Anyone who has shopped for a sleeping bag is all-too-familiar with temperature ratings of sleeping bags. Unfortunately, as many people have also discovered, sleeping bag temperature ratings are often wrong—and sometimes very wrong.
While it is always tempting to blame manufacturers for inaccurate labeling, there are many reasons a sleeping bag fails to keep everyone warm at a specific temperature. This article first explains how manufacturers obtain a sleeping bags temperature rating. Then the article explores other factors that affect a bags temperature rating. Additionally, you might wish to read other articles about sleeping bags on this site:
What Sleeping Bag Temperature Ratings Mean
Less than ten years ago, a sleeping bag temperature rating was the manufacturer’s “best guess” how warm the bag was for the “average person.” They arrived at this rating by giving bags to testers or employees, who then tested the bags in various environmental conditions or in an “ice box.”
However, the “average person” is a rare person. You see, the reason a temperature rating was never fully accurate is because people sleep at different body temperatures. If you are a “cold sleeper,” then you are going to need a significantly warmer bag then someone who is a “hot sleeper.”
Hot Sleepers v. Cold Sleepers
So what are hot and cold sleepers? The best way to understand the concept is to use sleeping at home in your bed as an example, in a house heated to 65 degrees.
Hot Sleeper – A hot sleeps with few if any sheets. They might use sheets at first, but as the night goes on, the sheets magically disappear. Also, hot sleepers gives off significant heat. If you have a bedtime partner and they complain about how warm you make the bed, then you are probably a hot sleeper.
Cold Sleeper – A cold sleeper goes to bed with every blanket piled on top of them, yet still wakes up cold. This happens because the person has a slow nighttime metabolism. Their body produces little body heat, so many blankets are need to stay warm.
Europe Rides to the Rescue with EN 13537 Sleeping Bag Temperature Regulations
Perhaps recognizing the inherent problems with prior sleeping bag testing standards, Europe in 2005 adopted what was then known as EN 13537. This regulation laid out a standardized testing regime for sleeping bag temperature ratings. The regulations laid out guidelines as follows (quoted from Wikipedia):
Upper Limit — the temperature at which a standard male can sleep without excessive perspiration. It is established with the hood and zippers open and with the arms outside of the bag.
Comfort — the temperature at which a standard female can expect to sleep comfortably in a relaxed position.
Lower Limit — the temperature at which a standard male can sleep for eight hours in a curled position without waking.
Extreme — the minimum temperature at which a standard female can remain for six hours without risk of death from hypothermia (though frostbite is still possible).
These ratings are taken assuming that the subject is using a sleeping pad, tent and is wearing one base layer of thermal underwear.
For the purpose of these measurements, a “standard man” is assumed to be 25 years old, with a height of 1.73 m and a weight of 73 kg; a “standard woman” is assumed to be 25 years old, with a height of 1.60 m and a weight of 60 kg.
While these regulations started in Europe, they quickly spread to the United States. Several specialty gear retailers, specifically REI, started requiring sleeping bag manufacturers who wanted their bags sold at REI to have their bags undergo testing to determine the sleeping bags rating using EN 13537 methodologies.
Once REI started the process, other retailers—especially online—started to require the same thing. And so now, most but not all temperature ratings you see for sleeping bags arise from testing using EN 13537 methods.
How the EN 13537 Test is Performed
So what are EN 13537 methods? Well, it involves placing a heated mannequin that wears a hat and a thin layer of upper/lower thermal underwear inside a sleeping bag. The sleeping bag is then placed on top of a thin closed-foam sleeping pad. The mannequin is then wrapped with sensors. Once the mannequin is ready, everything is slid into a “cold box.”
The test involves examining how quickly the sleeping bag loses heat. This rate of heat loss then goes into determining the various upper/lower limits of the sleeping bag, as well as helping to determine the “comfort zones” of the sleeping bag.
In short, the EN 13537 test establishes a baseline.
The test is not meant to provide a specific temperature rating—as in, the bag will keep everyone warm to a specific degree temperature. Instead, it simply provides a baseline so consumers can better compare one sleeping bag to another.
Yet the Same Problems Remain
Despite the rather scientific test provided by EN 13537, the same old problems remain when it comes to measuring how well a person stays warm inside a sleeping bag during real-time use.
In particular, you run into these problems (or perhaps I should say variables) that can really throw off how well a sleeping bag keeps a person warm at night when sleeping outside.
Hot Sleepers and Cold Sleepers
Whether a person is a hot sleeper or a cold sleeper. Typically, men are more likely to be hot sleepers and women are more often than not cold sleepers. Yet this isn’t set into stone (I’m a guy and a very cold sleeper). Thus, a “cold sleeper” still needs to remain vigilant that they buy a sleeping bag that’s on the “upper end” of the “comfort range” of the EN 13537 ratings system.
What Sleeping Pad Is Used
Tests using the EN 13537 use a thin closed-cell foam pad. This pad provides relatively low insulation. For campers desiring more comfort than a thin foam pad, by going with a thicker and more comfortable sleeping pad, the camper also gains additional warmth. Remember, much of a sleeper’s warmth is lost to the ground. A thicker sleeping pad goes a long ways towards “bumping up” how warm a sleeping bag is in actual use.
Read the Sleeping Pad Buyers Guide article to learn more about sleeping pads and what kind to get for specific activities.
The EN 123537 test assumes the use of a tent, which is common sense. But how big of a tent? What are the tents wall made from? Is the rain fly used? How many people in the tent? These are all unanswered questions. And it makes a big difference.
For a solo camper, a smaller tent—especially one that uses a rain fly—will be much warmer than a larger tent. I’ve always been amazed that my own smaller backpacking tent, a Big Agnes Copper Spur, keeps the tent a good 20 degrees warmer than the outside air temperature when I have the tent fully closed off. Those thin nylon walls really do make a difference in retaining heat. But it’s a lot easier for one person to “heat up” a small tent than a larger tent.
The EN 12537 test uses thermal underwear, which is a good clothing choice to test. Such underwear keeps the wearer dry and doesn’t leave the sleeper covered in perspiration during the night.
If you want to stay comfortable and warm in your sleeping bag, never wear cotton clothing. The reason for this is very simple. Cotton clothing absorbs water, which when you sleep is perspiration. This water coats the sleeper over the course of a night with a thin film of water. While having a thin film of water you’re your body is great for cooling off, it is terrible for trying to stay warm.
Thus, the moral of the story is this. Never sleep in a sleeping bag while wearing cotton clothing. Sleep in the buff, or in thermal underwear, or even in quality fleece clothing. Just leave the cotton clothing at home.
If you want to sleep in your clothes (and I’ll confess, I do), sleep only in clothing made from moisture wicking material, such as fleece or polypropylene. I sleep in lightweight fleece pants, nylon underwear and a lightweight fleece shirt. On real cold nights, I may also use a fleece hat.
Read the Fleece Clothing Guide to understand why cotton is just awful for camping and hiking.
The EN 13537 testing standards went a long ways towards removing the “total guesswork” out of determining how warm a sleeping bag actually is.
However, it is still only provides a baseline for comparison among sleeping bags. The EN test, all by itself, does not peg a sleeping bag to a specific temperature bag rating.
As such, campers should use the EN test as a rough baseline to follow. Then add in the other variable, such as hold/cold sleeper, size of tent, type of sleeping pad used, etc…to better fine tune how warm of a sleeping bag to buy.
Finally, I’ll suggest one more thing. It is always better to buy a sleeping bag that is a “bit too warm” than a “bit too cold.” Remember, you can “open up” a bag that is a bit too warm to stay comfortable. But there are no effective methods to make a cold bag warm. Thus, to be safe, buy a bag that is a “bit too warm” than a “bit too cold.”
For general three season use, I suggest sleeping bags that either have a comfort rating or specific temperature rating to a minimum of 15-20° degrees.