The Best -40 Sleeping Bags: Extreme Weather Calls for Extreme Gear


-40 sleeping bags represent the most hardcore sleeping gear obtainable, and for an expedition into the back country or mountains amidst the deep-freeze, it’s extremely important to have, because isolated areas take additional time to reach, should anyone need aid – this rig could save your life. Being one of the most critical pieces of equipment for winter temperatures, great care should be taken to select the model that will best suit your needs and has the correct temperature rating for the areas you’ll be using it in. Don’t forget to check out our article about less extreme, more general-purpose cold weather sleeping bags. In this article, we discuss features of extreme cold-weather equipment in detail, but if you’re in a hurry to look at our 3 picks of what we think are the best -40 rated sleeping bags, by all means, jump ahead.

So who needs a bag like this anyway?

  • Backcountry Hikers
  • Mountain Climbers
  • Winter Campers
  • Winter Sports
  • Anyone Planning to Spend and Extended Amount of Time Outdoors in the Winter

Sleeping Bag Temperature Ranges Explained

Temperature ranges for sleeping bags reflect outdoor temperatures and are a guide to selecting the right one for the purpose needed. It is a good idea to select one that is about 5 to 15 degrees below what you think will be the temperature needed. For example, if you are planning an Alaska expedition, you would check the average weather temperatures. With temperatures ranging from 10 degrees to -30, you would want to select one rated at -40 to ensure you remain warm.

What are the temperature ranges for modern sleeping bags?

Temperature ranges depend on how you want to use the sleeping bag with most ranging between +10 to +50 as most people camp in the summertime. Sleeping bags rated at +10 to +35 are the next most common and is suited for spring, fall, and summertime use. +10 and lower is for winter temperatures.

Are the temperature ratings accurate? Does a -40 bag really keep you warm in -40 temperatures?

Yes – if used properly. You can absolutely count on the temperature rating to be accurate. Keep in mind though that a sleeping bag is one piece of equipment needed for sleeping outdoors and every person has their own comfort level. Sleeping pads should always be used when camping outdoors. It provides a layer of insulation and acts as a barrier between you and the cold ground and provides greater comfort and warmth. However, sleeping bags rated -40 will keep you warm in that range.

You also will want a shelter that can withstand high winds and provides a barrier between you and the cold. The temperature ratings are based solely on the lowest temperature the sleeping bag can still perform without the aid of a sleeping pad or shelter. Understanding how all of your equipment works together can help you stay warmer even in temperatures slightly lower than the rating. This is not something you should try to determine when selecting a sleeping bag because there is not a way to accurately determine the difference. Having the additional equipment simply offers greater peace of mind that you will remain warm.

Will a -40 sleeping bag be too warm in the summer?

Yes. too-hot-to-sleep Summertime temperatures at night can range from 40+ degrees and require a sleeping bag better suited for the summertime. Also, most cold weather sleeping bags have minimal if any venting. Venting is something you definitely want in your sleeping bag during the summertime to release the heat. Also, the density or rating of the fill is an important factor in temperature ratings with higher fill or density used in cold weather sleeping bags.

Materials Used in Construction

The shell and liner nearly always consists of ripcord nylon threads. Additionally, some threads may have a thin film of GORE-TEX, which also protects the bag from getting wet. Fill is either synthetic or natural, like goose down feathers or cotton.

Other sleeping bags could have a shell and liner made of polyester or cotton, but they are not as durable as nylon. Polyester shells and liners are more likely to rip than nylon. Cotton becomes heavy and the fill shifts around the bag causing lumps. Cotton bags are used in the summertime and are generally the lower priced sleeping bags because of its low performance. It is not nearly as durable as other materials used in higher quality sleeping bags.

Materials Used To Fill Cold Weather Sleeping Bags

Most sleeping bags use synthetic materials which consist of nylon as well but are balled up instead of individual strands. Other sleeping bags use down, which is not recommended because once down becomes wet, it does not perform as well. Some manufacturers are now using water resistant treatments on the down to alleviate this issue. Down ranges from 400 to 900 fill with the lower numbers for warmer weather. Also, higher down ratings have a greater loft (thickness) than lower rated ones. If you use a down sleeping bag, make sure the fill is rated at 600 or better and has been treated for water resistance.

Synthetic fills are also used because they provide effective insulation even when wet. Most cold weather camping environments tend to be in “wet” environments due to snow and/or rain. Synthetics are also a wise choice for people that have a tendency to perspire when sleeping as the moisture can affect the insulation ability of down.

Which materials are used in the coldest rated bags?

A bag rated at -28 or greater is for extreme cold. The materials used for the shell and liner are ripcord nylon, with a higher thread count for the shell and slightly lower for the liner. This material is quite durable and is not prone to ripping. Nylon ripcord is also extremely windproof. Shells are coated with DryLoft or Epic which is a thin layer similar to GoreTex that creates a sleeping-bag-fillingwaterproof barrier.

The fill is predominately down because it insulates better than synthetics. Manufacturers have begun treating down with DryLoft to negate the only negative aspect of down, which is reduced insulation when wet. Synthetic fill is used equally because it can be wet or dry and still deliver effective insulation for the bags temperature rating.

Synthetic Fill vs Natural Fill

What is the difference between synthetic bag fillers and natural fillers like goose down? It comes down to weight, insulation, cost, and performance. Goose down is priced higher than synthetic fills for several reasons. The primary reason is that down, unlike synthetic, requires more physical material. A higher down fill rating uses the most down and is the most costly. Down is lighter and provides more warmth but does not perform as well when it becomes wet.

Advantages and Drawbacks of Synthetic Fill


  • Still warm when bag gets wet
  • Hypoallergenic
  • Less costly


  • Bulky
  • Breaks down faster
  • Heavier

Advantages and Drawbacks of Natural Fill


  • Lightweight
  • Better insulator than synthetic
  • Compresses down for easier packing


  • Poor warmth when wet
  • Allergen for some people
  • Special care is required for cleaning

Our Top 3 Best -40 Cold Weather Sleeping Bags

Marmot Membrain Down Sleeping Bag

marmot-CWM-membrain-down-sleeping-bagThis bag provides great warmth with its 800+ rated goose down fill, but it will also drain your wallet. Luckily for the human race, you will undoubtedly spend money to keep yourself alive, so while you’re blowing cash, you may as well be snuggled up in one of these. You will be encased in luxurious warmth and comfort with this mummy-style sleeping bag. The 30D ripstop nylon shell is coated with Pertex Shield 2L laminate which is a membrane that is both breathable and waterproof.

It also features nine baffles running through the bags core keeping the down well controlled by preventing clumping when shifting or rolling over. The head of the bag contains six baffles providing greater comfort because it fits closer to you head. Your head releases heat making this feature quite important. The draft collar provides additional insulation because it is lined with down. Keep drafts out with the adjustable draw-cord hood to cover your head or even your face, leaving an opening for breathing. A draft tube runs along the zippers length to prevent drafts from entering the sleeping bag. The draft tube also contains a hidden pocket to ensure essentials like a wallet, car keys, tissue paper and other small objects remain within easy reach.

The bags liner is also constructed with 30D ripcord nylon that is treated with Pertex Microlight. This added touch has a softer feel and durable enough to prevent the down from poking the inside of the bag and keeps the down in the baffles. Your feet will remain warm with a wrap-around foot box which is an ideal feature in cold weather environments.

This bag is available in regular and long sizes. The regular length is 84 inches in length with a hip circumference of 64 inches and a 46 inch circumference at the foot. The long measures 90 inches in length, 66 inches at the hip and 48 inches at the foot. This sleeping bag also comes with a limited lifetime warranty and weighs in at 4 lbs with the stuff and storage sacks that are included.

Black Pine Backside

black-pine-backsideYou may forget your in extreme cold weather with the Backside -50 Down-X sleeping bag. You get the best of both worlds with the combination fill it utilizes. The 750 down fill is combined with a super lightweight X-fiber fill. Synthetic X fibers are thick and braided and can cover more surface area. The combination fill means you will stay warm and comfortable, even if the bag does get wet. It also reduces the weight compared to synthetic fill and is also more compressible. The down fill weight is 2.5 pounds as is the X-fiber for a total fill weight of 5 pounds.

The shell is comprised of high thread count, down-proof Tactel diamond ripstop nylon and coated with a waterproofing agent. Tactel Diamond ripstop nylon contains tightly woven microfiber filaments and threads to stop rain from entering the sleeping bag and keeping you dry. The wind is also unable to penetrate the shell ensuring you a warm and comfortable experience even when the wind brings frigid cold with it. The sleeping bags shell is two layers that are offset to prevent the seams from overlapping and to prevent drafts.

An over-sized draft tube provides assurances that the frigid air outside stays out and the warmth stays inside the sleeping bag with you. The box foot construction is a nice feature that provides additional insulation so your feet are never at risk from exposure and your feet will enjoy the warmth. The self-locking YKK zipper is not standard on most sleeping bags. It is a nice feature because you can sleep soundly without worrying that the zipper will slide down and expose you to outside temperatures

This bag weighs in at six pounds and is 88 inches long. It measures 35 inches across starting at the head and tapering down towards the foot at 12.5 inches. It packs down to 9 x 15 inches so you can easily pack it and hit the trail.

Goretex 5-Piece Improved Modular Sleeping Bag System

improved-military-sleep-systemYou can’t go wrong with this sleep system. Again, I’m biased because I’m a former US Marine, but even taking that out of the equation, unlike other winter sleeping bags, this bag can be used year-round! I should also mention that it’s an improvement over the original US Government issue sleep system. This rig is rated at -30, however, it is rated at -40 when the bivy cover is used as well. You may recall that the sleeping bag is only one component when camping, the sleeping pad is critical gear. With the bivy sac and a well-constructed sleeping pad, you will remain warm. It has a bivy cover that can be used for greater warmth.

The shell is ripstop nylon and water resistant. Another cool feature is the double insulation at the foot of the bag. The edge of the bags opening is quilted to form a lip that prevent drafts entering the bag. A synthetic fill provides the material to keep you warm on those cold nights. The zipper is a reversible double-pull slider so you can open the sleeping bag from end to end. They are also non-locking meaning it self-repairs if there is a jam or it gets snagged on the bag or other material.

The included bivy cover is made from a three-layer durable laminate fabric that is both windproof and waterproof. Snap and slide fasteners can hug the cover closer to the sleeping bag for greater insulation. It measures 85 inches by 28 inches x 35 inches and weighs 1 pound, 4 ounces.

The larger of the two stuff sacks has six 1 inch wide nylon webbing straps that can be adjusted. It can compress down to one cubic foot and the sacks circumference is reinforced with webbing that has been sewn in. It measures 30 inches long and 14 inches in diameter. It weighs 12 oz and comes in the foliage pattern. The smaller stuff sack has the same features but weighs only 8 ounces.

A 3 season sleeping bag is also included with this sleep system and features many of the same components as the cold weather bag. The temperature rating and longer length are the primary differences between the two bags, along with venting and a different color. Place the cold weather bag inside the 3 season sleeping bag for additional warmth.

The sleep systems cold weather sleeping bag measures 87 inches long and 23 inches or 35.5 inches wide and comes in urban grey. Total bag weight is 3 pounds, 10 ounces. The three season sleeping bag measures 92 inches long and 24 inches or 37 inches width and comes in a foliage pattern. It weighs 2 pounds, 7 ounces and is completely ridiculously perfect.


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Meet the Author

I'm a former US Marine, full-time father and husband, and part-time survivalist. I like old things, like music, furniture and time-tested survival technique, but there's always a place in my heart for new technology such as weapons and sleeping bag fill. I'm here to share my knowledge and experience, and I hope you'll share yours as well.

6 comments… add one
  • Morgan R. Jan 11, 2016, 1:05 am

    I just recently purchased this Marmot bag and I’ve got to tell you, the quality is insane! What a wonderfully made piece of gear – I love it.

  • HikingMom Sep 25, 2016, 11:20 am

    Hi there. We will be hiking to Everest base camp in October with kids. What bags would you recommend for adults and/or kids that could be used for this hike and the many more trips we will make, as the kids want the do the 7 summits (as high as they can on each one) and so we want to invest in sleeping bags that will keep us warm and last in the years to come. Any suggestions would be appreciated! Thanks

    • Mike Sep 27, 2016, 7:48 pm

      There are many nice bags for kids, but unfortunately, I don’t have any direct experience with them. I will tell you that the Modular Sleep System (military) is a superb setup. Full disclosure, I’ve never hiked Everest – you and your family have amazing ambition!

  • emilio Apr 10, 2017, 10:34 pm

    hi, thnks so much for your post . what about this seeping bag? ….Snowy Owl EX -60 Sleeping Bag…. let me know what do thing about it .

    i really like the military bag and i wanna buy one but im thinking about the weight, be cause it is 2 times more heavy than the other . at the same time its 3 times cheaper than Marmot…

    i wanna do denali, aconcagua and in the future the everest, if is possible 7 summits . . starting this year , i need something good for all this years .. all in winter with no guide. only a grup of 2 or 3 , someones by myself . . i dont have sponsors and so i need cheap and good things.

    thnks for all the information. bye =)

    • Mike Apr 11, 2017, 12:21 pm

      Emelio, I don’t have any experience with that bag, so I can’t really provide any input. Marmot makes nice gear, but I think you pay a lot for the name. I’ve also heard very good things about the Canadian version of the Military Sleep System, and I’m going to be checking that out for sure.

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