Fill power
The fill power of a duvet refers to the volume, in inches, filled with an ounce of down. The higher this number is, the more efficient and air-locking the down will be. It is also a good indication of the duvet’s ability to retain heat.
Duvet cover
A duvet cover should be made of a fabric that allows the duvet to breathe, with at least 200 threads per square inch to prevent the down from escaping.
Also, to prevent the down from piling up at the ends of the duvet, some models are equipped with airtight panes.