can windows crack from heat

Can Windows Crack From Heat?

Can windows crack from heat? We live in an old building, and after a series of increasingly hot summers this is a question close to my heart! Thermal stress from rapid heating and cooling can cause glass in windows to crack, but there are several simple precautions you can take to prevent it happening.


Can windows crack from heat?

Yes, windows can crack from heat. Heat from the sun in summer, or your own central heating in winter can cause thermal stress fractures in glass. Cracking from heat in summer is more commonly associated with windshields on cars and trucks than windows in your home, but it can happen in both places.

Since no one appreciates the sudden financial burden of replacing cracked windows, we’ve got some practical tips to keep the risk to a minimum.

What causes thermal stress cracks?

Thermal stress cracks occur when windows are heated up or cooled down very rapidly and unevenly. When glass is exposed to heat, it expands. When it cools down, the glass shrinks again. If one area of a window is expanding or shrinking quicker than the area next to it, the difference can lead to a stress crack. Alternatively, if the frame is expanding quicker than the glass, and the glass is fused to the frame, then this can also cause a stress crack to appear.

can windows crack from heat

Many people have already witnessed a thermal stress crack if they’ve filled a glass with hot water, or rinsed a hot dish under cold water. The sudden change in temperature causes the glass to shatter into the sink — making a mess and leaving the person confused and alarmed. Thermal stress cracks can also be seen when ice cubes are dropped into a glass of water.

Which windows are most likely to crack in heat?

Stress cracks can occur to any glass that comes into contact with rapid and extreme changes of temperature. They’re most common on windows which are:

  • old
  • already damaged
  • large
  • west or south facing.

Heat cracks in old windows

Modern glazing is so much more than just glass. It is tempered during the manufacturing process to make it stronger and safer to have in our homes. Tempering also changes the rate at which glass expands and contracts with exposure to hot and cold temperatures, so it is less likely to fracture. Old windows which pre-date modern glazing technology are more at risk of all forms of damage, including heat cracks.

Heat cracks in damaged windows

Windows which have already been chipped are more vulnerable to heat cracks. The chip is a weak point, and stress on it will more readily turn into a crack, which starts at or passes through the chip.

Heat cracks in large windows

Large windows are more at risk of these cracks as they can get uneven sunlight throughout the day. If one portion of the window is shaded while the rest is exposed directly to the sun, there will inevitably be an off-balance distribution of heat.

Heat cracks in south facing windows

In the Northern Hemisphere, west and south facing windows take the brunt of the sun’s glare in summer. This means they get hotter than any other windows in the home, and are more likely to crack if they are rapidly cooled down again.

How to identify thermal stress cracks

Thermal stress cracks are usually recognisable because

  • They start at the window frame, or run through a pre-existing chip.
  • If they start at the frame, the first inch or so of the crack runs perpendicular to the edge of the glass and is fairly straight.

They differ from impact cracks, as there is no impact site. When a glass window is hit with a solid object, such as a rock, the cracks tend to radiate out from an obvious impact point.

It’s not a bad idea to check the corners of your home windows every so often to spot any stress cracks before they have the chance to spread and cause more damage. Once a crack has become larger than six inches, the window needs to be replaced instead of repaired, which is a bigger, more expensive job.

What to do about heat cracks in windows

Windows with thermal stress cracks should be repaired if possible or replaced. If the crack is small and hasn’t spread very far, a repair with epoxy or adhesive may be possible. DIY glass repair kits can be purchased at your local home improvement store and they will prevent the crack from spreading. This tends to be the more cost-efficient route to take, if it is available.

If cracks are left unattended, the energy efficiency of your home can decrease. Once the crack is more than a few inches long, it requires specialist attention, and the pane may need replacing altogether.

Tips for avoiding window cracks due to heat

While you can’t control the temperature outside, there are a few easy habits to develop to prevent thermal stress cracks on your windows:

  • Shade glass if possible, to protect it from rapid and extreme changes in temperature.
  • Don’t wash hot windows with cold water.
  • Position garden sprinklers so that they can’t spray cold water onto hot glass.
  • Don’t slam windows and doors, as this can put unnecessary pressure on the glass. If a stress crack has begun, it can quickly spread across the window if the force was jarring enough.
  • If your house gets very hot inside, cool it down using the gradually, not as quickly as the air conditioning unit can possibly manage.

When cooling down a hot vehicle, the doors or windows should be left cracked to allow the air to circulate and distribute evenly. Make it a habit to give the car a few moments after it’s been turned on before blasting the AC. This helps prevent the glass from reaching a stressing point that could result in unsightly cracks.

Can windows crack from heat generated inside your home?

Yes, heating cold glass up very rapidly can also cause thermal pressure cracks. Once the weather begins to cool, homeowners often want to crank up the heat. When heating the home, the temperature should be raised gradually to avoid the cold glass from being expanded at a rate that causes it to crack under pressure. This is particularly true if you’ve left the heating off whilst going on a winter vacation, and you want to warm the house up when you get home.

Can windows crack from heat: Summary

Glass can be susceptible to extreme temperature fluctuations in summer and winter. Thermal stress cracks are a potential result of such changes. It can be an incredibly frustrating experience to find one in your windows, so it’s worth taking the extra care to prevent them in the first place. Once they appear, minor stress fractures can sometimes be treated using kits from hardware stores, but larger cracks need professional attention from a glazier.

Do you have thermal stress cracks in your windows?

Were you able to pinpoint the event which triggered them? Let us know in the comments box down below.

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