Which side is the condensation on?
This is the whole diagnosis, and it takes five seconds. Put your finger on the moisture and work out which pane it is sitting on.
- On the inside of the glass
- A ventilation and humidity problem, not a broken window. The house is producing more moisture than it is clearing — cooking, showers, drying washing indoors — and the glass is simply the coldest surface for it to land on. Replacing the window may reduce it, but if you change nothing else it can move to another cold surface instead.
- On the outside of the glass
- Counter-intuitively, a good sign. It means the outer pane is staying cold because the unit is not leaking heat out through it. Modern efficient glazing does this on clear still mornings and it burns off within the hour. Nothing is wrong.
- Between the panes
- The sealed unit has failed. The seal has let go, moisture is inside the cavity and there is no way to dry it out. This one is not a maintenance issue and cannot be cleaned — the glazed unit needs replacing, though often the frame does not.
A failed unit does not always mean new windows
Worth saying, because it is where some companies are less than straight with people. If the frame is sound and the hardware works, a misted unit can often be replaced on its own for a fraction of the cost of a new window.
It becomes a whole-window conversation when several units have gone at once, when the frames are the original single-glazed or early double-glazed generation, or when the hardware has worn to the point where the sash no longer compresses the seal. At that stage you are spending repeatedly on something that will need doing properly anyway.
Draughts: find out where it is actually coming from
Run the back of your hand slowly round the closed window on a windy day. A draught along the opening edge usually means the seal or the hinge adjustment has gone, which is often fixable. A draught from the join between the frame and the wall is a failure of the original installation and points to the perimeter seal.
The one that means replacement is a draught coming through the frame itself, or a frame that has moved enough that the sash no longer meets the seal along its whole length. Old timber and early uPVC both do this eventually.
Cold spots, and why the glass is rarely the culprit
If a room has a cold zone near the window but the glass itself is not cold to the touch, look at the frame and the reveal rather than the glazing. Older aluminium without a thermal break conducts cold straight through, and poorly insulated reveals do the same around the outside of the frame.
This matters when comparing quotes: an excellent glazed unit in a poor frame, or a good window fitted without attention to the reveal, will still leave you with the cold spot you were trying to fix.
What to do first, honestly
If it is internal condensation only and the windows are otherwise sound: improve ventilation before spending anything. Use the trickle vents, run extractors properly, and stop drying washing on radiators. That is free and it fixes a surprising proportion of cases.
If units are misted, hardware is worn, or you can feel the frame itself is cold, get someone to look at it. We will tell you when the answer is a replacement glazed unit rather than a new window — that is a smaller job for us and the right advice more often than people expect.
Published August 2026 · Written by the Custom Choice team, Peterborough