Space blankets are great for keeping body heat in.http://en.wikipedia.org/wiki/Space_blanket
In my younger years I used to go hiking without a tent (too lazy to carry one).
Just a lightweight flysheet, sleeping bag and space blaket as ground sheet folded over me.
The one I had was more like a groundsheet/ tarp with metal eyelets on each corner,
silver side and a blue side (from memory) and was bought from Aussie Disposals.