Owing to the cost of the fill, virtually all down bags are high quality, since it isn’t worthwhile to cut costs.
Shell Materials
Sleeping bag shells need to be lightweight, hard-wearing, breathable, resistant to wind and water, nonabsorbent, quick drying, and, for down fills, downproof (so the fill doesn’t leak out). Some modern synthetic fabrics have all these properties and feel comfortable next to the skin, making them suitable for the lining as well as the outer shell. Cotton and cotton-polyester shells have just about disappeared, since they’re heavier, more absorbent, slower drying, and harder to keep clean. After several months of continuous use, a polyester-cotton inner lining feels sticky and unpleasant—I speak from experience—but because a synthetic liner won’t absorb sweat or dirt, it stays fairly fresh as long as it’s aired occasionally.
The lightest fabrics allow the fill to loft fully and mold themselves around the body, maximizing the warmth of a bag. Many nylon and polyester fabrics are used, often ripstop versions, and there really isn’t much to choose between them. A typical high-quality nylon taffeta is 30 Max from Western Mountaineering, which weighs 1.4 ounces per square yard and has a denier of 30 (9,000 meters of fiber will weigh 30 grams). Some shells are made from microfibers (which weigh less than 1 gram per 9,000 meters of fiber). Although the denier is less than 1, this doesn’t mean microfibers are lighter than other fabrics; it means that more fibers are packed into a thread, making a fabric that is very windproof and water resistant because air spaces are too small for wind and rain to enter easily. The thread count of a fabric—the number of threads per inch—matters too. Good fabrics have a thread count of 300 or more.
My favorite shell fabric is 20-denier Pertex Quantum nylon, which weighs just 0.9 ounce per square yard yet is hardwearing, downproof, breathable, and very soft and comfortable. It’s not waterproof, but moisture quickly spreads out over its surface and dries. Quantum is used by a growing number of makers including Marmot, GoLite, Western Mountaineering, The North Face, Exped, and Rab.
Nonwaterproof shells are highly breathable, allowing body moisture through very quickly, which keeps the fill dry and ensures good loft. They are moisture resistant and dry quickly, but they won’t keep rain out for long. Waterproof-breathable shell fabrics are waterproof, but bags made from them usually aren’t, since water can enter through the seams unless they are sealed, which is rare. However, new construction methods make waterproofing seams possible, and fully waterproof bags are likely to become more common. The Swiss company Exped makes down bags with high-frequency-welded seams, while Mountain Hardwear makes down bags with baffles that are glued to the shell fabric. In both cases there are no stitch-holes through which water can enter, so these bags are waterproof.
There are several breathable waterproof or water-resistant shell fabrics, including Pertex Endurance, Gore Dryloft, and eVENT, plus proprietary ones like Mountain Hardwear’s Conduit and Sierra Designs’ DryDown. eVENT is new and so far is used only by Feathered Friends. Given how well it works in rain gear (see pages 153–54) it might well make the best waterproof-breathable sleeping bag shell. I’ve tried Endurance and Dryloft, and both are pretty good, keeping condensation drips from wetting the insulation of the bag. However, these shells add a little weight (they weigh from 1.7 ounces per square yard upward) and a lot of cost and don’t breathe as well as non-waterproof fabrics, as you’ll quickly find out when you try to pack one into a stuff sack—starting at the foot is essential, so air can be forced out the top opening. When damp, bags with these shells take longer to dry, too—it’s best to turn them inside out. In really damp conditions, I’d rather use a bivy bag over my sleeping bag. Any moisture is likely to occur between the sleeping bag and the bivy bag, leaving the insulation in the sleeping bag dry, and it’s easier to dry a bivy bag. Mostly, though, I find a nonwaterproof shell fine, since when it’s raining I always sleep under a tarp or in a tent. An interesting compromise would be a shell of EPIC by Nextec (see pages 145–46). This silicone-treated fabric is breathable and very water resistant but not fully waterproof. I haven’t tried a sleeping bag with it, but I imagine it would work very well. EPIC is used by Feathered Friends and Nunatak.
The color of your sleeping bag may seem to have no practical relevance, unless you want to sleep out unobserved. However, dark colors absorb heat and dry more quickly than light ones, and any moisture in the fill will dissipate faster. This heat absorption is noticeable to the touch. A black bag feels much warmer than a pale one when both are aired in the sun. Since light colors also show dirt more—one with a white lining looked unpleasantly grubby after a five-week trip—I prefer bags with dark linings.
Shape and Size
The most efficient sleeping bag is the one that traps warm air closest to your body. A bag with lots of room is a bag with lots of dead air space to heat. Most bags reduce this dead space by tapering from shoulder to foot. Most also have hoods to prevent heat from being lost through the head and at the neck. The resulting shape is called a mummy bag. It’s the standard shape for high-performance, lightweight sleeping bags and is very efficient at heat retention. Some warm-weather bags dispense with the hood and have a tapered shape sometimes called semirectangular. Actual rectangular bags are fine for warm-weather camping, though heavier than tapered ones. Because they usually have a zipper that runs down one side and across the foot, they can also be used as quilts on a bed. I don’t think they’re really a serious choice for backpacking though. If you find a close-fitting mummy bag restrictive, a broader mummy is a much better choice than a rectangular bag.
A sleeping bag that is too wide or too long won’t keep you as warm as one that fits properly, and the weight will be more than you need to carry. But a bag that is too small will be uncomfortable and won’t keep you warm in spots where you press against the shell and flatten the fill.
For these reasons, bags come in different lengths and shoulder, hip, and foot girths. Many companies offer two sizes in each model, while GoLite’s SmartFit bags come in three lengths and three girths. Finding a reasonable fit isn’t difficult, although very tall and very broad people may find their choices limited, and short people may end up with a bag that’s a little too long and slim ones with a bag that’s too wide. It’s worth climbing into a bag in the store to see how it fits before you buy it, even if you do feel conspicuous. A slightly roomy bag is better than a slightly small one, for both comfort and warmth. Make sure you can toss and turn and lie comfortably in the bag. Check too that the bag will accommodate any clothing you intend to wear in it, such as an insulated jacket in a cold-weather bag.
There’s a theory that most sleeping bags are designed for men and may be too roomy at the shoulders and too tight at the hips for many women. An increasing number of bags are now made for women, some with narrower shoulders, wider hips, shorter lengths, and more fill at the foot and across the chest, where many women report feeling the cold. EMS, Feathered Friends, Lafuma, Marmot, Outbound, REI, Sierra Designs, and The North Face all make women’s bags. However, when GoLite designed its SmartFit bags it measured a number of people and found no significant difference between men and women within girth categories—there were just more women in the trim category and more men in the wide. Wide men and wide women resembled each other in all girth characteristics, as did trim and regular ones. The conclusion was that women tend to sleep colder because their bags are too wide for many of them throughout their length. Close-fitting bags are warmer than wide ones, so this makes sense. The best approach is to find a bag that fits well, ignoring whether it’s labeled male or female.
A problem with a close-fitting bag is that it can feel restrictive. Some people like to be able to turn over inside a bag rather than with it and bend their knees and elbows. A good solution to this problem is stretch elastic seams and baffles, as found in MontBell bags and Sierra Designs’ Flex Bags. These bags hug the body closely yet give when you move so they don’t feel restrictive. Because the stretch baffles pull the
fabric inward, bags with them have a somewhat strange wrinkled look, but they work really well, cutting out cold air spots and reducing air movement in the bag. I’ve used bags that stretch made by the British company Mountain Equipment, and they are very warm for the weight and very comfortable.
An alternative and less effective method is to add a zippered panel to a bag so that the volume can be varied—roomy when it’s not too cold, close fitting when it is. Some bags, like Mountain Hard-wear’s Quantum Expander series, have these panels built in. In other cases, as with North Face’s Polarguard 3D-filled Expander Panel, they’re separate and can be zipped into a bag when required. Such panels add weight, though. North Face’s weighs 9.5 ounces, which seems rather a lot for something that reduces the warmth of a bag.
Construction
A mummy-shaped sleeping bag.
The method used to hold fill inside affects the warmth of a sleeping bag. Down fill has to be held in chambers, which give the familiar ringed or ribbed look, to keep it evenly distributed throughout the bag. To create these chambers, the inner and outer fabrics are attached in sections. The simplest and lightest way of doing this is with straight-through or sewn-through stitching (also known as quilting). This method is adequate only for bags designed for above-freezing temperatures because heat escapes through the stitch lines and cold air can blow in through them. Most of the lightest bags use sewn-through stitching.
KEY FEATURES: SLEEPING BAGS
A good fit. One that is too tight will be uncomfortable, and compressed loft will reduce its insulation; too much room can lead to cold spots.
An adjustable, shaped hood.
A tapered shape for efficient insulation.
A shaped foot box.
A two-way zipper for ventilation (optional).
A filled draft tube behind the zipper to prevent heat loss.
Key features of sleeping bags.
To cut this heat loss, the inner and outer shells can be connected by short walls of fine netting, called baffles, to make rectangular boxes—hence the name box-wall construction. If the walls are angled (offsetting the top and bottom stitches), it is called slant-wall construction; if they’re angled away from each other it’s trapezoidal construction. In V-tube or overlapping-tube construction the baffles form triangular compartments. All cold-weather down bags use some type of walled construction. Chambers should have enough down in them so they bulge slightly, creating that familiar well-padded look. If the down can move much in the chambers, it may migrate to the corners or to one side, leaving a cold spot.
Synthetic-fill bags can’t use baffle construction because the fill is fixed in layers known as batts. Continuous fiber synthetics can be used in single sheets in a bag, so no stitching is required. For cold-weather bags two or more continuous fiber sheets may be layered on top of each other. Batts of short fibers need stabilizing, so stitching is necessary. Sewn-through construction, as in down bags, is all right for warm weather bags, but for most conditions double-layer or shingle construction is better. In the first, two or even three sewn-through layers are layered, with the seams offset to avoid cold spots—an efficient method, though it produces a rather heavy bag. In shingle construction, slanted layers of overlapping fiber are sewn to both the inner and outer shells. Some bags are made with a combination of methods—a shingle layer over a quilted one, for example. Whatever the construction method, synthetic insulation should be stitched firmly to the shell at the edges to stop it from tearing away and leaving a cold gap. Down bags also usually have baffles along the sides that prevent all the fill from ending up on the top or the bottom. Some bags dispense with these on the principle that it might sometimes be useful to shift the fill to the top or bottom of the bag to give more or less warmth. I distrust such a construction because the down could move even when I don’t want it to. Another construction method I don’t like, used in both down and synthetic bags, places side baffles at ground level so that more of the fill is in the upper section. Other makers simply put more fill in the upper half of the bag. The rationale is that the fill under you is crushed anyway, so there might as well be less of it.
The problem with bags that have more fill in the top is they don’t account for sleepers who don’t keep their bags the “right” way up. The designers seem to assume that all users sleep on their backs and don’t move during the night, although studies suggest that most people turn over many times during the night. I often wake to find the hood above me because I’ve turned the bag completely over during the night. Just turn on your side, and you can lift part of the base of a bag off the ground. If it has less insulation than the top, you may then feel a chilly line down your back. I prefer to have the fill equally distributed.
The outer shell on most bags is cut larger than the inner; this differential cut means that the inner shell has a smaller circumference than the outer shell to allow the fill to loft fully, to cut out cold spots caused by loose extra fabric on the inside, and to stop the fill from being compressed by knees or elbows pushing against the inner fabric. The idea makes sense, but I have to say I’ve used bags both with and without it and not noticed any difference. These days virtually all mummy bags come with differential cut whereas rectangular ones don’t so they can be opened out and used as quilts.
Sleeping bag construction.
Bottomless Bags, Topless Blankets, Covers
If the fill under you is useless because it’s compressed, the logical thing is to do away with it altogether. Bags such as the down-filled 35°F (2°C) Big Agnes Horse Thief (1 pound, 8 ounces) do just that, having nothing more on the bottom than a sleeve for you to slide your sleeping pad into. Bags like this are usually meant for above-freezing temperatures because the design has inherent problems. Cold can creep in between the bag and the pad, as it has with all three bags of this type that I’ve tried. I also didn’t like not being able to sit up in the bags and pull them up around my chest. Sleeping on my side was awkward too. For low bulk and weight these bags can’t be beaten, but I find a full sleeping bag much more comfortable and psychologically reassuring. I like being totally enclosed in cold weather.
Blankets or quilts are even lighter than bags with no insulation in the base. Ray Jardine promotes these in his books, and for a while you could get his quilt designs from GoLite. Insulated quilts have no zippers and no hoods, just shaped foot-pieces. You spread them over you and tuck in the edges. They’re light, but to my mind—after, admittedly, only one very brief trial—they’re inefficient compared with a sleeping bag. Some people like them, however. A hiker I met on the Arizona Trail had made his own quilt by simply removing the zipper from a sleeping bag. Jake Schas was perfectly happy with his quilt and went on to use it on a Pacific Crest Trail through-hike. Nunatak, Integral Designs, and Lynne Whelden Gear make blankets. Nunatak’s 1-pound, 32°F (0°C) Ghost Blanket has 800-fill-power down and either a Pertex Microlight or an EPIC shell. Of course, a rectangular sleeping bag can be used as a blanket for two people. Integral Design’s 50°F (10°C) Primaloft-filled Prima Blanket is really one of these, since it has a zipper for conversion to a bag. There are instructions on how to make a quilt in Jardine’s Beyond Backpacking and on various ultralight Web sites.
Bottomless bags can be used as lightweight covers to boost the performance of a sleeping bag on trips that are colder than usual. Some companies offer zip-on covers for this purpose, and some bags even come with them. They have the same problem as having more fill in the top of the bag, however—they need to be kept above you to be any use. I should mention Stephenson, which introduced this type of bag back in the 1960s. Its Warmlite Triple bags come with either a 2-inch foam pad or a down-filled air mattress and two removable down tops of 820 to 890 fill power, a thin one rated to 25°F (−4°C) and a thick one to −10°F (−23°C). Combined, they are rated to an astonishing −60°F (−51°C). The tops attach to the bottom with double zippers, so there are no cold spots. I haven’t used one, but the bags look roomy, come in four sizes, and are designed
so you turn over in them rather than with them. The lining is a soft vapor-barrier fabric. Weights for the total system run from 91 to 123 ounces depending on the size and the type of pad, but you need only the thin top (16 to 20 ounces) in temperatures above 25°F and only the thick one (26 to 33 ounces) between −10 and 25°F.
Half Bags
An alternative to getting rid of the bottom of a bag is to get rid of the upper half and wear an insulated jacket. This type of bag, sometimes called an elephant’s foot, was first developed by Alpine climbers for bivouacking on narrow ledges. One of the few half bags is the 14-ounce 20°F (−7°C) Akula from Nunatak, filled with 800-fill-power down and made with a variety of shells. Shock cord runs over the shoulders to keep the bag up around your rib cage. I’ve never used one of these bags, but I imagine you’d want a thick insulated jacket with a hood and a good seal between the two items.
Design Features
Hoods
Because of the massive amount of heat you lose through an unprotected head and at the neck, a hood is very useful in all but the warmest weather. A good hood should fit closely around your head and have a drawcord with self-locking toggles that permit easy adjustment from inside the bag. Most hoods fit reasonably well, though few are easy to adjust from inside. Some have two drawcords so they can be adjusted from both sides. This seems unnecessarily complicated to me, especially when I’m trying to get out of a bag and can’t find the toggles. Some toggles and drawcords dangle in your face, which is irritating. Bag makers seem to assume that sleepers will lie on their backs staring upward with their hoods neatly framing their faces. (Sierra Designs is an exception; its designers actually picture people in different positions.) Most people don’t sleep like this (I often sleep on my front), and chances are you’ll end up with your face pressed into the side of the hood much of the time. If it’s badly designed, this will strain the fabric and feel uncomfortable. Getting into a sleeping bag and trying the hood is the way to find out if you can live with it or if it will drive you crazy.
Chris Townsend Page 33