Do emergency space blankets actually work?
Inspired by a space blanket being included in emergency winter kits, I wonder if there's any rigorous study about whether these things actually work and in what ways to best use them.
In my own experience, they do make nice reflectors. For example, making a tarp-leantoo and a long fire, a space blanket really helps warm the shelter. Same thing as a ground reflector, sleeping with one curled around me from underneath seems to reduce chill from the ground.
On the other hand, wrapping myself in one as shown on most packaging, as emergency insulation, did not work well. I tested this simulating an emergency situation in autumn, with a leantoo and sleeping bag nearby as safety backup. I wore thermals and a jacket and had mildly wet socks after a day hiking, as if in an emergency situation and 'sheltering in place' after some problem on a trail. I'm not sure if it helped or hurt for survival chances: the material was very noisy and fragile which made it difficult to use and rest in, it let chilling wind in and was not easy to secure around me, and worse yet (though maybe I was doing something wrong) it collected condensation inside and made me feel clammy and wet. I felt I'd have been better off curled up without it! If I recall correctly I gave up in the middle of the night as I kept waking up with chills and a moist clammy feeling.
So what are these things really meant to be used for, and how effective are they for those purposes?
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They have zero insulation value.
They have near perfect IR reflectivity and are vapour tight.
This is unlike most things you have used. Since they are dirt cheap (in the flimsy emergency form, use a couple and find their limits.
Test 1: On a cold windy day have someone spray you with water, then wrap one of these around you.
You will discover that the area where the blanket is warms in a hurry. Anywhere the wind gets in is still chilly. This is due to the wind blocking ability mostly. A piece of plastic tarp has the same effect.
Test 2: Take off your shirt, and let the blanket be next to bare skin. On the windy side of you where the wind is putting it in direct contact, you get cold -- by conduction to the outer surface. Any place it's not touching you is reasonably warm. Radiant reflection combined with a layer of still air.
Another worthwhile emergency item is a plain large garbage bag. Turn it into a pullover vest by cutting the corners off and a head opening. Pull over your head, over wet clothing, and tuck into your pants getting the bottom edge as far down as you can.
The heavier grade ones make very nice ground sheets for sleeping under the stars. Put them down silver side up. The emmisivity of the silver layer means that they don't radiate their heat into space. No dew on them.
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Do they work? Yes...but you need to know why...
The Key, Like Most Things, Is Understanding How They Work
The material on “space” blankets was actually developed by NASA for the purpose of use in space to protect astronauts from solar heat, and they’re very good at it. In a wilderness setting, they do have limitations with must be mitigated though. They are made of a thin plastic film with an ultra thin layer of vaporized aluminum and reflect low relatively low heat (radiant and convective- the are non-conductive and will melt if exposed to fire or electricity).
What Are Their Weaknesses?
Since they are made of aluminum (or aluminium if you are English), they don’t “breath” and so will not permit moisture to escape. By design, since they don’t breath, they will increase humidity so, if you are wearing wet clothing when you are wrapped up in them, you are adding to the moisture in the micro-environment which you are creating (thus feeling clammy after getting in with wet clothes). Increasing humidity can actually be a good thing so long as you keep it under control (try to have dry insulation in it) as it may reduce the amount you will sweat...unless there is a breeze. Having a blanket style is a big challenge here as, no matter what you do, you will face the challenge of warm air and moisture leaving, cold air replacing it, cooling your and moisture (imagine trying to stay warm all night in the same conditions with your mummy bag unzipped all the way).
They are, in the end, plastic and aluminum. Only this plastic is thinner than your kitchen wrap and the aluminum thinner than your kitchen foil. Together, they are much stronger than the two but that is still relative and they will tear pretty easily.
How Can I Make Them Work?
To make the most of them, you need to be as dry as possible when you start. This is tough since the point is to be an “emergency” blanket and you may be wet to begin with. You need to weight the benefit of your wet clothes- are they too wet to be effective as an insulator currently? They are probably too wet to be effective in the blanket. Your wet clothes may serve better as a barrier between the ground and you in the blanket, than actually on you- this is a judgement call though, and there is no hard and fast on it. Some may read this and say “Bullshit! Never take your clothes off!! You’re always better off with them than without them!” to which I say- I haven’t yet encountered an “always” scenario in the backcountry unless it was to keep the red stuff in the body and keep the patient warm and breathing.
Before you get hung up on the concept of laying on wet clothes with an emergency blanket over you, allow me to clarify. I do not recommend the blanket style (remember the open sleeping bag concept). I prefer the bivvy style, specifically those made by Survive Outdoors Longer (SOL). They have a tarpaulin-like outer layer which provides a bit of strength to reinforce the thing, the seams are sealed to ensure adequate insulation from wind and rain, and they work excellently for patient transport and evac with the added benefit of having only one opening for hot-cold air exchange- just like your sleeping bag; I picked one up at REI for about $5.
Summary
Finally, make sure you keep in mind that they don’t make you warm, they simply reflect whatever heat you put out, and hold in the moisture in the air- and they reflect external heat away so you will not be warmed by the sun when in them (their very specific design is to prevent that). If you’ve been out on a hot day and tried using one and got hot in the sun that isn’t proof that “Oh look! The sun did make it hotter!!”, you were warmer already, the ground under you was warmer, the air around you was warmer when you put it on- the sun didn’t make it warmer inside it because, you know...science. Like a sleeping bag, if you’re cold and wet when you get in it, you’re going to stay cold and wet for a while unless you do something to change it. They don’t do anything to change the laws of physics, so convection, reflection, conduction and radiation are still going to affect how you lose or retain heat, you just need to understand how they function in order for them to work to your advantage.
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If you've got a (waterproof) outer layer, that can be used to retain the blanket, by wearing the blanket under the coat. If you don't have a waterproof layer, the foil blanket will keep the worst of the rain off your torso. They do work reasonably well worn over a breathable waterproof layer, as the condensation ends up trapped between the waterproof and the foil.
In or with every kit of mine that contains a foil blanket is some duct tape, which can also be used to keep it wrapped round you
One reason I carry one when cycling in winter, or kayaking in almost any season, is that someone who's cold but basically uninjured (a mechanical in bad weather or a swim followed by a slow kit recovery) can wrap it round their body under their outer layer and carry on. That's not true of the bags, or group shelters, which I carry winter kayaking or hiking.
Foil blankets are so small and light that you can carry one when you don't have room for anything else (e.g. in a saddlebag). They're also disposable - or more usefully give-away-able.
In general, they're not designed for sleeping, so the rustle shouldn't be an issue. Sitting on the ground leaking against something with the blanket wrapped round your knees as well as your torso, it balances heat retention and water vapour more effectively than lying down with it wrapped round you.
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Yes, they do. Their task is to reflect as much of the heat radiation of your body back to it, while also inhibiting heat convection and conduction as good as possible without becoming bulky. They are great at this.
However, how helpful they are for you depends on your concrete situation.
Short term treatment of mild hypothermia or hypothermia risk
This is a situation where the person in question is only exposed to the cold for a limited time. Examples could be an injured person, somebody who got wet, persons who have been in the cold for a longer time when help/a warm shelter is close. In this case, the blanket should help to raise the body core temperature or at least aid maintaining it for a limited time span. This is actually where the space blankets excel, because things like moisture management do not play an important role during that duration.
In the hypothermia training I received while working with refugees landing on greek islands after rough boat rides on the Mediterranean sea in fall/winter, the space blankets were essential tool for exactly this usecase. If the person you want to treat has wet clothes, the advice was to put the blanket below the clothes. As with all warming items, you also want to cover the head.
As @WilderBurn mentions in their answer, the blankets do not produce heat themselves. This is why they can only be a supportive treatment for more severe hypothermia and other means are necessary in this case as well.
Long term hypothermia prevention
The case you described in your question is different - you are interested in what helps you survive a complete night in the cold, and maybe even allows you to sleep more or less comfortably. While the heat retaining properties of the blanket are still nice and helpful in this situation, it comes with a number of downsides which you also experienced: it captures water inside, it is noisy, it is hard to keep in place and it is not very durable.
In a survival situation, probably the lacking of breathability would concern me most. Even if your clothes are completely dry, you sweat out some water during sleep. The amount depends on how warm you are, how much you move and other factors, but it can easily reach 200ml or more - imagine pouring a glass of water in your sleeping bag or over your clothes. This is a problem which you could only solve with ventilation, which of course takes away many of the advantages of the blanket. However, you can still use it as a reflector to lie on, behind a fire (careful, it could melt and I don't know if they are flammable) or as a makeshift tarp keeping rain out and some of your warmth in. If you want to wrap yourself in it depends on the environmental conditions you are in - if its really cold or raining or I am wet anyway I wouldn't care about the downside, if I am good without it I would just use it as an insulation pad or save it for later.
Conclusion
While emergency space blankets work well at what they are designed to do, they are not designed for what you used them. They might be better than nothing in a survival situation like you describe, however there are better alternatives (e.g. the bivy style blanket suggested by @WilderBurn, real blankets with one reflecting side, the suggestions by this site or other emergency gear)
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