A friend of mine in Australia has heard from several of her friends that it's unsafe to pour boiling water on car windows in order to defrost them.

Online examples of people claiming it's unsafe include Lifehacker:

Remember to never pour boiling water on your windscreen as the thermal shock from the change in temperatures might result in cracks in the glass.

and this article / press release (not sure which it is) from the Central Western Daily: Hot water may shatter windscreens

THE NRMA has warned Orange motorists to take care when attempting to remove ice from their windscreens.

With the region experiencing night-time temperatures well below zero, many residents have a daily battle dealing with frozen windscreens.

Some people opt to pour hot water over their windscreen, but this could result in cracking or shattering, according to NRMA chief executive Rob Carter.

Mr Carter also warned motorists of the danger of existing chips in the windscreen.

"A seemingly insignificant chip in the glass can cause significant damage if hot or boiling water is poured onto the windscreen when it is covered in a layer of ice.

"In very cold weather, putting the air conditioner on hot can also crack the windscreen if there is a chip in the glass,” Mr Carter said.

To reduce the risk, the NRMA recommends a slow defrost or the use of a plastic card, such as a credit card, to scrape off ice.

And from the NRMA's website, Driving in the snow:

You should also lift wipers from the screen. Warm water may be used to remove ice from the windscreen and windows. Never use hot water as it may cause the windscreen to crack.

Is it unsafe to pour hot or boiling water on car windows?

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    Sudden changes in temperature are commonly known to cause glass (and other materials) to break. You should never put ice or a cold beverage into a hot glass, as it can cause it to break. – Flimzy Jun 24 '13 at 9:34
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    I don't see how it's a notable. Is anyone at all claiming that it is safe? – vartec Jun 24 '13 at 12:01
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    Anecdotally, I once defrosted my car's laminated windscreen with warm (not boiling) water and produced a two-foot crack. There's plenty of anecdotal evidence. Small (tiny?) chips in the glass may make this more likely. – RedGrittyBrick Jun 24 '13 at 20:59
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    @vartec: there is no need for someone to claim the contrary. The OP is just skeptical of the claim, and I have seen many people pouring hot water to defrost car windows. – nico Jun 25 '13 at 5:47
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    "To reduce the risk, the NRMA recommends a slow defrost or the use of a plastic card, such as a credit card, to scrape off ice." Too bad no one has ever invented something with the purpose of removing ice from windows... I'm guessing this is written by/for an audience that rarely experiences below-freezing temperatures. – JMac Aug 9 at 16:51
up vote 12 down vote accepted

When chipped: definitely

The key to answering the question is here:

Mr Carter also warned motorists of the danger of existing chips in the windscreen.

"A seemingly insignificant chip in the glass can cause significant damage if hot or boiling water is poured onto the windscreen when it is covered in a layer of ice.

"In very cold weather, putting the air conditioner on hot can also crack the windscreen if there is a chip in the glass,” Mr Carter said.

A quick search finds many sites warning against temperature changes when the window is damage. The top three results...

Example

Your windshield is more likely to crack—and cracks are more likely to spread—when the temperature changes.

[...]

You can’t control the temperature, so what should you do? First, keep a close watch on your windshield. Inspect it often. If you get a crack or a chip in your windshield, the hot temperatures can cause the chip or crack to expand more quickly.

Example

If a rock chips your windshield, be sure to get it repaired. Sudden temperature changes can cause small chips to start to crack which can lead to your entire windshield cracking, causing you to replace the entire windshield. Be proactive and get rock chips repaired as quickly as possible.

Example

Unfortunately, all it takes is a speed bump, a pothole, a strong slam of your car door, or an extreme change in temperature to turn your minor windshield damage into a major problem. In fact, even parking your car out in the bright sunlight can make your existing damage worse!

So when there is a pre-existing crack or chip, pouring boiling hot water on a frozen window can definitely cause cracks to form and/or expand; it is even very likely to do so. And if water gets into the crack, and then freezes, this will definitely make the damage worse.

  • Bear in mind that most modern windshields are laminated and chip/crack damage only affects the top layer. Any "danger" about this is more about the crack interfering with your vision rather than the whole slab of glass falling apart. – Snow Aug 10 at 15:39
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    @Snow That still counts as breaking the window, and you must still get that window replaced, having to pay the repair bill for it (or at least the insurance excess). And of course poring hot water is done when the car is stationary and you are outside of it. So when OP asks "Is it unsafe to pour hot or boiling water on car windows?", they are of course not talking about your personal safety or risk of any bodily harm, and instead asking "is it unsafe for the window to pour hot water on it". – MichaelK Aug 10 at 15:44
  • This is kind of a partial answer, as it doesn’t say for sure what happens with unchipped windows. If anyone comes up with a better answer, I might accept that instead. – Andrew Grimm Aug 12 at 21:02
  • @AndrewGrimm No-one can say for sure what happens with chipped or unchipped windows. If you were expecting an answer of the sort "100.000% of the times someone pours hot water over a window it will break" or "100.000% of the times it will not break", you have by reading this sentence realized that is not the sort of answer you would get. Here is one example of a window not breaking. Does that mean a car window will never break? No, since people report windows breaking supposedly spontaneously, due to faults in the glass or mounting. – MichaelK Aug 12 at 21:53
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    The way the question was asked though... "can pouring boiling water on frozen car windows break them", the answer is — as shown by the references — a definite "yes", in case the window is compromised. Does that mean you can pour hot water on your own car window and be sure it will not break just because you cannot see any visible damage? Well... can you be sure the glass is in tiptop shape, with no hidden defects, chipping under the hidden parts, or a faulty mounting that has put it under tension? You cannot be that. – MichaelK Aug 12 at 22:00

It's not just boiling water, per se, but the extreme expansion of the glass if boiling water would be thrown into an ice-cold windshield that would cause the windshield to crack, break or shatter (which would be the most common reason, probably, throw extremely hot water onto a windshield). The glass can clearly handle the heat, itself, as evidenced by how hot the glass and cars interiors can get during brutal summer heat. The modern design of the windshield is that of layered/laminated materials. The inner layer protects the unit from shattering, but, in this case, you have different materials bonded together that will behave differently (being different matierals) to a sudden, radical temperature change.

Even though to the naked eye a windshield appears to simply be a sheet of regular glass, laminated safety glass is actually composed of three layers. The inner layer is constructed of poly-vinyl butyral, and it is sandwiched on either side by clear tempered glass.

CarWindshields.Info: How a Windshield is Made

DON'T: Pour hot water on the vehicle's windshield and windows to melt the ice. The extreme temperature change can cause the glass to break.

The Glass Doctors - 8 Dos and Don'ts for De-icing Your Windshield

Here's an actual explanation from a Stack Exchange user, but I couldn't track down this information as an answer, so I can't link to the question -

When we pour boiling water on the glass, the outer part rapidly heats to around 80-100 °C (let's say 80), while the inner part may remain at -20 °C, thanks to the inner PVB sandwiched layer, for a comparatively long time (tenths of a second).

During this time, if we consider an area maybe thirty centimeters across, the heated zone undergoes a thermal expansion to the tune of 100 K (-20 to 80) by 9 millionths per K per unit of length.

Nine millionths may seem paltry, but multiplied by 100 K and further multiplied by 300 millimeters become 270,000 millionths of a millimeter - that is, one quarter of a millimeter.

Given the modulus of elasticity of seventy thousand megaPascal, this deformation should equal a "pressure" of around six hundred atmospheres: i.e., the same quarter-millimeter-per-thirty-centimeters deformation can be obtained by applying pressures of the same order of those eighteen thousand feet underwater.

It is not too surprising that in such a condition the PVB layer would come unraveled, and/or the outer layer of glass shattered, especially if there's already a flaw or chip to provide a weak point.

Lifehacker: Harness the Power of Physics to Make Winter Driving Easier

In the late 1980s, I watched a sorority resident try that on her pristine, undamaged-appearing but frozen car window (parked in the open area behind the dilapidated rat-hole I was living in) with a carafe full of hot water from a coffee maker. The entire windshield shattered and it looked almost like there was never one there.

On the plus side, she could see just fine where there was once ice.

  • There should be a lot of youtube videos on that. Car windows are flimsy and sudden change of temperatures will cause that, but will it also happen if it's 100 degrees outside? – SCFi Aug 9 at 18:36
  • The physics are simple enough: safety glass in under a lot of internal forces by design. This is so that when such glass finally do break, it shatters into small pieces instead of big dangerous shards. If you subject them to a very steep temperature gradient, you will have one side of the glass that is hot and expands, while the other is still clod and do not expand. This can sometimes — depending on circumstances — cause enough internal stress at to overload the glass and make it shatter. – MichaelK Aug 9 at 19:58
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    How can we be sure you're a "dude"? en.wikipedia.org/wiki/… – Daniel R Hicks Aug 9 at 21:47
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    @SCFi It can. My wife broke (as in spidering cracks throughout) a windshield by attempting to wash the car on a 90+ degree (F) day. The car had been parked in the sun, so everything was scalding hot, and the hose with the water had been in the shade, and so the water was still quite cool. As soon as the water began pouring on the glass, the cracks rapidly formed. Importantly, the glass did have one small, preexisting crack in it, so it might not have happened if the glass had been unblemished. But from this, it seems cold water on hot glass has largely the same effect as the opposite. – cpcodes Aug 28 at 20:55
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    @AndrewGrimm - I would, as well, and considered that, but I can't see a propaganda agenda against boiling water on cold windshields that enrich an interest group at the expense of consumers. – PoloHoleSet Aug 29 at 14:41

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