'Loft' is how much the fill of a sleeping bag expands from its compressed state after unpacking to form an effective layer of trapped air able to be warmed by body heat for insulation. For insulation, most sleeping bags are filled with natural materials such as down or feathers, a mix of both or with synthetic fibres. Whether the fill is natural or synthetic is the key factor in weight, price and performance.
Fluffy down (the under-plumage of ducks and geese) is by far the best natural insulator. It is light, easily compressed to pack small and its loft recovers fully when shaken out. Initially costing much more than synthetic fills, down bags will last for years and years if looked after well. The problem usually trotted out is that they lose most of their ability to hold warm air when wet and are difficult to dry on the trail; the simple answer is to keep your bag dry. With modern outer fabrics, waterproof stuff sacks and quality tents, it really is not difficult to achieve. Down bags can deliver greater warmth, less weight and lower bulk than similar performing synthetic bags though the gap narrows steadily. Down is often mixed with less effective feathers to reduce cost so make sure you know what you are looking at.
Synthetic fibres come in quite a variety of forms and effectiveness. Overall, they are cheaper, heavier, bulkier and do not last as long as quality down. However, they maintain insulation when wet and will dry fast. Synthetic fills are made up from layers of fibres sewn to fabric to keep them in place. Some of these fibres are hollow, acting like animal fur in trapping warm air, helping to account for price differences along with varying construction techniques in apparently identical bags. Others mimic the qualities of down closely and feel very similar.
Being jammed for months on end in a stuff sack reduces any material’s ability to loft over time. After each trip, air sleeping bags well and try to hang them up or, at least, store them in a large mesh or cotton bag. Plastic bags mean that any moisture retained could encourage mould and an unpleasant smell when you come to use them next. A sheet liner will absorb body oils and sweat reducing the need to wash bags and, by keeping them clean, help maintain their efficiency.
The best advice for cleaning sleeping bags is the simplest – follow the manufacturer’s instructions. All have comprehensive advice on offer and should give in a good result.