A good sleeping bag is a cocoon of warmth comfort. You zip in for the night, grubby and exhausted. You emerge rested and refreshed, ready to the trail like a butterfly. The wrong sleeping bag, on the other hand, is a cold sack of discomfort that will haunt you with fatigue. It’s nicer to be the butterfly.
 
HOT OR NOT You won’t sleep well unless you’re warm. Most manufacturers list temperature ratings for their bags, which are meant to indicate the minimum temperature at which you’ll be able to sleep comfortably. But beware: There is no industry standard for setting these numbers and companies are free to label their bags as they want. They’re often overly optimistic. People also have different metabolisms; some doze like a furnace while others snooze like an ice cube. If you sleep on the warm side of bed, temperature ratings will generally be accurate. If you sleep on the colder side—and most people fall into this category—you’ll need to buy a warmer bag. As a general rule, plan on buying a bag rated 10 to 20 degrees lower than the expected conditions.

DOWN TIME Goose down provides more warmth for its weight than any other insulation. Down-filled sleeping bags are lightweight, incredibly compressible, and have a remarkably long life span. Properly cared for, a down bag will stay as warm as the day you buy it for a decade or more. But down performs poorly when damp; it clumps together and loses a substantial amount of its warmth. If it gets soaking wet, it becomes nearly worthless—and takes a really, really long time to dry.

The quality of down is measured by its “fill power,” or the number of cubic inches that an ounce of goose down takes up under standard laboratory conditions. Fill powers range from 550 to 900, though the price rapidly escalates for 700 and above. A common misconception is that a sleeping bag with higher fill power is warmer. Not exactly. A lower fill-power bag rated to the same temperature will be just as warm, it just needs to contain more down—and a few additional ounces of weight—to achieve the same level of toastiness. Expect to pay around 20 percent more for a down bag than a synthetic-fill version.

POLYESTER POWER Synthetic insulation handles moisture well and is less expensive than down, a good choice for soggy conditions or a tighter budget. It provides warmth even when damp or wet and dries very quickly. But it is bulkier and heavier than down, and will lose a significant amount of warmth after it flattens out from a few seasons of regular use. Virtually all synthetic-fill sleeping bags use some form of polyester. Less expensive bags ($50-$100) use multitudes of short fibers that can pull apart over the years, clumping up and forming cold spots. More expensive versions ($100-$200) use continuous filaments, a single strand woven together that won’t separate. PrimaLoft and Polarguard Delta are two brand-name insulations widely considered to be the warmest—and most compressible—for their weight.

FIT TO BE SNUG  The closer a sleeping bag fits, the warmer it will be. When fully zipped in, your feet should almost, but not quite, reach the tail of the bag. If they are pushing against the end, they will compress the insulation and compromise warmth. But there are tradeoffs, especially if you are claustrophobic. You don’t want the bag to be so tight that your movement is constricted. The key dimension to look for is girth, the circumference of the bag around the chest, which generally varies between 57 and 63 inches. Also look closely at the hood. The best ones wrap naturally around your head and do not chafe your face when pulled tight.

Courtesy of the Appalachian Mountain Club