Sleeping bag fill

A sleeping bag’s fill insulates you from the cold environment: it traps body heat and prevents the exchange of warm and cold air. It’s simple really, although it’s easy to lose sight of this when boggled by technical details about loft, feather/down composition, fill weight and synthetic suspension systems. At its most basic, it comes down to two choices: down (natural) or synthetic fill.

Good quality down is still the most efficient sleeping bag insulation available, with the best warmth-to-weight ratio and amazing compressibility. With proper care, down lasts a long time, maintaining its performance better than synthetic fibre. On the flip side, down is expensive and not exactly easy-care. When traditional down is wet it loses most of its insulation value, and it can take a long time to dry. Some manufacturers (including One Planet) apply a durable water-repellent (DWR) treatment to their down at the final stage of production. This can reduce the amount of water absorbed by the down in damp conditions, allowing it to retain higher loft (insulation) and dry faster.

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Synthetic fill is also an excellent option. It is easier to clean, maintains greater insulation when wet and is less expensive than down. However, it is also slightly bulkier, heavier and may have a shorter user life.

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It is worth noting that sleeping bag fill works most efficiently when clean.


Down forms the lightweight undercoat of aquatic birds’ plumage, insulating them in extreme cold. The plumes and clusters in good quality down create hundreds of thousands of tiny air pockets, where air is trapped and warmed by body heat. By utilising this ‘golden fibre’ in sleeping bags, humans can trap heated air close to their body to maintain warmth. The quality of the down in a sleeping bag makes a huge difference to the performance of the bag, in terms of warmth, weight, durability and compactness.

  • What is loft?
    Loft is a measure of the ‘fluffiness’ of down – or, to be more scientific, the volume taken up by a given weight of down. The higher the loft (or fill power), the more room it takes up and the more efficiently it traps air. For a given weight of down, the higher the loft, the larger volume it will fill, meaning a smaller weight of down is required to fill each sleeping bag baffle. The test for measuring loft entails placing a given weight of down in a cylinder with a specially calibrated weight on top. The weight is allowed to fall slowly on to the down for a period of time. The position at which the weight stops, supported solely by the down is the loft: the higher this reading, the loftier the down and the better its insulating power. The loft is calculated before the bag is even made, using new, clean down. As time passes, age and cleanliness will have an impact on the down’s loft.
  • Composition
    The composition of down fill is described by the ratio of down to feather. To measure the down/feather mix, a sample is separated into four parts: down plumes, down fibre, small feather and ‘other’, with each represented as a percentage (for example 90 per cent down, ten per cent small feather). Generally speaking there is a correlation between a high percentage of down and high loft; however, this is not always the case.
  • Species
    There is a widespread belief that down from a goose is better than that from a duck even though the performance characteristics are due to loft rather than the originating species. Why? The roots of the misconception lie in the fact that until recently the highest lofting duck down available in commercial quantities was 750 loft, so anything ‘loftier’ than this came from a goose. This led to the outdoors industry (including One Planet) promoting goose down as superior as the best insulating down came from geese. (As an interesting side note, the highest performing down comes from the Eider Duck.)
  • Fill ratio
    This is the proportion of down placed in the top and bottom of the bag. Most people move in their sleep, so loading the top of the bag with lots of extra down isn’t necessarily effective. A common fill ratio is around 55/45, meaning 55 per cent of the fill is in the top of the bag where it provides maximum warmth.


Synthetic sleeping bags are a good alternative to down sleeping bags. They are non-allergic, provide some warmth when wet and are easy on your wallet.

However, there is a huge variation in quality in the synthetic sleeping bag market place: as a general rule you get what you pay for. A good quality synthetic sleeping bag will also be compact and last longer than an entry-level bag.