Where the moisture is tells you whether you have a ventilation issue or a glazing failure.
Condensation on windows is diagnosed by location. Inside, outside and in between are three completely different situations with three different responses — and only one of them is a fault with the window.
Moisture from cooking, washing and drying clothes condenses on the coldest surface in the room, which is usually the glass. Better ventilation, extraction at source and background trickle ventilation are the fixes.
Warmer glass helps too, which is why upgrading old units reduces internal condensation even though the moisture source hasn't changed.
External dew on the outer pane on cold clear mornings means very little heat is escaping through the glass to warm it. It looks like a problem and is actually evidence of efficient glazing.
This is the only one that is a fault. The seal has failed and the sealed unit needs replacing — usually without changing the frame.
Send a photo and we'll confirm whether it's a sealed unit replacement.
No — closing them usually makes internal condensation worse by trapping humid air.
They reduce internal condensation because the glass is warmer, but ventilation habits still matter.
Replacing the sealed unit is the reliable fix; see our guide on misted double glazing.