orange down insulation

Introduction

Down insulation

Synthetic insulation

How insulation works

Breathability

Measuring insulation

More guides

Introduction

Insulated clothing is a core component of a layering system.

This guide will help you choose the best insulation for your needs, focusing on the differences between down and synthetic materials, how insulation works, and how insulation effectiveness is measured.

Down insulation

Down is the soft, fluffy underlayer of feathers from geese or ducks.

pile of down insulation

Synthetic insulation

Made by brands like PrimaLoft and Polartec, synthetic insulation mimics down but is man-made.

Synthetic insulation

Best use

  • Synthetic: Ideal for damp, cold conditions (like UK winters).

  • Down: Best for dry, cold climates (like alpine environments), though high-quality down or hydrophobic down can still perform well in the UK if managed properly.

Water-resistant down

climber wearing a Rab down jacket

How insulation works

Insulation traps air, which reduces heat loss by preventing the transfer of heat away from your body. The more air a material can trap, the warmer it will be.

Breathability

Breathability of down & insulated jackets

close up of layering system

Measuring insulation

Down fill power

Fill power refers to how much space a set amount of down occupies. Higher fill power means better insulation and compressibility. It’s measured in cubic inches per 30g of down.

  • 500 fill power: standard quality

  • 600 fill power: good quality

  • 700 fill power: high quality

  • 800 fill power: Extremely high quality

  • 900 fill power - incredibly rare

  • 1000 fill power: Rare, seen in specialised gear like Patagonia's Encapsil Belay Parka.

The total weight of down used in a garment also affects warmth; more down means more warmth, regardless of the fill power.

Ethical sourcing of down

down fill

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