Sleeping bag temperature ratings: comfort, limit and extreme explained

Three temperatures on one label

2am, in the tent. The thermometer on your watch reads 3 °C, the bag promised ‘0 °C’, and yet your feet are cold. The bag did not lie: you simply read the wrong number. A bag tested to the standard carries three on its label, and they do not say the same thing.

These three figures come from ISO 23537-1, the current version of which dates from 2022. It sets out the requirements, test methods and labelling for adult sleeping bags intended for sport and leisure, down to a limit temperature of -20 °C; it does not cover military bags, expedition bags for extreme climates or bags for children [1].

• The comfort temperature: the lowest at which you sleep well, in a relaxed position [2]. Decathlon specifies that it is set for a standard woman aged 25, weighing 60 kg and 1.60 m tall [3].
• The limit temperature: the lowest at which you can sleep through the night curled up to protect yourself from the cold [2], set for a standard man aged 25 and 1.73 m tall [3].
• The extreme temperature: a survival temperature, for emergencies only, at which you can stay curled up in the bag for six hours, with an increased risk of frostbite [2] and hypothermia [3].

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How it is measured: a heated mannequin in the bag

The ratings do not come from a trial night in a tent. In a laboratory, a thermal mannequin fitted with heaters and sensors is placed in the bag, on a foam mat, and a series of measurements assesses how well the bag insulates at different temperatures [2]. Decathlon has these tests carried out by an independent laboratory, AITEX [3].

The method has two virtues: it is repeatable, and it allows bags from different brands to be compared. It also has its limits. The mannequin does not sleep, has no supper before bed and may look nothing like you. Expedition bags, finally, are beyond its scope: Rab points out that the standard does not measure performance below -20 °C [2].

Why two ‘0 °C’ bags are not equally warm

Here is the catch on the label: nothing obliges a brand to use the same rating in a bag’s name. Decathlon shows the comfort temperature on all its sleeping bags [3]; Rab names its models after the limit temperature [2]. Decathlon’s MT500 ‘5 °C’ trekking sleeping bag, for example, has a comfort temperature of 5 °C and a limit of 0 °C [3].

A bag called ‘0 °C’ after its limit will therefore sleep colder than a bag called ‘0 °C’ after its comfort rating: on the MT500, the gap between the two ratings is 5 °C. Before comparing two prices, compare the same two figures: comfort with comfort, limit with limit. And be wary of a bag that shows a single temperature without saying which one, or to which standard.

Choosing your bag’s temperature for the season

Start from the coldest night you expect, where you will actually sleep: at altitude or at the bottom of a valley, the night can be much colder than the forecast for the nearest town. Then choose a bag whose comfort temperature is at that level or below. The limit temperature is your floor: get close to it and the night will be a long one.

If you feel the cold, work from the comfort rating; if you sleep warm, the range between comfort and limit is still bearable, curled up. Either way, never choose a bag by its extreme temperature: it is a survival temperature, not a sleeping temperature [2].

The mat, your clothes and the rest

The standard’s temperatures assume the mannequin is lying on an insulating mat [2], and a serious one: Decathlon gives an R-value of 4.8 for the test mat [3], while Exped speaks of R 7 and recommends at least 6.5 for a bag to deliver its rated warmth [4]. On bare ground, the earth draws heat out through the crushed insulation, and no bag keeps its promise: the mat matters as much as the bag, and its insulating value is printed on a label too, as our guide to sleeping mat R-values explains.

A tent sheltered from the wind, a dry layer of clothing for the night, a woolly hat: anything that keeps the heat in adds to the bag. In winter, in a campervan, a good bag also lets you turn the heating down, as our guide to campervans in winter explains.

A good habit to get into: read both figures, comfort and limit, and choose your bag by the first.

Guillaume Hoarau

Camping, wild camping, bushcraft and hiking

Guillaume Hoarau writes for Outfarer about camping, wild camping, bushcraft and hiking. A survival-skills enthusiast and a collector of old field manuals, he likes clear explanations, lists that earn their place and other people’s stories, which he tells well.

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