Condensation between the panes of double glazing is the most obvious sign the seal has failed. Once moisture gets between the glass, the insulating gas is gone and you're effectively paying to heat the outside. Replacement is the only real fix.
Draughts are next. If you can feel cold air coming from the frame — not just through the locks — the seals have shrunk or the frame has warped. New multi-point locking systems on modern windows pull the sash tight against the seals. Old single-point espagnolettes don't.
External noise getting worse is another indicator. Older double glazing typically used 4mm glass. Modern A-rated units use 4–6–4 or thicker with a wider gap. The difference in noise reduction is noticeable, especially on busy roads like the A57 or A49.
Difficulty opening and closing usually means the hinges or the balance mechanism is failing. Modern friction-stay hinges are adjustable — a simple tweak can fix this. But if the frame has warped, adjustment won't help.
Finally, the age test. Most double glazing from the 1990s and early 2000s has a 15–20 year lifespan on the seals. If your windows are 18+ years old and showing any of the above, you're probably better off replacing than repairing.
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