Sleeping Bag Liner

The layer that keeps a sleeping bag clean

A sleeping bag liner is a thin sack that goes inside the bag and takes the sweat, skin oil and grit that would otherwise soak into the insulation. It washes in a machine; the bag it protects mostly does not, or not often without losing loft. That is the main job. Warmth, a smoother feel against the skin and a usable layer on hot nights all come after it. The range runs from silk-feel and nylon sacks to fuller covers, in single and wider sizes, most of them packing down to something that disappears in a rucksack lid.

Does a liner add warmth?

A little, and less than the numbers printed on some packaging suggest. A liner traps a thin film of still air and cuts draughts around the shoulders, which is noticeable on a marginal night but is not a substitute for insulation. If the cold is the real problem, the honest fix is a warmer bag: the loft in a down sleeping bag does far more than any liner can, and a liner then keeps that loft clean rather than propping it up.

Which material for which trip?

• Silk feel: the lightest and smallest packed, slippery against the skin, and the one that stops a bag feeling clammy in summer.
• Cotton: heavier and bulkier, absorbent, comfortable for van and campsite use where pack size does not matter.
• Nylon: hard-wearing and quick to dry, the sensible choice if the liner is going to be washed often on a long trip.
• Fleece: the warmest option and the bulkiest, closer to a second layer of insulation than to a hygiene liner.

What size and shape should it be?

A liner needs to match the bag it lives in. A mummy-shaped liner in a rectangular bag bunches at the foot; a rectangular liner in a mummy bag folds into ridges under the knees. Length matters as much as width, since a short liner drags off the shoulders every time you turn over. Check the cut of your sleeping bag first, then match it, and take the larger size if you sleep with a fleece on.

Is a zip worth having?

A zipped liner opens out flat, so it can be used alone as a summer sheet and vented at the feet when the night turns warm. It also makes getting in easier, which is the part that irritates people about closed sacks. The trade is weight, a little bulk and one more component that can fail. Closed sacks stay simpler and lighter, and they slip inside the bag with less fuss.

How is a liner washed and stored?

Wash it cool and often, on its own, and let it dry fully before it goes back in the stuff sack, because a liner packed damp will pass that damp straight into the bag. Store both loose rather than compressed at home. On the ground side of the setup, nothing a liner does replaces insulation underneath you, so pair it with a sleeping mat that suits the season. The rest of the kit that goes with it sits in sleeping equipment.