A sleeping bag only achieves its full thermal performance if the hood can be closed, leaving just a small opening for breathing. The marginal bulge around the opening (a well-padded chamber surrounding the cord) improves comfort and protects forehead, nose and cheeks.
2. Thermal collar
If you prefer to leave the hood open, make sure that the sleeping bag has a wide, heavily padded collar. This forms a shawl around the neck and stops warm air from getting out and cold air from flowing in.
3. Zip overlap strip
The zipper is a potential weak spot where cold can enter the bag. Therefore, a strip behind the zip forms a seal; the thicker and wider, the better. The stiffness prevents the zip slider from getting jammed in the material.
If the zip goes as far as the feet, you can completely fold out the sleeping bag and use it as a blanket. In this case, a two-way zipper is useful.
5. Foot section
The area around the feet needs to be spacious enough to avoid that the feet and toes compress the filling, as this can lead to unpleasant cold bridges.
6. Lining material
Nylon (polyamide) or polyester are the ideal lining. Cotton is too heavy and needs a long time to dry – which could be fatal on an expedition.
7. Outer material
Polyamide or polyester are mainly used for the outer sleeping bag shells. Impregnated material, microfibre or a coated material protect against condensation or everyday use hazards.
Better too long than too short. If your sleeping bag is too short, the filling at the top and bottom compresses and reduces the thermal efficiency of the bag. However, if the sleeping bag is too long, a lot of dead air needs to be heated.
Restless sleepers need slightly wider sleeping bags. Otherwise, elbows, bottoms or knees cause filling clots. If narrowness is not a problem, body-contour sleeping bags are preferable. They heat up quicker and weigh less than the wider models.
For temperatures below 5 degrees Celsius, only thermal constructions without stitched seams are used. Make sure the filling is evenly distributed between top and bottom; otherwise cold spots occur when you turn in your sleep.