The Western Mountaineering Hot Sac VBL is a sleeping bag liner to increase the warmth of your winter bag on the frostier winter trips. At just 4.5 ounces of weight, cold weather expeditions can hardly go without one of these puppies. It fits on the inside of your sleeping bag, up to 6'6" of length. The ripstop nylon face fabric plays nice against the inside of your bag, while the reflective metal coating on the inside stops heat loss from evaporative cooling. Whoa, science, I almost can't handle it.