Down is the diva of fills. No other fill has such excellent insulating properties as down, no other fill is so light and can be packed so small, and no other fill is as sensitive.
Down is a special kind of waterbird feather; land-dwelling birds, such as chickens, have no down. Its structure is similar to a snowflake and it has a three-dimensional form. This three-dimensional structure and the ability always to take on this form means that down can contain a large amount of air relative to its weight. A good down feather has about 2 million fluffy down barbs which lock into each other. This can form a layer which blocks in warm air, thus providing protection from the cold. Down is very springy and can be squashed or compressed, then shaken vigorously to get back the former shape. This makes it easily compressible. These characteristics mean that down makes the ideal insulating material. Down fills are especially used in clothing which is worn in a dry cold climate.
There are different down fills which each have different warming properties. These differences are made clear by the ratio of each fill, which refers to the percentage of down to supporting feathers of the fill’s total weight. Down creates warmth, but not until the supporting feathers create the necessary loft to insulate the warmth in the garment. A quality characteristic of down is the fill power, expressed in cubic inches (cu.in). This measurement shows volume per ounce (28.35g) of down. The higher the fill power, the better the quality of the down and the higher the thermal rating of the garment.
Guide to fill power:
Fill power > 700 cu.in: top class
Fill power < 600 cu.in: very good
Fill power < 500 cu.in: good
Down fills insulate only when dry. When they are wet the down feathers and supporting feathers lump together and the warmth-insulating loft collapses. They also take quite a long time to dry. It’s best to dry down clothing in the sun or in a tumble drier with 3-5 tennis balls .