Well, that’s not entirely true. They just aren’t good at being blankets. They fell out of favor quickly in hospital use, but survivalists still advocate for their use in wilderness settings. There’s at least one ultramarathon that makes participants carry one at all times. Why do they do this?
Space blankets came into vogue during the 60s and 70s, as a response to the “space race”. Anything that had to do with interstellar travel was popular, and out of that technology boom came the thin plastic coated with metal on one side.
They work quite well in space, since they’re small, weigh very little, and reflect radiated heat. Well, they work well on equipment, and only for keeping it cool. Heat from the sun is effectively reflected by the blankets. The problem is, humans don’t radiate much. Most heat loss is by convection, and the space blanket does little for this outside of being a wind break.
An even funnier consideration is referenced in the paper as well. The blankets only reflect heat on the size that has been metallized. Since this is usually only one side, and often clear plastic is used, you have a 50/50 shot of putting it on backwards if you’re not careful, and not reflecting any heat back to you. All of the negatives of looking silly without any of the benefits.
Now that the usefulness of the metal has been show to be invalid, let’s talk to the other putative benefits of space blankets. They can be made into shelters, rain catchers, water-repellent devices such as ponchos and boot liners, and all sorts of first aid items, from wraps to slings to wound dressings. Truth be told, a space blanket would probably work in that situation. A trash bag of any decent thickness would work just as well, and cost a whole lot less. You could get them in fancy colors if you wanted to use them as a signal in snow, and they come in multiple sizes.
Sure, the reflective side of the blanket could serve as a signal, but you’ve already got a mirror on you. (You DO have a signal mirror, don’t you?) If you’ve got one in your pack, don’t throw it out just yet though. You could always use it in a pinch, as long as it hasn’t become stuck to itself by prolonged storage. You might want to check before the next hike though.
So if your friend finishes a road race and is worried about getting cool too fast, give them a jacket. And if you’re in an emergency department and the patient is cold, give them a warm blanket.
Hypothermia and the use of space blankets: a literature review.