Cracked Double Glazing — Repair or Replace?

Impact cracks (a stone, a ball) are obvious. Thermal cracks (a curved crack starting mid-pane on a sunny day) come from uneven heating — often when foil or paper is stuck to the glass and creates a hot spot. Stress cracks (fine straight lines from the edge) suggest movement in the frame or building. All three point at replacing the sealed unit, but only stress cracks might also require frame inspection.

Windscreen-style resin repairs don't work on domestic double glazing — the sealed unit is under different stresses. The only sensible option is a full replacement of the affected pane (the sealed unit), which leaves the frame in place. This is not a full window replacement and doesn't require FENSA notification if you're replacing like-for-like.

A replacement sealed unit is typically £80–£200 for a standard casement pane, or £150–£350 for larger openings and door lights. Toughened, laminated or patterned glass adds to the cost. Fitting is usually included by a local window company; the whole job takes 30–60 minutes per pane.

If a unit needs replacing anyway, take the opportunity to upgrade: warm-edge spacers (better condensation performance), Argon fill (better U-value), low-E coating (if the old unit didn't have one), and laminated inner pane for security on ground-floor windows. The uplift over a like-for-like replacement is typically 15–25%.

Most home-contents policies cover accidental glass breakage as standard — check your policy wording. Impact damage (children, sports balls) is usually covered; thermal and stress cracks often aren't. A quick call to your insurer before booking the repair is worth the effort.