It’s 3 a.m. and it’s cold. Not just outside, but in the tent. In fact, it’s now creeping into your sleeping bag. You’re wearing everything you brought, including your sunglasses, extra underwear and yesterday’s dirty socks, but the cold keeps coming. Short of getting up and running back to the car screaming, you’re out of ideas. You lie there on the fringe of full-on shivering, hoping you don’t wake your tent mate with your slow, spoon-like advances in the search for warmth.

What really gets you is that you’re in a sleeping bag rated to 20 F, and it’s only 25 F outside. You should be fine, right?

The answer is a definitive “maybe.” In fact, there are so many variables that affect how warm you sleep while camping, that the rating is just a fraction of the equation for warmth. And if that’s not enough, there is no standard way to rate sleeping bag warmth. However, the EN 13537 Standard that is now used by many sleeping bag manufacturers, including Therm-a-Rest®, is a great step in the right direction.

“EN13537 is the only agreed-on calculation for translating insulation value to a temperature range, based on extensive physiological research,” says Jim Giblin, Therm-a-Rest’s Sleeping Bag Category Director.

So let’s take a moment to explain what the EN test procedure is, what its limitations are, how to interpret the numbers, and how you can use them to make sure you get the best bag for your needs.

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. An ASTM standard was used previously here in the US, but was primarily designed to make controlled comparisons of material performance, and wasn’t built to provide a specific rating per se. The numbers most manufacturers used came from that test, but were pretty subjective, and have no doubt resulted in misleading bag ratings and some awkward, unplanned spooning.

While there are some incredibly specific requirements for both tests, broadly speaking, both involve tucking a sensor-rich, heated manikin into a sleeping bag, and placing them in a cold chamber on a basic foam mat. Each manikin 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 manikin. In a nutshell, the test is looking for key benchmarks like when the manikin’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, 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, 1.73 m tall (5’ 6”) and weighing 73 kg (160 lb). A “standard woman” is assumed to be 25 years old, 1.6 m tall (5’ 2”) meters tall and weighing 60 kg (132 lb).

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 the latest graphic that the Therm-a-Rest team has developed with the definitions of the ranges indicated:

Comfort Range

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.” Sounds comfortable, right?

Transition Range

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

Per the EN Standard’s language: “In this range, a strong sensation of cold has to be expected. There is risk of health damage by hypothermia. A sleeping bag should only be used in this range in an emergency.”

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

EN Rating

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. You’ll also find Therm-a-Rest sleeping bags categorized by season­, offering another way to zero-in on the bag that’s right for you.

“It took me years of pushing the limits of my sleep systems to figure out how to get the most efficient and comfortable sleep in the backcountry,” says Giblin. “Everyone develops a unique system over time.” This sentiment is clearly reflected in Giblin’s unique designs for Therm-a-Rest, integrating the components of a sleep system’s components to maximize their overall comfort.

So, if you’re new and don’t have a few decades of experience under your belt, here are Giblin’s top 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 NeoAir® XTherm™ mattress will be a very different experience than sleeping on a flat foam pad.”

2. Fit

“It’s important to be able to move around in your sleeping bag, but you don’t want so much room that you spend all your energy heating empty space. 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.”

3. Clothing

“I always have dedicated sleeping clothing that I keep dry. They include long underwear and long sleeve top with hat, gloves and socks as option for pushing my bag’s performance. I tend to adjust the thickness of these depending on potential weather conditions, and I’m not afraid to put on even more layers if the need arises, making sure I maintain the room I 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.

Choosing the Right Sleeping Bag: Part 2—Down Vs. Synthetic>>

Originally published November 10th, 2015.