Double glazing is built to last a long time, and many sealed units carry on working well for twenty years or more. There is no fixed expiry date, though. How long yours lasts depends on how well it was made, how it was fitted and how much weather it faces.

The useful thing to know is that a window is made of two parts that wear out at different rates: the frame, and the sealed glass unit inside it.

The glass unit usually goes first

A double glazed unit is two panes of glass held apart by a spacer bar, with a seal around the edge. That seal keeps damp air out of the gap between the panes. Over time, sunshine, frost and the constant expansion and contraction of the glass take their toll, and eventually the seal can fail.

When that happens, moisture gets into the cavity. You will usually notice it as mist or droplets between the panes that will not wipe away.

Signs your double glazing is past its best

  • Mist or condensation between the panes. This is the clearest sign of a failed seal.
  • White marks or streaks on the inside faces of the glass. These are left behind when trapped moisture dries out.
  • A pane that feels much colder than the others. It can mean the unit is no longer insulating as it should.
  • Cracks or chips in the glass. Even a small crack lets moisture in sooner or later.

Draughts and windows that are hard to open are worth checking too, but they are usually down to the frame, hinges or seals around the opening part rather than the glass.

The frame often outlasts the glass

uPVC, timber and aluminium frames can all last for many years if they are looked after. So when the glass fails, it is worth checking the frame before deciding you need new windows. If it opens, closes and locks properly and is not rotten or badly damaged, replacing only the sealed unit is usually the better value.

A new unit is made to the size of your window and fitted into the existing frame. You keep the windows you already have, and you only pay for the part that has worn out.

Getting the most from your windows

A little care goes a long way. Keep the drainage slots at the bottom of the frame clear, clean the frames gently and keep the moving parts working smoothly. Our guide to making your windows last longer covers the basics.

If a unit has already failed, you can read more about replacing misted double glazing, or get in touch for a free, no-pressure quote.