2006 Revision by Mike Martin

Missing from our reviews will be an assessment of whether or not the sleeping bag performs adequately at temperatures near its manufacturer-reported temperature rating.

There are too many variables involved that influence user comfort, including not the least, intrinsic metabolism and physiology, which vary greatly from user to user. Consider also that an individual user may sleep warm one night while sleeping cold another night, even when environmental conditions and sleep systems are equivalent, and the mere concept of temperature ratings becomes somewhat unjustifiable. Add to all of this the effects of a user’s daily caloric budget (calories consumed versus expended), clothing worn, and amount of moisture in a sleep system (e.g., from condensation of perspiration vapor), and you have a process of such complexity that it has baffled the world’s best physiology modelers for years.

Thus, to “rate” a sleeping bag is simply an exercise in comparing the relative performance of similar systems under a fixed set of conditions. Further, to compare a manufacturer’s rating to real-world performance based on experiences of one – or even a handful – of product reviewers would not only be unfair to the manufacturer, it could possibly lead to a false conclusion by a reader that did not fit the physiological profile of our testing corps or did not experience the same conditions that we experienced in our tests.

Other magazines, other writers/reviewers, and countless numbers of people on Internet forums are quick to say “this bag was miserably cold at 10 degrees above its temperature rating”. Hey – did you ever stop to consider that maybe you screwed up and didn’t maximize the performance of the bag because of human error? Or maybe your body was trashed – metabolically deficient, dehydrated, or mildly hypothermic – and you weren’t astute enough to recognize that? Granted, we recognize that some manufacturers offer some outrageously inaccurate ratings – and we’ll certainly let you know when those bags come through our queue – however, we also recognize that just because the bag may not have kept one person warm at a given temperature, it may have caused another to overheat.

Consequently, the best we can do is to measure the loft and key performance features of the bag, comment on our subjective experience near the temperature rating of the bag, and ask you as an informed consumer to compare it to the loft and temperature ratings of bags from other manufacturers in order to make a wise decision. Nevertheless, we recognize that some guidance in correlating a bag’s loft to its warmth is useful to consumers. Table 1, below, represents our best effort at such a correlation:

Table 1, Single Layer Loft and Estimated Temperature Rating
Temp (°F) Loft (in) Temp (°C) Loft (cm)
50 1.2 10 3.1
40 1.5 4.4 3.8
30 1.8 -1.1 4.6
20 2.2 -6.7 5.6
10 2.6 -12 6.6
0.0 3.0 -18 7.6
-10 3.5 -23 8.9
-20 4.0 -29 10

Note that the loft figures in Table 1 are for single-layer loft. This is the thickness of the insulation on top of the sleeper; any insulation under the sleeper is ignored. Also, bear in mind that in addition to the physiological factors mentioned above, construction details of the bag, as well as environmental conditions can influence the warmth of a bag tremendously. Don’t expect to sleep out comfortably in your 3.5-inch thick drafty quilt in a sub-zero blizzard atop Mt. Rainier after a day of climbing 7000 feet and consuming 1000 calories of dehydrated goop, just because Table 1 says it should be warm at -10 degrees! Use this table only as a starting point to determine the warmth of your sleep system. Experiment under safe conditions to find out what works for you.