A sleeping bag is something that few campers would dare go without. It’s what keeps you warm in cold weather, and provides an extra layer of comfort and insulation against the hard ground below.
But how do you know which one will be right for you? Whether you buy a sleeping bag that’s too warm for the summer, or not warm enough for those colder nights, you’re in for an unpleasant camping trip. That’s why sleeping bag temperature ratings are so valuable – and why we took a whole article to explain what they mean and how they work.
Sleeping Bag Shape
Before we get into what the actual sleeping bag ratings mean, let’s talk about how the shape of your bag can impact warmth. For starters, there are three different styles that you can choose from: mummy, rectangular, and semi-rectangular.
Mummy sleeping bags are by far the most common design that you’ll find, and they’re also the warmest. As the name implies, a mummy bag will wrap around you like a cocoon, effectively “trapping” you inside. The good news, though, is that you aren’t the only thing that’ll be trapped – any heat that you generate will have a hard time escaping as well. Especially if you’re wearing the hood, it’ll be very difficult for air to travel from the inside to the outside, or vice versa.
Rectangular sleeping bags are probably what many of you had as kids. They’re wide, spacious, and cozy, making them the ideal option for people who value comfort. Since they don’t trap heat as well as mummy bags, they mostly used for summertime camping, when heat retention is the last thing you need.
And finally, semi-rectangular sleeping bags land somewhere between the other two. They have a shape that’s similar to a mummy sleeping bag, while retaining more of the interior space and comfort that a rectangular bag provides. Arguably the least common style of sleeping bag, the semi-rectangular design is still great for folks who want something a little warmer without completely sacrificing comfort and mobility.
Sleeping Bag Fill
Another thing worth mentioning is the insulation used to keep you warm inside the sleeping bag. At the end of the day, your two options are going to be down and synthetic.
Down is my favorite, simply because you don’t need very much of it in order to achieve a great amount of warmth. It is more expensive, but it’s also more lightweight, making it the superior option in just about every way. Unfortunately, it does lose its insulating properties when wet, and it’s not suitable to people who are allergic to down. But if these aren’t applicable to you, I would suggest going with this fill, if it’s something you can afford.
Synthetic, on the other hand, is heavier and doesn’t insulate quite as well. The bonus is that it is cheaper and hypoallergenic, so it’s suitable for a wider variety of people. Not to mention, it will still keep you warm even if it gets sopping wet. It just typically isn’t the choice backpackers go for, because it is heavier and bulkier than down.
EN and ISO Ratings
Alright, now let’s dive into the ratings themselves. You’ve probably seen each of them before, if you’ve ever gone shopping for a sleeping bag. If not, EN (European Norm) and ISO (International Organization for Standardization) may be unfamiliar to you, so I’ll give a basic overview.
Up until 2017, EN was the primary way that sleeping bags were rated. At that point, ISO was developed, but the way that they measure temperature ratings are so similar that they can be talked about interchangeably. Essentially, ISO is just an updated version of EN, as a way to better standardize the methods by which sleeping bag temperatures are rated.
So, How Does it Work?
But how can you gauge the effectiveness of a sleeping bag with any amount of objectivity? Well, compared to the days when they were tested by shoving someone inside a meat locker, I’d say the current method has gotten pretty advanced.
Nowadays, a manikin is used instead of a human, which is already a lot more objective in its own right. This manikin is full of heat sensors, and dressed in a base layer that any camper might find themselves wearing. After being placed inside the sleeping bag, the manikin is put inside a cold room on top of a closed cell foam sleeping pad.
Over the course of time, various types of data are pulled from the manakin, such as the temperature that its heat level stays constant, the point that heat starts to accumulate, and the point when heat is lost. All of these thresholds are put together to create ranges that encompass comfort, transition, and risk. And the transitions between these ranges are referred to as comfort, limit, and extreme, respectively. Here’s what that means:
The comfort range is a concept that really just applies to women. Since women tend to feel colder than men, with all else being equal, sleeping bag developers found it helpful to create this category. It’s a way for women to determine whether they’ll be comfortable or not while they’re lying in a relaxed position (i.e. not curled up in a ball shivering to death).
On the other hand, the transition range is directed at men who are starting to feel the chill. In these temperatures, men will still be at thermal equilibrium (not cold enough to be shivering), but they’ve curled up into fetal position in order to conserve heat. The performance limit of your sleeping bag will likely fall in this range.
And finally, we have the extreme range, which is a place I hope you never find yourself. At this point, your sleeping bag is just barely keeping you from cold related injuries, and even then, it might not prevent against hypothermia. It’s recommended that you only use your sleeping bag in temperatures like these if it’s an emergency, and you aren’t able to find any warmer options.
Keep in mind that these are ranges and not exact temperatures. Temperature raters do their best to provide a general idea of when you’ll be comfortable or uncomfortable, but ultimately, our individual bodies are very different from each other. If you run hot, then you can likely get away with sleeping in a colder environment than recommended. Likewise, the opposite is true if you’re someone who always feels cold.
Other Influencing Factors
Aside from your personal temperature fluctuations, there are other factors that influence how cold or hot you’ll feel in the sleeping bag. For example, do you have a liner? Are you damp? What kind of mattress or sleeping pad are you using? What are you wearing? Are you used to sleeping outside?
These are just a few of many things that can impact the quality of your sleep. However, despite all of the variables, the EN/ISO ratings system is still the best way to get a thermal baseline. After all, it is a range that’s often created with your specific height and weight in mind. So, unless you plan on pushing the sleeping bag temperature rating as far as it can go (such as sleeping in 20 degree weather in a bag that’s rated for 20 degrees), I think you’ll find that it’s more than sufficient.
Build a Sleep System
Of course, your sleeping bag isn’t your only option for staying warm during the night. Just because your sleeping bag has a lower limit doesn’t mean you can’t go beyond that with the help of some other gear.
Building a sleep system will allow you to withstand a brutally cold environment with minimal effort. Here are a few components that you can work off:
Sleeping Bag Liner
As the name implies, a sleeping bag liner will line the interior of your sleeping bag. While warmth is a common reason for using one of these, some folks like to have them just to add a bit of extra comfort. Many are made of silk, a material that feels nicer than the synthetics used on the inside of a sleeping bag.
Aside from that, though, liners also preserve the longevity of your sleeping bag. Liners won’t wear out as quickly when you wash them, so it’s not as big of a problem when they get dirty.
Underneath your sleeping bag, you should always have either a sleeping pad or an air mattress. Not only will they provide extra insulation, but you’ll also be happy to put some cushion between you and the hard ground.
More than anything else, it’s important that you have a dedicated set of dry clothing to wear to bed. If your current outfit is damp from rain, snow, or sweat, you’d be better off taking it off before hopping in your sleeping bag.
I like to keep my hat and socks on during the night, and depending on how cold it is, I may even wear gloves. Some people suggest wearing long underwear and a long sleeve top, but there are different variations you can use. Personally, I just slide into a pair of sweatpants and throw on a fleece jacket, which feels like a soft blanket wrapped around me.
Sleeping Bag Fit
All of this talk about temperature ratings doesn’t mean much if you got a sleeping bag that doesn’t fit well. It’s important to be able to move around inside, but if you got one that’s too big, you’ll be wasting heat by trying to warm up empty space. On the other hand, if you got one that fits too tightly, the insulation will start to compress, and you’ll develop cold spots.
Final Pro Tip
Hopefully you have a better understanding of how sleeping bag temperature ratings work, and what you can do to stay warm at night. With that in mind, though, here’s our final pro tip:
Give yourself a buffer.
Which is to say, don’t cut it close by dancing on the edge of the extreme range. The sleeping bag might be rated for 20 degrees, but that doesn’t mean you should actually try to use it when it’s that cold outside. Hypothermia is no joke, and if you’re far away from civilization, it’s best not to play with fire. Or would it be ice, in this case?
I’d suggest giving yourself at least a 10 degree buffer. Take a look at the weather forecast where you’ll be camping, and determine how cold it’s supposed to get. If it’s supposed to dip down to 35 degrees, grab a sleeping bag that can keep you comfortable down to 25 degrees or lower. It’s always better to be too warm than too cold. You can always take layers off or unzip your sleeping bag when you get too hot, but your options are far more limited if you get too cold.