Degrees of Comfort

In the dark ages of sleeping bag development (read 25 years ago) temperature testing was somewhat...inexact. One early developer recalls utilizing a local meat locker in Southern California to test bag prototypes. Other companies relied on expeditions to test sleep systems—which worked great as long as the bag worked as planned. Testing standards have become more sophisticated since then, but the question remains, are sleeping bag temperature ratings any more accurate?

There are two competing standards for modern sleeping bag temperature ratings. One is ASTM- American Society for Testing and Materials (ASTM). ASTM provides standards for everything from uniform plumbing codes to the testing of guinea pigs for contact allergens (really). Of the organization's 12,000 plus internationally accepted standards, ASTM F 1720 provides methods for measuring the total thermal insulation of a sleeping bag. In the first component of the test, a naked manikin is placed in a sleeping bag on a cot. To better model real life, a second test was developed to allow "accessories" in the test—the manikin is placed on a sleeping pad (then on the mandatory cot to ensure air-flow), and dressed in an array of jackets, pants, hats, and coats. The results of the tests (with a variety of bags) are then graphed. ASTM doesn't provide temperature rating predictions, but manufacturers can use their results to determine whether a bag will keep a person "comfortable" at a particular outside temperature.

Over the past decade, many U.S. companies have adopted the EN (European Norm) as the sleeping bag testing protocol of choice. The EN 13573 test is similar to the second ASTM test—a full-sized thermal manikin with heaters and temperature sensors is placed in a bag on a cot in a climate chamber. Then the manikin is heated to simulate body warmth. The air temperature is measured in the climate chamber and on the skin surface of the manikin. From these measurements, the insulation value of the sleeping bag is calculated. Oddly, the EN test standard mandates that the dummy is dressed in a track suit, knee socks and face mask—generally not something you see every day, even in the backcountry (with the possible exception of Germany and Switzerland!).

The EN testing trumps ASTM as it provides a range of temperatures that give insight into how a sleeping bag will perform in different conditions, for different genders. The test provides three ratings per bag. The T-comfort rating is the lowest outside temperature in which a "standard" woman can sleep comfortably. The T-limit is the lowest outside temp for which a "standard" man can sleep comfortably in the bag. The T-extreme is a "survival only" rating....meaning that that in the t-extreme temperature range, the bag would probably keep you alive. (People who select bags by the T-extreme rating are generally ultra-light thru-hikers and alpinists who plan on using their down jackets, gloves, hat, and insulated pants to ensure adequate warmth during the night.)

The EN and ASTM certifications are just two ways that companies help provide information for consumers. Other key factors in determining how well a bag will perform in certain conditions include loft and fill-weight (traditionally the most reliable way to determine temperature rating, as more loft/fill generally equals more warmth). Manufacturers agree that while the EN test is a good indicator for bags rated to keep you warm at 0 degrees F, the accuracy diminishes for sub-zero bags. If you're shopping for a super-cold weather bag, go with the loft/fill rule of thumb.

Published temperature ratings help to determine how warm a bag will keep you, in ideal conditions. But there are other factors, such as how you use the bag, which are even more important. A tent will add about 10 degrees in warmth (providing you remember to zip up the door and vents!). Sleeping pads are equally critical in maintaining body heat. Direct contact with the ground provides the most heat loss—sleeping on a foam pad to insulate your body from the ground can easily add 5 to 10 degrees to your "comfort" level.

Says one industry expert, "how the bag is used is a huge component—much more important than any single temperature rating. We use the tests to help us design better bags, but there are also a lot of other components that lead to a good night's sleep while camping."