The key element that helps people choose their bag is its rating, (season, comfort or extreme), and unfortunately it is the easiest thing in the outdoor world to overstate. One person that passes the night without waking at -10 in a particular sleeping bag does not a -10 sleeping bag make! We have tested many of the bags in our range ourselves, in locations from the high Himalaya to the back garden with the kids. The ratings given are our best approximation to appropriate ratings for them in use by a man of average fitness and build that has had an average day on the hill, is well fed and is sleeping naked in a tent with 1 other person and having an undisturbed nights sleep.
Kid's sleeping bags can be found in the Children's Gear section.
So what makes the difference between a budget synthetic sleeping bag and a top quality one? The most important element is the fill. Standard fibres are rods which allow air to accumulate around them, hollow fibres also allow air to accumulate inside them, multi-hollow fibres allow even more air to accumulate inside and around them, spiral and zig-zag fibres do the same but also provide a degree of loft (ie they make the bag expand and trap even more air). So the more complex the fibres, the more air trapped for less fibre and the warmer, lighter and more compactable the bag will be. The simplest fibres can be found the in Aztec Swallow where the high qualityAjungilak bags use a range of clever and complex fibre shapes. See the Swallow and the Snugpak range for budget sleeping options, and for warm weather bags.
The way the fibres are held in place is important too, most manufacturers use a mixture of glue and fibre mats. Nanok on the other hand place the fibres in air-trapping stitched mats. The mats help further the trapping of air, whereas glue can inhibit it and make the bag slightly heavier and harder to loft. Sleeping bags are made up of layers of fibre mats. If the stitching of each mat lies one on top of the other this can cause cold spots, so offset stitching is usually used between each layer (this has been known to occassionally realign in transit which means that the cold spots can reappear). Nanok use multi directional insulation, ie the stitching on each layer sits at 90 degrees its neighbour thus definitely reducing cold spots. The thinner the mats the more you have to use, the more mats you have to use the more air you trap. Nanok's Comfort and Performance ranges are our most popular synthetic bags, finding a great balance between warmth, weight and price.
The use of very light weight shell fabrics are a great way of saving a bit more weight and bulk, and here Ajungilak score very well. Some manufacturers reduce weight by making their bags narrower, which can lead to more crushing of the synthetic fibres so that they can't trap as much air and also a less comfortable night. A folded bag will often have one straight side and one curved side, this making the bag cheaper to produce but less spacious. Ajungilak use a two piece construction in true mummy shapes for comfort at low weight. Women and smaller men will want to look at the Ajungilak Tosca range. A smaller bag is not only lighter, there is less extra air space around a small person and hence greater efficiency.
Nanok use a 3D construction in their bags, which means that they have sidewalls for less crushing of materials and more room for you. They have put stiffer fibres in the sidewalls to support the shape a bit better too. The lower material of the bags are more durable whilst the upper surfaces are extremely light. A final clever idea is the positioning of the zip at the top of this sidewall to keep it away from the cold ground. All in all each of these little gains should add up to a warmer, lighter, more compressible and comfier range of bags.