The main components of a double-glazed window are a frame, usually made from wood or UPVC, and individual glazed units inserted into the frame.

The lifespan of a double-glazing unit depends on several factors, from the quality of the materials used in its manufacture to which way your house faces. Typically, it will be somewhere between 12 and 25 years.

The most common reasons for failure are:

· Expansion and contraction due to temperature changes

· Large differences between internal and external temperatures

· Seals degrading or getting damaged

· Damage from impact

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Causes of Damage

Some windows are more vulnerable than others, for example, a tall south-facing window will be more likely to crack than a short north-facing one. The thickness of the glass, surface scratches and the location of radiators can all make a difference.

Temperature variations across window surfaces can cause cracking due to thermal stress. This may occur when part of the window is in the shade while the rest is in hot sunshine. Tinted glass can actually make this worse.

Misty glass is probably the most common problem with double glazing. It means the seal has failed, allowing water vapour in the air to get inside the unit. You’ll not only have reduced visibility, but the insulating effect of the double glazing will also be affected.

Variations in temperature between the inside of your house and the air outside, cause the water vapour to first, form condensation then, evaporate. When the water evaporates, it leaves mineral deposits behind which, over time, build up on the surface of the glass, leading to a permanent reduction in visibility.


When a double-glazed unit cracks, it is not something that develops gradually so is usually a surprise. It needs to be addressed as soon as possible for reasons of safety and security as well as performance. Misting, on the other hand, can develop slowly over time. Its effects gradually become apparent and, while the insulating properties of the window may be affected, there is no need to rush to repair it.

You’ll find various supposed quick-fix solutions online and they may well help prevent more water vapour getting inside your window, but the damage will have already been done. Once mineral deposits, dirt, dust, insects etc. have got onto the inside surfaces of your glass, you won’t be able to remove them.

The good news is that repairing a damaged window is often less expensive than you might think. The only time you need to replace a complete window is when the frame is damaged. In all other cases, the problem can be solved by replacing the damaged glass unit. This can be done quickly and easily by an experienced window fitter.