What Happens to Your Windshield When the Temperature Changes?

Getty Images

Article QuickTakes:

• Why windshields crack
• Ways to prevent a windshield crack in extreme temperatures
• What you can do from the inside of your car

Modern automotive windshields are made of what’s called laminated safety glass, which just means there’s a protective layer between sheets of high-strength glass. This “lamination” gives strength to this structural component of your vehicle and prevents outright shattering in a collision while still allowing the glass to expand in heat and contract in the cold. A rapid swing in temperature, though, puts enough stress on the windshield to cause it to crack. Here’s why and how to prevent it.

Windshield Vulnerabilities

To keep weight down, a car’s windshield is typically thicker in the middle than it is at the edges. This means the perimeter of the glass heats up and cools down more quickly than the center and is, therefore, more susceptible to cracking. What’s more, the edges of the glass rest against metal, rubber, and sealant, which expand with heat and further stress these weaker areas of the glass.

Small pits and chips in the glass only exacerbate the problem, as they allow water to penetrate the first layer of glass and sit atop the vinyl. As the temperature rises or drops below freezing, those water molecules will expand but put pressure on the outer layer of glass, turning that little nick into a lengthy crack.

Ways to Prevent Temperature-Related Window Cracking

First things first, address chips in the glass as soon as possible. You can generally pick up a repair kit for under $20 at your local auto store.

Apart from that, the best course of action is to take extra care when the mercury rapidly rises or drops. If you can, park your car in the shade in the summer to keep the glass and its metal housing from excessive heat. (This should also prevent accelerated wear on your paint.) And refrain from washing your car with cold water on really hot days or vice versa.

If you live in a climate that sees frosty winters, use a plastic ice scraper—never a shovel or knife—to clear the glass of any accumulated ice and snow.

What you can do from the inside of your car

When you step into a sweltering interior, make sure to direct your air-conditioning vents to blow away from the glass, and be careful about slamming your doors, as the vibration could set off a microscopic pit. Go gentle on the heat too, adjusting the defroster settings so as not to shock the glass with blasts of hot air. Also avoid ever pouring boiling water on a frozen-over window.