Understanding EN Temperature Ratings

The EN 13537 Standard is a standard that is used to measure the temperature rating of a sleeping bag. Here is how it is tested and what it means...


The Test

Used exclusively for hooded sleeping bags, the EN 13537 Standard was developed in Europe (EN stands for European Norm) and is a standard for all sleeping bags sold there.

There are some incredibly specific requirements for the test, broadly speaking, it involves tucking a sensor-rich, heated mannequin into a sleeping bag, and placing it in a cold chamber on a basic foam mat. Each mannequin wears what most of us would sleeping – a thin base layer (the standard actually says “track suit” believe it or not).

As temperatures drop in the cold chamber, corresponding measurements are taken from the mannequin. In a nutshell, the test is looking for key benchmarks like when the mannequin's heat accumulates in the sleeping bag, the range where its temperature remains relatively steady, the point at which heat begins to be lost, and then when it is lost at a rate where the bag is deemed no longer effective, and continued use would put the user at risk. The results are tallied and the ranges of Comfort, Transition and Risk are established, along with the defining limits of these ranges, Comfort, Lower Limit and Extreme, respectively.

One key advantage of the EN test that you’ll note in the graphic below, is its inclusion of a standard for men and women. A “standard man” is assumed to be 25 years old, 173cm tall, with a weight of 73kg. A “standard woman” is assumed to be 25 years old, 160cm tall, and weighing 60kg.

Contrary to conventional wisdom however, the designation of male vs. female is really unnecessary, as recent research has shown that the idea of women sleeping colder than men is a myth. Your ability to generate and retain warmth is more a function of your mass and density, not your sex. Although this is often correlated to gender, a man and woman of equal size and density would have virtually no difference in metabolic rates.

Interpreting the EN 13537 Sleeping Bag Standard

It’s important to note that unlike the old ratings of a single temperature rating that applied only to a fraction of people, the EN 13537 Standard is read in ranges of temperatures, not a specific rating, ­reflecting the very subjective nature of warmth for a wide spectrum of body types. The numbers included along the continuum are merely points of reference between ranges, helping you to estimate where you might find the sleeping bag to provide ample insulation.
Here is a graphic with the definitions of the ranges indicated

Comfort Range (Women's Comfort)
This is the temperature range where a “standard woman" is comfortable. According to the EN standard, she is “not feeling cold,” in a “relaxed posture.”

Transition Range (Men's Comfort Limit)
Here, a standard man is “in a situation of fighting against cold (posture is curled up inside the sleeping bag), but in thermal equilibrium” and not shivering. That means that somewhere within this range is likely the performance limit of your bag.

Extreme Range (Extreme Survival)
This is the minimum temperature at which a "standard woman" can remain for 6 hours without risk of death, but the risk of frostbite is still possible.
At this temperature you will be cold, you will be uncomfortable, you will probably not sleep, and there is the risk of serious injury.
A sleeping bag should only be used in this range in an emergency. Do not buy a sleeping bag based on this temperature!

Remember though – these are just ranges. That’s where reality steps in and puts standards in perspective.

Important Considerations
The stated temperature range should be used as a guide only. Ratings will vary based on whether you are a warm or cold sleeper.
Other factors that will determine a sleeping bag's performance are your metabolism, age, health, fatigue, diet, whether you are wearing socks, thermals, headwear, what type of sleeping mat is used, and how exposed you are to the conditions.

Standards are Still Just Standards

Basically, standards would be great if the world was limited to the variables inherent in the test. Fortunately, we’re all way more diverse than that, as are the places and conditions in which we camp. As individuals, how we feel, what we eat, when we eat it, hydration, what we’re wearing inside the bag and, perhaps most important, how well we fit in the sleeping bag, all have dramatic effects on how warm we sleep. Zoom out to include your environment outside the bag and you’ll find even more variables: How warm is your mattress? You can dramatically improve or reduce a sleeping bag’s warmth with the insulation provided by your mattress. Is it windy? Significant convective heat loss is not accounted for the EN test. What if you’re damp? Are you sleeping with a hot water bottle? Are you used to sleeping outside? It’s a pretty endless list of things that come together to affect how warm we sleep in the backcountry.

However, despite all the variables, the EN standard is still the best way to help you figure out a baseline performance level for a sleeping bag. It reflects a range, and one that accounts for your body size, giving you a great idea of how any given bag will perform in the real world.

So what's the best way to keep warm?

Here are some recommendations for finding a sleeping bag with the warmth you need, along with some tips for adapting your system to achieve warmth across a range of temperatures:

  1. System
    Your sleep system includes your sleeping bag and your camp mattress. The warmth of your mattress will have a significant impact on how warm you sleep. Using the same sleeping bag, sleeping on a good quality mattress will be a very different experience than sleeping on a flat foam pad. The flat foam mat may offer excellent thermal insulation but probably wont be as comfortable.
  2. Fit
     If a bag is too small, you’ll end up compressing insulation and creating cold spots. Find a bag that fits your width as well as your length. Excess space and material in a bag will loosely drape over your body and conform to your shape ,(like bedding) so there is effectively no loss in efficiency, but you may end up simply carrying more material than is necessary if your bag is an overly large one.
  3. Clothing
    Always have dedicated sleeping clothing that you keep dry. They might include long underwear and long sleeve top with hat, gloves and socks as option for pushing my bag’s performance. You can adjust the thickness of these depending on potential weather conditions, and don't be afraid to put on even more layers if the need arises, making sure you maintain the room you need inside to prevent compression of the sleeping bag’s loft.
  4. Right Temperature Range
    Use those EN numbers! They are a great help in finding a bag that will meet your needs. While it’s obviously important to get a bag with ample warmth, it’s also important not to get a bag that’s too warm; if you sweat, you’ll wake up cold. However, also keep in mind the versatility you can add to a bag by the system you build around it.

Hopefully you’ve now got a good idea of what the EN13537 Sleeping Bag standard is and how to use it in finding your next bag. Now all you need to do is be sure to make the time to get out and sleep in it.


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