How to Tell If Your Window Seal Has Failed

Signs: misting or moisture between the two panes that cannot be wiped off, deposits or streaks on the inner faces, and (in cold weather) frost forming inside the sealed unit. Fix: replace the sealed unit (see cracked double glazing guide). This is the most common 'failed seal' complaint and the easiest to fix.

Signs: audible draught on windy days, black or torn rubber gasket visible around the opener, hand test detects cold air where the sash meets the frame. Fix: replace the gasket run — a low-cost repair a competent DIYer can do in an afternoon.

Signs: hairline gap between the frame and the wall (inside or outside), water ingress on interior sill during driven rain, damp patch on the plaster below the window. Fix: rake out and re-silicone with a low-modulus (LM) product. Untreated, this causes damage to plasterboard and cills.

Signs: cold spot around the very edge of the glass pane, visible gap between glass and bead, occasional water dripping down the inside of the pane on cold mornings. Fix: professional bead replacement — the beads need to come out cleanly, the gaskets replaced and the beads clipped back in without damage.

If multiple seals have failed on the same window in quick succession, or if the frame visibly bows in the middle when the opener is closed, the frame itself has moved — usually because the fixings into the wall have loosened. That's not a seal repair — it's a frame refix or, on older windows, a full replacement.