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Sleeping bag temperature ratings: How important they are and how to choose the right one

Choosing the right bag depends on conditions and personal preferences

Sierra Designs Nitro 20 Degree Sleeping Bag - $319.95

Sierra Designs Nitro 20 Degree Sleeping Bag - $319.95

Sierra Designs

Shopping for a sleeping bag can be overwhelming. There are numerous things to take into consideration: synthetic or down, mummy or rectangle shape, temperature rating, and packable weight and size. All of these options contribute to the most important (and basic) function of any sleeping bag: providing a comfortable sleeping experience after a long day of adventuring in the outdoors.

For me (and I suspect most people), sleeping comfortably and well hinges on not being too hot or too cold. No one wants to wake up in the middle of the night shivering or sweating. So how do you decide which kind of sleeping bag to buy and how important, really, is the temperature rating?

The short answer is: Pretty important, but like most things in life, making the best decision you’re able to requires weighing multiple factors and combining those results with your personal preferences. In order to do that, you first need to have a basic understanding of how temperature ratings are calculated in sleeping bags.  

EN/ISO temperature ratings

If they are rated (and not all are), sleeping bags will have either an EN (European Norm) or ISO (International Organization for Standardization) rating. The good news is that the tests conducted by both use essentially the same criteria: a heated mannequin dressed in a base layer and hat on a foam sleeping pad. The test results produce the following ‘limit’ temperatures - which are what you will find printed on the inside of your bag. 

Upper limit

Upper limit refers to the highest temperature that an adult male would sleep comfortably without excessive sweating. 

Comfort limit

Comfort limit uses the temperature that an adult woman would need in order to achieve a comfortable night’s sleep.

Lower limit

Lower limit takes into account the lowest temperature an adult male would be able to sleep in comfort. 

And the extreme limit is the absolute lowest temperature an adult woman could survive. This rating shouldn’t be used for purchasing bags in general as this it assumes a very high risk of hypothermia.

These ratings are most often displayed in a combo graphic of text that goes from highest to lowest temperature. One of my Rab bags lists the comfort limit as 39F, the limit of comfort as 30F and the extreme as 1F. What this means is that the average adult woman would find the bag comfortable down to 39F while the average male would probably be able to use the same bag in temperatures as low as 30F. The extreme limit of 1F is the lowest possible temperature at which the bag should be used at all (but only in cases of emergency).

The rating systems’ use of male and female is based on scientific data that shows that women tend to feel cold at higher temperatures than men do and adult males often “run hotter” than adult females. One reason for this, according to studies,  is that lower muscle mass to body surface usually makes a person feel colder. 

Another reason (if you’re into science) which was just published in an October 2021 study suggests that the whole women feeling colder than men thing is actually an evolutionary characteristic related to having and caring for children (and it extends to other species besides humans).

In order to figure out which temperature range you want, consider where you’ll be using the bag. If you’re sleeping outside in the summer and staying at a low elevation (not climbing any mountains) a bag rated at 32 degreest Fahrenheit and above should work just fine. If you’ll be somewhere where night temperatures could drop below freezing or on an extended multi-month trip, a 3-season sleeping bag that falls into the 20 degrees to 32 degrees Fahrenheit range would be better suited. And, if you’re doing some serious mountaineering where there will be snow, you’ll need a true winter bag rated for temperatures below 20 degrees Fahrenheit.

Many people I know always add at least a 10 degree buffer zone when deciding on what bag to take where. For instance, if they think they’ll be camping in 32 degrees Fahrenheit weather, they’ll opt for a 3-season bag as opposed to a summer bag. You should also obviously take into account your own body temperature tendencies. 

Summer/low elevation: 32 degrees Fahrenheit and above

Not all bags have temperature ratings and the common wisdom is to assume a bag without one will fall into the summer/low elevation category – especially those made by mid-range or budget brands. The old adage “you get what you pay for” is often true for sleeping bags.

My go-to bag in this category is the Rab Neutrino Pro 200 with a 32F comfort rating.

Rab Neutrino 200 Sleeping Bag - 349.95

Rab Neutrino 200 Sleeping Bag
Rab
moosejaw.com
$349.95

Neutrino Pro 200 Down Sleeping Bag - $400.00

Neutrino Pro 200 Down Sleeping Bag
rab.equipment
$400.00

The Neutrino Pro is a down-filled, mummy shaped bag with an internal collar and drawstring. It packs down very small (12.6 x 6.2 inches) and weighs 21 ounces. Made with a water resistant and durable outer fabric, this bag remains warm even during mornings when there is condensation or dew.

I used mine on numerous bikepacking and backpacking trips, including a five month sojourn through May to September across Central Asia from Mongolia to Turkey. Many mountains and desert regions played a part in this adventure, and, while there were a few stifling nights in Turkmenistan where I slept on top of the bag with a frozen water bottle, I obtained adequate comfortable sleep during the majority of the trip.

Most notable were a few below freezing nights on the Mongolian steppe where, though I had to don my Patagonia insulated coat as well, I was still able to sleep.

3-season sleeping bags: 20 degrees Fahrenheit to 32 degrees Fahrenheit

The Big Agnes Star Fire UL 20F Down bag, which features body mapping and vertical sidewalls to trap warmth close to the body, is a popular 3-season sleeping bag among my hiking and bikepacking friends.

Big AgnesStar Fire UL Sleeping Bag: 20F Down - $499.95

Star Fire UL Sleeping Bag: 20F Down
big-agnes
backcountry.com
$374.96

As is the Sierra Designs Nitro 800:

Sierra Designs Nitro 20 Degree Sleeping Bag - $319.95

Sierra Designs Nitro 20 Degree Sleeping Bag
Sierra Designs
amazon.com
$319.95

One of the best features about the Sierra bag is its self-sealing foot vent for when you’re a bit too toasty. You simply pull your knees up, then insert your toes in between two overlapping layers of fabric to expose them to the outside air. When you pull them back in, the flaps close back up without drafts – or so I’ve been told by many users of this bag (sleeping bag functionality is often a topic of camp breakfast conversations).

As for the Star Fire, two of its biggest selling points include it’s warmth to weight ratio and especially form fitting, cozy hood. Some of my male friends who have this bag do say that the dimensions (60 inches at the shoulder and 54 at the hips) don’t allow for people with bigger frames to sleep on their sides so if that’s you, this particular bag may not suit your needs. 

Winter/cold weather sleeping bags: 20 degrees Fahrenheit and below

Because I’m one of those people who finds a brand that I like and sticks with it, my favorite winter sleeping bag is the Rab Neutrino 800 which has a -8F temperature rating.

Rab Neutrino Down Insulated Lightweight Sleeping Bag - $399.95

Rab Neutrino Down Insulated Lightweight Sleeping Bag
RAB
amazon.com
$386.21

This is a mummy shaped bag with a water resistant outer shell great for alpine adventures and mountaineering. I took this bag with me on my K-2 trek and was happily warm while sleeping in one of the most beautiful (and chilly) places I’ve ever camped.

I also polled some of my mountain trekking friends and one winter sleeping bag mentioned quite a few times was the Northface Inferno 0F degree bag.

The North Face Inferno 0 Sleeping Bag - $520.00

The North Face Inferno 0 Sleeping Bag
The North Face
rei.com
$520.00

According to those who recommend it (and other REI online reviewers) one of the best things about this bag is that it is roomier than other mummy style ones while not sacrificing warmth. One reason for this is that the 800 fill down is “enhanced with a hydrophobic finish that improves warmth by allowing it to dry faster and repel moisture ten times longer” than your normal run of the mill down. Good on Northface for managing to improve on mother nature. 

All the sleeping bags included in this article are mummy style bags with down filling. In general, mummy bags are warmer because there is less space inside to heat and the warmth that your body generates remains trapped close to you.

The fact that they are all down-filled bags correlates with me being a hiker, trekker and bikepacker who usually carries all my gear on my person. Down is lighter than synthetic materials and packs smaller. Down bags always have a higher warmth to weight ratio than those made from something synthetic.

Down, measured by ‘fill power’ or its ability to loft (fill space) refers to the plumage underneath a duck or goose’s feathers. This means that every ounce of down inside my Neutrino 800 sleeping bag fills 800 cubic inches of space. The downsides (ha) of down include price and the fact that if your down sleeping bag gets excessively wet it will take a long time to dry (and won’t insulate well when wet.

Synthetic insulations, on the hand, tend to weigh more and take up more space. They also break down (lose their warming capabilities) every time they are stuffed into a sack which makes them less durable than down bags.

If you’re worried about how the geese and ducks fare in this scenario (as I am), know that most manufacturers, including Rab, The Northface, and Big Agnes, have adopted the Responsible Down Standards (RDS) in the making of their sleeping bags. So, go confidently forth into the wilderness with your down-filled bags and cheers to a good night’s sleep.