Good points JAK and Paul.
My problem would be lack of insulating value, noise, and lack of breathability with 'space blankets' as a whole.
There are many types of 'space blankets' now on the market; and one must clarify the specific one they are posting about to get any reansonable answer IMO. For instance there are the cheapo flimsy, tear if you look at it twice type that are solely one use types. Yes, you could get more than one day's use from one if careful, but you cannot predict a sudden wind gust shredding it against something. So you cannot trust this type in the field for extended use IME.
The next type are the AMK type, which are much stronger; though a bit on the small side to me. They have a shiny side and an orange side with printed outdoor tips on it. These I've used for season long use without any issues, and always pack one. 3.5oz IIRC.
The third is the most, IMO, robust of the bunch but weighs in at 12ozs in 5 x 7ft. It's the 'All Weather Blanket' by MPI Outdoors from NH. It's a multi layered fabric blue on one side and reflective on the other. Again, this is a smallish one to use as a tarp, or tarp and ground cover to sit on in fornt of a small cookfire IME. If they made a 10 x 10FT it would weigh more than a pound and a half IMO and negate it's intended usefulness. It does make a good Winter ground cloth, have'nt tried it for wrapping under my hammock as of yet. Pretty stiff, so that would be the issue for conforming under a hammock IMO.
There is a 'Space Blanket' sleeping bag marketed in the mylar film, at least I have one, still in orginal package; that is for emergency bivy use IMO. You'd have to jump in near nekked <img src="/forums/images/graemlins/blush.gif" alt="" /> in order to not wet out your clothing, and it has no foot vent, though that's easily solved <img src="/forums/images/graemlins/smirk.gif" alt="" /> It would be a one time use thing as it would be thin, fragile and bulky after the first use not to mention wet <img src="/forums/images/graemlins/tongue.gif" alt="" /> it's there if I want to fill it with leaves and make a matteress someday <img src="/forums/images/graemlins/grin.gif" alt="" />
Insulation must be thick enough to prevent convection of air currents to keep you comfortable.
PEPPER SPRAY AIN'T BRAINS IN A CAN!