Why did my new window crack?

Understanding stress cracks in glass

You remodeled your living room and installed a large picture window. And now, you discovered a long crack running across it! It doesn’t look like anything, or anyone, hit it. So what happened? And what should you do?

Understanding stress cracks

Here’s the scientific reason for the crack: Thermal stress cracks — commonly known as “stress cracks” — typically occur in windows when a thermal gradient causes different parts of the glass to expand by different amounts. At some point, the stress of the expansion may overcome the strength of the glass, causing a crack to form.

Here’s a simpler way to look at it: Stress cracks in windows are similar to what might happen if you poured cold water into a hot baking dish that you just took out of the oven. As the cold liquid touches the hot dish, there’s an uneven increase in the thermal expansion of the walls of the baking dish. If the expansion is significant enough and the dish is thin or compromised by a nick or weak spot, the baking dish may break.

Now think about your windows and where they are located on your home.

Stress cracks more often occur in large windows that are beneath overhangs or are recessed behind a protruding outward room. The shadow lines created by the overhang or wall sets up a quickly changing stress factor from the glass that is in the sunny, hot area that contrasts with the glass in the cool, shaded area.

If the stress gets too be too much, the glass expands a lot in the heat or shrinks in the cold, it can crack.

“Stress cracks are often a function of the architectural design of the building as much as the glass in your window." “They tend to show up more in newly-installed windows than in older windows. But stress cracks are not necessarily time-related. It may have more to do with the shading of your home.”

Common causes for stress cracks

If the wind blows down a large shade tree in your yard or a new house goes up next door, it could change the amount of shade on your window and create conditions for thermal stress cracks. Seasonal factors, like dramatic overnight temperature changes, are a common cause of stress cracks as well.

When areas are experiencing hot temperatures during the day and then much cooler temperatures at night.” “It’s often a seasonal factor.”

Identifying stress cracks

Thermal stress cracks in windows are easy to identify because they start perpendicular to the glass edge. “It’s easy to identify a stress crack if you can see the edge of your glass,” he said. “The crack will extend about a half-inch straight away from the glass edge.”

But that’s only true of the first half-inch of the crack, after that it may spider in any direction. Windows with cladding or trim may obscure your view of the glass edge.

-Thermal stress crack (see photos for example) — cracks at a perpendicular angle. May be caused by sudden temperature swings or shading changes on a building.

-Impact crack — cracks in a starburst pattern that radiates from a central point. Hitting a window with a baseball or golf ball could result in an impact crack.

-Pressure crack — cracks in a pattern similar to the curve of an hourglass. Insulating glass — windows with two panes of glass that have air inside them — may get pressure cracks if they are installed at too high or too low of an elevation level or if there are drastic pressure system changes in the weather.

Can I prevent stress cracks?

The short answer is: not really. Stress cracks are a naturally occurring result of expanding and contracting glass due to temperature changes. However, installing thicker glass and choosing glass features based on your climate and sun exposure needs may help decrease your risk.

In existing homes, installing thicker, stronger replacement panes of glass — 4mm glass instead of the standard 3mm glass — can help prevent stress cracks. For home additions or new construction projects, work with your architect to best dictate sun exposure on your windows.