Because they don’t taper to conform to your body, rectangular bags are the least heat-conserving of all the shapes. Likewise, they weigh the most and consume the most pack space, though they also tend to be the least expensive. This shape is best if you want your unzipped bag to double as a comforter.
These durable nylon and polyester fabrics feature stout threads woven into the material in a checkerboard or diamond pattern to prevent and reduce tearing.
More tapered than a rectangular bag, but not as body-hugging as a mummy, this shape sacrifices some weight and heat conservation for more tossing-and-turning room.
Made from fine, hollow fibers reduces weight and bulk while increasing loft and warmth.
Nylon and polyester taffeta fabrics are less durable but more supple than ripstops.
Taffeta Inner Lining
Weighs little, warms quickly, and lets sweat vapor pass through.
An insulation-filled tube runs alongside the zipper to keep out drafts. The tube should overlap the zipper’s teeth, where it’s needed.