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Broken Glass In A Double Glazed Window

There are various reasons, sometimes a combination of which can cause the glass to crack or break.

It is not just down to impact as you might expect, but factors ranging from temperature to atmospheric pressure or weak points created during manufacturing, can all cause glass to crack or shatter.

Broken Glass

Broken Glass in Double Glazing

In this article we will look closely at some of the reasons for breakages, as well as looking at how to get repairs carried out and how to improve upon safety and take precautions against breakages.


Why does glass in double glazed windows break?

Although it is sometimes referred to as the double glazed unit ‘exploding’, what actually happens is that the glass implodes. This tends to happen in the UK between October and February due to the colder weather outside.

If an inner pane has broken it is most commonly because the inside pane is being heated by the home, while the outer pane is cold from the drop in temperature outside.

When the outer pane is the one to crack, this is usually due to a very cold pane being quickly heated by direct sunlight. In both cases the glass cannot cope with the thermal stresses and will crack along the length of the frame, or even shatter completely.

Other factors that may contribute to breakage include a weak point when the sealed unit is manufactured. If the glass is scratched when the lead or bevels are being cut around with a knife, it can create a weak point that will suffer and crack under thermal stress.

If the unit was manufactured during a period of very low atmospheric pressure, it could also suffer under high pressure conditions once installed.


Does the whole window need replacing or just the sealed unit?

As double glazing is a sealed unit, you cannot simply replace one pane; however, there is no need to replace the entire window. Replacement sealed units are considerably cheaper than buying the whole window, and you will find many local suppliers and window fitters able to carry out the work for you.


Where to get replacements?

You can find window fitters through your local business directories or online; there are some companies which offer an emergency glazing service and will respond within 24 hours.

If the security of your home, or the safety of its occupants has been compromised by the breakage then it would be worth tracking down one of these suppliers.

If you are in less of a rush, and the window isn’t in desperate need of replacement, it’s more sensible to contact several companies to get quotes. If you are able to do this, the section below on safety glass will give you an idea of what to ask for from the suppliers.


Can I install them myself?

Installing a replacement sealed unit into existing frames should not really be considered a DIY job unless you have some sort of relevant experience.

It is quite a difficult job to remove the broken unit without damaging the frame when removing the beads. It is not something you should try yourself if you are not confident, or you may find that you need to buy a whole new window after all.


Preventing breakages

The best method of preventing breakages is to install toughened or reinforced glass. Human impact is still one of the most common causes of breakages, and building regulations even cover the use of safety glass; requiring it to be installed in any ‘critical locations’ where there is a perceived risk of human impact.


Safety glass is broken down into three groups:

1. Toughened Glass (Class A) The glass is strengthened by the use of thermal chemicals during manufacturing. It gives an extra level of safety as it breaks into small pieces upon impact rather than breaking into shards.

2.  Laminated Glass (Class A, B or C). This type of glass incorporates a plastic interlayer between the panes of glass. It is entirely transparent so does not affect the view through the window, and as well as adding strength to the unit, if the window is broken any shards of glass will be left stuck to the plastic and will not cause any risk.

3.  Georgian Wired Glass (Class C). This is held firm in an accident by an embedded wire mesh, and this is a fire glass rated material.



The use of safety glass in the home is very important, particularly when young children are living in the property.

Building regulations state that safety glass should be used in doors and adjacent windows, as well as any window which is below 800mm above the floor.

Burglar breaking glass

Burglar breaking glass in double glazing

All safety glass installed in a home should be clearly stamped with the British Standard Test Reference BS6206, along with the Letter L for laminated, or T for toughened. It should also be marked with the registration number of the company which supplied the window.



The panes of glass in a double glazed window can be broken. Sometimes it is caused by human impact, while on other occasions it may be due to a change in temperature or atmospheric pressure outside.

Once a pane has broken, the only thing you can do is replace either the sealed unit or the entire window.

It is worth remembering that if the security or safety of home has been compromised by the breakage, there are firms out there offering emergency glazing services and you should be able to get the window replaced within 24 hours in an emergency.

Thankfully, building regulations now stipulate the use of toughened or laminated safety glass in ‘critical locations’ within our homes; places such as doors, or low windows that could be susceptible to impact by toddlers or the elderly.

Look out for the British Standards Test Reference BS6206 when inspecting glass, and if you are replacing a broken unit it is worth asking suppliers what types of safety glass they have available.

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