Cold and winter sleeping bag temperature ratings – those under 10 degrees fahrenheit - are not assigned using the new EN13537 sleeping bag standards that many North American sleeping bag manufacturers now use. The new standard has proven unreliable for bags rated below 10 degrees, so most manufacturers rate their bags using the same techniques they used before the new sleeping bags standards were introduced.
That’s a problem for several reasons:
- Experience with the new EN13537 standard showed that manufacturers overestimated the warmth of their three season bags by about 10 degrees. For example, a 20 degree bag from a few years ago would be closer to a 30 degree bag when tested with the new standard today. Therefore, it’s probably wise to add 10 degrees to a cold weather sleeping bag that is rated by the manufacturer. For example, a bag rated for 0 degrees is probably closer to a 10 degree bag, and a bag rated for -20 degrees is probably closer to a -10 degree bag.
- When manufacturers rate their own sleeping bags, they’re almost always using a MENS rating and not a WOMENS rating. One of the benefits of the EN13537 sleeping bag standard is that it showed that women sleep much colder than men. So if you’re female, you should probably add another 5-10 degrees to the rating of a cold weather bag unless it’s a women’s specific model.
- The sleeping bag temperature ratings of cold weather bags listed by most manufacturers assume the you are wearing long underwear (tops and bottoms), socks, you’re sleeping on a sleeping pad with a high R-value (5+ is a good winter minimum), and your sleeping bag is not exposed to the wind (you’re in a tent or a bivy sack.) If you don’t do all of these things in winter, you will sleep colder even if your sleeping bag rating is 100% accurate.
Implications for Winter Backpacking and Camping
I lead a lot of winter backpacking trips in New Hampshire’s White Mountains for the Appalachian Mountain Club and I always interview people who sign up for my trips to see if they have the required gear, experience, skills, and physical conditioning required to complete the hike safely.
At night, winter time temperatures in the White Mountains range from 0 degrees down to -20 below. Since I can’t forecast the weather when I schedule my trips – often months in advance – I require that all of the people on my trips have -20 degree sleeping bags.
Assuming they have all the other gear and clothing required, a sleeping bag with a -20 degree bag will keep a man comfortable down to -10 degrees, and a woman, down to about -10 to -5 degrees. When screening participants, I also try to find out who manufactures their sleeping bag, because manufacturers such as Western Mountaineering and Marmot tend to rate their sleeping bags more accurately, either because they overstuff them with high quality goose down or they test them in refrigerators (called cold rooms.)
However, if someone with a sleeping bag with a 0 degree rating applies to get on one of my winter backpacking trips, I will turn them down. If they’re male, this equates to a 10 degree sleeping bag or a 15-20 degree sleeping bag if they’re female, and that is simply too risky for winter nights in the Whites. On a -15 degree night, they’ll be worse than uncomfortable.
Winter Sleeping Bag Temperature Ratings
- Cold/Winter sleeping bag temperature ratings are assigned by manufacturers and fall outside the new EN13537 sleeping bag standards.
- The temperature ratings on most cold weather/winter sleeping bags are probably 10 degrees too warm.
- Women need winter sleeping bags that are 5 to 10 degrees warmer than men.
- Most winter sleeping bag temperature ratings assume that you are wearing long underwear including socks, sleeping on a sleeping pad or pads with a high combined R-value, and you are in a tent or bivy sack.
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