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A down sleeping bag consists of two bags with down in between. However, if you don't want all the down to collect at the foot of the bag, we have to add baffles at regular intervals around the bag to restrict the movement of the down. These baffles are wh
A down sleeping bag consists of two bags with down in between. However, if you don't want all the down to collect at the foot of the bag, we have to add baffles at regular intervals around the bag to restrict the movement of the down. These baffles are what gives a bag that curious ribbing which looks like quilting. The baffles do not have to be as strong as the cover material: they are often a very light mesh. That's enough to stop the down moving. However, this mesh can be very light - so light it can be easily ripped if the bag is mistreated, so treat your bag carefully, especially if you have to wash it.
Synthetic bags also have baffles, but these are far less important. The synthetic filling is made from very fine fibres, but these tend to be lightly bonded together somehow, so the filling comes in sheets or bats. However, 'sleeping bags are quilted', so even synthetic bags are quilted. On the other hand, padded ski trousers usually aren't.
Different brands and different models have different baffle designs. The marketing guys make a lot of noise about their special designs, but I don't think there is much between them really - except that increasing the number of baffles and narrowing the tubes usually makes for a warmer bag - up to a point. On the other hand, more baffles means more cost and weight. Complex mixed baffle arrangements sometimes just conceal the fact that the shell was made in two smaller parts.
If you stretch the inner bag sideways somehow you still want some insulation: you don't want the down squashed flat. To achieve this goal good sleeping bags have a 'differential cut'. Basically, this means the outer layer of fabric is larger than the inner layer - think concentric circles. This is a Good Thing, if not essential, and all manufacturers proudly proclaim they use it on some of their bags, as though they are special. In practice, if you have one of those vary narrow tapered bags you do need it, but if your bag has a reasonable amount of room in you don't really need the differential cut. Just make sure the bag is not stretched tightly over your knees.
Some bags don't have baffles: the outer layer of material is sewn straight through to the inner layer. A tube for the down results, but the region along the stitching is very thin, has negligable insulation, and will leak heat quickly. For this reason the sewn-through design is not found on anything but very cheap bags designed for warm conditions or for use as an inner sleeping bag (Antarctica and all that). They are not really suitable for bushwalking. But check before labelling a bag as 'sewn through': some of the very light bags may look as though they are sewn through, but in fact there is a very small wall there. Some cunning sewing has been used. But check carefully and suspect the worst.