Key takeaways
- A down jacket uses goose or duck down for the best warmth-to-weight ratio and packability.
- Fill power (e.g., 650, 800, 900) measures down quality/loft; fill weight is how much down is inside.
- Down is warmest and lightest but loses insulation when wet, unless treated (hydrophobic down).
- Best as a static warmth layer in cold, dry conditions — for camp, belays, and rest stops.
How a down jacket works
A down jacket is filled with down — the fluffy plumage beneath a bird’s outer feathers — which lofts into a thick layer of trapped, still air. Because that air does the insulating and down weighs almost nothing, it gives the best warmth-to-weight ratio of any insulation, and it compresses small for packing.
Fill power and fill weight
- Fill power — the loft/quality of the down (650, 800, 900); higher = warmer per gram and more packable.
- Fill weight — the actual amount of down inside; more = warmer overall.
A light, warm jacket pairs high fill power with adequate fill weight.
A climber keeps an 800-fill down jacket stuffed in their pack and pulls it on the moment they stop moving — at belays and on the summit — when their body isn’t generating heat, then stows it again before climbing on.
Down’s weakness: water
Wet down clumps, loses loft, and stops insulating, and it dries slowly — so down shines in cold, dry conditions. Hydrophobic down helps, but for wet climates a synthetic jacket may be smarter. See down vs synthetic insulation.
The bottom line
A down jacket delivers unmatched warmth for its weight and packs down small, making it the go-to static warmth layer for cold, dry conditions. Judge it by fill power (loft quality) and fill weight (amount), keep it dry, and it's hard to beat — but in persistently wet weather, a synthetic jacket may serve you better.
Frequently asked questions
What is fill power in a down jacket?
Fill power measures the quality and loft of the down — how many cubic inches one ounce fills (e.g., 800 fill power). Higher fill power means the down traps more air per gram, so a higher-fill jacket is warmer for its weight and packs smaller. Fill weight, separately, is the actual amount of down inside, which sets total warmth.
Why is down warm but bad when wet?
Down insulates by lofting into a thick layer of trapped air. When it gets wet, the clusters clump and collapse, losing that loft and most of their insulating value — and down dries slowly. Hydrophobic (water-treated) down resists this somewhat, but down still performs best kept dry.
Down or synthetic jacket?
Down is warmer for its weight, more packable, and longer-lasting but fails when wet and costs more; synthetic insulation keeps warming when damp, dries fast, and is cheaper but is heavier and bulkier. Choose down for cold, dry conditions and weight savings; synthetic for wet climates and budgets. See our down vs synthetic comparison.
Sources
- Down insulation & fill power — The Mountaineers
- Insulation basics — American Hiking Society