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Sleeping Bag Ratings
Ratings | Loft | The Human Factor | Sleeping naked? | Roominess | Pad
When shopping for sleeping bags, manufacturers assign temperature ratings as an indicator for what conditions they feel are appropriate for their bags. Unfortunately, there are no defined standards for for assigning ratings in the U.S. Therefore, comparing sleeping bag ratings from different companies is somewhat like comparing apples and oranges... both fruit but they may still be very different.
Different manufacturers use different methods to rate their bags. For example, I believe Feathered Friends uses "survival" ratings which means a bag will go to a certain temperature and you will likely survive but you're not going to be comfortable. Western Mountaineering, on the other hand, uses comfort ratings which means you'll be comfortable to a certain rating. That doesn't mean Western Mountaineering is better than Feathered Friends, it just means that ratings can't be directly compared and may not be the best indicator of performance in the field.
There are now standards for sleeping bag ratings in Europe which are based on mannequin tests. Some U.S. manufacturers may be adopting them. This Mammut document is a complete description of the ratings system with background information. More information can be found by searching for EN 13537.
Until ratings standards come to U.S. manufacturers, rather than comparing temperature ratings, a better statistic may be to look at loft, the vertical height which a bag's top layer attains due to its internal insulation. It is the trapped air, after all, that's going to keep you warm. If one bag lofts to 2" and the other to 3", the 3" bag will be warmer.
If you do go with loft, be sure you are checking the height of the loft on the top of the bag. What's below you won't count once your body weight compresses the insulation. Some bags have continuous baffles that allow you to move the down from the "bottom" of the bag to the top thereby extending the range of the bag a bit.
The Human Factor
Of course, the biggest factor is the person using the bag. Some people sleep cold and may need a 0 degree bag in 20 degree weather. Others sleep warm and may only need a 32 degree bag in 20 degree weather.
Experience helps determine how much of a bag you will need. Some people are comfortable going with a bag that goes to their expected temperature range knowing that during a cold snap, they may spend an uncomfortably cold night on occasion. Others, aren't willing to take that chance and hike with bags rated much colder than their normal hikes on the off-chance they encounter some temperatures colder than expected. They end up carrying much heavier and bulkier bags. Long-distance hikers for whom weight and pack space is very important, tend to fall in the former category.
You'll hear some people say that bags are warmest if you sleep naked (or nearly so). If there's room in your bag for wearing extra clothing without compressing the loft in the bag, then it will be warmer to wear additional clothing in the bag as long as those layers don't constrict your circulation. If those extra layers are going to compress the bag's loft, then you're probably going to be warmer with the bag's loft than with your clothing's.
This doesn't mean you should go buy a roomy bag. Remember, it's your own body heat that's going to warm the bag in the first place. The more "extra" room in your bag, the more your body has to work to heat the extra space. The more your body works, the more energy you'll need to get through the night. I you don't have enough energy, you'll get cold in the middle of the night or worse, you won't warm up to begin with.
Once you figure out your sleeping bag, don't forget that you'll also need a pad with good insulative qualities. Closed-cell is better than open, etc...
Read my write-up about pads.
Last updated, January 3, 2016.
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