Despite often being mistaken for feathers, down is really the soft clusters underneath a goose’s feathers that insulate them in cold weather (who would have thought?). The lofted down filaments overlap and create small air pockets that trap warmth or body heat.
Wherever you find a down garment, you’ll usually find a fill number (such as 650-fill down or 800-fill down). Often mistaken for QUANTITY values, the fill rating actually indicates the QUALITY of the down within that jacket, sleeping bag, or what-have-you.
The fill power rating, usually somewhere between 500 and 900, indicates the number of cubic inches one ounce of lofted down occupies—each ounce of 700-fill down occupies 700 cubic inches. One ounce of 900-fill down takes up 900 cubic inches … You get the picture. Higher fill down has more of that air space between the filaments, so it lofts higher, compresses better, and weighs less. With that in mind, if you’re of the fast-and-light school of thought, aim for somewhere over 800.
The purer the down, the higher the rating. Simple, right?
Another common misconception is that the fill number tells you how warm the jacket is—not true. The new breed of lightweight down sweaters usually have upwards of 800-fill down but won’t do you much good below zero degrees Fahrenheit.
To get a better idea of warmth value, look for the temperature rating on a sleeping bag or an ounce measurement for a jacket.
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Tags: fabric, gear guides, outdoor clothing, puffy jackets, technology
