How to choose a sleeping bag temperature rating

Knowing how to choose a sleeping bag with the right temperature rating can mean the difference between a good night’s sleep or a bad night’s sleep—and in extreme cases, it can even mean the difference between life and death.

Unfortunately, you can’t always take a particular sleeping bag’s temperature rating at face value. If a sleeping bag is rated at 23°F (-5°C), that doesn’t necessarily mean you’ll be warm enough in your bag when the nighttime temperature drops that low.

Confused yet? You’re not the only one. But you’re on the right track for deciding to research how to choose a sleeping bag based on its temperature rating.

Here’s what you need to know.

The big mistake campers make when reading a sleeping bag’s temperature rating

The biggest mistake you can make is assume that a sleeping bag’s temperature rating is the minimum temperature that the bag can keep you warm. In fact, for many sleeping bags, the temperature rating is merely a company’s best estimate of the lowest temperature that the average person can sleep comfortably in that particular bag.

For example, a person who gets cold easily might not be able to sleep comfortably in a sleeping bag with a 23°F (-5°C) temperature rating, even if the actual temperature outside is only 30°F (-1°C). However, someone who sleeps warm might be able to sleep comfortably in that same bag when the temperature outside dips to 23°F (-5°C).

Why you should look for sleeping bags with ISO or EN ratings

If you do enough browsing around, you’ll undoubtedly come across several bags with ISO or EN ratings. These ratings are set by the International Organization for Standardization (ISO) and the European Committee for Standardization (CEN), which are international standards organizations that set guidelines for a variety of products, including sleeping bags.

An ISO- or EN-rated bag typically shows two ratings: a comfort rating and a lower limit rating. In some cases, you’ll also see a third “extreme” rating.

The comfort rating is the temperature where the average cold sleeper can sleep comfortably in the bag. Women typically feel colder than men, which is why the comfort rating is used on women’s sleeping bags.

The lower limit rating is the temperature where the average warm sleeper might stay comfortable, which is always lower than the comfort rating. It’s typically used on men’s sleeping bags.

The extreme rating isn’t on every ISO- or EN-rated bag, but if you see it, you should know that it’s the absolute lowest temperature where someone could theoretically survive in the bag without dying of hypothermia.

How ISO and EN testing works

It’s worth knowing a little bit about the testing so you understand where these ratings are coming from. Here’s what happens: A heated manikin wearing a base layer with sensors all along its body is put inside a sleeping bag in a controlled environment. As the temperature is lowered, the sensors take readings to determine how well the sleeping bag retains heat.

The lower the temperature gets, the more difficult it becomes for the manikin to maintain its core body temperature within a comfortable range. When that happens, the test is stopped and that’s how the companies arrive at the temperatures for the comfort, lower limit and extreme ratings.

Why ISO- and EN-rated bags matter

If you see a bag with an ISO or EN rating, that means it’s been put through standardized testing. That doesn’t necessarily mean it will perform better than a bag without an ISO or EN rating, but it does mean the company has gone through the effort to test its products according to international standards.

It also means you can compare its temperature ratings equally against other bags with ISO or EN ratings. This is helpful because it means you can shop around and find the bag that best suits your needs without having to worry about different companies using different standards.

What about sleeping bags with no ISO or EN rating?

You’ll find some bags on the market that might say “temperature” with no specific sign of an ISO or EN rating. If it doesn’t say ISO, EN, comfort limit, or lower limit, then you can be sure that the bag has gone through standardized testing.

In some cases, it’s because the company didn’t want to incur the cost of getting an ISO or EN rating for their bag. But in other cases, it’s because the bag doesn’t meet the minimum requirements for an ISO or EN rating. Bags designed to be used in extreme cold weather and children’s bags are two types where you can expect not to see ISO or EN ratings.

So, if you see a bag without an ISO or EN rating, that doesn’t necessarily mean it’s a bad bag. It just means you can’t compare its temperature ratings equally with other bags on the market.

Other variables that affect sleeping bag warmth

A sleeping bag’s temperature rating can only tell you so much about how warm it will actually keep you. There are so many other things that can influence how well a bag keeps you warm—all of which you need to consider.

Seasonal temperatures

Are you planning to use your bag in the summer or winter? If you’re using it in the winter, will you be camping in the mountains where it might be colder than at lower elevations? These are all things to consider when trying to determine how warm of a bag you need.

Most sleeping bags are classified as summer, three-season, or winter bags. Use these ranges to make your best judgement on which type of bag is best for you.

Summer range: 32°F (0°C) and up

Three-season range: 5°F (-15°C) to 0°F (-1°C)

Winter range: 5° F (-15°C) and below

Insulation type

The type of insulation in your bag will also affect how warm it keeps you. The most common types are down and synthetic, but there are other options as well.

Down is the lightest and most compressible insulation, making it a good choice for backpacking and ultralight camping. It’s also one of the more expensive options. The main downside to down is that it loses its insulating properties when wet.

Synthetic insulation is less expensive than down and still does a good job of insulating even when wet. The main downside to synthetic insulation is that it’s not as compressible as down, making it a less popular choice for backpacking and ultralight camping.

Other insulation options include:

• Hollow fibre
• Fiberpile
• Polarguard

Down fill power

If you’re choosing a down sleeping bag, you’ll want to know what its fill power is. This is essentially a measure of the loft, or fluffiness, of the insulation. The higher the fill power, the more loft—and the warmer the bag. Bags with a higher fill power are also more expensive.

Fill power is measured in cubic inches per ounce (in³/oz) and is usually between 400 and 900. Bags with a fill power of 700 or higher are considered high-quality bags.

Bag shape

Sleeping bags typically come in three different shapes: mummy, barrel, and rectangular. Its shape can affect heat retention because of how much dead space there is inside the bag.

Mummy bags are the most form-fitting type, which have a hood for your head and taper at the bottom to provide more warmth and insulation. They’re also the lightest and most compressible.

Barrel bags are a bit more spacious than mummy bags and have more room around the legs. This makes them more comfortable for some people but also means there’s more dead space inside, which can affect how warm you stay.

Rectangular bags are the most spacious and have the most dead space. They’re not as good at retaining heat but can be more comfortable for some people.

A mummy bag will generally be warmer than a barrel or rectangular bag because it has less dead space for your body heat to escape. However, some people find mummy bags claustrophobic. If you don’t like feeling constricted, a barrel or rectangular bag might be a better choice for you.

Two other popular sleeping bag types include quilts and double sleeping bags. Quilts are essentially a mummy bag without the bottom, which can save weight and space. They’re typically used with an insulated sleeping pad to provide warmth. Some people prefer this because the down or synthetic that they’d be lying on in a sleeping bag would be compressed and not provide as much insulation.

Double sleeping bags are designed for two people and usually have a zipper down the middle so you can zip them together. They typically have more dead space than a single bag, which can affect how warm they keep you. However, they do give you the opportunity to share body heat with a partner, making them a good option for couples.

Liner material

The material of the bag’s liner also contributes to how warm it will keep you. The most common materials are cotton, polyester, and nylon.

Cotton is a comfortable material but doesn’t insulate as well as synthetic materials. It also takes longer to dry if it gets wet. Polyester and nylon are synthetic materials that insulate better than cotton. They also dry more quickly if they get wet.

Baffles

Baffles are the sewn-in walls that divide the bag into different chambers. They help to keep the insulation in place and prevent it from shifting around. Baffles can be vertical or horizontal.

Vertical baffles are more common because they’re easier to construct. They also tend to be warmer than horizontal baffles because there’s less dead space for your body heat to escape.

Horizontal baffles are less common but provide more room for you to move around inside the bag. They’re not as good at retaining heat, but some people find them more comfortable.

Draft tube

A draft tube is a strip of material that runs along the zipper of the bag to prevent warm air from escaping through the opening. It’s usually made of synthetic insulation or down.

Draft tubes are an important part of a sleeping bag’s design because they help to keep you warm. Without a draft tube, warm air would escape through the zipper every time you moved around in the bag.

Draft collar

A draft collar is a band of material that goes around your head and neck to prevent warm air from escaping through the top of the bag. It’s usually made of synthetic insulation or down.

Hood

Most mummy and some barrel sleeping bags have a hood that you can cinch down around your head to prevent warm air from escaping. Sleeping bag hoods are important are far more important in three-season and winter bags because they help to keep your head and face warm.

Shell material

The shell material may be less important than other variables, but it can still affect warmth. The most common materials are nylon and polyester. Ripstop nylon is more breathable than taffeta nylon and will help regulate your body temperature better. This is especially important if you tend to sleep hot.

Keep in in mind that a thicker shell will generally be warmer than a thinner shell. However, a thicker shell will also add weight and bulk to your bag.

Weight and bulk

The weight and bulk of your sleeping bag are also important factors to consider. If you’re backpacking, you’ll want a bag that’s as light and compact as possible. If weight isn’t an issue, then you might want to choose a bag with more insulation for added warmth.

Air mattress or sleeping pad

Your sleeping bag and air mattress or sleeping pad both work together to keep you warm. A good air mattress/sleeping pad will insulate your body from the cold ground and provide additional comfort.

When considering a mattress or sleeping pad, make sure to look for its R-value, which is a measure of how well it insulates. The higher the R-value, the better the pad will be at keeping you warm.

How to choose a sleeping bag temperature rating that’s best for you

Now that you know how sleeping bag temperature ratings work and all the other variables that factor into it, I’ve got three final tips for you to help you make your decision.

1. Give yourself a temperature buffer. This means that you should choose a bag that’s rated for temperatures anywhere from 5 to 15 degrees colder than the lowest temperature you expect to encounter. This will prevent you from experiencing any “rude awakenings” in the middle of the night in case the temperature drops unexpectedly.
2. Ask yourself whether you’re a warm or cold sleeper. Everyone is different. I sleep warmer than Elise, but that probably has a lot to do with me being a guy and Elise being a gal. In general, women tend to sleep colder than men. But there are always exceptions to the rule.
3. Look at cottage industry retailers. Contrary to popular belief, the best sleeping bags (and gear in general) aren’t necessarily the ones with big brand names like Mountain Equipment Coop, Patagonia, and Marmot. Sure, they’re good, but some of the best sleeping bags out there are actually made by smaller, lesser known companies.

How to make an existing sleeping bag warmer

If you already have a sleeping bag, but are wondering whether you really need to get another one that’s warmer, you might be pleasantly surprised to find that there are all sorts of things you can do to beef up its warmth. We use all of these techniques and find they can make a huge difference.

Layer up

First things first: How are your clothing layers? Make sure you’re sleeping in thermal underwear and/or a base layer made of synthetic or wool material. This will go a long way towards keeping you warm throughout the night.

If the temperature is really going to test your sleeping bag, consider adding a fleece layer or even a down jacket on top of your base layer. This can make a big difference, especially if you’re trying to eke out every last degree of warmth from your bag.

Use a sleeping bag liner

A sleeping bag liner is an inexpensive and easy way to add a few degrees of warmth to your bag. It’s basically a glorified sheet that you insert into your sleeping bag.

Liners come in all sorts of different materials, but we recommend one made of silk or another synthetic material. Down is great for adding warmth, but it’s also more expensive than synthetic insulation. If you’re on a budget, a synthetic sleeping bag liner will do the trick.

Add a bivy sack

A bivy sack is a waterproof and breathable shell that you can use to protect your sleeping bag from the elements, which is ideal if you’re camping under a tarp or other open area. It’s essentially a one-person tent that goes over your sleeping bag.

Bivies are great for keeping you warm in cold and wet conditions because they help prevent your body heat from escaping. They also add a layer of protection from the wind, which can be a major source of heat loss.

Use a foam pad with your inflatable pad

The ground can be very cold and is one of the biggest sources of heat loss, so it’s important to insulate yourself from it as much as possible. If you’re using an inflatable sleeping pad, consider using a foam pad like the Therm-a-Rest Z Lite on top of it for extra warmth.

We loved this trick when we went winter camping in -22°F (-30°C) this past February. Many winter campers tend to place their foam pad on the bottom and their inflatable pad on top, but we discovered that the reverse is far better for warmth. We (and many other campers) think that it’s because the air inside the inflatable pad gets very cold, so with a barrier between you and that air, you’re more likely stay warm.

Warm up your bag by the fire

If you still have a fire going before getting ready for bed, consider turning your sleeping bag inside out and carefully hanging it or holding it next to the fire to warm. This must be done with extreme caution—particularly next to an open fire—to prevent sparks from reaching the bag burning holes into it.

We find this technique best when we’re hot tenting in the winter with a wood stove because there are no sparks flying around. The heat also rises, so it’s easy to hang the bags at the top of the tent and let them warm up a bit.

Use a hot water bottle

This is a simple trick that can go a long way. The idea is to boil or heat some water in a pot before turning in for the night, pour it into your water bottle, and sleep with it in your sleeping bag.

The hot water will provide a consistent source of warmth for potentially several hours. Just make sure it’s not too hot before getting into bed with it, and check that the lid is fastened as tightly as possible to prevent unwanted leaks!

Tip: Check out our guide on how to stay warm in a tent.

Sleep well… and warm!

We hope this gear guide helped you learn how to choose a sleeping bag temperature rating that best suits your needs. If you’re at all curious, here are the following sleeping bags we own and use depending on the season.

• Elise uses the MEC Mallard 23°F (-5°C) Down Sleeping Bag (Unisex) in the late spring, summer, and early fall seasons here in Ontario, Canada.
• I prefer a quilt in the warmer months, so I use the Flex 15°F (-9°C) Quilt from Katabatic Gear.
• We have a custom-made double quilt we like to use every so often in the warmer months, which we got from Enlightened Equipment.
• In the late fall, winter, and early spring, we both use our own MEC Talon Windstopper -22°F (-30°C) Down Expedition Sleeping Bag (Unisex).

Overall, we’ve had good experiences with all the bags and quilts mentioned above. We use them with our Therm-a-Rest sleeping pads for added warmth.

Sleeping bags are one of the most essential pieces of camping gear. Make sure to do your research, take your time, and consider those variables when settling on your final choice!