Those shiny silver space blankets we see wrapped around disaster victims in movies are available everywhere these days. And for good reason, they can be a great addition to your "just in case" kits. But you've got to know their benefits and limitations. My family recently spent a beautiful week-end in October camping. In order to try out the efficacy of this material, we purchased space blanket sleeping bags. They were essentially larger sheets taped together at one edge and the bottom to create a traditional sleeping bag shape.
The weather had been getting down in the 30s at night and none of us had sleeping bags rated even close to these temperatures. Our tent is a ginormous screen filled monstrosity, as well, so the breeze blew right through. I acknowledged that we might end the night in our van, or at Wal-mart buying warmer sleeping bags.
But the first night we endured sleeping with our cute but useless summer sleeping bags on the outside and the mylar bags on the inside, right against our pajamas. The kids complained the plastic smelled funny, and this was the LOUDEST night I'd even spent camping. Every move threw a barrage of crinkling around the tent. (Our camping neighbors didn't seem to hear it, though.) We were warm enough with a sleeping bag included, but by morning sweat and condensation left us damp inside our "plastic bags." The second and third nights we put the mylar bag on the outside of our sleeping bags and found it to be warmer, dryer, and quieter. The bags appear fairly sturdy. The thin tape ripped on a couple of bags, but we repaired it with duct tape and saw no more problems. I can't image being comfortable using a smaller flat space blanket, I imagine we'd be chilly.
I'd love to try this experiment in "worse" conditions but a controlled environment. We may find ourselves "surviving" a night in our suburban backyard this fall. Most likely, I'll be surviving and the family will be inside watch movies and eating popcorn. I'll let you know how it works out.