Temperature Ratings Explained

When looking for a sleeping bag the two main things most people consider are cost and warmth. Next might be whether it’s made with down or synthetic insulation, and the weight or pack size of the bag. Cost, weight and insulation type are pretty uncontentious, but ‘how warm is a sleeping bag’ is arguably as confusing now as it has ever been.

The good ole days

The early days of buying sleeping bags was pretty simple: you bought a bag based on a recommendation from a friend, shop staff, or a magazine. Occasionally you might chance it on a new model from a big name manufacturer. However, as the industry grew and new users wanted to buy sleeping bags, manufacturers needed a simpler way to advertise how warm their products were. This led to the development of many different methods for testing the warmth of sleeping bags, from simple lab tests to complex ones, to field tests, even just measuring the thickness of the bag (which doesn’t work). There were simple rules of thumb too: 400 grams of down meant a summer bag or ‘two season’ bag; 800 grams of down was a winter bag or ‘4-season’ bag. Despite the growing complexities of sleeping bags, these simple rules have stood the test of time better than many other tests.

Eventually, number-specific temperature ratings arrived as the ‘most accurate’ way to describe the warmth of a sleeping bag. This is where the real controversies begin.

The bad ole days

The problem with lots of different tests used to measure sleeping bags is that you can’t necessarily compare one bag with another. The same bag tested in three different ways might get three different ratings, so how do you know warm it will really be? Historically, comparisons between brands from the same country generally weren’t too difficult, but if comparing between manufacturers from different countries it was basically impossible. Inaccurate temperature ratings came to a head when much cheaper sleeping bags were released to the market with temperature ratings that were seemingly fabricated. Suddenly, high street shops and supermarkets were selling sleeping bags with the same claimed warmth as specialist bags that cost ten times more.

Standardisation

This situation was rectified when a sleeping bag testing standard was developed, which was influenced strongly by Mountain Equipment and our experience of testing bags. While not legally binding, the standard meant that if manufacturers released bags onto the market that had wildly inaccurate temperature claims then they could be reported to trading standards. The standard, EN 13537: “Requirements for Sleeping Bags” was introduced in 2002 and was a big step forward in trying to standardise the temperature ratings given to sleeping bags, meaning that consumers could compare sleeping bags from different manufacturers with relative ease. The 2002 standard has since been superseded by the EN 13537: 2012 standard, and the current standard is now ISO 23537. By becoming an ISO test it is now recognised globally, and in Europe and North America ISO 23537 results are now commonplace on sleeping bags. The testing standard itself undergoes fairly regular reviews, and there are occasional meetings where testing labs, academics, retailers and brands meet to discuss possible improvements or developments. Mountain Equipment have been a constant part of these meetings for the past 20 years and we remain part of the UK’s guiding committee – one of only three members - for the standard.

ISO 23537

ISO 23537, and EN 13537 that went before it, uses a person-shaped heated manikin composed of numerous different zones each equipped with their own temperature sensors and power sources. The manikin is used inside a climate-controlled room and lies on a standardised sleeping mat. He wears a set of standard pyjamas and a cold weather mask which lends him a frankly terrifying appearance. This is made worse by the data from the manikin being fed to a computer by cables which often come straight out of the manikin’s eyes.

Before a test, sleeping bags are allowed to loft and are kept at a constant temperature and humidity to improve repeatability. The manikin is put inside the test sleeping bag and because the room is cold and the manikin is warm, the power required to keep the manikin warm is measured to determine the thermal resistance (warmth) of the sleeping bag. The test is carried out until steady-state or equilibrium is achieved, which takes a few hours.

Once the manikin test is completed, the thermal resistance numbers are correlated to a series of temperature ratings given in the standard. This conversion is based on testing with real life people going back many years. The standard produces three temperature ratings:

• Comfort temperature. This is where a ‘standard woman’ with relaxed posture is just not feeling cold.
• Limit temperature – This is where a ‘standard man’ with curled up posture is just not feeling cold. This is the number which most shops and brands make the biggest deal out of.
• Extreme temperature – This is where risk of health damage to a ‘standard woman’ by hypothermia occurs. This number can basically be ignored unless you have a masochistic bent.

The standard has removed a lot of the big discrepancies between manufacturers and is fairly repeatable, to within a couple of degrees. It relies on a manikin that provides a consistent heat source unaffected by diet, feeling tired, mind-set, or other factors that might affect the thermoregulation or sensations of a real person, and so it is better than some of the field trials that went before. Also, unlike some other lab tests, it measures a whole sleeping bag. However, like many testing standards it is not perfect, and it’s important to remember that the test measures the thermal resistance of a bag on a manikin which might not represent you or the way you sleep, and in an environment that is not necessarily like the one you sleep in.

Drawbacks of the ISO 23537 test

There are a few drawbacks of the ISO 23537 test, and three of them are explained below:

Manikin shape

The manikin used in the test is roughly person-shaped, but actually varies a little bit depending on which lab carries out the test. The problem is that any tight spots on the bag around the manikin will result in the insulation getting compressed and the bag being perceived as colder than it really is. Of course, the same thing would happen with a real person, except a real person would pull their arms in if they are cold and will curl up, making themselves smaller. As a result of this, the manikin test often favours larger-fitting sleeping bags, and you will often see poor ISO 23537 results for bags cut with a tailored or slim fit. Unless the manikin accurately represents your size then the insulation offered by the sleeping bag will differ: an overly-large sleeping bag allows air to circulate around you, making you cold, while a bag that is too small will be constrictive and compress the insulation around you, again making you cold. It is for this reason that if you are trying to make a good and informed decision on buying a sleeping bag then it is a very good idea that you try it for size the way you would a jacket or pair of climbing shoes. This is a reason why we have numerous different fit options, different length options, and women’s iterations in many models.

Winter sleeping bags

Another important thing to remember is that the ISO 23537 test excludes the testing of “extreme climate zone expedition” sleeping bags. This is perhaps deliberately quite vague, and while officially the test doesn’t apply to bags which have a measured limit temperature colder than -24 °C, we think that below approximately -15 °C the accuracy of the test begins to break down. For sleeping bags containing over 1000g of down, or very thick synthetic bags, the test can produce results that differ significantly from the experiences of real users, and the warmer the sleeping bag, the more the discrepancy increases. As a result, if you are buying a bag for very cold conditions then you need to do it the old fashioned way: talk to experienced friends or shop staff, do your research, and buy from a trusted brand who might have carried out testing in addition to that of the manikin test. These methods are likely to be more reliable than trusting temperature ratings produced by ISO 23537. Also, use some common sense: if a bag contains 1200 grams of down it is likely to be warmer than one containing 800 grams of down unless their construction or quality is very different. 

The effect of face fabrics

When sleeping in a hut or in a sheltered tent the face fabric (the fabric on the outside of a sleeping bag) does not really affect the warmth of the bag, assuming that the bag remains dry. This is because there is very little air flow over the sleeping bag in these conditions. However, if sleeping outside with no shelter, or in a draughty tent with a gale blowing through it, there is a lot of air flow, and this means that face fabrics are suddenly very important in determining how warm the sleeping bag is.

The ISO 23537 test is carried out with air moving directly down on to the bag at about 1 kmph. This is a faster air flow than most users might experience if in a good tent, in a hut, or in a bivvy bag, where air is relatively still. This difference doesn’t matter for most sleeping bag face fabrics which keep the wind out. However, if the fabrics used on the sleeping bags are very air permeable then the ISO 23537 test tends to produce poor results. So why would you want a sleeping bag fabric that produced a poor temperature rating? The problem is that in most real life conditions an air permeable fabric does not make a bag seem cooler, and there are plenty of advantages to these fabrics: they are exceptionally breathable, very comfortable, fast drying, and usually the most lightweight. Air permeable fabrics also have a remarkable ability to dry wet layers inside them, which is a real benefit to those on multi-day trips. Many of our pro team use sleeping bags with exceptionally lightweight and air permeable fabrics because they work so well in real life conditions.

What does this all mean for ISO 23537?

ISO 23537 is the best rating system that has ever existed for sleeping bags and shouldn’t be dismissed despite a few shortcomings. However, it is worth remembering that the test was developed to determine if bags were far from their advertised rating. Comparing two bags that are within a few degrees’ rating of one another on ISO 23537 merely shows that they are similar in warmth; it doesn’t necessarily show that one bag is a few degrees warmer than the other.

Something that is rarely discussed is just how useful ISO 23537 is for sleeping bag designers like us. It is an invaluable tool for development work and we conduct numerous tests throughout a new product’s development cycle. The information we get from the test is exceptionally useful, and often highlights information that a human test subject might not have noticed.

Which temperature rating to use?

If buying a Mountain Equipment sleeping bag then we suggest you buy based on our Good Night’s Sleep temperature. We think that this rating is the most accurate on how warm a bag will keep you warm in real-life situations. In most cases our Good Night’s Sleep rating will tally closely to that of the ISO 23537 Limit temperature, but for certain bags the discrepancy might be greater. Our Good Night’s Sleep temperature has been developed considering not only the ISO test’s result, but is also based on extensive experience of manufacturing sleeping bags and extremely exhaustive testing in the laboratory, during cold chamber testing, and it is influenced by user feedback from countless expeditions stretching back sixty years. We also guarantee our Good Night’s Sleep temperature, allowing you to choose with confidence. For more information on our Good Night’s Sleep guarantee please see here: https://www.mountain-equipment.co.uk/pages/a-good-nights-sleep-guaranteed

Information overload?

While we suggest you base your sleeping bag purchase most closely on our Good Night’s Sleep temperature rating, you should of course consider whether you particularly feel the cold or otherwise. This individuality is very important and is overlooked by temperature ratings. In the same way, a person who is ill, exhausted, hungry or sleeping in the open is much more likely to be cold than someone who’s fit, well-fed and sleeping in a sheltered spot, and so our Good Night’s Sleep temperature should be used as a guide, but it’s ultimately up to you to decide how warm a bag you think you might need.