Why Did My New Window Crack?

Cracks in a new window can be extremely frustrating. After all, you just had it installed. How do you make sure it doesn't happen again? Should you choose a different manufacturer? What caused it? Well, it depends, but you can't place all the blame on the window.

Why New Windows Crack

There is a scientific reason for the crack that has suddenly appeared on your new window. Thermal stress cracks, or "stress cracks" as they're more commonly known, occur in windows when an extreme change in temperature, in a short distance, causes different parts of the glass to expand at different rates. Eventually, the stress from the expansion overcomes the strength of the glass, causing it to buckle and crack.

Think of it like pouring ice cold water into a hot glass baking dish you just took out of the over. As the cold water touches the glass of the baking dish, it causes an uneven increase in thermal expansion. And if the contraction and expansion is significant enough, and the dish is thin or has a flaw, it will break.

Is It a Stress Crack?

Identifying a stress crack is easy, if you can see the edge of the glass. Thermal stress cracks start about a half inch away from the edge and run perpendicular to the glass. After that, the crack may spider in any direction. Just be aware that cladding and trim can hide the edge of the glass, so you may not see them right away.

If the window is hit with a ball, for example, it will get a starburst-like impact crack. The point of impact is the center of the crack. Pressure cracks, on the other hand, have a pattern that resembles the curve of an hourglass.

Identifying the Reason for the Crack in Your New Window

Now that you know what causes cracks, you need to examine the window. Larger windows are more prone to cracking than smaller ones, particularly if they're beneath overhands or are recessed behind a protruding wall. This is because the shadows from these overhangs create an extreme change in glass temperature -- the shady side is cool and the sunny side gets hot. And once the expansion and contraction of the glass becomes too much, it cracks.

In this instance, the architectural design of the home is as much to blame as the glass. However, this means new windows are more prone to this type of stress crack than older homes.

Stress cracks aren't necessarily related to the age of your windows. In some instances, changes in the way your home is shaded can crack your windows. If the wind blows down a large tree, or if a new house goes up next door, for example, previously shaded areas of your home will now be in full sun, causing the thermal stress cracks. Changes in the season and dramatic temperature changes are also common causes.

Preventing Stress Cracks in New Windows

If you're planning a new addition or construction project, preventing stress cracks in new windows starts at the architectural stage. Work with your architect to plan and control the amount of sun each window receives. If you have windows that will experience dramatic temperature changes, choose glass that's better able to resist cracking, such as tempered, heat-strengthened or tinted insulating glass.

Heat-strengthened and tempered glass windows are similar, but there are a few differences. Both types are heated and cooled (quenched) in the same way as steel, but at different rates, giving them different levels of strength. Tempered glass is the stronger of the two, but it is more expensive.

Because sharp temperature changes naturally cause stress cracks, there really isn't much you can do to prevent them. However, you can limit your risk by using tempered or heat-strengthened glass and by doing your best to minimize the temperature changes the glass is exposed to.