| Insulation | Down (goose or duck) |
| Strengths | Warmest for weight, packable |
| Weakness | Loses warmth when wet |
| Rated by | Fill power |
A down jacket is insulated with down — the soft, lofty plumage under bird feathers — giving the best warmth for its weight and packing down very small. Down’s weakness is moisture: it loses loft and warmth when wet, so it excels in cold, dry conditions and is often paired with a shell or treated to be water-resistant.
Warmth and limits
Rated by fill power, down is the warmest insulation for its weight, but loses warmth when wet.
Down vs synthetic
For damp conditions, a synthetic jacket is safer — see down vs synthetic insulation.
Frequently asked questions
Down vs synthetic jacket — which is better?
Down is warmer for its weight and packs smaller, ideal for cold, dry conditions and saving weight; synthetic keeps insulating when wet, dries faster, and costs less, ideal for damp climates and active use. Many people own both and choose by conditions.
What is fill power?
Fill power measures the loft (fluffiness) and quality of down — how many cubic inches one ounce fills. Higher fill power (e.g. 800-900) means more warmth for less weight. It rates the down's quality, not the total amount of warmth, which also depends on fill weight.
Are down jackets warm when wet?
No — wet down clumps and loses its loft, and with it most of its insulating power, which is down's main drawback. Hydrophobic (treated) down resists this somewhat, and keeping down dry under a shell is key. In persistently wet conditions, synthetic insulation is safer.
Sources
- Insulation basics — The Mountaineers