How to choose a sleeping bag

Most sleeping bags sold in the US now carry an EN 13537 or ISO 23537 rating, which is a standardized laboratory test using a heated mannequin in a controlled chamber. It produces two numbers you will see on the label, and they mean very different things.

The comfort rating is the temperature at which a standard cold sleeper can sleep comfortably in a relaxed posture. The limit rating is the temperature at which a standard warm sleeper can sleep curled up for eight hours without waking from cold. Many brands advertise the limit number because it is lower and sounds better. It describes surviving the night, not enjoying it.

Buy on the comfort figure, and give yourself about 10F of margin below the overnight low you expect. A 20F bag used at 20F is a bag you will be awake in. The same bag at 30F is a bag you sleep in, and that margin costs very little when you are already choosing between ratings.

Two further caveats. The test is conducted with a sleeping pad underneath, so the rating does not apply without one. And the standardized mannequin is not you -- metabolism, hydration, what you ate and how tired you are all move the real figure by several degrees in either direction.

Work through these in sequence. Each one narrows the field enough that the next is easy.

This is the second decision and it has a clean answer for most people: down if you carry the bag and your trips are mostly dry, synthetic if either of those is false.

Down is warmer per ounce and packs far smaller, which is why backpackers use it. Fill power -- 650, 800, 850 -- measures the loft of the down rather than the amount of it, so a higher number means the same warmth in less weight, not a warmer bag. Two 20F bags at 650 and 850 fill are equally warm; the 850 one is lighter and more expensive.

Down's failure mode is water. Wet down collapses and insulates almost not at all, and it takes a long time to dry in the field. Treated or hydrophobic down helps at the margins and does not change the basic picture. Synthetic fill is heavier and bulkier for the same warmth, costs less, and keeps a usable fraction of its loft when damp.

The practical rule: single overnights and dry climates favor down. Multi-night trips without a chance to dry out, canoe trips, anything coastal, and anything with a child who might spill something, favor synthetic. The full tradeoff including hydrophobic treatments is in down vs synthetic sleeping bag.

A bag warms you by trapping air that your body heats, so a bag with too much space inside is a bag you spend the night heating.

A sleeping bag does not work alone. The insulation underneath you is crushed flat by your body weight and contributes almost nothing, which is why bag ratings are tested with a pad and why ground insulation is the usual reason people are cold. Before buying a warmer bag, check your pad -- what R-value sleeping pad do I need explains how to read that number.

Quilts take this logic further by removing the compressed underside entirely and strapping to the pad instead. They are lighter for the same warmth and they are less forgiving: a quilt that is not sealed to the pad lets in drafts, and that takes some practice.

A liner adds a few degrees, costs little, and is far easier to wash than a down bag. If you are between two ratings, a liner is a reasonable way to buy the warmer-weather bag and extend it. Do not expect the full claimed gain.

Almost all of a sleeping bag's lifespan is decided by storage. Stored compressed in its stuff sack, the fill -- down or synthetic -- takes a permanent set and loses loft, which is warmth. Store it uncompressed, in the large mesh or cotton sack it came with, or hanging in a closet.

Wash it rarely and correctly. Body oils and sweat degrade down over years, so a liner is the cheap preventative. When it does need washing, use a front-loading machine, a down-specific or mild detergent, and tumble dry on low with dryer balls for several hours. A down bag that is not fully dried will clump and smell.

In the field, air it out every morning. Twenty minutes draped over the tent in the sun removes most of the moisture you put into it overnight, and that is the difference between a bag that performs on night four and one that does not.