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Is it harmful for our health? How thick should the foam be around the body to be okay down to 0degC with warm clothes underneath? Is there enough foam for this in a 500ml can (average man)? Or two?
I thought this could be a great cheap and compact alternative to expensive and big sleeping bags.
Note that a waterproof and windbreaker cover should certainly be used around the foam.
No, it's not going to be feasible:
You'd need a lot of cans. To approximate the volume of a sleeping bag and thermal mat I'd say at least around 5-10 cans. For this weight/volume you can just as well bring a proper sleeping bag/tarp/bivvy.
The spray foam is not stable in itself. It will form bubbly clouds of foam, so you'd need a mold. This is not easy, out in the wild - and if you can build a mold you can just as well build a shelter to keep you warm.
Time to set: as noted by @whatsisname standing around for 10-30 minutes while the foam is hardening out will not be very pleasant.
Lot's and lot's of waste: a ton of non-degradable foam and a lot of empty spray cannisters.
Last but not least it might not be very healthy. I know of people who got rashes when they got this spray foam stuff on their skin. I'm not at all sure how long you'd need to wait until all the aggressive and unhealthy solvents have left the foam - it could take a lot longer than it does for it to harden.
It's a novel idea, but it's not going to work in any practical sense.
Most of those foams take about 15 minutes to setup, and you'll have to come up with some way to fabricate a mold while you spray it. It will be about an hour before you could safely enter your foam sarcophagus, and if this is to be used in an emergency situation, that's probably not time you're going to want to stand around and wait for the stuff to cure.
From my experience using the stuff for sealing purposes, I think you'd be lucky to get a cubic foot (about 30 liters) out of a can. The best R-value I can find for Great Stuff brand spray foams is about 2-3. Compare that to something like an example sleeping pad. The linked pad has an R-value of 4.9, and an inflated volume of about 2 cubic feet. As we can all know, a single sleeping pad is not going to be enough to keep you warm at the freezing point, even if you use it as a blanket. You'd have to use 4 cans just to simulate the insulation capabilities of a typical sleeping pad. You'll need many, many more to complete your sleeping vessel.
Additionally, you're going to end up with a solid, bulky mass of foam that you are going to haul out, and the cured foam is not biodegradable and it will be like managing a Styrofoam cooler making a mess everywhere, as well as all the empty aerosol cans.
There might be other foams that could work, but you won't be able to get them from the hardware store, and they won't cost only 4 bucks/can.
