They are not meant to insulate, but to block wind and water. They can raise the temperature around your body a few degrees, which is enough to survive in some cases. I actually like the combination of a wool blanket with the space blanket outside of that. It is very warm, and it let me sleep in temperatures around 15 degrees Fahrenheit. Not as good as a down sleeping bag, but better than shivering to death all night. Laying on a good bed of pine straw, it was surprisingly warm.