Down is an amazing natural fibre that has an unbeatable warmth to weight ratio. It's compressible, resilient and very long lasting if properly maintained. The major drawback of using down-filled clothing is it's inability to retain loft and insulate when wet. To optimize the performance of your down-filled garments: 1) clean it regularly and, 2) add the appropriate Nikwax water-repellent finish.
Dirt and body oils will contaminate down causing it to clump together and lose loft. Dirt also attracts water and reduces the beading-up of droplets on the fabric's surface. Wash the garment in Nikwax Downwash non-detergent soap. Detergents can leave water-attractive residues that will not rinse out. They also contain agents that can wash away the natural oils of the down. Never use liquid detergents or fabric softeners.
When machine washing always use a front load machine-top loading agitator machines can damage the garment. Tumble Dry the bag at the heat recommended on the care tag. Large bags will require several hours of tumble drying. Make sure the bag is completely dry before storing. An old (clean!) training shoe can be included to knock the down about so that it regains its loft.