Down fill
Down fill is the soft, fluffy under-feathers of ducks or geese, used to insulate a sleeping bag. It traps a lot of warm air for its weight and packs small, but it loses insulation when it gets damp and takes longer to dry.
Where you'll see it
It is named on sleeping bag labels and product pages. A fill power figure, if listed, describes how well the down springs back and traps air, rather than how warm the whole bag is. The fill weight tells you how much down the bag holds, so read the two together.
What to check
- The fill weight and fill power, as listed on the label or the maker's page.
- Whether the maker gives advice on keeping the down dry, since damp down loses warmth.
- The packed size, to check it suits your car or rucksack.
- Whether the comfort rating is listed, because the fill type alone does not show how warm a bag will be.
Related terms
- Loft: the thickness of fill that traps warm air, which down provides.
- Synthetic fill: the alternative that copes better when damp.
- Comfort rating: the figure to compare bags by.
Read more: sleeping bag temperature ratings explained.