A friend recently recounted to me his unplanned bivy on a Patagonian wall. He and his partner shivered all night, with nothing but thin bivy sacks. They would’ve killed for the three-quarter-length THE NORTH FACE ELEPHANT’S FOOT ($129, thenorthface.com). This super-durable (Oxford nylon), waterproof/breathable (HyVentTM DT fabric) bag warms with Climashield® NEO synthetic insulation, weighs just over a pound, and stuffs into a 7-by-11-inch sack. The Elephant’s Foot has an opening generous enough to slip over insulated boots and cinches down via an adjustable shock-cord closure — it’s big warmth for minimal space/weight.