Philip J. Smith's Blog: The Winter Sleeping Bag FAQ: Practical Advice for Winter Backpackers

Winter sleeping bags have specialized features that help prevent heat loss in colder temperatures.

What is a winter sleeping bag?

A winter sleeping bag has a temperature rating of 0 degrees fahrenheit or lower. Remember, 32 degrees 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?

The best winter sleeping bag manufacturers are made by: Feathered Friends and Western Mountaineering which specialize in cold weather bags and have numerous models and temperature ratings to choose from. Mountain Hardware, Marmot, Montbell, and Valandre also have excellent reputations, but they're in a different league than Feathered Friends and Western Mountaineering, which are head and shoulders above the rest.Feathered Friends, incidentally, is one of the few sleeping bag manufacturers to make women's-specific winter sleeping bags as well.

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

Mummy bags tend to be lighter weight because they're formfitting, 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.

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 a 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.

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 a 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 bodyweight 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, sleeping with a sleeping bag liner, and so on. It is best to experiment with these in your back yard or within walking distance of your car before you need to count on them in a wilderness survival situation.

However, 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.

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″ 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 a wider shoulders or chest measurements so you can hug your extra gear at night to keep it warm or tuck it behind your back 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 .

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.

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 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 let's 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 very 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 reapply 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 warm. 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.

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