To stay really warm, designers generally agree, with few modifications proprietary to each manufacturer, that a cold weather bag must have the following features: zipper draft tubers and shoulder collars, ergonomic hoods that cup and insulate without being claustrophobic, and a temperature rating of zero or below (in the winter it is far better to err on the side of warmth).

What insulation do designers prefer? Believe it or not, the response was virtually unanimous. Down, with a 650 power-fill rating is best for weight to warmth ratio and for longevity and would be the perfect insulation if it could retain its loft when wet. Unfortunately, down is useless when wet. Synthetics are best if you are worried about moisture compromising the insulation --Lite Loft, Primaloft 2, Polarguard HV, Polarguard 3D (more expensive but very soft and lofty) and Lamalite (heavy, stiff and bulky, but it works well even when soaked) are most often mentioned as favored insulations in winter bags.

How should you buy a winter bag in terms of sizing? Buy it long. Most experienced mountaineers recommend chucking regular sized bags and opting for bags that offer at least an extra 8 to 10 inches of space at the foot once you're nestled comfortably inside. That's so there will be adequate space to store cameras, water, boots and such--items for which freezing would be a detriment to either you or their performance. In addition, bags with a wider cut can offer more warmth. While a narrower bag (small volume) may be more efficient if you sleep naked or in a single layer of underwear, a wider bag (large volume) will allow you the ability to add clothing to yourself without constricting the bag.

For most people, however, that's not a viable approach, which is why some manufacturers, including Cascade Designs, Mountain Hardwear and Marmot offer the system approach. This type of program allows you to purchase the bag you want and then if, and I do mean if you decide at some point you want a bag that is a bit warmer, you can purchase an upgrade and zip it in or slide it in (depending on the manufacturer's specific design features), adding up to 20 degrees of additional warmth.

"Testing instruments such as the copper mannequin provide clo data using a very controlled environment. The mannequin is a fixed size and does not roll. The comparative data that's collected is great for evaluating the comparative insulating power of bags used by the mannequin," adds Williams, "but how does it address the variables of body size (a small body in a large bag) or movement (nighttime rollers who constantly crush insulation), metabolism, and the other factors of the field?"