How to choose the right sleeping bag

26 June 2008 16:16

You need the right sleeping bag to stay warm and cosy in a tent, so follow these tests to make sure you have a comfy night.

Insulation
In order to keep you warm, sleeping bags are packed with air-trapping insulation, made from either natural down or a synthetic material. Down is the most efficient insulation and so it produces sleeping bags that are light and compact. The better the down quality, the smaller and lighter the bag will need to be. But down loses its insulating qualities when wet, and it is expensive. Synthetic insulation performs better than down when wet and it is far lower in price than down. But synthetic bags are heavier and bulkier than equivalent down bags.

Down
Eider duck down is the best, but it’s also the most expensive. Goose down is almost as good, while duck down is a cheaper alternative. The amount of feathers that you get with the down affects performance. The quality of a down is therefore given in terms of how much down and feathers it contains. The best possible down is 96/4 (96% down, 4% feathers) but usually a 90/10 mix is used on the best bags, with 70/30 being common in lower-priced bags.

To simplify, the better the down, the more expensive, but also the more efficient it will be so you’ll need less to stay warm.
Fill power gives a gauge of how efficient the down is at trapping air and therefore keeping you warm. The higher the number, the better – and a fill power of 750 is the best generally available. However there are also different test methods, so a 750 fill power down tested in Europe may rate as 800 when tested in the US, for exactly the same product.

Fill weight tells you how much down there is in the bag. The better the down, in terms of fill power, the less you will need to stay warm – so a lower fill weight is required if the best down is used. By comparing the fill weight and fill power, you can judge how warm bags will be.

Synthetic insulation
Among the best synthetic insulation is Polarguard 3D, and this is almost as efficient as down, with the advantage that it maintains performance when wet. Other synthetic insulators are lower-priced, less efficient and produce bulkier and heavier bags, but are ideal if you need to save cash or use bags in the wet.

Hood & neck baffle
When it is really cold you’ll need the hood to fit closely around your head. The best hoods fit neatly and can be easily tightened down or loosened off. Sleeping bags work by trapping air next to the body. A shoulder baffle or neck baffle helps to keep that warm air inside the bag and around the chest.

Side zips
In order that you can get into your bag easily and have a degree of temperature control, side zips are provided that extend along the whole length of the bag. These are often two-way so that you can open them at the bottom to allow your feet to cool down. Half-length side zips make a bag lighter, but this may make the bag too hot to use in mild weather. Look for a substantial baffle to prevent draughts and cold spots.

Construction
The better the construction method, the less opportunity there is for the insulation to migrate and leave cold spots. Down bags have a system of internal fabric baffles that are designed to trap the down where it is most needed.
Box wall construction produces a light bag but it can allow cold spots and does not hold down in place as well as more complex methods such as trapezoidal baffles or shingle (like overlapping tiles on a roof) construction.
Synthetic bags are usually made from one or two layers of insulation attached to the inner and outer materials. If this is stitched through, cold spots can develop. Shingle construction is sometimes used in better-quality synthetic bags to avoid cold spots.

Packed size
The smallest packed size will come from a down bag that offers minimal insulation, so you need to make sure you are taking temperature ratings into account.

Dimensions
Bags vary a great deal in size, so check the sleeping bag data and then try the bag on for size in the shop. A close-fitting bag is the most efficient. Some bags designed for women are shorter and narrower.

Weight
When comparing weights take into account the temperature rating, materials and construction method as each can affect performance as well as the weight.

Shell and lining
Nylon is the standard fabric used for the shell and lining of sleeping bags, as it is durable, fast-drying, breathable and reasonably comfortable. Polycotton is sometimes used for the lining as it is very comfortable, but it is slow-drying and can feel clammy if not washed regularly.

Temperature ratings
All the bags listed are given temperature ratings. These indicate the temperature at which the bag will keep you comfortable and may be referred to as the ‘lower comfort limit’ of the bag. So a bag rated as -5 deg C should keep you comfortably warm when the surroundings are at -5 deg C. But use these only as a rough guide, as how warm you feel will depend on your metabolic rate, body fat, food intake, exertion levels and environmental conditions such as wind and humidity. Ratings also assume a sleeping mat is used. Women may require a sleeping bag that is rated between 5 and 10 degrees warmer than men when used in the same conditions.