Sleeping Bags: Choosing the right sleeping bag

Sleeping bags come in a variety of shapes, sizes and materials and so choosing the right bag may sound simple, but it can be as complex as it is critical for your comfort, or even survival.

Sleeping bags keep the person warm by trapping and holding a layer of non-circulating air next to the body. This air, which is warmed by the body heat, forms a barrier between the person and colder air outside the bag.

However things aren't quite as simple as this, as technology has ensured that the outdoor pursuiter has a bewildering choice of bag design, fillers, ratings, shell materials and features.

Shape

In terms of shape, there are basically two styles of sleeping bag – Envelope and Mummies.

Envelope bags are rectangular in shape. They can usually be fully opened out and be used as a quilt. They are designed to be roomy but they do let a lot of body heat escape. Some models have hoods, but this is more for comfort (and somewhere to put a pillow) than to aid heat retention.

Because they open out fully, they are well suited to being zipped together to make a double sleeping bag. When using two bags to make a double, you should ensure they are either both left-zip, or both right-zip. Although a left-zip and a right-zip bag will zip together, you would end up with both zip baffles on the same side, so one of you would get a draft.

The main advantages of this style of bag are that they tend to be more comfortable with a lot more legroom. The disadvantages are that they are inefficient insulators and tend to be heavier and bulkier. Because they are inefficient insulators, you are unlikely to find one rated as more than two-season.

Mummies are narrow, close-fitting bags designed to save weight and maximize heat retention. They start narrow at the feet, get wider toward the shoulder, and then taper to an insulated, fitted hood. The majority of sleeping bags are mummy-shaped.

The slimness of the bag saves space and weight when carrying it and the hoods retain a lot of warmth in the body (one of the main places where body heat is lost is through the head). However the narrow shape can feel prohibitive to some people and restrict their ability to sleep.

Fillings

There are two kinds of fillings used in sleeping bags – Down and Synthetic

Down is the thin, fluffy undercoating found just beneath the outer feathers of geese and ducks. This natural fibre is a superb insulator. Goose down is preferred to down from ducks. It is lightweight and fluffs up really well to trap air and is extremely compressible to pack away to a small size.

Down has excellent "drape" properties, settling around the body and eliminating the draughty gaps sometimes left by stiffer synthetics. It offers tremendous warmth for surprisingly little weight (thus offering a superior "warmth-to-weight" ratio).

However, down is a poor performer when wet, taking much longer to dry. Also, down bags are expensive. Although more expensive, remember that its resistance to deterioration makes it good value in the long term.

Down is graded according to fill power — meaning the number of cubic inches one ounce of down will displace. The higher the number, the better the insulation.

Synthetic fillings are heavier than down but are cheaper, easier to clean and have better performance in wet conditions. Synthetic materials are basically plastic threads, normally a continuous long, single strand. Usually the threads are hollow, reducing their weight and enabling them to trap more air, giving good insulating properties and reducing weight. Synthetic fillings dry fairly quickly and are non-allergenic.

The disadvantages of synthetic fill are that it is bulkier than down (so takes up more space when carrying and is heavier), the filaments gradually degrade over time, and the filling tends to be stiffer so does not drape over the body contours as effectively as down.

Many bags now combine the benefits of down and synthetic. They have a combination of fibres with different crimp systems and surface finishes. Some are crimped, some are curled and some just left as they are. The random nature of the process results in a product that closely resembles the structure of natural down and the way it performs. They have excellent loft recovery abilities when unpacked and all of the advantages of synthetic filling.

It is important to note that the weight of filling in a bag is not an accurate indicator of its thermal performance as the quality of fillings varies from manufacturer to manufacturer. A high-quality filling may cost a bit more but gives better warmth to weight ratio.

Layers

Every sleeping bag has a shell (the outer layer) and a liner (the inner layer that touches the skin). In between is the filling or insulation. In some bags, a metalised layer is integrated between the filling and shell to reflect back body heat. This can improve the performance of a bag by up to 15%.
The goal in bag construction is to avoid cold spots. The insulating layer is held between the shell and the lining, and the even distribution of that insulation depends on how it is held in place. Cold spots occur where the stitching is used to hold the insulation in place.

Cheap sleeping bags represent low value because they are stitched through all three layers making a cold spot on every stitch line. Quality sleeping bags prevent this by using baffles.

This means that the walls of the baffles are angled so each section partially overlaps the adjacent baffle. With synthetic bags, sheets of insulation are offset-quilted and layered over one another to prevent cold spots. Some synthetic bags are arranged with a Z-shaped system, which staggers how the insulation is arranged and assures that stitches on the shell (top) of the bag do not connect with stitches on the liner (inside).

A "continuous baffle" is one that wraps around the entire bag. The advantage with this is that the user can shake the bag and shift more filling to the top layer (for cold nights) or bottom (for warm evenings).

Shell (or outer layer)

Shell Fabrics are also an extremely important part of the effectiveness of a bag. A close weave on the outer helps keeps water and wind out. The most common fabric used today is lightweight nylon, which also offers low bulk. Ripstop is often used on the outer because it incorporates a reinforced, fibre mesh to prevent tearing and, because of the increased strength gained, an even lighter material can be used.

Pertex uses many thousands of bundles of microfine yarn within its construction. The capillaries created between these yarns actively draw moisture away from the source and spread it across the surface area allowing it to more readily evaporate. As the moisture is evaporating from the fabric, and not from the occupant, chilling is dramatically reduced.

Lining (or inner layer)

Some bags use cotton as a lining, as this is very comfortable to sleep in. The penalties for using this are extra bulk and weight, and it is slower to dry. The majority of bags use man-made materials for the lining, such as nylon and tactel nylon, or poly-cotton. This is due to the superior wicking qualities of these fabrics helping to keep you dry, and therefore warmer.

Temperature Rating

When you buy a duvet or quilt, the manufacturer supplies a TOG rating for it. Nearly all sleeping bags come with either a season rating or a temperature rating.

Season Rating

This gives the purchaser a rough idea of the time of the year the sleeping bag can be used comfortably. However, the season rating cannot be too exact as toleration of cold temperatures varies in each individual.

The season ratings basically mean:

• 1 Season: Summer only
• 2 Season: Late Spring, Summer and early Autumn
• 3 Season: Spring, Summer and Autumn
• 4 Season: All year (although it may be a bit warm in the Summer)