Cracked Double Glazing on the Inner or Outer Pane: What It Means

When only one pane of a double glazed window cracks, the window usually still keeps the weather out, but the sealed unit has lost its seal and will need replacing. Tape the crack, keep the window closed, and arrange a new unit. The good news is that only the glass unit changes; the frame and hardware stay.

How to tell which pane has cracked

Run a fingertip lightly along the crack from inside. If you can feel it, the inner pane has cracked. If the inside surface is smooth, the crack is in the outer pane. A torch shone at an angle can also show which layer the crack sits in. It helps to tell the glazier which pane is affected and whether the crack reaches the edge.

Is it still safe?

A single crack in one pane is not usually an immediate danger, as the other pane still closes the opening. The risk grows if the crack spreads, if the window is in a door or at low level, or if the pane is on an upper floor above a path. If glass is loose or pieces could fall, treat it as urgent and have it made safe.

• Put wide clear tape along the crack on the cracked side
• Keep the window closed and avoid slamming it
• Keep children and pets away from low level or door glass
• Call for urgent help if the pane is loose or falling

Why the whole unit is replaced

The two panes are bonded to a spacer bar with a continuous edge seal. Once one pane cracks, that seal is broken and damp air starts to get between the panes. Replacing a single pane within a unit is not a reliable repair, so the standard fix is a new sealed unit made to the same size, with the right glass for that position.

That is also a chance to upgrade, for example to toughened glass if the old unit should have had it, or to obscure glass for a bathroom.

Why panes crack on their own

Not every crack follows an impact. Thermal stress, where part of a pane heats much faster than the edge, can crack glass, as can a frame that pinches one corner or a unit that was set on poorly placed packers. Our guide on windows that crack on their own goes into these causes.

Getting it replaced

Send a photo of the whole window, a close up of the crack and the corner of the glass where any etched mark sits. Mark will confirm the glass type needed and give a fixed price. The unit is then measured, made and fitted, with the beads removed and replaced so the frame is untouched.

Insurance and responsibility

Many home insurance policies cover accidental damage to glass, sometimes as a separate glass cover section. Check your policy wording, take photos of the crack and keep the quote for the replacement unit. If a neighbour's child or a contractor caused the damage, their insurance may be the right route instead.

In rented homes, broken glass is generally a matter to report to the landlord or letting agent promptly, who will then arrange the repair unless the tenancy says otherwise. In flats, the window may belong to the building rather than the flat, so check the lease. Whichever route applies, the repair itself is the same: a new sealed unit fitted into the existing frame.

Call Mark on 07535 916982 or request a callback.