Why Is My Double Glazing Misting Up? Causes and Fixes
18 April 2026 · 6 min read
Foggy glass you can't wipe away is a classic sign of a failed sealed unit. Here's what's actually happening and what your options are.
What misting actually means
If condensation appears between the two panes of a double-glazed window — where you cannot wipe it off from either side — the sealed unit has failed. Double glazing works by trapping a layer of dry air or inert gas between two panes. When the edge seal breaks down, moist air gets in and condenses against the cold glass.
Misting on the inside surface of the glass (which you can wipe) is different — that is usually a ventilation or humidity issue in the room, not a failed unit.
Why sealed units fail
Sealed units have a finite lifespan. Age is the most common cause, but failure is accelerated by trapped water in the frame drainage, south-facing heat cycling, and poor original installation. Once the seal goes, the insulating performance drops and the window looks permanently foggy.
It is rarely a sign that the whole window needs replacing — usually only the glass unit itself has failed.
Can it be repaired?
You cannot reliably 'clear' a failed unit — drilling and drying methods sold online tend to fail again quickly and leave the glass looking marked. The proper fix is to replace the sealed glass unit while keeping your existing frame, which is far cheaper than a whole new window.
A glazier measures the exact unit size and specification, orders the replacement, and swaps it in — often in a single short visit per window.
Should you upgrade while you're at it?
Replacing a failed unit is a good moment to upgrade the glass — for example to a modern A-rated, low-emissivity unit that keeps more heat in, or to toughened or laminated glass for safety and security. The labour is the same; only the glass specification changes.
Ask your glazier to quote both like-for-like and an upgraded option so you can weigh the long-term energy saving against the cost.
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