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Fill Power

Fill power is the rating system for down insulation in sleeping bags. It rates the ability of natural goose down to trap air, which restricts circulation, keeping warm air in and cold air out. Down is an extremely efficient insulator because it consists of very fine hair-like fibers. An ultralight sleeping bag with a rating of more than 700 will provide excellent warmth but will be much more expensive to purchase than a lower rated sleep system.

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