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Are aluminum foil and emergency mylar blankets equally effective as heat reflectors/insulators or is one better than the other?

And if there is a difference, what's causing it?

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  • I've answered but could add a bit if you say why you might use foil as a substitute
    – Chris H
    Oct 21, 2021 at 10:05
  • As an aside, I’m a fan of the emergency bags - less wind/water gets in. No weight penalty.
    – Jon Custer
    Oct 22, 2021 at 1:29
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    @JonCuster I've got a foil bag, never used it, but carry it on remote winter rides. Foil blankets are lighter/smaller than that; you can give them to someone in need easily - and they'll accept it willingly because these are known to be disposable; also the blankets are easier if you need to keep going - I'd rather tape up a blanket than cut up the bag if that's what's needed; being able to just drape it round someone is helpful. We didn't use my foil blanket the other night for someone who broke his collarbone (a spare jacket instead) but getting him into a bag would have been hard
    – Chris H
    Oct 22, 2021 at 8:14
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    @ChrisH - sure thing, but the bag will help a hypothermic/injured person stay warm inside the tent. Great as a sleeping bag liner as well. But I'm not taking a roll of aluminum foil anywhere (unless roasting corn in the fire).
    – Jon Custer
    Oct 25, 2021 at 15:18
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    Isn’t an emergency Mylar blanket just plastic sheeting with a metal layer (usually aluminium)? They can be made lighter, thinner and more durable while having the same reflective properties as a pure aluminium sheet.
    – Michael
    Oct 28, 2021 at 8:31

1 Answer 1

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Aluminium foil is far heavier for the same coverage, bulkier and less strong. Snagging on something sharp can easily tear mylar, foil far more so. Getting and carrying foil in big enough sheets is impractical.

So the main difference is practical.

Thermal differences will be slight, but foil will be worse. This is because while both are mainly a block against radiative heat and wind chill, aluminium is a good conductor of heat so cooling the outside will cool the inside almost as much. This is made worse by foil moulding to the body more. So you end up not trapping slightly warmed air so well.

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    I think you might be being a bit pessimistic on the efficacy of aluminum foil, especially since aluminum foil is used elsewhere as a thermal barrier. Oct 21, 2021 at 23:08
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    Aluminum reflects ~95% of heat. And it's thin enough that the good conduction aspect is somewhat insignificant (not enough thermal mass). The radiant aspect is much higher than the conductive aspect. See this article. There is very little difference from a heat perspective between the two (a few %). It is almost entirely a practical difference. As @fyrepenguin points out Aluminum foil is used as a thermal barrier all the time. Look in any science related lab :)
    – noah
    Oct 22, 2021 at 0:17
  • @noah the conduction in question is through the thickness of the foil, meaning that the thinner it is, the more heat is conducted (along a strip of foil, conduction would be reduced for it being thinner). The mylar film is coated with Al so the radiative performance is basically the same - that's why I said thermal differences will be slight as (you're right) radiative effects are the main ones, and both block wind (2nd most important heat effect)
    – Chris H
    Oct 22, 2021 at 8:01
  • @fyrepenguin Aluminium is one of the best readily-available thermal conductors . It's also a good reflector which is why it's used to coat mylar foil blankets. Yes, Al foil can be used to reduce heat loss, if done properly by trapping warm air, reflecting heat, or probably both. In the case of keeping people warm, "trapping warm air" includes reducing wind chill. All I say about the thermal performance is that Al foil won't be as good as aluminised mylar film. It would be better than many other materials
    – Chris H
    Oct 22, 2021 at 8:05
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    ...In an unusual emergency where you had kitchen foil but not a foil blanket, the foil could have some use, but it would be in narrow strips which wouldn't be great for covering someone up. Wrapped round the torso under the outermost clothing it would help a bit, especially a double layer, crumpled to introduce a bit of a gap between the layers. Cyclists used to use newspaper for this on long descents in the mountains. Deliberating carrying foil instead of an emergency blanket would just add weight and reduce strength so would be pointless
    – Chris H
    Oct 22, 2021 at 8:09

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