The clusters of very fine feathers, or fluff, beneath a duck’s tougher exterior feathers make up the composition of duck down. Down has incredible insulation power because it has a high loft. This creates pockets of air that trap body warmth, maintaining your temperature.
The quality of the down is measured by its loft - also known as fill power. The higher the loft, the more insulation it provides per ounce.
Duck down performs best in dry, cold environments where damp will not limit its performance.