Size, Shape and Fit
Your sleeping bag's shape and fit greatly influence its warmth. Form-fitting bags contoured to fit a human body are called mummies because, like your mother when you were a kid, they hug you close and keep you warm (actually, they were named for the preserved bodies of dead Egyptians, which they resemble in shape, but I'd rather fall asleep thinking about my definition than the real one.) More expansive bags, for those of substantial girth or those who like more wiggle room, are known variously as wide mummies or barrels (I can see my mother frowning); even more capacious bags are known as semi-rectangular bags. Fully rectangular bags (named, of course, for really square moms) are for slumber parties and warm-weather car-camping. They're too bulky and heavy for backpacking.

Snug-fitting bags are generally warmer than loose-fitting ones, in part because there is less cold air and icy sleeping bag surrounding you when you first climb in, so the bags warm up faster. More importantly, a snug-fitting bag, combined with an effective hood, helps prevent your movements from pumping warm air out of the mouth of the bag, then drawing cold air in. A bag that’s too snug, however, will give you claustrophobia, so be sure to slip inside the bag at the shop before buying. Most good shops will let you try on a bag if you take off your shoes and look civilized. Many winter campers like to buy an extra-long bag to provide room at the foot for items that should be kept from freezing, such as water bottles and boots.

Several more details are worth considering. A hood is an integral part of all good mummy bags. It's designed so that tightening a drawstring cinches down the bag's mouth until only your face is exposed. In addition to a hood, good winter bags often have an insulation-filled collar that closes down over your shoulders and around your neck to further reduce the escape of warm air. Most bags are supplied with full-length zippers to make it easier to get in and out of them. That zipper can be another avenue of heat loss unless it's protected by a draft tube, a long, insulation-filled tube on the inside of the bag that covers the zipper. Cold feet seem to be a perennial problem on chilly nights. Better bags often have extra insulation in the foot area. Don't worry unduly about the other details of construction, such as which particular baffle system is used in a down bag, or whether a synthetic bag is described as having shingle or double-overlapping quilt construction. All of the methods used by reputable manufacturers work just fine.

Most sleeping bags have an outer shell of porous nylon that makes no claim to be waterproof. Even as a novice, I knew that sleeping unprotected in the rain would guarantee a soaking wet bag. But what about in winter? Shouldn't it be possible to sleep in the open, since it would be so cold that snow wouldn't melt on the bag? Such a tactic, if successful, would save the weight of a tent during winter climbs.

In 1977, Joe Kaelin and I set out to test that theory on our first effort to climb a major route in Rocky Mountain National Park in the wintertime. Already impressed with the cold, we bivouacked at the base of the face under a boulder, then started up the climb at first light. Darkness caught us only halfway up the route, and we searched futilely for some kind of sheltered bivouac site. At last we gave up, scooped out two body-sized ledges in the midst of an unprotected gully and crawled into our bags.

At midnight I awoke feeling that my sleeping bag had grown tremendously heavy. Spindrift pouring down the narrow slot above us had completely buried our bags. I pushed away the snow as I best I could from inside the bag and tried to go back to sleep, but in vain. Two hours later, with sleep impossible and our bags rapidly becoming drenched, we began soloing up the face by headlamp. We summited a few hours after sunrise.

As we had so convincingly demonstrated, enough heat escapes through your sleeping bag to melt snow or frost lying on the shell fabric. You don't need to do anything as foolish as exposing your bag to a non-stop spindrift cascade to get your bag damp in the winter. Frost sifts down from tent walls, while spindrift blows in each time the door is opened. Meltwater drips from snowcave roofs while wind carries snow into the entrance. Even in summer, condensation dripping from tent walls can gradually dampen your bag. A sleeping bag with a waterproof-breathable or water-resistant shell will help keep your bag dry. Such bags are also a bit warmer because of the windproofness of the shell, but I wouldn't buy such a bag just for that reason.

If you and your mate are like otters, who like to sleep half on top of each other, you may be interested in buying sleeping bags that can zip together into one giant love-nest. One bag must have a right-hand zipper, the other a left. In addition, the zippers must be the same size. Most sleeping bag manufacturers offer bags that will zip together. Another option for highly compatible couples is to buy a semi-rectangular bag that can be unzipped completely so it lies flat like a comforter. The unfolded bag is then zipped to a simple cotton or nylon sheet with a zipper along its perimeter. The result is, again, a giant love-nest, but this time one that has insulation only on top. The sheets that make this transformation possible are known variously as doublers, couplers or couplets. The advantage of a doubler is that it lets you carry sleeping gear for two that weighs just a pound more than sleeping gear for one. Cora and I find that doublers work well down to about 40 degrees or so. Below that, we sleep cold because it's impossible to cinch down the mouth of the bag to prevent the escape of warm air.

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