Brushed Inner Lining
The bag's lining is brushed for extra softness, like a flannel sheet. Some folks like the extra coziness and wicking properties, while others complain that brushed linings are oppressive in hot weather and snag on dry skin and ragged toenails.
The outer shell is bigger than inner lining and allows insulation to loft as much as possible.
The fuzzy plumules found under a goose's outer feathers and inside many sleeping bags. Down offers unbeatable loft, warmth, and compactibility, but it does not insulate when wet.
This water-resistant/breathable shell fabric will help your bag shed wind, condensation, light rain, and spilled soup, but doesn't breathe as well as most taffeta or microfiber shells. It's ideal for those who frequently sleep under tarps, in snow caves, or in generally wet conditions.
These synthetic insulations will perform similarly to others, but are heavier and bulkier than newer versions, which is why they typically are found in less expensive bags.
An insulated hood that can be cinched down around your head and face is an essential feature for reducing heat loss in all but the most temperate weather.
A piece of material with velcro that secures the zipper and prevents unplanned midnight unzippings.
Lite Loft, MicroLoft, Primaloft, Primaloft 2, Thermolite Extreme
These synthetic insulations come closest to the softness and warmth of down per pound, but they lack the durability. Known as "short-staple" fibers, they are lighter and compress better for packing than Hollofil, Quallofil, and Polarguard.
Lighter, more supple, but less water resistant than DryLoft (and also less expensive), microfibers are tightly woven nylon or polyester fabrics that breathe well, yet still turn back wind and resist light moisture.
The most popular sleeping bag design for backpackers, a mummy bag has a close-fitting hood and a tapered cut from shoulder to toe. The taper reduces dead air space for maximum thermal efficiency–your body has less volume to heat–and eliminates needless extra ounces.
A lightly-insulated sleeping bag with an extra-roomy cut that's used to boost your bag's temperature rating by 15-20 degrees F. It can also serve on its own as a lightweight summer bag.
The same stuff used for jackets is found in some ultralight warm-weather sleepers.
More durable than the short-staple synthetic fills, these synthetics have long, continuous-filament fibers that make them more durable, but also heavier and bulkier. Polarguard 3D is the best, and most expensive, of these fills.
The latest, greatest version of Polarguard 3D, using a slightly different polymer design, a higher-denier fiber, a bigger void inside the fiber, and thinner walls. What's it all mean? You get a bag that's 10 percent warmer and slightly lighter, with better loft and durability.
This synthetic insulation will perform similarly to others, but is heavier and bulkier than newer versions, which is why it typically is found in less expensive bags.
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.