Down consists of very fine, fluffy filaments that interlock to trap air. The air trapped in down fibers provides insulation. There are different types of down and some down is more effective than others. The most common measure of down effectiveness is the Fill Rating which measures the ability of the down to resist compression (lofting power of the down). The fill power is measured by placing the down in a container, adding weight to compress the down, and then removing the weight. The down will spring back and the amount that it uncompresses is the fill rating.
Down with a higher fill rating is more resistant to compression, has higher loft, traps more air, and provides more insulating value. Higher loft down is both warmer, more compressible, requires less down, and is lighter for the amount of warmth that it provides. Items with a high fill rating are warm, more compressible, lighter, and higher cost.
- 800-900: Superior level of warmth and compressibility but high cost
- 600-700: Very good
- 500-550: Good
- 400-450: Medium quality
Most goose down has a fill rating of 400-550 because it comes from immature geese raised for meat consumption. Higher quality down comes from mature geese bred specifically for their down. These geese were not raised for food, they take time to mature, and this type of down is more scare and more expensive.
Properties of Down
Down is a fantastically efficient insulator due to its light weight and compressibility. Down is much lighter and more compressible than synthetic fill materials. However, down loses all of insulating ability if it gets wet. Synthetic fill material normally retains about 80% of its warmth with wet. If you choose down you must keep your sleeping bag dry.
Some manufacturers treat the down. Kelty has a product called DriDown and they claim that this type of down dries faster than normal down.