Mummy sleeping bags are warmer and lighter than rectangular and barrel or hybrid bags, and usually include a hood. This sleeping bag is meant to fit snug to the body and to move less with the user. They are ideal for serious campers who might face extreme weather conditions. They are also usually more compressible and take up less space than other bags.
The majority of sleeping bags are made with a polyester outer shell. Some bags use heavy cotton, which is durable, but adds bulk and weight.
Lower–priced bags generally use a polyester lining similar to the polyester outer shell. These are lighter weight and cooler on the skin. Some bags use cotton flannel, which is durable, warm and comfortable.
By sewing the inner lining and outer shell together; the fill is secured, preventing it from shifting. The disadvantage of this stitching is that there is no fill along the stitched-through line resulting in cold spots.
Used on more premium bags, this method provides the benefits of quilting but uses a double layer of insulation with each layer being stitched to the outer using offset stitch-lines. This means no cold spots!
Some bags will have zippered vents along the side or near the foot box to allow you to regulate temperature and allow air circulation.