Take a cue from marathoners. The Adventure Medical Kits Heatsheets Emergency Bivvy is made of metalized polyethylene—the same stuff cold-weather runners use at the finish line. It packs down to the size of a kiwi fruit and weighs less than four ounces—but this 84-by-36-inch lifesaver reflects 90 percent of your body heat.
After a trial run, one tester reported, “It’s far from an electric blanket, but the warmth it traps inside—even when I was sitting in a snowfield—is impressive, especially given the wispy material.” $16; 3.8 oz.; adventuremedicalkits.com