Choosing, using and caring for a sleeping bag

The fill of a sleeping bag traps air. The higher and fluffier it rises – this is called “loft” – the more still air it holds and the better it insulates. The fabric outside and inside keeps the fill in place and slows drafts. Baffles and seams make sure the fill does not shift and no cold spots appear.

Under your body the fill is squashed so much that it barely insulates there. The sleeping pad takes over that job. A very warm bag on a thin pad can therefore feel colder than a simple bag on a good pad. Sleeping bag and pad are one system.

Moisture is the second big enemy. It comes from outside, for example rain or condensation in the tent, but also from inside: your body gives off water vapour every night. If you breathe into the bag, extra moisture from your breath ends up in the fill. Down loses more of its warmth than synthetic fill when this happens. That is why it matters so much to carry the bag dry and air it in the morning.

Many sleeping bags are tested to the EN ISO 23537 standard. A heated mannequin lies in the bag on a sleeping pad under defined conditions, and three temperatures are derived from the measurements: comfort, limit and extreme. Roughly, the comfort rating describes the temperature at which a person who feels the cold fairly easily does not feel cold lying relaxed. The limit rating describes down to what point a person who does not feel the cold easily can just about sleep curled up without feeling cold.

The extreme rating is not a sleeping temperature but a warning range in which there is a risk of hypothermia. Never choose a bag by this figure. Instead, compare the comfort or limit rating with the lowest night temperatures you expect and allow a margin. If you feel the cold easily, go by the comfort rating.

The standard values only apply under the test conditions. How warm you actually are also depends on how tired and hungry you are, how damp the air is, whether wind blows through the tent and how good your pad is. Not every bag is tested to this standard. Without standardised ratings, manufacturer figures are hard to compare and should be treated with caution.

Down or synthetic? Down is lighter and packs smaller for the same warmth but is sensitive to wetness. Synthetic fill is heavier and bulkier, keeps more insulation when damp and is often easier to care for. In damp climates or when you have little chance to dry things, synthetic is often the more relaxed choice.

Fit affects warmth more than many people think. If the bag is too big, your body has to warm up a lot of cold air as well. If it is too tight, you squash the fill from the inside. Hood and draft collar at the neck should close well, because otherwise a lot of warm air escapes there. There are shorter bags made for children – an adult bag that is far too long stays cold at the foot end.

When sleeping, it helps to: shake the bag out first so the fill lofts. Lay it on a suitable pad. Wear a hat and put on dry socks. Do not breathe into the bag; keep mouth and nose outside. A liner protects against dirt and can add a little warmth. Blankets over the bag are possible as long as they do not heavily squash the fill.

In the morning let the night's moisture escape: open the bag, air it or lay it briefly in the sun if possible before packing it again. After the trip, dry it completely at home. A bag forgotten while damp can turn musty, and the fill suffers.

At home, do not keep it in the small compression sack but loosely in a large breathable bag or hanging up. Months of compression can reduce the loft for good. The small stuff sack is only meant for the trip.

Wash it only when really necessary and exactly as the manufacturer says – with the recommended detergent, the stated temperature and the described drying. Down in particular must dry right through, otherwise it clumps and leaves cold spots. The manufacturer's care instructions always take precedence over general tips.

Someone who is very cold at night does not just sleep badly but can also become hypothermic. So always plan with a margin: a bag that suits the coldest night you expect, a pad that insulates enough and a dry layer of clothing to put on. On snow or frozen ground you need far more insulation underneath than in summer.

A wet sleeping bag insulates much less well. So pack it waterproof, for example in a dry bag or a plastic bag inside the pack, and watch for condensation in the tent. Good ventilation reduces the moisture that collects on the inside of the tent fabric and drips onto the bag.

If someone shivers hard, seems confused or very drowsy despite the sleeping bag, that is a warning sign of hypothermia. Then protect them from wind and wet, put on dry layers, insulate them well from the ground and get help via the local emergency number (112 in the EU). By the way, a sleeping bag can also help at home if the heating fails.