# Could expanding air in a closed, overheated car really cause the windows to burst?

A friend told me story today about the time he put his car in longterm storage (read: several months) in a giant open-air lot near Phoenix, AZ. The attendant told him to be sure to leave the windows cracked a quarter-inch because the air inside the car would expand so much in the Arizona sun that a window might burst.

I called bullsh*t! (On the attendant, not my friend.) Surely even the most airtight car can not contain a gas like a scuba tank, especially over the time it takes for a car to gradually heat up in the sun. Even if the door and window seals hold tight, the A/C ducts vent to the outside somehow, don’t they?

So what say you Dopers? Was the attendent nuts (or possibly a car burglar) as I suspect?

I say the attendant is nuts or at least isn’t using logic. There is nothing magical about a car sitting in a car lot as opposed to being parked somewhere else during that time. I am sure that millions of people in the Phoenix area leave their car outside much of the time. If residents of Phoenix routinely do this, then I might believe it. However, I have never heard of it so I don’t. If it did, it couldn’t be because of air pressure. I would WAG that it would be because of uneven heating of the glass.

Yes, leaving your windows open can reduce the chance of them cracking somewhat – but it has nothing to do with pressure build-up.

If the car gets very very hot inside, the possibility exists that the inside of the glass will be very hot and the outside of it will be cool – especially if you get in and drive off quickly. The inside of the windshield will therefore expand more than the outside, and that can create enough stress to start a crack.

No even close. Even if the car were perfectly hermetically sealed (which it sure as hell isn’t), you’re not going to develop enough overpressure to pop tempered glass. Let’s assume the pressure starts out at one atmosphere (14.7 PSI) at, say, 25 degrees C. If the inside temperature rockets to 50 degrees C, the pressure would double to 29.4 PSI–high, but not nearly high enough to burst the windows out. In any case, real-world cars have multiple leaks that would prevent any significant pressure rise.

It’s a not completley unknown thing but I honestly don’t know anyone who does that here. Cracking your windows does invite theft so I’d be more likely to keep them closed. I sometimes keep the back window in my truck open since the topper keeps anyone from getting in that way but I don’t leave it open for example when I leave my truck at off airport parking for several days.

FWIW I agree with you. AC vents can’t hold pressure but closed windows mean superheated air in the passenger compartment can’t flow out. I suspect any burst windows are from the glass expanding, not the air inside. I’ve seen cracks in my windshield grow on extremely hot days so I don’t think it’s too farfetched.

I suspect that’s more due to the thermal expansion Larry Mudde mentioned than air pressure.

I know you know, but you meant:

14.7 PSI --> (14.7 PSI)(323 K/298 K) = 15.9 PSI

Crap. I knew I knew I forgot to normal to absolute temperature. Stupid, stupid! :smack:

Or normalize, even. Sheesh.

I could swear I read somewhere that car doors have a flapper valve that prevents an uncomfortable (to the ears) spike in pressure when you slam a door. That valve would open way before the pressure would get high enough to burst the windows.

I apparently didn’t make it clear that’s what I was saying.

With the case of a windshield cracks getting larger, is it possible it’s more likely because of the natural twisting of the body and frame of a vehicle over time?

I’m inclined to doubt it. Windshields are set in with a rubber molding, which would tend to insulate the glass from mechanical forces. Although, I suppose hitting bumps hard enough might make the cracks bigger.

i had a window in my 1979 subaru gl wagon do exactly this… it sat outside sealed up and one of the windows shattered as it sat. all the windows were tinted as well if that makes a heat diffrence… but It DOES happen, so maybe there was no burglar.

Actually most windows now are set using some type of resin, not rubber seals. On our cars the bonding agent is called PUR (off the top of my head, I don’t recall what the PU stands for, but the R is resin) Thick black gooy glue like stuff. Not flexable at all once it has set up.

About 20 years ago a friend’s brand new Ford had the window shatter in the summer heat. It did look like it “popped out”.

Ever noticed the puff of air that pushes your car door open, after you’ve had it parked in the hot sun with the windows up? No? That’s because there isn’t any.

Now push on the window. Even if you pushed gently, you were applying more force than any heat buildup pressure would ever reach.

Occasionally car windows do spontaneously shatter. All the cases I know of were on hot days. The one I saw personally did send most of the glass fragments outside of the car, I’m not sure why. Perhaps it was assisted by a small pressure buildup. But pressure doesn’t burst the glass.

It happened to the rear window of my car one summer day. I wasn’t there when it happened, but I combed the passenger compartment looking for any projectile that could have caused the damage, but did not find any.

My mom always opens the windows before turning the air on in a hot car, because she swears up and down that back in the day she once got in a car with her father, turned the air on high, and the back window shattered. Could that really happen? (Not with just the car sitting in the heat, but a car full of hot air with a sudden blast of air conditioning? The air wouldn’t ever have been cold, I guess, so it wouldn’t be that.)

Nope, can’t happen. Think to the last time you got in a really hot car. When you first turned on the A/C just how cold was that very frist blast of air? (hint: Not very). If the rear window broke, it was not from the A/C blowing ice cold air as soon as the engine started.
Now with that said, rolling down the windows, to exhaust the superheated air from the car, will allow the A/C to cope faster. So rolling down the windows for a block or so will allow you to get comfortable quicker, but isn’t necessary to save the windows.