Types of Sleeping Bags

Sleeping bags work by trapping warm air created by the body to prevent it from cooling off. A sleeping bag should also be breathable (that is, allow perspiration vapor to escape). The larger the airspace inside the bag, the less efficient the insulating properties of the bag. There are three main styles of sleeping bags: the mummy-shaped bag (so named because its shape resembles an Egyptian mummy), the tapered bag, and the rectangular bag. Each design has different insulating properties because of its shape, and each, depending on the fill inside, will do a better job of keeping the person inside warm.

Regular vs. Mummy Sleeping Bags

Efficiency has almost everything to do with shape, or more specifically, with how little or how much airspace there is to heat in the bag. The more airspace, the more there is to heat. Tapered and mummy-shaped bags reduce the amount of airspace inside the bag; therefore, they are warmer than rectangular bags.

The mummy has become the high-performance, lightweight standard for sleeping bags. Because the amount of space inside a mummy bag is limited by its shape, it's the style most often chosen for trips of all kinds. It's practically de rigueur for those expedition-style camping trips that border on daredevil adventures into the frozen wilderness.

Mummy bags completely encapsulate the occupant and may seem constraining when compared to rectangular bags. Mummy bags should also feature closing hoods that tighten up around the head with the pull of a drawstring. All this in the name of limiting airspace? Is it worth the trouble?

Well, yes and no.

A tight-fitting bag will keep you warmer than a looser one will. But if you're doing most of your outdoor sleeping in spring and summer, you won't need extra-heavy-duty temperature protection. And don't forget region and topography. Temperatures at higher elevations will be cooler than in low-lying areas. If you're traveling in Alaska or New-foundland, temperatures at night may also be cool in the summer.

As with all camping gear, the acid test is finding the perfect fit for your body type—meaning your width and length, to put it plainly. Climb into as many sleeping bags as you can before settling on the one sleeping bag you want to live with for years. Check out how it feels, and how your feet feel in the “foot box” at the end of the bag. Mummy sleeping bags designed for use in extremely cold weather will seem the most constraining, simply because they are designed to protect the person inside from the danger of hypothermia. Warmth will have a lot to do with what insulation the bag is filled with, too; in the quest for the perfect sleeping bag, shape is only one ingredient in the mix. Equally important for backpackers is weight. Sleeping bags can weigh anywhere between just over a pound and a half to six pounds, depending on the amount of insulation and season the sleeping bag is designed for. Sleeping bags designed for summer use will be lighter than sleeping bags designed for winter. Down bags weigh less than synthetics when compared to each other's insulating capabilities.

If you toss and turn in your sleep, plan to venture into the outdoors only when it's warm, and entertain the possibility of sleeping in close proximity to others, then a rectangular bag may be for you. Many of these sleeping bags are zippered all around and can be joined together without much trouble. Some mummy sleeping bags offer this option too, but because they have hoods, they don't join together as conveniently.

Rectangular sleeping bags have no hoods at all, and in the face of colder weather don't offer the same insulating properties as mummy sleeping bags. Shape is also a factor: There is more dead airspace to warm inside the sleeping bag because it isn't tapered, which means less efficiency in keeping you warm. Some manufacturers offer a compromise between the mummy and the rectangular bag: A tapered bag with a fold-down hood that isn't as wide as a rectangular sleeping bag, but not as constraining as a mummy sleeping bag.

In addition to their reduced efficiency, rectangular sleeping bags are often heavier and bulkier. Thus they are a less-than-ideal choice for backpackers, who often encounter colder temperatures.

There is an obvious trade-off between the heat-efficient mummy sleeping bag and the inefficient, but roomier, rectangular sleeping bag. Recreational campers will not have much need (or appreciation) for the insulating advantages of the mummy bag. If you fall into this category, remember to try on your sleeping bag before purchasing it. Any sleeping bag should feel as though it would be warm and cozy in cool weather, but also allow you to loosen it up or fold it down when temperatures heat up.