# Aluminum foil breaking windows? **URL:** \ **Category:** Factual Questions\ **Created:** [June 11, 2011, 10:36am UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096 "2011-06-11T10:36:22Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![sohvan](https://avatars.discourse-cdn.com/v4/letter/s/96bed5/32.png) [@sohvan](https://boards.straightdope.com/u/sohvan)\ **Post date:** [June 11, 2011, 10:36am UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/1 "2011-06-11T10:36:22Z")
It’s been very hot here recently, and my room temperature is getting close to 30°C or 86°F at times. I don’t have airconditioning, so my options are rather limited. I thought about coating my windows with aluminum foil for a temporary solution, as the sun shines straight inside through most of the day. I do have curtains, but they don’t seem to help much with the temperature. However, I heard this myth that using aluminum foil on the inside of multilayered windows can cause the window to break. I’ve even heard that some landlords have forbidden aluminum foiling windows for this reason. Is there any truth to this, or is it just a myth? Are there any other means of controlling the temperature, preferably ones that wouldn’t cost too much. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [June 11, 2011, 12:53pm UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/2 "2011-06-11T12:53:01Z")
I never heard those stories. In my apartment in So Cal I taped some aluminum foil across one of my windows (although my main motive was not the heat, but rather to keep the morning light out of my eyes when at my computer, and I did not cover the window completely). It did not cause me any problems, and the landlord, who must have seen it, did not object. I don’t think mine were “multilayered” windows, though. Is that the same as “double glazed”? ---
**Author:** ![ExcitedIdiot](https://avatars.discourse-cdn.com/v4/letter/e/ea5d25/32.png) [@ExcitedIdiot](https://boards.straightdope.com/u/ExcitedIdiot)\ **Post date:** [June 11, 2011, 12:59pm UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/3 "2011-06-11T12:59:44Z")
Aluminum foil only reflects about 90% of light, so the other 10% is converted to heat. So I suppose it’s possible that this heat could crack a window. You could use [Mylar](http://en.wikipedia.org/wiki/PET_film_(biaxially_oriented)), which relfects over 99% of light. ---
**Author:** ![sohvan](https://avatars.discourse-cdn.com/v4/letter/s/96bed5/32.png) [@sohvan](https://boards.straightdope.com/u/sohvan)\ **Post date:** [June 11, 2011, 1:07pm UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/4 "2011-06-11T13:07:39Z")
> [@njtt](#): > > I don’t think mine were “multilayered” windows, though. Is that the same as “double glazed”? I’m not sure on the exact terminology. Double glazed means there’s two panes of glass very close to each other within a few millimeters, right? So this would be more like having an outer and an inner double glazed glass with a 10cm (4 inch) air distance between the two to provide more insulation. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [June 11, 2011, 2:08pm UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/5 "2011-06-11T14:08:30Z")
I don’t know what this concern might be. I wouldn’t hesitate to cover my own windows with foil on this basis. “Mylar” is a trade name for polyester resin, which is translucent and often transparent. It reflects only a few percent of light, due to the change in index of refraction, like glass does. But it reflects much less light than glass does, because its index of refraction is very roughly only half as far from that of air. You may be thinking of aluminized Mylar polyester film, but that reflects light the same way aluminum foil does. They can make it shinier, though, as it is smoother on a very small scale, so it may be somewhat better a reflector. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [June 11, 2011, 2:51pm UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/6 "2011-06-11T14:51:03Z")
I’ve coated many windows in foil with no problems, but those were the plain type. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [June 11, 2011, 2:59pm UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/7 "2011-06-11T14:59:28Z")
I would think that the concern from your landlord is that it looks like crap from the outside. My mom has a different solution. She hangs a kind of bamboo role down shade on the outside of some of her windows that get hot. But since you are renting, that may not be an option ---
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [June 11, 2011, 4:19pm UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/8 "2011-06-11T16:19:40Z")
Depending on how much money you’re looking to spend, they sell rolls of mirrored stick-on film at home improvements stores, that’s what we used in our kitchen and other rooms that get a lot of sun, and it has made a dramatic difference while still letting enough light in. ---
**Author:** ![ExcitedIdiot](https://avatars.discourse-cdn.com/v4/letter/e/ea5d25/32.png) [@ExcitedIdiot](https://boards.straightdope.com/u/ExcitedIdiot)\ **Post date:** [June 11, 2011, 7:58pm UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/9 "2011-06-11T19:58:43Z")
> [@Napier](#): > > “Mylar” is a trade name for polyester resin, which is translucent and often transparent. It reflects only a few percent of light, due to the change in index of refraction, like glass does. But it reflects much less light than glass does, because its index of refraction is very roughly only half as far from that of air. > > You may be thinking of aluminized Mylar polyester film, but that reflects light the same way aluminum foil does. They can make it shinier, though, as it is smoother on a very small scale, so it may be somewhat better a reflector. According to [Wikipedia](http://en.wikipedia.org/wiki/BoPET)Mylar is Biaxially-oriented polyethylene terephthalate. Maybe Mylar is just a name brand of many products, but BoPET is what I was referring to. It’s used when growing plants under artificial light, so as not to waste precious light. ---
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [June 11, 2011, 8:30pm UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/10 "2011-06-11T20:30:12Z")
BoPET is also what **Napier** was referring to. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [June 12, 2011, 7:05am UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/11 "2011-06-12T07:05:33Z")
What about those silver sunshades people use in cars? They don’t cause any problems, and I think they’re similar to foil, showing foil should be ok. I also tried using those sunshades, so I could take them down in winter. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [June 12, 2011, 9:41am UTC](https://boards.straightdope.com/t/aluminum-foil-breaking-windows/585096/12 "2011-06-12T09:41:03Z")
> [@yoyodyne](#): > > BoPET is also what **Napier** was referring to. So… if Little Bo Peep lost her sheep, does that imply Little BoPET lost her shit?