Down bags use duck or goose down, which consists of the small, fluffy feathers birds grow next to their skin for insulation. Ounce-for-ounce, no man-made fiber can match the warmth-to-weight ratio of this remarkable material. Highly compressible and extremely lightweight, down is the preferred choice for traveling small and light. The bags on backcountry.com display a temperature rating in the title. This is the EN, or European Norm, rating which is a standard used throughout the industry. This number is used to give you an idea of the lowest temperature at which the bag will keep you warm.
Down bags have a fill rating that indicates the amount of volume one ounce of down occupies. The higher the fill-rating, the loftier the down. It provides more warmth with less weight.
Mummy-shaped bags taper towards your feet to reduce volume and hold warmth closer to your body. These bags can feel restrictive to some. A semi-rectangular bag can offer more room to move.
Down insulates less when wet. Some bags feature hydrophobic treatments on the outer fabric or on the down itself (like DriDown) to keep the down insulation dry.