and what it costs
A misted double glazing unit has a failed seal: the moisture is inside the unit and can't be cleaned out. The durable fix is replacing the sealed unit in the existing frame for £80 to £250, under an hour per window, no new frames needed.
What's actually happened
A sealed unit is two panes bonded to a spacer bar, with desiccant inside the spacer and an inert gas (or dry air) in the gap. The perimeter seal keeps the weather out for typically fifteen to twenty-five years. When it fails, moist air cycles in and out with temperature changes, the desiccant saturates, and condensation fogs between the panes. The mist you see is the symptom; the failed seal is the disease, and it doesn't heal.
Why the cheap fixes don't work
Drill-and-dry 'repairs' (holes, valves, drying agents) can clear the fog temporarily. They don't restore the seal or the gas fill, so the unit re-mists, typically within one to two years, and now has holes in it. You will have paid half the cost of the real fix for a delay. This is one of the few cases in home maintenance where the proper job is also the cheap job, done once.
The real fix, properly done
Unit replacement is undramatic: the glazing beads come off, the failed unit comes out, the new made-to-measure unit goes in, beads back. Twenty minutes to an hour per window. The frame is untouched, the room is undisturbed, and the new unit carries its own ten-plus-year seal warranty. The survey measures thickness as well as size (24mm and 28mm don't swap) and reads the glass spec from the spacer stamp, so the new unit matches the rest of the house.
What it costs in Teesside
£80 to £150 for standard sizes, £150 to £250 for large panes or specialist glass, less per unit when several are done in one visit. Teesside's 1990s and 2000s glazing is at peak seal-failure age, so multi-unit jobs are the norm: a typical three-to-five-unit house runs £300 to £600 all-in. Compare that with the £6,000-plus replacement quote the window company offered for the same fog.
Preventing the next one
You can't prevent seal failure, but you can avoid accelerating it: don't pressure-wash window edges, keep drainage slots at the frame bottom clear, and fix failed seals early, a wet unit rots timber frames and corrodes hardware over time, turning a £150 glass job into a joinery one.