Where the condensation is tells you what is wrong. Between the panes means the sealed unit has failed. On the inside face means the frame is cold — usually an old aluminium window with no thermal break. On the outside face means the window is working extremely well.
If the fog is inside the glass and you cannot wipe it off, the edge seal on the sealed unit has failed and the desiccant is saturated. It cannot be dried out. The sealed unit needs replacing — typically £75–£190, keeping the existing frame.
Warm, moist air hits a cold surface and gives up its moisture. If the surface is the frame, the frame is too cold — and on a pre-1990s aluminium window with no thermal break, it always will be. Bare aluminium conducts heat straight through, so in January the inside face of the frame is nearly at outdoor temperature and it will run with water no matter how much you ventilate.
No repair fixes this. It is the frame. A modern thermally broken frame keeps the inner face warm and the problem disappears — which is the single most common reason people replace old aluminium windows.
If the condensation is on the glass rather than the frame, the answer is usually humidity and ventilation: drying washing indoors, unvented showers, a fish tank, an unvented tumble dryer. Extract at source, use the trickle vents, and heat the room a little more evenly.
External condensation — dew on the outside of the glass on a clear, cold morning — means the outer pane is cold, which means the window is not letting heat out to warm it. It is a sign of a high-performing low-E unit and it is a compliment to the glass, not a fault. It burns off by mid-morning.
It depends where. Between the panes: the sealed unit has failed and needs replacing. On the inside of the frame: the frame is cold — almost always an old aluminium window with no thermal break, which cannot be fixed by any repair. On the outside of the glass: the window is performing very well and there is nothing wrong at all.
Not on the frame. The polyamide thermal break keeps the inner face of the frame warm, so it does not drop below dew point. Condensation on the glass of a modern window is a humidity and ventilation problem in the room, not a window fault.
Reduce the moisture (extract at source in kitchens and bathrooms, vent the tumble dryer, don't dry washing on radiators), increase the ventilation (use the trickle vents — they exist for exactly this), and warm cold surfaces. If the frame itself is streaming, the frame is the problem and no amount of ventilation will fix it.
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