Measuring a sleeping bag’s temperature rating is one of the least exacting and consistent sciences out there. The formula for a bags temperature rating, and a good night’s slumber, involves far more variables than bag design and materials. Are you male or female? What, if anything, did you have for dinner? What are you wearing to bed? What type of shelter (if any) and mattress (very critical) are you sandwiched between? Even your mood (did you succeed on your climb or fail miserably?) can affect you sleeping experience.
But that inconsistent science is changing. Europe led the charge towards standardized sleeping bag testing and reporting roughly ten years ago. Known as EN (European Norm) 13537, this standard has slowly been being adopted by North American brands ever since. The trend has gained considerable steam of late due to the urging of one very large and influential retailer (REI).
Distilled down, EN 13537 provides each bag with a temperature range after testing and/or certification at one of a handful of locations around the globe (The University of Kansas is the North American facility). Using nomenclature that I’m not that all thrilled about as I think it is unnecessarily confusing and not all that accurate, this range was built around three temperatures: “Comfort” (the warmest temp above which you be too warm), the “Limit” (what in the past was the bag’s lower normal limit), and the “Extreme” (the absolute coldest temperature you could take this bag down to and still survive).
This verbiage has morphed a bit, however. Now, think of the Comfort rating as a women’s Limit, think of the original Limit rating as the bags lower limit for a man, and think of the Extreme rating still as the absolute lowest emergency rating, but one that is based on a women’s physiology. Got that?
OK … here’s a simple review example. You and your significant other both need to buy new bags. First off, the two of you need to decide what is the general warmth rating of a bag you’ll need. Let’s say 15-degrees Fahrenheit. Now, in store or online, the men should look at the selection of bags (down, synthetic, mummy, etc.) that have a Limit rating of 15-degrees. Women need to look at the group of bags with a Comfort rating of 15-degrees. (Forget the “Extreme rating altogether). Now with everyone using the new unisex temperature range, what this will result in is that women with get a warmer bag—which they need—relative to the guy’s bag. It’s that simple.
The move to the EN 13537 rating has born other interesting fruit. Overall, it was discovered that many, if not most bags, were overrated in the past. Whether that was from manufacturers wanting to under-report their weights in a “trail weight” obsessed world, or for some other reason, bags temperature ratings and weights are now more accurate—but perhaps most importantly—vastly more comparable.