What is Down? Definition
Down is a layer of insulation found under the exterior feather of birds. It holds more insulative capacity and is lighter than normal feathers. Sleeping bags that offer a mix of feathers and down will be bulkier and less warm than if the same sleeping bag was filled with down only.
Down Fill Power or Loft
Loft rating or fill power of a sleeping bag is a measure of the loft or volume of the down and its ability to regain its volume after being compressed. The higher the fill power the greater the loft, the better the quality and the more efficient (warmth vs. weight) the down will be. Technically, it is the volume (number of cubic inches) that 1 oz of down will fill under specific laboratory conditions. Down is usually available in 550 through to 900 fill.
Down is the best insulation for lightweight camping and travel because of its loft, compressibility and durability. However, down loses it's capacity to insulate once wet. Hydrophobic or DWR down is a treatment that is applied to down to prevent down from absorbing water and allow it to dry out faster.
See our in-depth Article: Buying Advice: how to choose the right sleeping bag.
See more definitions in our Outdoor and Travel Gear Glossary.