A bird’s down is a layer of fine feathers located beneath the tough exterior feathers that help keep it warm.
Those down feathers can keep you warm, too, if you buy a sleeping bag or coat that uses them for insulation. The loose structure of the feathers traps air, which helps insulate against heat loss.
In the United States, any product labeled “100% Down” must contain only down feathers. In contrast, products labeled “Down” can contain a mixture of fiber and feathers. Also, products labeled as “Goose Down” must contain at least 90% goose down, 10% goose feathers.
Down insulation is rated by fill power, which is the number of cubic inches displaced by a given ounce of down (in3/oz). Eider-down has the highest fill power, at 1200. However, even down with a fill power as low as 550 still provides reasonably good insulation.
Higher fill-power downs insulate better than lower fill-power downs of the same weight.
Most sleeping bags and coats range from about 400 to 900 fill.
Down rated 500–650 fill is warm enough and light enough for most conditions.
Down rated 800–900 fill is very lightweight and suitable for frigid weather.
Cared for down will retain its loft up to three times longer than most synthetic fill; however, there are downsides:
1 – When down gets wet, the thermal properties are virtually eliminated.
2 – Down forms clumps when wet and will mildew if left damp.
3 – Down will absorb and retain odors.
4 – Most down is collected after the birds are killed for meat, but in a few countries like Poland, Hungary, and China, the live-plucking of birds still takes place. The cruelty of this method should not be tolerated, so buy from companies that do not support this or by a synthetic fill.