Winter Sleeping Bag FAQ

Winter Sleeping Bag FAQ

A winter sleeping bag has a temperature rating of 0 degrees Fahrenheit or lower. Remember, 32 degrees Fahrenheit is the temperature where water freezes, so 0 degrees is plenty cold. Winter sleeping bags also tend to have extra features not found in sleeping bags designed for warmer weather including draft collars, draft tubes, zipper guards, continuous baffles, and sophisticated shell fabrics. If this is all mumbo jumbo to you, no worries. I explain how to choose a winter sleeping bag and what all of these features are, below.

Who makes the best winter sleeping bags for backpacking?

If you plan on backpacking in winter, weight and packed size should be high on your list of selection criteria for choosing a winter sleeping bag. The lightest weight and most compressible winter sleeping bags are made by Feathered Friends and Western Mountaineering which specialize in cold weather bags and have numerous models and temperature ratings to choose from. NEMO, Mountain Hardware, Marmot, and Valandre also have excellent reputations, but they have a much smaller selection of bags available. Feathered Friends, incidentally, is one of the few sleeping bag manufacturers to make womens-specific winter sleeping bags as well.

Make / ModelTemp Rating (F)Down Fill PowerWeight
Feathered Friends Snowbunting EX 009002 lbs 13 oz
Feathered Friends Ibex EX 009003 lbs 1 oz
Feathered Friends Murre EX 009002 lbs 10 oz
Western Mountaineering Kodiak MF08502 lbs 12 oz
Mountain Hardwear Phantom 008502 lbs 10.6 oz
NEMO Sonic 008003 lbs
Mountain Hardwear Phantom Gore-Tex 008503 lbs 5 oz
Marmot Lithium 008002 lbs 9.5 oz
Marmot Paiju -5-58003 lbs 11 oz
Valandre Shocking Blue Neo -58502 lbs 14 oz
Feathered Friends Widgeon EX -10-109003 lbs 4 oz
Feathered Friends Eider EX -10-109003 lbs 7 oz
Feathered Friends Artic Finch EX -10-109003 lbs 6 oz
Western Mountaineering Bristlecone MF-108503 lbs 15 oz
Western Mountaineering Lynx MF-108503 lbs 2 oz
Western Mountaineering Lynx GWS-108503 lbs 5 oz
NEMO Sonic -20-208003 lbs 8 oz
Marmot Col -20-208004 lbs 6 oz
Valandre Odin Neo-228503 lbs 9 oz
Feathered Friends Peregrine EX -25-259003 lbs 14 oz
Feathered Friends Plover EX -25-259003 lbs 11 oz
Feathered Friends Ptarmigan EX -25-259003 lbs 12 oz
Western Mountaineering Puma MF-258503 lbs 7 oz
Western Mountaineering Puma GWS Expedition-258503 lbs 12 oz
Western Mountaineering Cypress GWS Expedition-308504 lbs 14 oz
Feathered Friends Snow Goose EX -40-409004 lbs 3 oz
Western Mountaineering Bison GWS Expedition -408504 lbs 10 oz
Mountain Hardwear Phantom Gore-Tex -40-408504 lbs 10 oz
Marmot CWM -40-408004 lbs 15.5 oz
Valandre Thor Neo-408504 lbs 1 oz

Which is better in winter, a mummy sleeping bag, or a rectangular sleeping bag?

Mummy bags tend to be lighter weight because they’re form-fitting, making them better for winter backpacking when you want a lightweight, compressible sleeping bag. Rectangular sleeping bags can be just as warm, but they’re more appropriate for winter camping, not backpacking, because they’re not as form-fitting, they’re heavier, and have less efficient hoods to keep your head warm.

Can you use a quilt instead of a sleeping bag in winter?

It really depends on how cold you want to go. Most people switch from quilts to mummy sleeping bags at about 20 degrees Fahrenheit because sleeping bags are much better at blocking cold drafts than quilt-pad attachment straps. A quilt can certainly work down to 0 degrees if you augment it with a down hood (effectively replicating a mummy bag hood and draft collar) to keep your head, neck, and upper chest warm. Below zero, you can try stacking quilts but I highly recommend testing out this strategy at home before trying it someplace where it counts.

What’s the best insulation, down or synthetic?

If you’re interested in winter backpacking, your best bet is to get a down sleeping bag with the highest down fill power (see below) you can afford because it is ounce-for-ounce lighter weight, and more compressible than a winter sleeping bag filled with synthetic insulation. Winter backpacking gear is very heavy when you add up the weight of insulated boots, a high-capacity backpack, a four-season tent, winter sleeping pad(s), a liquid fuel stove, fuel, snowshoes, crampons, etc. so it really does pay to save as much weight and pack space as you can by using the lightest and smallest winter sleeping bag that will keep you warm.

Which is better, waterproof down, or regular untreated down?

So-called waterproof down isn’t actually waterproof, but water-resistant. If you dunk it in water, it will fail to insulate, although it will dry faster than untreated down. Therefore, if you plan on sleeping in snow caves or out in the open in a bivy sack, waterproof down is probably a better option since the down will dry faster if the fabric shell of your bag gets wet. If you’re sleeping in a cozy tent with a waterproof floor, it probably doesn’t matter that much, except if you’re on a multi-day trip where the accumulation of perspiration passing through your insulation and out the breathable shell of your sleeping bag can accumulate in the down, degrading its loft and ability to retain warmth. Waterproof down would be better in those circumstances.

What is down fill power?

Down consists of fluffy filaments that are a lot like human hair. A single ounce of average quality down contains about 2 million of these filaments which interlock to keep warm air in and cold air out. This layer is very springy so you can scrunch it up by compressing it, but it will spring back into shape almost immediately. Fill power measures the lofting power of goose down which is its ability to trap air. To measure fill power, one ounce of down is compressed in a small glass cylinder. When the weight is removed, the down’s ability to spring back can be measured. Down with a higher fill power rating is more resilient to compression, lofts better, and can trap more air. Besides being warmer, this also means that sleeping bags or parkas with higher fill ratings require less insulation by weight to provide the same level of warmth than an item made with lower quality down.

Is there a warmth difference between goose down and duck down?

No. Fill power is measured the same way across different animals and species. In other words 750 fill power goose down provides the same level of insulation as 750 fill power duck down.

Is there a cost difference between different down fill powers?

Yes. The higher the fill power, the more expensive it will be. Prices have dropped in recent years however since there is a worldwide surplus of down, but the higher fill powers remain the most expensive.

What’s the best down fill power for a winter sleeping bag?

The lightest weight, most compressible winter sleeping bags are insulated with 800, 850, 900, or 950 fill power down. A 950 fill down power bag is top of the line.

How trustworthy are winter sleeping bag temperature ratings?

While warm weather sleeping bag ratings have become much more objective in recent years with the adoption of international temperature rating standards and third-party testing, the same can’t be said about winter sleeping bag temperature ratings. While the EN13537 temperature rating published for warm weather sleeping bags rated to 10 degrees and higher has proven to be reliable, studies have shown that it is not a statistically reliable way to rate the temperature rating of sleeping bags rated below 10 degrees, including all winter sleeping bags.

Instead, sleeping manufacturers rate their own sleeping bags by having people sleep in cold rooms, basically walk-in freezers, to see if they stay warm at different temperature settings. This can generate very subjective results depending on who does the testing, whether they’re male or female, what their body weight is, how well a sleeping bag fits them, the warmth of the long underwear they’re wearing, whether they’re wearing a hat or not, what sleeping pad they’re using, when they last ate, and so on.

So who can you trust? Your best bet is to buy a winter sleeping bag model that’s been on the market for a while and has a loyal following of people who will attest to the accuracy of its temperature rating.

Feathered Friends and Western Mountaineering have built their business around word-of-mouth testimonials, which is why people prefer their winter sleeping bags over all others. No one else provides the range of selection or technical features provided by these two manufacturers.

Can you use a 0-degree winter sleeping bag instead of a -20 degree winter sleeping bag?

There are all kinds of tricks to extend the range of a 0-degree sleeping bag for colder weather, especially if gear weight or bulk is a big concern:

  • by wearing insulated clothing inside your sleeping bag in order to boost its effective temperature rating
  • sleeping with hot water bottles
  • sleeping with chemical hand warmers
  • using a sleeping bag liner
  • stuff all of your spare clothing inside so your body has to heat less open space
  • eat a heavy dinner with lots of fat and protein before going to bed
  • and so on…

It is best to experiment with these in your backyard or within walking distance of your car before you need to count on them in a wilderness survival situation.

If you are on a major expedition and your guide requires a -20 degree or -40 degree bag for sustained frigid weather, you should consult with them about using a winter sleeping bag rated for warmer temperatures. Many guides rent cold weather bags, which can save you the expense of buying one for one-time use.

Another option is to cancel trips where you know the weather will be too cold for your sleeping bag. As a winter backpacking trip guide, my partners and I routinely cancel trips where the weather forecast calls for -20 nighttime temperatures since it’s really no fun to sit outside and melt drinking water in weather like that. There’s no shame in bailing on a trip when it’s too cold outside to enjoy yourself.

Do women need warmer winter sleeping bags than men?

Women tend to sleeper colder than men, so it’s recommended that they get a sleeping bag that is 10 degrees warmer when buying a unisex or men’s sleeping bag. For example, women should buy a -10 degree winter sleeping bag in order to stay as warm as a man in a 0-degree winter sleeping bag. However, that’s not the case, if you buy a winter sleeping bag designed for a woman. A women’s 0-degree winter sleeping bag should keep you as warm as a men’s 0-degree sleeping bag.

What’s the difference between men’s and women’s sleeping bags?

Women’s sleeping bags are often shorter in length, narrower in the shoulders, and wider in the hips. They may also have extra insulation over the chest, in the hood, and in the footbox, since women have a harder time keeping their head, hands, and feet warm.

How do you size a winter sleeping bag?

Sleeping in a winter sleeping bag is different from sleeping in a three-season or summer sleeping bag because you need to sleep with some of your gear from freezing overnight (boots, water bottles, electronics) or because you need to wear additional insulated clothes in your sleeping bag like a down parka and down-filled pants.

To size up, people typically get a longer-sized sleeping bag or one that has a wider shoulder girth. Shoulder girth measures the circumference of the sleeping bag at shoulder height. When fitting a winter sleeping bag, you want to minimize the amount of extra interior free space that your body has to heat up while not compressing the loft of your insulation by getting a bag that’s too tight.

For example, a 5′ 10″ tall man might opt to get a 6′ 6″ long bag instead of a 6′ long sleeping bag in order to store some of his gear in the sleeping bag foot box. Alternatively, you can opt to get a bag with wider shoulders or chest measurements so you can hug your extra gear at night to keep it warm or tuck it behind your back (if you’re a side sleeper.) That’s always been my preference, instead of getting a longer-length sleeping bag.

How do you determine your needs and preferences? Try on lots of sleeping bags while wearing your overnight gear, even if it means buying bags and returning them to retailers if they don’t fit.

What are the most important features to look for on winter sleeping bags?

After fit, the most important features are having a draft collar, draft tubes, zipper guards, and a well-fitting hood. It’s also important to get a sleeping bag with a breathable external shell that can vent perspiration but is water-resistant. Sleeping bags with waterproof/breathable shells are often less breathable than those without a waterproof/breathable external fabric.

What is a draft collar?

A draft color is an insulated tube of insulation that covers the top of your chest and back and seals in all of the warmth below it in your bag so it can’t escape. It prevents what’s called the “bellows effect”, where the warm air around your legs and core is forced out through the top of your sleeping bag when you move around at night. You can achieve a similar effect by wrapping a weather or down jacket around your upper chest or neck as well.

What is a draft tube?

A draft tube is an insulated tube of insulation that runs along the zipper and prevents cold air from leaking in your sleeping bag or warm air from leaking out. Some sleeping bags like the Western Mountaineering Puma -25 have two interlocking draft tubes, one above and one below the zipper.

What is a zipper guard?

A zipper guard is a piece of fabric or stiffened fabric tape that runs along a zipper and prevents it from snagging on the inside lining of your sleeping bag. It’s an important feature of a sleeping bag since it eliminates snags which can prevent you from closing the zipper in frigid weather.

What is a sleeping bag baffle?

A baffle is a fabric tube containing down or synthetic insulation. They’re usually oriented horizontally or vertically in sleeping bags.

What are continuous baffles?

Continuous baffles are fabric tubes filled with down insulation that usually run horizontally around a sleeping bag. They let you shake the down inside them to move it where you want it, usually to the top of a sleeping bag, or down the sides. Found in high-end quilts and sleeping bags like those from Feathered Friends or Western Mountaineering, continuous baffles are a highly desirable feature for some people that lets you move the insulation to the parts of you that are cold. For others, it’s a curse, because the down insulation can shift where you don’t want it to go, creating cold spots.

What are block baffles?

Blocking baffles, also called side block baffles or V-block baffles are used in high loft, winter expedition sleeping bags to prevent down from shifting from the top of a sleeping bag to its sides.

What are the pros and cons of sleeping bags that have waterproof breathable exterior shells like Gore-tex?

Unfortunately, they’re mostly cons. The idea of covering a sleeping bag with a waterproof/breathable shell fabric is appealing because it would mean that you don’t need to carry a bivy sack to sleep in a snow shelter or worry about getting internal condensation on the outside of your sleeping bag when you touch your tent’s walls at night. But experience has shown that covering the exterior of a sleeping bag with a waterproof breathable shell tends to trap more perspiration inside the insulation of a sleeping bag than one a much lighter shell fabric.

Contrary to what you’d expect, waterproof/breathable fabrics are actually far less breathable than most of the non-waterproof shell fabrics used on the exterior of sleeping bags today. You can get the same waterproof benefit by spraying a DWR coating on the outside of these lighter-weight, more breathable fabrics, which will repel water droplets that fall onto the outside of the bag, causing them to bead and roll off, just like a rain jacket. Most sleeping manufacturers already do this at the factory. But if the DWR coating wears off, you can apply it at home using Nikwax TX Direct or similar products.

In addition, most sleeping bag manufacturers don’t tape or seam seal all of the seams in their bags, which is really required for true waterproofing. Think about all of the tiny needle holes in the baffling of a down bag. Taping or seam sealing them all would be very costly. In contrast, most bivy bags made with waterproof/breathable fabrics have taped seams or can be easily sealed with seam sealer. You really can’t do the same with a sleeping bag.

What is the best external fabric for a winter sleeping bag?

You want an external shell fabric that is tough enough to be durable and has good breathability, with a tight enough weave and/or a DWR coating that will make water roll off its surface. Pertex Shield and microfiber calendared nylon are good examples of fabrics with these properties.

How should a winter sleeping bag hood fit?

This is an area of personal preference, but you want a hood that will fit around your head without any air gaps that leak warmth. It should be easy to adjust (many aren’t), move with you if you roll onto your side, and not become saturated with water vapor when you exhale at night through your mouth. The best way to determine hood fit is to get inside a sleeping bag and try it out. It’s important to get a hood that you can spend 12 hours or more in comfortably since winter nights are so long.

How important is a high R-value sleeping pad?

A winter sleeping bag without a high R-value sleeping pad is like a sheep without wool. Without one, your body heat will be sucked into the cold ground. We recommend using a sleeping bad with an R-value of 5 or better. R-values are additive, so you can also use two sleeping pads, one stacked on top of the other to achieve a higher composite R-value. Many winter backpackers will carry a foam pad, which can be used as a seat when cooking or snow melting, in addition to an inflatable sleeping pad for comfort. When choosing winter pads, we recommend using ones that have been rated using the new sleeping bag standard, such as the Therm-a-Rest XTherm, which has an R-value of 6.9

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About the author

Philip Werner has hiked and backpacked over 7500 miles in the United States and the UK and written over 2500 articles as the founder of, noted for its backpacking gear reviews and hiking FAQs. A devotee of New Hampshire and Maine hiking and backpacking, Philip is the 36th person to hike all 650 of the hiking trails in the White Mountain Guide, a distance of approximately 2500 miles, completing a second round in 2021. Philip is the author of Backpacking the White Mountain 4000 Footers, a free online guidebook of the best backpacking trips in the White Mountains in New Hampshire and Maine. He lives in New Hampshire.

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  1. As a side sleeper I just don’t get on with sleeping bag hoods & hated awaking to a soggy mess in my face; to 20F I generally get along with a quilt + Blackrock beanie + merino Buff; below this it’s the winter SB + down balaclava; I find it a much more comfortable & warm setup……but each to there own.

    • I’m always wearing a hat in a winter sleeping bag or a hoodie over my base layers. Quilts are just too drafty for me below 20 degrees and take too long to rewarm between bio breaks.

    • I am a side sleeper too and have found that it works well if I just roll the sleeping bag with me so that the face opening stays over my face and the hood over my head. I used to think that the zipper would make that uncomfortable so I didn’t do that, but that turned out not to be the case.

  2. At one point early in my backpacking career, I switched completely over to quilts and got rid of all my traditional sleeping bags I first started with. I backpack year round in the east and I used the quilts down to very low teens. Not always comfortably. I found the whole attachment system fiddly (I used Katabatic quilts). You had to set it up each night, if you got up to pee you had to reattach them. If you just wanted to situp, you had to unclip them and would be hit by a blast of cold air in the back. I remember an upcoming trip where it was forcast to be 0 or slightly below. I just thought, there is no way I trust this quit at those temps, so admittedly a spur of the moment decision I ordered a 0 degree western mountaineering sleeping bad expressed shipped 4 days before the trip. I was nice and toasty warm in the bag when I woke up and it was -2 out. It was so nice to be able to just get in the bag, with no fuss. It was also my first really high quality traditional sleeping bag and I was impressed by the build. After that I slowly started selling my quilts and replacing them with bags. I still do prefer hoodless bags down to the mid 20s and just wearing a beaning or down hood. I have a couple enlightened equipment bags, one that fully opens, and another with a closed foot box, but both can be totally zipped up so no drafts period. I just don’t miss the futz factor of quilts, the extra few ounces is worth it. I also got a traditional 15 degree WM bag that I will use if I’m expecting temps into the mid to low 20s.

  3. Me again. Long time visitor, now have questions two days in a row: Anyway…Here it goes:
    From a past article I learned sleeping pad R values are cumulative. How about sleeping bags? Seems a +20 degree bag inside a +40 degree bag would be warmer, but by how much? Is there a formula or rule of thumb?

    • Not really – nothing reliable. The problem is that the inner bag is compressed by the weight of the outer bag so you lose loft (warmth). You’d be much better off using a winter bag with a sleeping bag liner, since they use a different insulation approach, like fleece. In any case, always experiment in the backyard before you take it on a trip to make sure it does what you think or hope it does.

    • There is no formula. The problem is that you have two outers hindering the airflow. It has been tried and done (a summer bag inside a 3-season bag might be cheaper for some people than having three different bags), but always had compromises.

      1) Weight – you now have much more fabric outers and inners, probably at least30-50% more weight than a simple winter bag, with better baffles.
      2) Breathability – the several fabric layers hinder it greatly. Depending on conditions and temp. gradients, it might even trap moisture inside the bag, where it would condensate and freeze.
      3) Cost – most of such “doubles” I have seen used a simpler baffle construction because of costs, while a better buffle single bag would have been better.
      4) It’s just for the convenience of buying two bags for four seasons, but not really. You are much better buying dedicated winter bag, especially for the weight. The weight saving is really worth a lot, especially with all the other winter gear…
      5) If your winter outings are infrequent, you could still get a “cheap” winter bag from such as Decathlon (Makalu from Simond, their – formerly independent – mountaineering brand). Would be lighter and probably much better than frankensteining two bags or even quilts together.

    • Before I had saved up for a winter bag, I nested a 15 degF down mummy inside a 15 deg F semi-mummy (BA encampment). This worked well down to 0 Deg F and I always felt I had a safety margin. The key was the relative shapes; the inner down bag had room to loft inside the outer bag. Bulky, heavy solution that necessitated a pulk and therefore limited the trip selections. Also a PIA to get in and out of when nature called. But it worked well. Of course this was tested out in back yard and then car camping in the Whites.

      • @Tom, I did the same, when I was a cheapskate (no offence, just describing my finances back then)! :)

        But it was just a lot much more gear (and volume) inside an already overcrowded winter backpack, with all the things that are needed (much more fuel, bulky winter stove, belay jacket, more climbing equipment, etc…).

        Even doubling up on two pretty lightweight bags uses up a lot of volume and weight compared to a single good winter bag (even more so, since winter bags often don’t need full zippers, just 1/3 would do – heck, hardy mountaineers and explorers of old used just an elephant’s foot!)*

        *) elephant foot as a 2/3 height sleeping bag that you used in combo with your belay jacket – down jacket on body, 2/3 bag below. Saves the weight of the jacket, essentially, since it’s double-use.

  4. I wouldn’t discount EN13537 ratings for winter bags! There is a caveat that people are very subjective in feeling cold and in their sleeping habits, sure, but it gives much better comparison than random manufacturers’ (especially cottage ones, sorry) claims. I have seen too much fuzzing around with the numbers to not like the EN13537 standard. Sure, your feelings of cold might differ (everybody sleeps differently), but whenever I see a manufacturer NOT using it, a warning light lights up. Their numbers might be completely legitimate – or not. Maybe I’m an average sleeper, but I found the EN numbers pretty accurate for me, down to -20C (-4F) so far. The biggest difference is the baffle construction,

    It’d be nice to see a paragraph on different down Fill Power measurements: USA vs EU, if it’s still a thing (used to be). US FP numbers tend to be around 7% higher than equivalent EU FP numbers, e.g. the same 800 FP EU down would be rated as 850 FP in the US, which is not insignificant difference.

    Since there are some really excellent winter bags made in the EU (after all, for example Polish climbers are famous for their winter ascents of 8000s) like Cumulus, it’d be nice. Most of the EU made bags also use RDS labeled down.

    I’d also mention that higher FP tends to last less than lower FP down, with repeated stuffing cycles.

    Finally, sorry, but the article plugs two cottage brands much too much to be considered neutral. They might be great indeed, but it is still plugging particular brand(s), feels like an advert at times (I am sure it’s probably not and they are indeed good, it just feels that way with the wording you use, since there are plenty other great bag manufacturers with decades of experience making expedition clothing and sleeping bags that actual 8000s expeditions use).

    • The independent labs who do the EN13537 tests discount the ratings because they’re not statistically reliable for bags under 10 degrees. This is science, not a whim. When I see an EN13537 rating on 0 degree or colder bag, my assumption is that some marketing person doesn’t understand the limits of the standard or the company is trying to sham its way to credibility by using the standard to fool potential customers who don’t know better.

      I can also assure you that Western Mountaineering and Feathered Friends are not cottage companies but mid-sized companies that specialize in sleeping bags. This isn’t some guy in a log cabin with a sewing machine. Western Mountaineering, for example, has had international distribution in Europe for decades. Feathered friends also has quite a large customer base in the US, although they stick to direct sales instead of wholesale like WM.

      Of course, Valandre is also European company (French).

      • Thanks for the reply! Got any links please? “Not statistically reliable” as in not comparable at all, or just not completely accurate to real world? It would be interesting to read!

        Sure, any test that doesn’t include actual humans, just a hot/cold plate might be very different from actual real world use, but I still feel it’s more reliable for comparison of bags between different manufacturers. Or not? :) Since I know how I feel in e.g. my bags in the winter in some conditions, it might help me know how I’d feel in a similar bag from another manufacturer in similar conditions, without delving into comparing different baffle construction and other things. Of course there are many variables, but so far the EN standard has held for me pretty well *for comparing relative performance* between manufacturers, *when I know my baseline from how I feel in some EN rated bag*. Same could be perhaps done by comparing FP and baffle construction, but with so many variables, a generic picture like the EN standard works for me pretty well.

        Sure, for actual use, the “real feel” numbers from some manufacturers might be better, but these can’t be really compared amongst different brands, you’d still need a baseline.

        And sorry, I did not mean to disparage the article or the FF or WM bags :)
        They might be great for all I know (I don’t know them), it was just the article wording that put me off slightly, like in the continuous baffles paragraph. I just felt it was not quite neutral enough, but of course it’s your article and your website :)

      • It’ll take me a while to dig up. The test establishes a baseline so bags from different manufacturers can be compared using a common standard. The testing conditions simulate cold room conditions which is how most tests were performed before the standard passed. Whether they are a good measure of actual outdoor conditions is debatable (humidity, wind, dehydration, gender, etc.)

      • Thanks, that’d be interesting to read, if you can find it! I’ll stick with the EN in the meantime for comparisons (and of course comparing baffle construction, down amount and fill power), with the usual caveat that it’s just an artificial standard and the temps need to be cross checked to experience :)

        Unfortunately, some (none of those in that list of yours though, as far as I can tell) manufacturers abuse even the EN standard with clever marketing, subtly promoting the T(limit) as their bag’s normal comfort range! Seems to be limited to the Chinese synthetic bag importers and rebranders only, but good to know – even with a standard, you can still be “creative” with how you market and present the numbers… Boldly presenting a synthetic bag limits as -10C (14F) when it is it’s T(extreme) rating (i.e. you might not die of hypothermia with it at -10C) on their website (even if they sometimes provide the EN numbers in the small print) is a common marketing ploy. Of course that has nothing to do with EN standard but everything to do with clever marketing…

        I’d always cross check the manufacturer’s claimed numbers with bags of similar fill, baffle type (!) and so…

      • I’ve seen it abused by some American brands, who report the identical temperature ratings for men’s and women’s version of the identical bag even though there’s no difference between the two models (men and women by definition can’t have the same temp rating). When I’ve called them on it, I get a huge runaround. They’re not on this list, naturally. Western Mountaineering does publish EN13735 ratings on their bags, but only in Europe because it’s required, by law, I believe. I suspect it’s a Western Mountaineering subsidiary responsible for this, not the American branch of the company, but they do correlate exactly with their advertised temperature ratings.

      • Here’s an article by the head of the KSU test center which tests most US-made sleeping bags.

        I covered the roll-out of the EN13537 standard quite extensively and was in touch with members of the standards committee, including Dr McCullough, the author of the article cited above. In a personal communication from 2009, Dr McCullough wrote to me that “that a +/- 10% tolerance in measuring the insulation value could result in a 20 degree F difference in the limit temperature rating for a highly insulative bag.”

        This was all 11 years ago so it took me a while to dig this up. I did a bunch of interviews at the time as well and the consensus was that the EN13537 ratings for cold weather bags had a lot of statistical variability between standard temperature testing centers.

      • I think the most usual abuse of the EN rating I saw was with one OEM re-brander of Chinese synth bags, where they simply took the T(Extreme) ratings and advertised them boldly on the web as the bag’s sleeping temperature (!). With small-print stating the true ratings and a very confusing graphic explaining it. While technically right, T(extreme) is obviously quite extreme: “the minimum temperature at which a standard female can remain for six hours without risk of death from hypothermia (though frostbite is still possible). These ratings are taken assuming that the subject is using a sleeping pad, tent and is wearing one base layer of thermal underwear”. But that was intentional misrepresentation by their marketing department.

        Still, I find it helpful for comparison, if used correctly – obviously, it has its own caveats…

  5. Of course, breath & condensation, as you mention, is indeed the killer of any down (even synthetic, but less so) winter bags. I tend to prefer a warm hat or even a separate down hood, neck baffle tightly closed and leaving the bag’s hood fairly open to prevent any condensation from my breath wetting the breast area of the bag. You can easily thoroughly wet and clump all the feathers in the breast baffles of mostly any bag by breathing into the tightly closed hood. Some mountaineers recommend using sacrificial balaclavas or headscarfs that you can shake the ice out of in the morning. I thought of making a DIY tyvek collar there, but it probably wouldn’t stay in place and might just exaggerate any condensation from inside, like membranes do.

    As for the rest of the bag and longer expeditions, Kirkpatrick tried (a long time ago, though) a combo of down bag and synthetic overbag, with the reasoning that any condensation (inevitable in such conditions, especially in single skinned mountaineering tents) would, depending on the thermal gradient, fall upon or end up in the synthetic overbag, where it could be shaken out much more easily. A concept I’d like to try out, if we get any good winters again.

    The main problem with the membranes as outer material is that they are very dependent on a suitable thermal gradient and conditions. If the freeze point falls between the membrane and the down, you will just get lumps of ice inside the bag’s outer fabric, which is not nice at all. So they might work pretty well, but only in a very narrow range of conditions.

    • How should a winter sleeping bag be stored? I store mine in a loose fitting storage sack (approx 2’x3′) that doesn’t pose much restriction to the bag itself. To be honest I keep it stored in this sack all summer. Is this healthy for the bag?

      • It’s the perfect way to store it. Loose and fluffy, out of the sunlight.

      • Loose and fluffy it definitely the right way to go :)

        I use a space under the bed, but depending on your locale and bag use, that might be suspectible to mold. It’s always better to store it *horizontally*, though, whenever possible. Some of these mesh storage bags are made to hang vertically in the closet, but that’s not what you want to do.

  6. For a high quality sleeping bag, I very strongly recommend

    PHD is a UK-based company that offers both off-the-shelf and “design your own” sleeping bags (the link above is to the design your own section – it’s quite good fun!).

    My experience is that their pricing for a bespoke sleeping bag isn’t much different to retail options, but YMMV.

    I’ve got two of their sleeping bags and I can attest to the quality and customer service.

    They also offer a wide range of down clothing – I’ve never bought any so I can’t comment.

    • “bespoke” for our US audience, means “custom-made”

      • Thanks, Philip :)

        Yes, “bespoke” = “custom-made”.

        If you want to know more about the “mumbo jumbo” then you can use PHD’s online “design your own sleeping bag” tool to select the fill power, inner and outer fabric, stuff pattern, baffles collar, etc – they give helpful descriptions of what they mean and the purpose.

        The online tool updates the weight of your sleeping bag as you add or remove features (and matched pretty closely to the actual weight when I put my sleeping bags on the weighing scales at home).

        Like other cottage manufacturers they rate their sleeping bags using “real world field tests”. In response to Philip’s excellent (as always!) comment: I’ve been happy with their rating and they have a loyal following, including some high profile explorers and mountaineers.

      • They do have an excellent reputation.

      • And I should add that I’ve bought their products with my own money at full retail price. I have no affiliation with PHD.

  7. I recently picked the Therm-a-Rest Polar Ranger -20 Down Sleeping Bag at REI and will give it a whirl on a pemi loop in January.

  8. I recently climbed Kilimanjaro, and we rented “5-season sleeping bags” from the guides. Nighttime temperatures reached as low as -20 at altitude (before the summit ascent).
    I plan on climbing the other “Seven Summits”. What are the drawbacks of my just going for a -40 degree sleeping bag?

  9. My issue with winter is what to do when the bag is too warm, because weather, altitude, change, or you’ve moved into a snow shelter that’s tons warmer than outside. A winter bag zipped open makes an awesome quilt. And I never get drafts because winter bags are so big, and well stuffed they seal great, but there’s a danger of the outer edges getting soaking wet if you’re not in a bivi sack.
    Last winter I bought a Nemo -20 bag with thermo gills, based on a section hiker review, what a relief, the gills make a huge difference, sleeping outside in the open in top to bottom 150 wt merino inc hat, gloves & socks I was toasty warm at -9 degrees, then later I slept in the exact same place, same pad at 44 degrees in quilt mode with the gills open. Definitely the extreme upper limit, for me.
    Given that I could easily have gone colder I’m able to span 60 degrees with this bag. If I knew how to sew really well I’d add gills to my +20 bag.

  10. You have mentioned sleeping bag liners. Is there one in particular that you recommend? I noticed that there are now wool ones out there. I’m looking to extend the range of a 0 degree bag for winter camping in MN.

  11. what is typically the warmest ambient temperature that a -20 degree F bag can be used before becoming uncomfortably warm? Also a 0 degree F bag?

  12. I’m new to winter camping. Right now I’m testing in my backyard as nights get colder and colder in Canada. I have a WM Bristlecone which is a -23c bags and the nights are currently 0c – 05c. This is obviously nothing for the bristlecone. I move a lot while sleeping and go from side to back sleep all the time. As understood, I notice that the down under me gets squished but I’m still very confortable because my mat (megamat) is warm where I stand. But I also notice that if I move around I essentially move a section of squished down from a warm section of the mat to a cold part of the mat essentially creating a temporary cold spot. I do not feel like the upper part of the sleeping bag will ever get cold because it is so damn lofty. Do I understand well that the limit of my sleeping bag will be when these cold spots of squished down will hit much colder surfaces as the winter gets cold ?

    • If you’re sleeping on a megamat, which is a very high R-value sleeping pad, you won’t feel the cold through the bottom. The limit of your sleeping bag will still be the down on top and your ability to secure the draft collar/hood over your chest and around your head.

  13. As a long time winter camper, mostly on skis, I feel I’ve finally found theist winter (-20 F.) down bag for the money. It is an LL Bean winter bag with themes good features of any winter bag I’ve ever used or even seen in mountaineering shops.

    ->The “Bean bag” is 750 fill DWR treated “responsibly harvested” goose down.
    ->It is size Long which is good for storing things tha must be kept warm overnight.
    ->It came with captive ends elasticized draw cords on the hood and draft collar.

    All these make it an excellently designed winter bag and under it goes my REI FLASH All Season R 5.3 insulated air mattress at 18 ounces. With my light down jacket and down pants I’m sure it would be warm to -40 F. (in a tent).

  14. A timely article for me, Philip. Thanks. What is your opinion of vapor barrier liners for winter use? In using a lightweight rain jacket and a home-made waterproof nylon half-sack, I’m been a little clammy, but warm so far.

  15. Forgot–I use a WM 20 degree bag and have gone down to low teens with the set up.

    • I played around with VBLs for a while and just decided I liked carrying an appropriately insulated bag instead. The problem that arises is the flash-off as soon as you get out of your bag to pee. All that moisture evaporates and you get colder than when you started. I don’t bother. If I’m cold I just put on all of my clothes inside my bag or under a quilt, but that rarely happens anymore. When it gets below 20 at night, I usually carry down pants for camp or for sleeping, which also helps tremendously.

  16. I have had pretty good luck in winter stacking quilts and very good luck with a quilt over a mummy bag to increase insulation. A couple guys that thru hiked the PCT in winter have some advice in the book they wrote about it. They discussed using a synthetic quilt on top since the condensation occurs on that level which I have done several times and it seemed to work well…hard to quantify. It IS important to size the layers appropriately and Mr. Werner is correct… should experiment in your back yard, close to the trailhead, or when car camping in case it goes horribly wrong.

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