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Mummy vs Rectangular Sleeping Bags -- Based on Data from over 1,350 Actual Owners
"Rectangle" sleeping bag are the traditional-type sleeping bags, and they get their name from their basically rectangular shape. “Mummy” sleeping bags get their name from the fact that they resemble an Egyptian mummy case.
Each type has its own characteristics, strengths and weaknesses which are described below based on over 1,300 actual owner experiences.
Comfort
Compared to rectangular sleeping bags, mummy bags by design offer less room inside, which can result in some people feeling constricted. In addition, a person in a mummy bag must usually lie on his or her back. A rectangular bag, by contrast, offers more freedom of movement and a variety of sleeping positions.
Advantage: rectangular
Warmth
If you will be sleeping outdoors in sub-freezing temperatures, a mummy bag is often superior. When mummy sleeping bags are fully closed they typically leave only one’s nose and mouth exposed, thereby trapping air inside which acts as an effective insulator. Mummy bags also conform to the body's contours which reduces the amount of heat the body must generate to maintain a constant temperature. (Mummy bags can also often be used in much warmer temperatures without being too hot.)
Most rectangular bags are best suited for temperatures above freezing, but some are capable of keeping you warm as much as ten degrees below freezing.
Advantage: mummy
Backpacking / backcountry camping
Mummy sleeping bags are often lighter and less bulky than rectangular bags which makes them ideal for backpacking and backcountry camping. Some lighter and smaller rectangular bags can be suitable for short- to medium-distance hiking.
Advantage: mummy
Car camping
Rectangular sleeping bags are ideally suited for car campers where bag warmth, weight and size are not as high a priority as they would be while backpacking the backcountry.
Advantage: rectangular
Price
The more expensive mummy bags can out price the more expensive rectangle bags by several hundred dollars. The lower end mummy bags, however, are often similarly priced to most rectangular bags.
Advantage: rectangular