Sleeping Bag Temperature Ratings

Sleeping Bag Temperature Ratings

by Michael Hodgson

Publication Date: February 1996

Not too long ago, while in Alaska on assignment, I had occasion to spend a night or two testing a synthetic sleeping bag rated to 20F. Though it was raining, the bag stayed relatively dry. Sure, I was tired from a 16-hour trek in chilly conditions. But feeling chilled at 35F while inside the bag, while wearing dry Thermax underwear, dry wool socks and a Polartec hat was not my idea of accurate temperature rating - this from a person who can regularly slumber comfortably at IOF in a 20F rated down bag. What's up with temperature ratings.

According to Fred Williams, president of Moonstone, temperature ratings as they are currently used are pure bunk and should be abandoned. "It is not uncommon to have users judge the minimum comfort limit of an identical sleeping bag up to 20 degrees apart," said Williams. "I know of at least one case where two users judged the same sleeping bag more than 30 degrees apart." Even without the variables of environmental factors, clothing worn, food eaten, pads used, body size, fitness, etc., the perception of 'comfort' varies widely.

"Testing instruments such as the copper mannequin provide accurate 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," added 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 other factors of the field?"

Gary Schaezlein, owner of Western Mountain Sports, manufacturers of Western Mountaineering down sleeping bags, agreed 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," said Schaezlein. " In fact several years ago, we were copper mannequin tested and found our ratings were 20 degrees too warm-our 20F bag tested to OF, for example. The bottom line with a bag is it must keep you warm."

Moonstone also reported that they overfill their bags to ensure warmth. I have heard 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 percent of the time either.

One company, when asked how they were rating their hags, 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 customers only catalog-compare price, stuff size and weight-making for a more difficult sell for the retailers.

Still, as long as the consumer knows 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 insulation punch per unit thickness than others." Tests conducted at Kansas State University indicated that a Lite Loft bag (no manufacturer listed) which had been abused until it had lost 27 percent of its original loft still managed to maintain 94 percent 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 a unbiased, meaningful, understandable and standardized rating system.

In the meantime, 3M insulations, makers of Lite Loft insulations, will stand behind the temperature ratings of bags made with Lite Loft, no matter who the manufacturer is. If a Lite Loft hag does not live up to its temperature rating and carries the "Sleep Warm Warranty" hang tag, 3M will replace the bag for a warmer one with no questions asked. Call (800)770-9276 for more information.

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 seams or other obvious cold spots? How do they compare with the other bag(s)? How much insulation is in the bag? Bigger bags require more insulation to provide equal insulating qualities than smaller or more narrowly 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. The only exceptions are those bags intended for the user to be able to shift down from top to bottom to adjust for temperature fluctuations).

Award-winning outdoor journalist and author Michael Hodgson is also the technical editor for Outdoor Retailer magazine.