Sleeping Bag

Sleeping bags, read as a system rather than a single number

A sleeping bag does not produce heat. It slows down the heat a body already makes, and how well it does that depends on the loft of the filling, the shape of the shell and what sits underneath. The range runs from cotton lined three season bags to goose and duck down builds for cold nights, in mummy and rectangular cuts, so settle the season first, then the filling, then the shape.

Down or a cotton lined synthetic filling?

Down traps air in clusters that spring back after being squashed, which is why a down sleeping bag packs smaller and weighs less for the warmth it holds. The trade is behaviour when damp: wet down clumps, loses its loft and takes a long time to dry. Cotton lined and synthetic fillings are bulkier to carry, but they keep some structure when they take on moisture and they shrug off rough handling in a car boot. Goose down generally forms larger clusters than duck down, which is why the two are priced apart.

Which shape suits the way you sleep?

A mummy cut tapers from shoulder to foot and adds a hood, so there is less air inside for the body to warm and less draught around the neck. It suits people who sleep still on their back. A rectangular bag gives room to bend a knee or roll over and opens flat as a quilt in summer, at the cost of dead space that has to be heated. Someone who thrashes about in a mummy bag sleeps colder in it than in a roomier cut.

What should you check before buying a sleeping bag?

• Zip baffle: an insulated tube behind the zip line, without which the teeth become a cold strip running the length of the body.
• Hood and neck collar: a drawcord that closes around the head and a second one at the shoulders stop warm air being pumped out every time you turn over.
• Foot box: a shaped end lets the feet sit upright rather than flattening the filling above the toes, where cold is felt first.
• Shell finish: a tightly woven outer resists condensation dripping off tent walls and keeps down clusters from working through the fabric.

Why does the mat underneath matter so much?

Body weight crushes the filling between sleeper and ground, and crushed filling holds no air. The ground then draws heat away all night whatever the bag does. That is why a self inflating mattress belongs to the sleep system rather than counting as an extra. A foam sleeping mat does the same job for less bulk, and either explains why a warm bag on bare ground disappoints people every autumn.

How do you keep a sleeping bag in shape?

Compression is for the walk, not for the cupboard. Months squashed inside a stuff sack leaves the filling flat at the folds and it never fully recovers. Store the bag loose in a large breathable sack, air it after a trip and wash it as rarely as the dirt allows. Sleeping inside a sleeping bag liner takes the skin oils and grit that would otherwise reach the filling, and the liner goes in the machine instead.