Down or synthetic insulation: what matters on a damp trip?

Down and synthetic insulation both trap warm air. The best choice depends on packed size, likely moisture, drying opportunities and budget—not simply which label sounds more technical.

Compare the trade-offs

• Down: usually excellent warmth for its weight and very compressible, but needs careful protection from wet conditions.
• Synthetic: generally bulkier for comparable warmth, but retains more insulating ability when damp and usually dries faster.

What fill power actually means

Down fill power measures loft: how much space a standard amount of down occupies under a test. A higher number indicates greater loft for that amount. It is not a jacket temperature rating. Fill quantity, construction, fit and coverage also affect warmth, so an 800-fill jacket is not automatically warmer than every 600-fill jacket.

Choose for the way you travel

For a compact walking kit, down may save valuable pack space if you keep it dry. For damp trips with limited drying options, synthetic insulation may be more forgiving. I would compare complete garments while wearing the layers that will sit underneath. With second-hand gear, check for clumped fill, empty sections, damaged stitching and care history. Keep either type protected inside your pack and put the warm layer on before you get chilled at a stop.

Read next: Keeping hiking gear dry: pack liners, dry bags and ratings.

Sources and photo credit

Information checked 29 September 2026.

• REI expert advice

Photo: Matti Blume. Original photograph; CC BY-SA 4.0. Resized for web display.