People differ greatly in their metabolism. Cora, for example, sleeps comfortably beside me in a bag with nearly double the amount of insulation mine has. She uses a bag rated to 0 degrees for summer backpacking even though the temperature almost never dips below freezing, and she's rarely too warm. It should be noted that Cora gets cold faster than a skinny dipper in January. At the opposite extreme, I've slept in a tent at 17,000 feet on McKinley in a sleeping bag rated to -15 when the temperature outside the tent was -40. I got some sleep, but I can't say I was terribly comfortable, and I was wearing every scrap of clothing I'd brought, including four pile hats and hoods. Each person also varies in their need for insulation from time to time. If you go to bed wet, cold, exhausted, poorly fed and dehydrated, you'll need a lot warmer bag to be comfortable than if you go to bed warm, dry and full to the brim with a final cup of hot chocolate.

Nearly all manufacturers supply a temperature rating with their bags. These ratings are based on the manufacturer's estimate of what the average person will need on an average night when the moon is full and Pisces is rising in the east. There's usually nothing too scientific in how they arrive at their guess, and even if there is, your own experience may differ. However, most people can make an educated guess, based on comparison with companions at home and during car-camping trips, whether they sleep warmer or cooler than average. People who always feel like a Popsicle should buy a bag rated to 5 or 10 degrees colder than the average low temperature they expect. People who find themselves frolicking up the trail in shorts when everyone else is swaddled in goose down can probably get away with a bag rated down to 5 or 10 degrees warmer than the average low.

Next Page: Vapor Barrier and Radiant Barrier Liners

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