Degrees of Comfort

The label on the sleeping bag said it was good to "20 below," so why were you shivering through the night when the air temperature only dipped to 15°F?

Welcome to the esoteric, arcane, and downright confusing world of sleeping bag temperature ratings. No other topic is guaranteed to generate as much cynical laughter among outdoorspeople, unless it's Smoky the Bear's role in forest fire policy.

You would expect that when purchasing a bag that claims to be rated to 30°F it will keep you warm down to 30°F, right? After all, you expect as much from a 40,000-mile tire purchased for your car, and that tire costs a lot less—and arguably performs a more vital function—than a nylon cocoon stuffed with spun fiber or goose down.

Yet, in the field—or forest, or atop a glacier—that 30-degree bag can leave you cold well shy of its stated performance rating.
Savvy sleeping bag buyers should treat temperature ratings not as immutable absolutes, but rather as suggested guidelines that can assist them in finding the ideal sleeping bag.

Think of a 30-degree rating, for instance, as applying to the typical camper using the bag under typical conditions, then think of yourself as anything but typical (which you knew already!). Combine your knowledge of how you are different together with the manufacturer's (maybe optimistic) rating, and you can make a satisfactory sleeping bag choice. Here's how:

The Correction Factor

You've already figured out the minimum overnight temperature you'll likely encounter. Now check yourself against the following 10 factors that can influence how warmly or coldly you sleep, and adjust the minimum temperature appropriately.

Don't get overzealous and add up a massive correction factor, especially if you camp in warm climates (above 32°F) anyway.

Sleep style. Adjust upward or downward by 5 to 15 degrees depending on whether you "sleep warm" or "sleep cold."

Acclimatization. If you're slow to adjust from a cushy room temperature of 68 degrees to life in the cold outdoors, then correct downward 5 to 10 degrees (for example, instead of a 30-degree bag, get a 25- or 20-degree bag).

Food intake. Do you eat enough when you recreate outdoors (no adjustment necessary) or do you use your trips as opportunities to diet (correct downward 5 to 10 degrees)?

Hydration. Adjust downward by 10 degrees if you are not a faithful guzzler of water and sports drinks. The enormous volume of water lost through sweat and the mere act of breathing can mess with your body's heating and cooling system.

Tiredness. The occasion you are really tired will be the time you most need a good sleep, so make a generous correction for this—as much as 5 to 10 degrees downward—if you take long trips where cumulative sleep deprivation would be dangerous.

Bag fit. Can you use a close-fitting bag without feeling constrained? If not, correct downward by 5 to 10 degrees. Can you sleep with the hood cinched down to a small peep hole around your nose and mouth? If not, correct downward by 10 to 20 degrees in really cold climates.

Dampness. Do you camp in damp conditions, such as wet coastal climates, or go on river trips where despite your best efforts bags get damp? If so, correct downward by 5 to 10 degrees if you'll be using a synthetic bag, and 10 to 20 degrees downward for down.

Body movement. Tossing and turning in a bag acts as a bellows to blow warm air out. If you're a thrasher, then correct by adjusting downward 10 to 20 degrees.

Wind protection. Sleep in a four-season tent (adjust upward 5 degrees), a three-season tent (no adjustment), or underneath the stars (downward by 10 to 15 degrees). Wind has much less effect if the bag shell is a very tightly woven microfibre or a laminate such as Dri-Loft or Stormlight.

Storage. A bag that's been used often for years (correct downward 5 to 10 degrees) and stored improperly (correct downward 10 or more degrees) loses loft and therefore performance. Sleeping bags should be removed from their stuff sacks as soon as possible and stored unrolled and loose. For storage advice, see Lofty Thinking.

You should now have a good idea of how much of a correction factor to apply to the minimum expected temperature of the bag you're looking for. If this puts you into a ridiculously low-rated bag, like 30 below, and a correspondingly low remaining bank balance, consider buying a slightly higher temperature rated bag and supplementing its performance by wearing clothes and booties to bed, using a bivy sack to eek out an additional 5 degrees of warmth (more in drafty environments), or using a vapor barrier (definitely an acquired taste!).