What Causes Double Glazing to Crack?

Spontaneous cracking is an occupational (although rare) hazard of many double-glazed windows – particularly those at the cheaper end of the market.  At any time of year, a window might suddenly decide to collapse inward, causing an unsightly (and heat-inefficient) shatter effect.

What causes this peculiar phenomenon to occur?  While it might seem like the window cracked on its own, there is always an underlying reason at play. In this article, we’ll shed some light on the subject.

cracked window

How does double glazing work?

In order to understand why a double glazed window might crack, it’s worth considering how double glazing is constructed.  Two sheets of glass are placed either side of a vacuum – or a layer of inert gas like argon – through which heat has difficulty moving.  This creates a barrier which helps to contain (or repel) heat, and thereby keep your home at a stable temperature – and your energy costs down.

A double-glazed window is a sealed unit, which means that the pressure within it is constant, in contrast with the air pressure on the other side of the glass.  This means that the glass will be under constant, very mild pressure.  You might notice the glass of a double-glazed window slightly deform inwards, depending on the pressure outside.  In most circumstances, the glass will be strong enough to withstand this pressure.  But in some cases, it won’t – and stress cracks in the window can appear.

It’s revealing that the peak time for double glazing breakages is during winter.  This is a time of year that we experience colder outdoor temperatures – and we try to compensate for this indoors by using central heating.  This creates a big difference in temperature on either side of the glass.

What factors increase the risk of a double-glazed window breaking?

Aspect ratio

Square (or even round) windows are at the lowest risk of suffering a break.  Why might this be?  The answer lies in flexibility. Shorter panes of glass have less opportunity to flex than larger panes.  A tall, narrow window, then, will be at the greatest risk of cracking.

Manufacturing error

As commonplace a technology as double glazing now is, it’s still something that requires precise engineering in order to get right.  There are many different practices and environmental factors at the point of manufacture which can produce short-lived windows.

Scratches

Creating windows involves, inevitably, cutting glass, but it’s important that these cuts occur only precisely where they’re meant to. Even a tiny, imperceptible scratch at the centre of a pane of glass can create a weak point which the elements might later expose.  This risk is greater when the cuts required are more complex, such as in bevelled glass.

Temperature and humidity

We’ve already mentioned how breakages become more likely when the temperatures on either side of the glass contrasts greatly, but what about the temperature of the gas inside the unit?  If a double-glazed window is created on a hot summer’s day, when the atmospheric pressure is low, then it’ll be at greater risk during the winter.

The same is true of the moisture within the unit.  Double glazing manufacturers use a special substance known as a desiccant to absorb all of this moisture and prevent condensation droplets from forming inside the window.  But this process, too, changes the pressure within the window, and so windows created on hot, humid days will be at greater risk of cracking when the temperature drops.

Not all desiccants are created equally, and some manufacturers look to slash prices by opting for cheaper alternatives.  Such desiccants will absorb not only water, but large amounts of nitrogen – which exacerbates any changes in pressure.

Strength of the glass

Of course, the odds of a window cracking can be reduced if the glass is of sufficient strength.  A thicker sheet of glass will be far more resistant to stress cracks than a thin one, so it’s often worth choosing a 6mm pane instead of the more common 4mm. When making this decision, however, you should be aware of where the stress is likely coming from. That will depend on how your house is arranged.

In order to reduce the impact of these environmental factors, it’s vital that manufacturers employ environmental controls in their factories.  While it’s impossible to eliminate the problem altogether, it is possible to reduce the likelihood of a break to the point of negligibility.  A good manufacturer will offer a lengthy guarantee against (seemingly) spontaneous window breakages.

Heat coming from outside

When it comes to stress on a pane of glass, it’s useful to think not only in terms of quantity, but of concentration.  Dispersing heat across the glass will reduce the impact that a concentrated beam of sunlight might have.

In the UK,  sunlight comes from the south.  This means that during winter, south-facing windows are at greater risk of cracking.  Overnight, the glass will cool (in some cases to sub-zero temperatures) – and then a beam of sunlight will suddenly appear.  The centre area will experience a sharp rise in temperature – but for the corners, which are still in shadow, this rise will be much milder.  This temperature difference across the glass will place it under much greater stress, and increase the chances of the window cracking.

In order to disperse the heat on those cold winter days, it’s worth placing a pale-coloured blind or curtain inside the glass.  This will help to reflect heat back onto the glass, helping to more evenly distribute the heat.

Heat coming from inside

Winter is also a time where windows are expected to cope with large changes in temperature from within, as central heating kicks in.  The biggest threats to your windows from within come in the form of acute, local thermal shock; typically from a radiator or fireplace right next to your window.

If you’ve got an under-window radiator, then it’s worth providing some protection in the form of curtains which fall behind the radiator, and protect the window from thermal shock.  You’ll also want to ensure that there is adequate airflow to the window from the rest of the room.

Looking for new windows? View our sliding sash windows, conservation casement windows, or our high-performance casement windows.

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23 Comments

  1. James Stepney

    Hello. My wife put foil on our window to block out the sun a few weeks back for our son so he may sleep. We took the foil down around 10 days ago with the windows fine. However, a heatwave since then has occurred and in that heatwave the inside temp reached 34° at night. We came home 3 days ago and the glass has a crack across with points going up and down as well. We have been honest with our estate agents about the foil when asked and they said the foil was to blame. I fail to believe this as the cracking appeared 1 week after the foil was removed? Could you please shed some expertise on the matter if possible.
    Many thanks
    James

    • Windows and More

      Windows and More

      Good afternoon James,
      Thank you for contacting us with your enquiry. I have to admit that I haven’t heard of a glass failing in the circumstances you describe. My advice would be to contact a glazing manufacturer like Pilkington as they may be to assist you further.
      Kind regards
      Alan

  2. Marie Matlock

    I have just returned from a two week holiday to find the interior pane of a double glazing unit cracked all over looking like crazy paving. The glass is in a conservatory window, could this be due to extremes of temperature?

    • Windows and More

      Windows and More

      Hi Marie,

      Thank you for contacting us and please accept my apologies for the delay in our reply!

      In most cases windows shatter due to an impact however there can be issues with rapid temperature change.

      Is there any evidence of impact damage?

      Kind regards

      Alan

  3. My double glazing frames have cracks showing from corners – can you tell me why this might be and how I can fix it?
    Thanks

    • Windows and More

      Windows and More

      Good afternoon,

      Thank you for contacting us.

      if you read through the article its gives a few examples of what can cause damage to the glazing, Can you tell me what material the windows are made from and how old are they?

      Many thanks

      Alan

  4. David Phillips

    Hi I have full double glazed front door the other day someone tried to kick the door in and as a result the inside pane has cracked. The police are saying he couldn’t have damaged my door because it has cracked on the inside. Just wondering if you have any advice for me as the police are being a bit useless thanks

    • Windows and More

      Windows and More

      Hi David,

      Please accept my apologies for the delay in our reply and I am sorry to read about your attempted break in.

      I haven’t heard of an incident like yours before where the inner glass has cracked when the outer took the impact. It does sound unusual however it might be worth contacting a glazing specialist or maybe try Pilkington glass.

      Kind regards

      Alan

  5. Hello, we had a new double glazed window installed two years ago during renovations. we returned from a week’s holiday at the end of August to find the inside pane has cracked. The blind was kept down when we were away and I’m not sure if this was a factor for the spontaneous cracking. Should our builder or the window supplier be our first port of call to get it fixed?

    • Windows and More

      Windows and More

      Good afternoon Steph,

      Apologies for the delay in our reply.

      I would initially speak to who installed them to see if they supply any guarantees for the work carried out and then contact the window supplier to find out the same, as glass should not crack spontaneously.

      Kind regards

      Alan

  6. any thoughts on why a double glazed unit has shattered. it happened last week so no unusual temperatures and definitely no impact. the unit is 8 years old. would you consider this a warranty claim, as i feel the installer will just blame me for the breakage.

    • Windows and More

      Windows and More

      Good afternoon Brian,

      Please accept my apologies for the delay in our reply.

      Initially i would speak to the installer however also check if the glazing/window came with a manufacturers guarantee.

      Kind regards

      Alan

  7. Hi,
    I have a small cracked circle from the seal of my double glazing window and as the months gone by the small cracked circle has form a straight crack line across the window. Please can you inform me what the cause is and what can be done to repair it! Many thanks

    • Windows and More

      Windows and More

      Hi Sandra,

      Thank you for your question. If this is a new window I would advise you speak to the installer and/or the window supplier for their comment first.

      If it’s an older window then I suggest you speak to a local glazing specialist.

      Kind regards

      Alan

  8. Kesley

    Hi, I have put some black poster paper on the inside of my sky light windows to block the sunshine that is south facing…..
    Would the glass panes get to hot as black or will they be ok please?
    Thanks

    • Windows and More

      Windows and More

      Good morning Kesley,

      Please accept my apologies for the delay in responding to you.

      I hope that you have got the answer you need however i would always recommend that for any window blinds that you always purchase ones specific for windows to ensure that should anything occur, you have fitted the correct type should you have need to claim on your guarantee or home insurance.

      Kind regards

      Alan

  9. My Windows were put in 1 year ago, this morning a crack has appeared on the inside pane from top to bottom in the middle of the glass, no idea why this has happened no frost no heat wave.
    The glazing company that did the work are coming tomorrow to sort the problem so that’s good.

    • Windows and More

      Windows and More

      Good morning Malc,

      Hopefully you have had the issue resolved however please accept my apologies for the delay in responding to you.

      Kind regards

      Alan

  10. Herbert Hookem

    Hi my name is Herbert we came back after been away for 4 days my wife went up stairs she shouted come quIck the big double glaze window was shared all over but still intact outside was ok it is only 4 years old I rang the company were it was purchased from they did not have a clue what cause this and said it doesn’t come under the guarantee could you in light me if I can claim of them thank you Herbert

    • Windows and More

      Windows and More

      Good morning Herbert,

      Firstly please accept my apologies for the delay in responding to you.

      Hopefully the issue has been resolved however if it has not, i would always advocate that the supplier/installer should be advised of the situation so they have a opportunity to make good under there guarantee. If the issue is still unresolved you may need independent advise.

      Kind regards

      Alan

  11. Joe Mcglone

    I have read all the above comments about windows shattering in the winter but not in the summer.
    In UK we are experiencing high temperatures at present and yesterday was the hottest day of the year so far, locally it was 32.5 * outside. At 5pm one of our “Origin” bi-fold doors spontaneously shattered on the inside, the whole pane crackled but remained in place. We were in the same room and one door was open and nothing impacted the glass. My doors were installed three years ago. My installer contacted the manufacturer but was told the 20 year warranty against manufacturing or installation defects would not cover this matter !
    Interestingly I was talking with a relative 110miles away he told me his velux type window in his roof shattered on the inside pane yesterday. It does happen in the summer.

  12. Jo Hall

    Hi there. Two of our small windows in our conservatory have severely cracked, causing large shard to smash off. We thought originally that the first one may have been from sort of impact we hadn’t witnessed. However, with the second one, I was stood in the kitchen next to the conservatory and heard a crash type sound. I thought a bird may have dropped something into the glass roof or something. When I looked, another window panel had spontaneously smashed. I heard it and witnessed to impact at all. I am now concerned as it’s my kids play room and I don’t want it happening when they’re in there. Any suggestions?

    • Windows and More

      Windows and More

      Good afternoon Jo,

      Thank you for your question and please accept my apologies for the delay in our reply.

      Glass only cracks or shatters when stressed or has had a impact but I would firstly contact the manufacturer as they have a better understanding of your product than we do and may be able to rectify it themselves or advise you how to do it yourself.

      Good luck and kind regards

      Alan

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