In addition to space blankets, you can get reusable tarps with a reflective coating on one side. For the most part, they are just like regular tarps with grommets for tie downs and are big enough to sleep under.
Would the reflective coating make any difference in insulation as compared to sleeping under a non-reflective tarp?
I have one, and have used it for years. As a warmth device it works primarily as a air movement blocker -- you would get the same effect from a chunk of plastic.
As a ground sheet it's wonderful: Silver side up, the low-e effect of the aluminum means that no dew condenses on it. This seriously reduces the amount of water your sleeping bag picks up squirming around under the stars.
I've never used it as a sky tarp. It's only about the size of a standard double bed.
Silver faced tarps are used to insulate tree seedlings on reforestation projects. Generally silver side up. Since they are substantially more expensive (but much heavier) than a Canadian Tire tarp, I suspect that the people using them have good reason.
That said, the air exchange under a secured tarp of tree boxes and under a tarp pitched for a snooze in the bush are substantially different.
Overall, my opinion: Not enough difference to matter.
