What is down?
The layer of soft, fine, fluffy feathers under the outer feathers of adult geese, ducks and other waterfowl.
What is fill power?
A measurement of the loft or "fluffiness" of down based on the amount of space one ounce of down occupies. Fill power is not directly related to warmth, but rather the warmth-to-weight ratio.
For instance, two products with different fill powers may offer the same degree of warmth. But the higher-fill-power product will be more efficient, meaning it will weigh less than the product with lower fill power. A lower-fill-power product requires more down to provide equal insulation.
Higher fill powers are best used for the most active applications, when weight and packability are significant factors.
How does down insulate?
Down of all fill powers traps pockets of air that warm and provide insulation. Dense, lower-quality feathers compress more than finer, more fluffy ones. So it takes a greater, heavier amount of lower-quality down to contain the same amount of air as higher-quality down will trap.
Goose down has a supremely high loft and fill power, which ensures superior insulation and volume retention after compression.
How is fill power measured?
In a clear plastic cylinder, a weighted piston settles upon one ounce of down. The volume, measured in cubic inches, occupied by this partially compressed down determines the fill-power measurement.
The higher the loft, the more cubic inches the down occupies. And the higher the fill power, the more "dead" air the down will trap to keep you warm.
Are there down alternatives?
Synthetic insulation, made mostly of polyester fibers, mimics the loft and warmth-trapping properties of natural down. Though the pros and cons of quick-drying synthetic alternatives and long-lasting natural down are widely debated among outdoor enthusiasts.
While authentic down is widely regarded as the superior insulator, synthetic down is less expensive and requires less maintenance. Most notably, in wet conditions, synthetic alternatives provide reliable insulation and are preferred to slow-drying natural down, which loses its insulating power when wet.