Everybody, please shake hands and go back to your corners. <img src="/forums/images/graemlins/grin.gif" alt="" /> We're friends here. (not directed at Blazer, only!!)

Blazer, yes, this subject does come up often and a search will reveal much....... however, part of the fun of 'ultralight' backpacking is the 'research and development' of gear for your personal tastes. I call it "back yard testing". If you don't get satisfactory answers from a forum, give your idea a practical try on the patio and report your results here. I did that with hammocks in my back yard and there I learned what really worked and what was hype. All those alky stoves you read about were invented that way too. So, go spend $3 on some mylar and give it a try. You might discover something the rest of us don't know.
My personal feeling is, an emergency foil/mylar 'blanket' will shred after the first evening, as a bag. Below 60 deg. it's going to be really cold. ( I do use them slung under my hammocks as a wind break.) A cotton bed sheet is heavy, compared to other materials, and deadlier than wet down....it will get wet from condensation due to the foil/mylar's inability to breath.
Give your idea a try....a few feet from your back door <img src="/forums/images/graemlins/cool.gif" alt="" />....and tell us what happened. Good science is about learning from successes as well as failures.

Cheers!
Paul