Rectangular versus Mummy shapes - Most sleeping bags you will see on our website are Mummy shaped, this is because a close fitting bag will be warmer for a given weight because there is less volume of air to move around carrying heat away with it, and a close fitting bag weighs less than other shapes because there is no waste of material.
Some people feel too restricted by a mummy bag and would rather carry a bit more weight and have a sleeping bag with lots of room in it. Rectangular or Tapered bags offer the user more room to move around and often the bag completely unzips around the feet so that it can be used as a quilt rather than a sleeping bag. Often tapered bags don't have a hood - as the focus is less on maximum heat for minimum weight, and more on comfort and freedom to move around.
Quilt - A flat quilted cover that offers complete freedom of movement when sleeping. The danger here is from draughts, and you need a warm sleeping mat to insulate you from the ground.
Shell Fabrics - Some sleeping bags have highly water resistant shell fabric, this reduces the risk of down getting wet from external water, but also reduces breathability. A common fabric used is Pertex Endurance. These bags are ideal for bivying when there is a higher likelihood that your sleeping bag could get wet. If you normally use a tent though its not thought necessary as it should be possible to keep your bag dry.
Waterproof Sleeping Bags - Crux Torpedo bags are made with a highly waterproof and highly breathable eVent fabric outer, with taped seams. These are specialist bags for extreme wet conditions.
Backless Sleeping Bags - these rely on your sleeping pad to keep your underside warm and they usually feature some method of attaching them to the pad, either a sleeve or straps. This construction means all the down is concentrated on the top and sides of the bag so none is crushed on the bottom rendering it useless as an insulator.
Because of the format of the EN testing backless bags can't be tested in the same way as other bags, therefore there are no ratings for them. However by comparing the Fill Weight and Fill Power of the down, and remembering that its all on the top and sides, its possible to infer a rating.
Backless designs are fine if you are fairly still in the night - if you toss and turn a lot draughts can be a problem.