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A Guide to the Layering System
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How to choose insulation for your layering system
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.
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Synthetic insulation
Made by brands like PrimaLoft and Polartec, synthetic insulation mimics down but is man-made.
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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.
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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
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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
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Final thoughts
Head in store for advice from our expert staff on the best insulation for your needs.
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