Buying a sleeping bag is, for some reason, harder than it should be. There are dozens of sleeping bags at any one outdoor store, and they all seem simultaneously different and the same. One of the main ways you’ll eventually choose your bag, though, is by looking at the temperature ratings. In this article, I’ll break down temperature ratings so you know exactly what you can expect from a sleeping bag in terms of comfort, warmth, and versatility.
Is there a sleeping bag temperature rating standard?
While standards do exist for testing and rating sleeping bags, companies aren’t actually required to test their sleeping bags, nor are they required to list a temperature rating. That said, almost every sleeping bag company does use a temperature rating, even if it’s not one of the two standards that exist.
The two standards – EN and ISO – aren’t so much different as one is old and the other is new. Older sleeping bags are usually tested to the EN standard, while new sleeping bags are tested to the ISO standard. Cheap sleeping bags from companies that don’t really care that much about quality don’t use any standard at all; they just test their sleeping bags in-house and give you a rating.
EN Sleeping bag temperature ratings
EN stands for “European norm”, and it was the first standard testing and rating system for sleeping bags. Not so much in use anymore, EN ratings are mostly found on older sleeping bags, and I would be a bit surprised if you saw one in-store that used them.
EN ratings were introduced in 2005 to help backpackers compare sleeping bags using an objective, universal standard. Before this, there was no way to know if a 30-degree rating on one bag was the same as a 30-degree rating on another bag.
EN ratings are presented in ranges rather than single temperature numbers. These ranges represent how a self-described “cold sleeper” will feel using a sleeping bag in different temperature ranges. It’s helpful to use ranges because temperature fluctuates from hour to hour; you can’t learn much about comfort on a night where temps range from 35-15 degrees if the sleeping bag just says “30”.
There are three temperature ratings used to describe the usability of a sleeping bag that has been tested to EN/ISO specifications: comfort range, transition range, and extreme range. Sometimes, the terminology changes a little bit, and there are also “upper limit” and lower limit” ratings, but the temperature ranges are the most helpful and that’s what we’ll focus on today.
The comfort rating is often used for women’s-specific bags, because women tend to be “cold sleepers” more often than men. Is it a bit weird that sleeping bags assume that all women are cold? Yeah, but we’ll leave that for the struggling stand-up comedian and continue on with how the ratings work, weird or otherwise.
Next up is the “transition rating”: the temperatures at which you can sleep, but maybe not comfortably, in a sleeping bag. If a sleeping bag’s comfort rating is 20-40 F, for example, its transition rating might be 20-0 degrees. You’ll be able to safely sleep, but you might be tossing and turning, feeling the cold much more than you’d like to. If you’re a hot sleeper, though, the “transition” range might actually be the one you feel best in.
At the extreme range, you can sleep, and you can survive, but it’s going to suck. Cold sleepers using a sleeping bag in temperatures that match the “extreme” range are going to be very cold, risking hypothermia. If you drift too much toward the lower limit, you’re inching toward the point of freezing. Below the extreme temperature range of a sleeping bag, you are risking frostbite, hypothermia, or even death!
Upper Limit Lating and Lower Limit Rating
The upper limit describes the highest temperature at which a standard male will feel comfortable (read: not sweating profusely). The lower limit describes the lowest temperature at which a standard female can sleep without dying.
These two ratings aren’t that useful when buying a sleeping bag, because the goal is to get good sleep, not simply avoid waking up drenched in sweat or avoid dying. But, many sleeping bag companies use those two numbers anyway, so it’s good to know what they mean.
ISO Sleeping bag temperature ratings
ISO ratings were introduced in 2017, and are pretty much just an update to EN ratings. The testing is pretty much the same, at least from a consumer perspective, so you don’t really have to learn anything new to understand ISO ratings. They use the same ranges (comfort, transition, and risk), and they’re used by the overwhelming majority of sleeping bag companies.
Are these standards useful? Do they have any weaknesses?
Sleeping bag temperature ratings are helpful, no doubt about it. When companies use the rating systems, customers are able to compare apples to apples. One sleeping bag with a 20-degree comfort rating will perform exactly the same as another sleeping bag with a 20-degree comfort rating.
That said, EN/ISO ratings have one glaring weakness: they don’t take real-world conditions into account. Wind, rain, humidity, the sleeping pad and tent you use, all the variables that can affect how well you’re able to sleep on a backpacking trip? They don’t factor into the ISO rating system used to evaluate a sleeping bag. That means you can’t take the 20-40 degree comfort rating as absolute truth – just something close to it.
The only other true weakness of the EN/ISO testing standard is that nobody is being forced to use them. It’s not like consumer electronics, plumbing, or other highly-regulated manufacturing sectors; when it comes to sleeping bags, sticking to the standard ratings is purely optional.
This means that cheap sleeping bags, those made by companies who are just trying to fill store shelves and not really concerned with performance, aren’t going to use them. They test their bags, sure, but we don’t really know how they test, which means we can’t compare them to sleeping bags that have been tested according to EN/ISO standards.
How to interpret sleeping bag temperature ratings: one question to ask yourself
Okay, so we’ve discussed the rating system for a sleeping bag, and what the ranges describe, but how do you actually choose a sleeping bag based on those ratings? After all, your sleeping bag is one of the most important pieces of your backpacking gear list. You can make it as complex as you like, but there is one essential question you need to ask yourself:
Am I a hot sleeper or a cold sleeper?
Knowing which kind of sleeper you are tells you which temperature ratings you should focus on: comfort or transition. Cold sleepers should use the comfort range, and hot sleepers should use the transition range.
There are other questions you can consider to help you choose your sleeping bag, but they’re based on the smaller variables and only you can answer them definitively:
- What kind of sleeping pad are you using, and how big a role will it (and r-value) play in keeping you warm?
- Are you planning on sleeping in pajamas, in your underwear, or in the buff? Usually, a sleeping bag temperature rating describes your ability to sleep comfortably with clothes on, not off.
- How humid will it be on your backpacking trip? Mid-summer camping is often humid, which means the right sleeping bag will be thin and breathable.
- How windy will it be on your backpacking trip? Cold temperatures are much colder when the wind is howling.
- Will it rain? Cold nights and wet nights combine for teeth-chattering sleep!
- Will you be sleeping by a fire to keep warm? The extra warmth of fire often keeps you from feeling cold, no matter the sleeping bag rating.
- How warm is your tent? You can often sleep comfortably with a bag that isn’t rated for colder temperatures because a good tent provides extra insulation.
- How many other people will be in the tent with you? Each additional person is additional body heat and insulation.
These questions are going to have different answers for every single backpacking trip, which makes it hard to make a purchase based on one trip alone. But, they’re still helpful, and knowing how you’d answer them most of the time can help you make a wise purchase.
How are sleeping bags tested?
How are these sleeping bag ratings decided, anyway? The EN and ISO standards provide specific instructions for testing a sleeping bag. We won’t get too deep in the weeds, but the test involves placing a dummy covered in sensors into a sleeping bag and taking readings at progressively colder temperatures.
The dummy is pretty high-tech, designed to have a natural body temperature of 98, just like a real human would. The sensors on the dummy measure how well the sleeping bag allows the dummy to retain its healthy body temperature. If the dummy can sleep comfortably at 20 degrees, it gets a 20-degree comfort rating. The dummy also wears clothes and uses a sleeping pad, to make the sleeping bag test more like real life.
It’s a little more complex than that, and the test is run many, many times to ensure accuracy, but those are the basics. Also, most sleeping bags are tested not just by the manufacturer but also by third-party labs for integrity and transparency. Adventure Journal did a great story on Therm-a-Rest’s sleeping bag ratings test a few years ago; check it out for more enlightenment!
Should you buy sleeping bags without EN or ISO ratings?
It would be wise to avoid buying a sleeping bag that doesn’t give you an EN/ISO rating. It’s not a sure sign that it’s a bad sleeping bag, but it does mean that company that made it isn’t really making an effort to give you the information you need.
I didn’t know that when I bought my first backpacking sleeping bag, one that I still use today. If I had known that back then, I probably wouldn’t have bought the sleeping bag, even though it was really cheap and felt like a higher-quality sleeping bag I couldn’t afford. I’m a hot sleeper, though, which means that any old bag will do for me because I’m going to wind up halfway out of it by morning anyway.
If you aren’t a hot sleeper, or if you’re backpacking in a place where cold nights can kill, the best thing you can do is get a sleeping bag that has been tested according to the EN/ISO standard. It’s the only real way to make sure your sleeping bag will perform as expected.