Sleeping Bag Systems
Spanning the temperatures with layers

Bag systems (bags with several layers you can add or pull off) have become more visible, but they do beg the question of actual performance versus acceptable compromise. After all, isn't a specific bag for a specific temperature and use far better and of greater value to you?

While manufacturers don't deny that bag systems are a compromise they assert that for many it is an intelligent compromise, and I agree. If you have a limited budget and are not at all sure if you might need a warmer or lighter bag as some point in the next year or two, you might want to consider buying into the system concept.

Sure, a specific bag that attacks a specific use is better than a system approach. After all, a system will generally weigh more and be more bulky. However, not everyone can afford the extra bags that might be needed. I have six different bags myself (4 down, 2 synthetic) and I could probably argue the case that I need a couple more.

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.

"It is not all that uncommon to have users judge the minimum comfort limit of an identical sleeping bag up to 20 degrees apart. I know of at least one case where two users judged the same sleeping bag more than 30 degrees apart!" says Williams. "Even without the variables of environmental factors, clothing worn, food eaten, pads used, body size, fitness, etc., the perception of 'comfort' varies wildly."

"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?"

Gary Schaezlein, owner of Western Mountain Sports, manufacturers of Western Mountaineering down sleeping bags agrees with Williams wholeheartedly.

"We just try to be honest with what we are putting out and provide for our customer in the worst of scenarios, not the most optimistic," says Schaezlein. "In fact several years ago, we were copper mannequin tested and found our ratings were 20 degrees too warm--our 20 degree bag tested to 0 degrees for example. The bottom line with a bag is it must keep you warm."

Hallelujah to that! Moonstone also reports that they overfill their bags to ensure warmth. Wise field advise that I have heard dispensed over the years is that you can always unzip a bag if you are too warm, but you're going to have a problem trying to add more down or insulation if the bag's too cold. Of course, this doesn't mean you want to buy a bag that you will be sweltering in 90% of the time either.

One company, when asked how they were rating their bags, went so far as to state that they had their bags tested by the copper mannequin a number of years ago, and simply extrapolated the information into their current line.

Of course, those companies who choose to be more optimistic with their temperature ratings often come out ahead when you only catalogue-compare price, stuff size, temperature rating and weight. As any knowledgable retailer will tell you, as long as you know how to compare bags properly, temperature ratings are of little significance and merely serve as a starting point or a place of reference for the eventual purchase--hopefully an educated one.

The first word of caution from Williams is "don't get too caught up in the loft game because some insulations pack more insulating punch per unit thickness than others."

Indeed, tests conducted at Kansas State University indicated that a LiteLoft bag (no manufacturer listed) which had been abused until it had lost 27% of its original loft still managed to maintain 94% of its original insulating ability.

Because of all the confusion, a number of heavy hitters in the industry, including Cascade Designs, Sierra Designs, REI, Marmot and 3M insulations, have banded together to attempt to hammer out an unbiased, meaningful, understandable and standardized rating system. Don't expect anything to happen in the next year however, since not everyone in the industry agrees that standardized temperature ratings is a good or even achievable idea.

What, then, are the most important criteria to consider when buying a bag? Look at the foot section, hood area and draft tube. How are they designed? Are there any obvious cold spots? How do they compare with the other bag(s)? How much insulation is in the bag? Consider that bigger bags require more insulation to provide equal insulating qualities as smaller or more narrow cut bags. On down bags, do the compartments feel firm ( a good sign that prevents cold spots forming since down shift is minimized) or soft and fluid (a poor sign since the down is free to 'fluidly' move around the tube creating cold spots--only exception are those bags intended for the user to be able to shift down from top to bottom to adjust for temperature fluctuations).

Winter Sleeping Bags
Features to Look For Before Buying

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.

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