Warmth

The warmth of a sleeping bag is provided by insulating material that traps dead air. How warm a particular bag is depends on the type and amount of this insulating fill, the thickness (loft) of the fill, and the bags's size, style, and method of construction.

Sleeping bags are generally categorized as recommended for summer, three-season, or winter expedition use. (See Chapter 15 for tips on sleeping bags for winter climbs.) Manufacturers give their bags comfort ratings, meant to indicate the lowest temperature at which the bag will be comfortably warm. Methods of rating the bags vary from manufacturer to manufacturer, so they can give only a general idea of how the bag will perform. Bags are usually rated to a minimum temperature of somewhere between 40 degrees and -20 degrees Fahrenheit (between about 5 degrees and -30 degrees Celsius).

How closely a rating matches you personally depends on whether or not you are in a tent, the clothing you are wearing, the ground insulation, and your body size, metabolic rate, and caloric intake. Look for a rating that matches your use. If you're mainly a summer backpacker, a bag rated to -20 degrees Fahrenheit will be too warm.

The warmest, lightest sleeping-bag design is the mummy bag—tapered toward the feet, hooded to fit over the head, and with a small face opening secured with drawstrings. Whatever the design, your bag should have a form-fitting hood or a semicircular piece with drawstrings to keep your head warm and prevent heat loss. Some bags incorporate an insulated collar to provide extra warmth around the shoulders, and some include extra insulation at critical areas, such as chest or feet. You can buy a thin over-bag to put around the sleeping bag during colder weather to improve the bag's performance. Also available are vapor-barrier liners (for inside the sleeping bag) that can increase your bag's comfort range by 10 or 15 degrees Fahrenheit. However, figure that whatever clothing you have on in the bag is going to get damp from body vapor that has nowhere else to go. These liners are best used at below freezing temperatures.

There are some basic procedures for sleeping warmer, regardless of the bag you are in. Undress inside the bag. Wear a wool hat and dry socks to bed. Secure the bag closely around your face. Breathe through a sweater to reduce heat loss from exhalation. You can put your head inside the bag to help warm them both, though the water vapor from your breath will likely make it damper inside.

The comfort of the bag offers a chance to warm up not only you, but some of your things. Small items, like mittens and socks, can come into the bag to dry. Don't try to dry larger items of clothing by wearing them to bed, because they will just keep you cold and make the bag wet. In very cold weather, you may want to bring boots inside, wrapped in plastic. You may also need to make room for a water bottle, to keep it from freezing.

TYPES OF INSULATION

The warmth, weight, and cost of a sleeping bag depend chiefly on the kind and quantity of insulation, which is either goose down or a synthetic material.

Goose down is still the most efficient insulation per unit weight. Down bags are warm, very compressible, retain their loft well, and are long-lasting. Disadvantages? High cost and water absorbency. A wet down bag loses most of its loft and insulating value, making it almost worthless until it can be dried out. A down bag takes so long to dry—more than a day in good conditions—that it can't be dried during wet periods in the mountains. This characteristic makes down a questionable choice in a wet climate, unless you take extra care to keep it dry.

Synthetic insulation is resistant to moisture, retains most of its loft when wet, and dries relatively quickly. Bags with synthetic insulation are less expensive than down-filled designs. These bags are still slightly heavier than comparable down styles and do not compress quite as easily, thus making a weightier, bulkier load. Synthetic insulation does not last as long as down, and loses much of its loft with use.

CONSTRUCTION

Down bags are made with one of three basic construction methods (fig. 3-9) to keep the fill uniformly distributed: sewn-through, slant-tube, or overlapping tube. In sewn-through construction, the inner cover is stitched directly to the outer, a simple and inexpensive method but one with substantial heat loss at the seams. Most down bags are of slant-tube construction, which eliminates cold spots at the seams. The most efficient design, over-

Fig. 3-9. Down sleeping bag construction: a, sewn through; b, slant tubes; c, overlapping tubes.

Fig. 3-9. Down sleeping bag construction: a, sewn through; b, slant tubes; c, overlapping tubes.

lapping tubes, is used only in the most expensive bags. In addition, many designs incorporate channel blocks, baffles that prevent down from shifting.

Synthetic bags are built using a variety of methods. There are two basic types of synthetic fills. The first is a long, stable polyester fiber, manufactured in batts that stand up well to use and laundering. A widely used construction method is the shingle style, in which sections of batting are sewn in the bag, overlapping like roof shingles to cover cold spots. The second type of fill is a short, stable polyester fiber that is quilted or sewn into fine scrims to keep it from wadding up or moving around. To avoid cold spots, these must be shingled, or quilted in double or triple layers with quilted lines offset (fig. 3-10).

Fig. 3-10. Synthetic sleeping bag construction: a, triple-offset quilting; b, shingle.

Zippers are the almost universal means of closure even though they sometimes snag the fabric or go off the trolley. They are backed up with a tube of insulating material to reduce heat loss through the zipper. Long zippers make it easy to get in and out of the bag and are a big help in ventilating the lower portion. If you want to zip two bags together, be sure one zipper is right-sided and the other is left-sided.

EFFECTS OF WETNESS

Do yourself a big favor and keep your sleeping bag dry, especially if it is filled with down. Compared with down bags, polyester-filled bags retain much of their warmth when wet and are easier to dry, but a wet polyester bag is still a wet bag and colder than a dry one. You can buy a Gore-Tex sleeping-bag shell that lets body vapor escapc

Fig. 3-10. Synthetic sleeping bag construction: a, triple-offset quilting; b, shingle.

while helping to protect the bag from condensation inside a tent or dripping water in a snow cave. Consider putting the sleeping bag inside a plastic sack before putting it on your pack, because most stuff sacks don't keep water out.

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