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Sleeping by Faith: Bag Temperature Ratings

Sleeping by Faith: Bag Temperature Ratings


by Ryan Jordan | 2002-07-04 03:00:00-06

Sleeping-bag temperature ratings as reported by a manufacturer should mean absolutely nothing to you.

Certainly such ratings have been the focus of controversy in the outdoor-products industry for many years. Some manufacturers (such as Western Mountaineering) have garnered a reputation for being quite conservative with their ratings while others are the object of outright ridicule among outdoor enthusiasts.

The reasons for such discrepancies? First, we are all physiologically different and the level of insulation required for comfort at 20 deg F for one person may be dramatically different from the level of insulation required for another.

Second, there are no industry standards for measuring sleeping comfort levels. One promising development, however, is the use of a copper, heat-controlled, sweating mannequin that mimics the rate of heat output required to maintain a certain temperature while in a sleeping bag. Such a tool, and a standard method that employs it, will be valuable for evaluating sleeping bags from different manufacturers. It may even prove to be valuable for evaluating comparative performance of bivy sacks, tents, sleeping pads, and clothing. Protocols are evolving for applying the technology to these other types of gear. However, these methods are controversial (as they always are when they are subject to input from competing manufacturers) and they are expensive to implement for small- and medium-sized companies without adequate research and development budgets.

So does that mean you should blame the company for false advertising if their claimed bag rating doesn’t meet your expectations? No! You need to take responsibility for your own choices and not rely on obscure claims such as temperature ratings that are impossible to validate, and you should take a hard look at sleeping bag design.

Save yourself time muddling through manufacturer specifications and sales pitches and recognize that there are only three things to consider when assessing the warmth of a sleeping bag:

  • LOFT. Higher loft means more warmth. The most important loft measurement is not that of the whole bag, but only that of the top layer, which remains uncompressed when the bag is in use. Ray Jardine, in Beyond Backpacking, claims that a two-inch thickness of upper-layer loft is suitable for a 20 deg F night, but 90% of experienced backpackers will tell you that you’ll need to supplement your long johns with a bit of extra clothing in order to achieve this level of comfort.
  • DRAFT CONTROL. Draft control features, such as draft tubes along the zipper, drawcord closures at the neck, and draft tubes at the neck, can go a long way to reducing heat losses. A sleeping bag with a hood will always be quite a bit warmer than a similar bag without one, simply because so much heat is lost through your head and neck.
  • INTERNAL VOLUME. It has been said to the point of cliché that the greater the girth (internal volume) of a sleeping bag, the colder it sleeps, simply because there “is more volume for you to heat up.” This is not exactly true. The real disadvantage to unnecessary internal volume is that spare volume in the back is not “dead air space” (as opposed to the dead air trapped in the sleeping bag’s insulation) and is prone to convection currents and drafts that can increase heat loss at the body surface.

And a few things to consider for sleeping warm:

  • FOOD. Adequate caloric intake is probably your body’s best defense against cold nights that are near the temperature rating of your sleeping bag, simply because metabolic heat generation (and calorie consumption) is the most important physiological warming process as your body begins to cool.
  • CAMPSITE LOCATION. Take advantage of terrain when selecting your camp. Low areas near valley bottoms are typically much cooler during the summer months than surrounding hillsides, ridges, and peaks due to the presence of a temperature inversion (where cooler air settles into lower elevation areas). Sleep above the temperature inversion boundary, which may only be a few hundred feet higher, and up to 10 deg F warmer.
  • WIND PROTECTION. Wind protection is vital to sleeping warm when you are camped in an exposed location. The use of a bivy sack or tent, and to a lesser extent, a well-pitched tarp, can spell the difference between comfortably warm and uncomfortably cool.
  • SLEEPING PAD. As the temperature drops below freezing, the importance of a sleeping pad increases greatly. Don’t expect to sleep warm on a 3/8-inch foam pad, even with a 0-deg F rated sleeping bag on a 15 deg F night. Also pay attention to ground cover: take advantage of lofty forest duff and avoid sleeping on heat-sapping bare rock or snow whenever possible.

The bottom line: use your head and learn about the design factors that contribute to a warm sleeping bag. Also, test and validate manufacturer claims in your own backyard or on short overnight trips before committing to a longer trip where nights are expected to be near the sleeping bag’s temperature rating. Above all, develop your own equipment and style for sleeping warm.

Or just forget about all of this, trust the manufacturer’s ratings, and sleep warm by faith!


"Sleeping by Faith: Bag Temperature Ratings," by Ryan Jordan. (ISSN 1537-0364)., 2002-07-04 03:00:00-06.