Duck down is the soft, fluffy layer of insulation found beneath the feathers of a duck. It is highly valued for its lightweight and excellent thermal properties, making it a popular filling for comforters, jackets, and sleeping bags. Duck down traps air, providing warmth without adding much weight. The quality of duck down is often measured by its fill power, which indicates how much space an ounce of down occupies. Higher fill power means better insulation and warmth. Duck down is generally less expensive than goose down, another popular insulation material, but both serve similar purposes in outdoor and home products.