Many premium manufacturers make down bags for all temperature ranges and conditions from -30 below to 60 above. Contrary to the belief of some, Goose down is not feathers, but the soft, fluffy underlayer that insulates geese and other waterfowl against cold and wet conditions. Down is purchased by wholesalers and manufacturers as a commodity, much like agricultural products and natural fibers. Down comes in a variety of processing refinements which in addition to referring to the cleanliness and purity, denote the most important factor in evaluating down, the loft. Kicking the Tires Loft basically refers to volume.
Greater volume means more trapped air, which in turn means warmer insulation.
A quick 'tire kick' comparison between two bags of different insulation ratings can be done by simply observing the height of a bag when lying on a flat surface. The beefier volume, height, or general 'fluff' denotes a warmer, higher quality insulation. Technically speaking, loft is measured as volume per ounce and is a direct reflection of insulative power. A 550 down fill bag, utilizes insulation that occupies 550 cubic inches per ounce of down. The higher the loft number the more efficient the insulation, therefore the less of it needed to fill a similar size sleeping bag, in turn, the lighter the overall weight of the bag.