TLDR

• Down:

  Best warmth-to-weight and packability; excels in cold, dry conditions; needs care around moisture.

• Synthetic:

  Keeps warm when damp and dries fast; better for wet, mixed, or high-output days; packs larger for same warmth.

Quick comparison

How insulation works

Both down and synthetic insulation trap still air between fibres. More loft = more trapped air = more warmth. Performance depends on:

• Loft per gram
• Ability to resist moisture and keep loft
• Shell fabric and construction (baffles, stitch-through vs box-wall)

Down insulation

What it is: Fine clusters from goose/duck under-plumage.

Key spec: Fill power (FP), e.g., 700, 800, 900. Higher FP = more loft per gram.

• Typical range: 550–900+ FP.
• Higher FP = lighter, more compressible for a given warmth.

Pros

• Exceptional warmth-to-weight
• Compresses very small
• Long service life if kept dry and stored uncompressed

Cons

• Loses loft when wet; slow to dry
• Needs more care (washing, drying, storage)
• Cost rises with fill power and ethical sourcing

Upgrades to look for

• Hydrophobic down treatments to resist moisture
• Box-wall baffles in expedition pieces to reduce cold spots
• Responsible Down Standard (RDS) or similar certification

Synthetic insulation

What it is: Polyester fibres formed as continuous filaments or short-staple clusters to mimic down.

Key spec: Grams per square metre (gsm), e.g., 40, 60, 100, 133, 200 gsm. Higher gsm = warmer.

Pros

• Retains warmth when damp and dries fast
• Better performance in steady drizzle, wet snow, or sweaty climbs
• Easier care; often lower cost

Cons

• Heavier and bulkier than down for the same warmth
• Can lose loft with repeated compression and washing

Upgrades to look for

• Recycled content (lower footprint)
• Zoned insulation (lighter in sleeves/sides)
• Continuous-filament types for improved durability

Performance by condition

Temperature and Effort

• Cold & dry, low effort (belaying, camp): Down maximises warmth with minimal weight.
• Cold & damp, stop-start (UK winters, hut trips): Synthetic handles moisture swings better.
• High-output (hiking fast, ski touring ascents): Synthetic tolerates sweat; pair with breathable shells.

Weather

• Sub-zero with clear skies: Down.
• Near-freezing with sleet/mist: Synthetic or treated down with robust shell.
• Uncertain shoulder seasons: Synthetic for reliability.

How warm is “warm”? Quick guide

Approximate mid-layer jacket warmth bands for typical use. Shell, fit, and metabolism vary.

• Light action layer: Synthetic 40–60 gsm or light down with low fill weight
• All-rounder: Synthetic 80–100 gsm or 700–800 FP down with moderate fill weight
• Deep winter/belay: Synthetic 120–200 gsm or high FP down with higher fill weight and box baffles

Construction details that matter

• Baffle design: Stitch-through = lighter but small cold spots. Box-wall = warmer, used in serious cold.
• Shell fabric: Tighter weaves and DWRs shed moisture and help protect loft.
• Fit: A close, non-compressed fit traps air better. Size for intended layering.

Care and longevity

Down

• Store loose, not stuffed.
• Wash rarely with down-specific detergent.
• Tumble dry low with clean dryer balls until fully lofted.

Synthetic

• Less fussy; machine wash cool with mild detergent.
• Air dry or tumble low; avoid fabric softeners.
• Store dry; moderate compression is fine.

Decision guide

1. Is it likely wet or sweaty?

• → Yes: Synthetic.
• → No: continue.

2. Do you need the smallest pack size and lightest weight?

• → Yes: Down.
• → No: either works.

3. Will you stop and stand around in the cold?

• → Yes: Down or high-gsm synthetic if damp risk.
• → No: Synthetic for active comfort.