I took one with me when I went camping for years and years, but never used it.
I'm sure they do. I bought some socks and gloves, white knit, maybe even synthetic which is normally not very warm, but these had "silver" metal strands in them and the gloves were so hot I couldn't wear them in most situations, in any weather I'd actually go hiking or campling in. The socks I haven't even tried yet. I"m sure they're warm as hell. It's the foil.