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
  • @vartec My friend said her mother used to use boiling water all the time in Japan. Link available if you insist. Also, a counter claim isn't required for notability: meta.skeptics.stackexchange.com/questions/1406/… – Andrew Grimm Jun 24 '13 at 12:07
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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 8 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
  • @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 2 days ago
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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 2 days ago

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