Canvasdale

Synthetic fill

By the Canvasdale team · Last checked

Synthetic fill is man-made insulation, usually polyester fibres, used in sleeping bags. It copes better than down when it gets damp, and it is often easier to look after, but it tends to pack larger for the same warmth.

Where you'll see it

It appears on sleeping bag labels and product pages, sometimes under a brand name for the fill. The name tells you what the bag is filled with, not how warm it is. Check the comfort rating for the warmth you need.

What to check

  • The fill listed on the label, and whether the maker says how it copes with damp.
  • The packed size, since synthetic fill usually takes up more room than down.
  • The bag's shape and length, since a bag that is too short squashes the fill.
  • The comfort rating, which tells you far more about warmth than the fill name does.
  • Down fill: the alternative insulation, which packs smaller.
  • Loft: the thickness of fill that traps warm air.
  • Sleeping bag liner: a thin layer that can be added to either type.

Read more: sleeping bag temperature ratings explained and, if one fits, the sleep system builder.