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What do Sleeping Bag Ratings Even Mean?

I have two sleeping bags. One is rated for 40° and the other for 15°. If I put both of them on, will I be comfortable sleeping at 55°? Wait ... what's the formula again?

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There is no standard, industry-wide method for determining warmth.I have two sleeping bags. One is rated for 40° and the other for 15°. If I put both of them on, will I be comfortable sleeping at 55°? Wait … what’s the formula again?

—Dmitry Dylov via rockandice.com

You are more likely to reverse engineer the secret formula for Coca-Cola, a recipe that has been locked in a vault since 1919, than to decipher the fuzzy
math used by sleeping bag manufacturers to rate their bags for warmth.

To be fair (an impulse shared only by me and dogs), bag companies do try to get it right, and have little besides money to gain by deceiving you. Trouble
is, sleeping bags aren’t ropes. There isn’t a standard, industry-wide method for determining warmth. Company X might say that their bag is 0°, while
Company Y might be more conservative and rate their similar bag to 15°. Marketing is also a factor—given a choice between two bags of similar
price we’ll buy the one with the lowest rating, just as we’ll purchase a rope with the highest fall-rating—so proceed with caution.

In Europe, where they regulate everything including the precise amount of hefeweizen that must be poured into a stein, they of course have a directive
for measuring sleeping bags. EN 13537 rates bags for upper-limit warmth, comfort, lower-limit warmth and “extreme,” the lowest temperature a “standard
woman” can be exposed to for six hours without dying. For the tests, the mannequin used to measure heat loss is suited up in thermal long johns, socks
and a cap. The man version of the mannequin is 25 years old, 5 feet 8 inches tall and weighs 161 pounds. The woman is also 25, a bit shorter and lighter,
DD and blonde. In other words, the ratings that result from the tests are wishful thinking. They aren’t widely used in the U.S., where we instead rely
on the honor system.

Now, your question about whether a bag inside a bag would be warmer is interesting. You might think that your sleeping-bag “turducken” would be twice as
warm, but because insulation relies on trapping air, when you put a bag in a bag, and the outer bag compresses the inner bag, you decrease the inner
bag’s insulating value. If the inner bag is really snugged in there, you might not gain much in warmth. You would be warmer unzipping the second bag,
draping it over the other sleeping bag, comforter style, and cozying up to that blonde Euro mannequin. Next!

This article was published in Rock and Ice issue 223 (January 2015).