As far as sleeping bags go, 3. 9 pounds is pretty light, especially when you're talking about a bag that will keep you warm in the dead of winter. On top of all the warmth, the Big Agnes Storm King 0 degree Sleeping Bag has a roomy rectangular design and can even be zipped together with another rectangular Big Agnes bag. The main reason for such a light weight is the fact that Big Agnes eliminated unnecessary insulation from the bottom of the bag. When you're sleeping on top of insulation, it doesn't keep you warm because it has no loft and therefore no air or heat is trapped inside. Instead, Big Agnes put a Sleeping Pad sleeve in its place, so you have the trapped air in the sleeping pad keeping you warm, and at the same time your pad stays underneath you. On to top of that, Big Agnes went with a hydrophobic down fill, so the Storm King will maintain insulating properties if it should happen to get wet.