Can Summer Heat Break Your Windshield?

We’ve been going through “weather whiplash” here in the Midwest. April was the seventh-coldest April in Kentucky ever since records began. Then, May followed with record-breaking high temperatures. (Both Louisville and Lexington beat their previous record highs.) Now, June is shaping up to be just as hot.

When temperatures are this high, it can be hard on your car. These high temps make a tire blowout more likely, especially if your tires are under-inflated. Overheating is also more likely if your engine fluids or low or your coolant is contaminated. And hot weather can even break your windshield.

That’s right—temperature alone can be the cause of a crack in your windshield! But how does this happen?

How Stress Cracks Happen

This type of crack—called a stress crack—can happen when a swing in temperatures causes windshield glass to expand (when heated) and shrink (when cooled) rapidly. This puts stress on the glass (hence the name “stress crack”). If the glass experiences enough stress, it will crack.

Sunlight can also cause stress cracks in a similar way. The heat from direct sunlight will heat up both the metal and glass of your car—but the metal will heat up faster. When the metal edges of your windshield heat up faster than the middle of the glass, this can cause stress that leads to cracking. These cracks often show up starting at the edges of your windshield.

Stress cracks are different from other types of chips and chracks because there is no obvious point of impact and no glass is missing from the windshield. Unlike chips from gravel, road debris, hail, and the like, stress cracks seem to appear out of nowhere. We’ve talked to many car owners who’ve said that their windshield cracked “for no apparent reason.” In fact, there is a reason—just not one you can see with the naked eye.

What You Can Do about Stress Cracks

Stress cracks are an unpleasant surprise, but the good news is that they are relatively easy to avoid (unlike other types of chips and dings). While you can’t control the weather or the changes in temperature, you can often control how much your car is exposed to it.

Here are a few tips to help you avoid stress cracks in your windshield:

• Don’t blast the air conditioning. It’s hot outside, and it can be tempting to crank up the A/C as soon as you get in your car. Try to avoid this as much as possible. By adjusting your A/C to cool the car more slowly, you’ve avoiding the extreme temperature changes (hot to cold) that put stress on your windshield.
• Park in the shade or a garage. Remember what we said above about direct sunlight? Parking in the shade or (preferably) in a garage will get your car out of direct sunlight and put less stress on your windshield. As a bonus, your car will be cooler and more comfortable when you get back in it. (This, in turn, means you’ll be less likely to blast the air conditioning!)
• Wash your car with warm water. After a winter of driving through slush and salt, you’re probably thinking of washing your car. But don’t wash it with cold water when it’s hot outside: take the extra time to wash it with lukewarm water to prevent the sudden temperature change from cracking your windshield.

Now that you’ve read the good news, let’s talk about the bad news: stress cracks can often be irreparable.

Many windshield chips and cracks can be repaired as long as they are small, uncontaminated by debris, and located away from the edge of the windshield. Stress cracks, however, are typically very long and often originate from the edge of your windshield. When that’s the case, the windshield needs to be replaced.