Quick answer: Fog or droplets between the two panes of a double-glazed window mean the unit's perimeter seal has failed and its moisture-absorbing desiccant is saturated — no cleaning product, dehumidifier or ventilation change can touch it, because the problem is inside a sealed cavity. Your two real fixes: a drill-and-dry repair (cheaper, cosmetic, mixed longevity) or replacing the sealed glass unit while keeping your existing frame (the standard fix — a fraction of the cost of a new window). First, though, make sure the mist really is between the panes.
It looks like a cleaning problem: a hazy patch that's foggy some mornings, droplets that come and go with the weather. You wipe both sides and it's still there — because it's not on either side you can reach. It's inside the unit, and it's telling you something specific.
First: which surface is actually wet?
Touch test, ten seconds. Wipe the inside face — if the mist smears, it's ordinary indoor condensation (fixable with humidity control — guide here). Wipe the outside face — if it clears, it's morning dew on efficient glazing, and it's actually a good sign. If neither wipe touches it, the moisture is between the panes and the unit has failed.
What failed, exactly
A double-glazed unit is two panes sealed around a spacer bar filled with desiccant, enclosing dry air or argon. The perimeter seal works for 15–25 years on a good unit, then age, sun and trapped water in the frame let humid air leak in. The desiccant absorbs it silently for months — the mist you can see means the desiccant is already full. That's why misting "suddenly appears": it's the end of the failure, not the start. It's cosmetic-to-thermal in consequence: the unit insulates worse (any argon is long gone) and the fog only spreads.
Option 1: drill-and-dry repair
A technician drills tiny holes, flushes or vents the cavity dry, and fits one-way valves. Pros: cheapest option, no glass replaced, often quick. Cons: the failed seal and saturated desiccant stay failed — the fix is cosmetic, results vary, and mist can return. Reasonable for buying time on many windows at once, or where the unit is an awkward non-standard size. Ask for a warranty on the result, in writing.
Option 2: replace the sealed unit (keep the frame)
This is the standard fix and the industry norm: the glass unit lifts out of your existing uPVC, aluminium or timber frame and a new sealed unit goes in — typically in under an hour per window, at a fraction of new-window cost. You keep frames, handles and reveals; you get fresh glass, fresh seal, fresh desiccant and current-spec low-E coatings, often better than the original. Get the new unit's spec in writing (low-E, argon, warm-edge spacer) and a 5–10 year unit warranty, which is standard.
Replacing the entire window because one unit misted is almost never necessary — that upsell pays for someone else's conservatory.
Making the next unit last
Seals fail early where water sits against them: keep frame drainage weep-holes clear, keep the frame rebates clean, and fix any external sealant gaps. And keep the glass itself clean enough that you'd notice early misting in the corners — early-stage failure is when repair options work best. (A robot-cleaned pane makes a between-pane fog patch impossible to miss.)
FAQ
Is misted double glazing a health issue?
The trapped moisture itself, no — it's sealed away. But it often coincides with indoor condensation and mould around the frame, which is worth fixing for health reasons.
Can I drill it myself?
Please don't — toughened glass can shatter at the drill, and standard glass cracks from edge stress. This is one genuinely-call-someone job.
My window is under 10 years old — who pays?
Check paperwork: sealed units typically carry 5–10 year warranties from the installer or manufacturer, and FENSA-registered installations have traceable guarantees. A unit failing that young is a claim, not a bill.
Misted units are the one glass problem no cleaning solves — but for every problem on the reachable surfaces, here's the maintenance rhythm that keeps glass (and seals) living their full life.