Why do down jackets lose loft, and what fill power should I choose?

Down loses loft when its clusters clump. The three causes are moisture (down holds water far longer than synthetics), body oils and dirt absorbed over time, and long-term compression in stuff sacks. Once clusters stick together, trapped air volume drops and so does warmth. Hydrophobic-treated down and strict care instructions (dry cleaning or specific wash cycles, thorough drying) slow this down.

Fill power selection: 550–650 cuin is the workhorse range for mid-priced jackets; 700–800+ cuin delivers more warmth per gram for premium lightweight pieces but costs significantly more per unit. For a brand line, 650 fill power with 80/20 down-to-feather is the most common commercial balance.