Fog/condensation inside my double glazing — can it be fixed?

Replaced two failed units this way — kept the frames, just new glass. Way cheaper than I feared.

Adding a note since this one comes up seasonally: the reason foggy double glazing can't be wiped clean is that the spacer and desiccant sealed inside the unit have saturated — so it's a whole-unit replacement, which I've broken down in can just the glass unit be replaced. And while you're replacing a sun-facing unit anyway, it's the ideal moment to upgrade to Low-E glass to cut the heat — same labour, better glass.

For anyone whose windows also feel draughty or let heat in (not just fogged): that's a different failure from a fogged sealed unit — it's the perished rubber gaskets/seals around the frame, and there's a newer thread on whether just the window seals can be re-done without new windows. Foggy-between-the-panes = glass-unit job; draughty with cracked rubber = a reseal. Useful to tell them apart before anyone quotes you whole new windows.

One thing that ties into this: when you're choosing new windows, the glazing and frame spec is exactly what governs condensation and heat, not just the opening type. A good insulated double-glazed unit with a thermal-break frame runs warmer on the inside face and resists the surface condensation you get on cheap single glazing or a non-broken frame. There's a thread on choosing aluminium window types — casement vs sliding vs tilt-and-turn — and the glazing to pair with them. (Condensation BETWEEN the panes, as in this thread, is a failed sealed unit and separate — but specifying good sealed units + thermal breaks up front avoids the surface-condensation version.)