Every fall, Dave Hardy, director of field programs for Vermont's Green Mountain Club, takes trail crews out for days in subfreezing temps. Naturally, one of his priorities is keeping his volunteers toasty. "They need warm sleeping bags, but one manufacturer's 10°F rating can feel like another's 20°F," says Hardy. "In practice, the rating is just a fuzzy guideline."
Traditionally in the U.S., a sleeping bag's rating indicates the lowest temperature at which the sack will keep the average sleeper warm (i.e., a 20°F bag should keep you comfortable down to 20°F). But no standard for measuring ratings exists in the U.S., so there's no control for variables such as metabolism, what you're wearing, and sleeping pad type and thickness. That's why the rating numbers–and their real-world meaning–can vary widely from one manufacturer to another.
REI recently announced plans to change this. By March 2010, all brands of sleeping bags sold there will carry a standardized rating developed and used in Europe (called EN 13537). It will make REI the first major American retailer to enforce a standardized rating system.
"We continue to get feedback from our customers that when they buy a 25°F bag and then buy another 25°F bag, they have very different experiences," says Rick Meade, product manager for REI. "So we thought, 'How can we fix this?' And we believe the EN standard is the most accurate standard out there."
The Euro ratings are based on a sleeper wearing one synthetic baselayer (top and bottom) and a hat, and using a closed-cell foam sleeping pad. Independent labs certify a bag's rating, generating three numbers that reflect what most campers already know–women sleep colder than men. Here are the three numbers you'll see:
- Upper Limit The highest air temperature at which an average man can sleep comfortably.
- Comfort The lowest air temperature at which an average woman can sleep comfortably.
- Lower Limit The lowest air temperature at which an average man can sleep comfortably.
Next up? Temperature ratings for apparel like puffy jackets are a possibility–and a move we'd like to see. A European standard already exists, and American researchers are hoping that catching up won't take so long this time. Elizabeth McCullough, director of Kansas State University's Institute of Environmental Research (a controlled icebox where companies send products for cold-weather testing), has proposed a U.S. standard. But, she says, "It would be a voluntary standard, and so far, no manufacturers are stepping up."