Sleeping bags require insulation to trap the heat that our bodies give off, and to prevent consequent loss of heat through convection. Without insulation, all of our precious body heat would escape into the atmosphere, leaving us vulnerable to a drop in the ambient temperature.

The main principle to bear in mind with all sleeping bags is that regardless of what materials they are made with, all are singularly cold and useless until heated up by a person. Therefore it is critical that you are feeling warm when you get into your sleeping bag: if you wait until you are chilled to the bone before slipping into your bag then you’ll be in for an unpleasant surprise. Even if you are feeling toasty, a sleeping bag is likely to feel cold immediately upon entering. But thrash around a little inside and the bag will soon warm up. There are two clear choices when it comes to sleeping bag insulation: natural and synthetic. Which type of insulation is most appropriate for your needs will depend on a number of factors including the weight you are prepared to carry, the amount of money you are able to invest, and in what conditions you plan to use the bag.

Deciding how warm a sleeping bag is makes for a tricky business. After all, everyone feels the cold differently, and even the individual will fluctuate in his or her resistance to the cold from day to day, depending on energy levels as well as psychological factors.

As I have already mentioned, sleeping bags are not inherently warm, so it is important to get into your bag whilst you are giving off plenty of heat. This is best done by consuming hot food or liquids whilst you are actually in the bag. Bear in mind that drinking alcohol will force precious core heat to rush to the skin. Lowering your core temperature in this way is unhelpful in low temperatures and can lead to hypothermia.

For the purpose of the table below, I have assumed that you have recently enjoyed a hot meal and have retired to your tent tired but content, wearing (when appropriate) a thin base layer, a hat, thin gloves, and a thin pair of socks.Your dry sleeping bag is insulated underneath with a sleeping mat.

Bear in mind the following points:

Every manufacturer has its own slightly different temperature ratings chart.

Down bag manufacturers tend to be quite conservative with their temperature ratings.

If you know that you ‘feel the cold’, buy a bag in the next bracket up from your intended use.

Buy the bag that will suffice for your most popular activity and uprate the bag with a liner or outer bag if you find yourself heading for a destination that is colder than your bag can cope with.

Natural insulation comes in the form of down kernels and feathers. Down is a superb insulator; these long-lasting balls of fluff are the warmest and most compressible filling that money can buy. Feathers, by contrast, trap very little heat. A decent down bag will possess at least 85% down; the very best bags boast 95% or more.

But there’s more to down than mere percentages. The other key consideration is the strength of the down, which is measured by placing a specified amount of down in a container and measuring how much room it occupies under pressure. The result is known as a fill power rating, and reputable manufacturers regularly test the down they receive from suppliers in order to ensure it meets their requirements. Down with a fill power of 550 is often used in mid-price bags. Anything over 650 is excellent. A few top-end bags claim a fill power of 750- plus. (Note that these figures are for down samples subjected to the European fill power test: some other countries employ a fill power test which delivers higher results for the same quality of down).

Yet down is not without its disadvantages.When wet it is useless, and it takes an age to dry out. Finally, down needs to be cared for very carefully.

Many people opt for the convenience of a bag filled with synthetic insulation. And with good reason. Most synthetic bags are filled with polyester filaments. These are designed to trap body heat just like down kernels, but without any of the natural fibre’s disadvantages. A synthetic bag will typically retain about half of its insulation value when wet, which makes it a more appropriate choice for damp climates such as the UK and Patagonia, as well as perennially humid environments like tropical rain forests. A synthetic bag requires little or no care when it comes to cleaning or drying out. And although a synthetic bag’s useful life is approximately half that of down, prices are correspondingly lower.

However, a bag containing synthetic fibres needs more filling than a down bag in order to work efficiently in a similar temperature range. So if low weight is your first priority then a down bag with a high fill power rating might be more suitable for your needs.

Another type of synthetic bag is made from fibre-pile, and is typically covered with a windproof nylon. Bulky and relatively heavy for a warm(ish) model, fibre-pile has the advantage of continuing to trap heat very efficiently even when saturated. Pile dries out quickly too, making it an excellent choice for mariners.

As we have seen, most types of sleeping bag filling lose some or all of their insulating qualities when wet. However, moisture can enter the bag not only from outside in the form of rain and snow but also from the inside.

One tip for keeping your bag dry is to understand the dew point and how it can make your sleeping bag damp from within. The dew point is the point at which your sweat vapour condenses into a liquid. Normally this occurs outside the bag after your sweat vapour – which can amount to a litre or more per night – has passed through the insulation. However, if your sleeping bag lacks sufficient insulation to stave off the ambient temperature, the dew point can creep inside the filling. If this happens then the bag will start to become damp. In really low temperatures, your sweat can freeze inside the bag, forming ice crystals within the insulation. Placing an ultralight, oversized pile or synthetic bag around a down bag in these conditions will help to prevent the dew point from moving into the down.

At any rate, some vapour is likely to be left in the bag at the end of the night and so regular - preferably daily - airing is essential in order to keep your bag dry and maintain the thermal efficiency of the filling.

It is also worth bearing in mind that you breathe out a lot of vapour through your mouth. If you choose to sleep with your head buried inside your sleeping bag, the area around your face will become very damp. I find that it is better to sleep with my face exposed, even if that means wearing a face mask or balaclava in cold weather.

Sleeping bags can get also get wet from rain, snow, tea spills, river crossings and sodden clothing. The latter two are easily dealt with: when travelling, pack your sleeping bag in a kayak-style waterproof bag, and avoid putting damp clothes into your sleeping bag. As for rain and snow, the first protection is a decent tent. If this is impossible due to the nature of the undertaking, then a bivi bag can provide an effective alternative.

When it comes to preventing tea spills whilst camping, it's definitely worth investing in an inexpensive, insulated plastic mug and lid. Many manufacturers use water-resistant nylon materials on the outside of their bags, so with luck you should be able to simply shake the worst of any spill from the bag as soon as any accident occurs. A small sponge is handy for mopping up the excess moisture.

At night you lose more heat through the ground than through the air, so try to ensure that you always lie on a foam mat (either closed cell or self-inflating open-cell). Place any spare clothing between the mat and the sleeping bag. In particularly cold temperatures fold your mat in half, place it under your torso, and put your legs on top of your empty rucksack. It’s easier to cool a warm body than heat a chilled one, so in cold weather utilise all the features of your bag. For example, tighten the shoulder collar and hood drawstrings in order to create a heat-trapping seal around your face and neck.

Getting up in the middle of the night for a pee is a quick way to get cold. But holding on until morning is little better as your body will have to divert precious heat to keep the urine in your bladder warm. So consider taking a clearly labelled, leakproof pee bottle to bed. (Ladies will find a funnel useful). In sub-zero conditions, either leave the full bottle in your bag or reach outside and empty it. Otherwise, you’ll end up having to defrost a litre of frozen yellow liquid in the morning! If you are still not enjoying a better night’s sleep, it may be that your bag simply does not have sufficient insulation to keep you warm. If your sleeping bag is roomy, buy a thick fleece liner or a thin one-season sleeping bag and slip it inside. Alternatively, buy an oversized one-season bag and put your existing bag inside it.

Sleeping bags come supplied in tight-fitting stuff sacs. These should only be used during a trip when a small packed size is important. Back at home and it is much better to store your bag in a large cotton sack, hang it in a cupboard or lie it flat under a bed. Keeping the bag uncompressed in this way will maximise its useful life.

A dirty sleeping bag works less efficiently than a clean one. Using a liner will help to keep the bag clean. Cotton liners are cheap and cheerful, but soak up sweat and can take a while to dry. Silk liners are more expensive, much lighter and dry more quickly.

When your sleeping bag becomes a health hazard, follow the cleaning instructions closely. Washing a sleeping bag takes time and effort, so choose a warm day well in advance of your next trip. Down bags are particularly troublesome to wash, and so many owners prefer to take advantage of the services of a specialist such as W.E. Franklin (tel: 0114 268 6161), who professionally wet clean down bags – and duvet jackets – for a reasonable price. On no account have your bag dry-cleaned; the chemicals used in the process can damage the bag and the fumes given off are toxic. Companies such as Mountaineering Designs (tel: 015395 36333) will re-cover old down bags in order to give them a new lease of life. Extra grams of new down can be stuffed in at the same time if required.

One of the first things to consider when choosing a sleeping bag is where you are going to use it. If the bag is mainly going to be used inside hostels, then a rectangular design with a L-shaped zip (which allows the bag to be thrown open quilt-style) will probably suffice. At the other extreme, a mountaineer wanting the most heat-conserving type of sleeping bag will almost always opt for a snug-fitting sarcophagus design. This type of ‘mummy’ bag reduces the amount of dead air in the sleeping bag, and so provides maximum warmth albeit at the expense of some comfort.

Backpackers and trekkers often opt for a tapered bag which offers some of the heatretaining benefits of the mummy shape but also allows room for manoeuvre inside the bag. Bags destined for service in colder climes will have features such as shoulder baffles, zip baffles, sculptured hoods, and intricately designed foot sections which help to keep toes warm. The main features of a typical winter sleeping bag are illustrated below:

We’ll discuss the different types of sleeping bag insulation in a moment. But whatever filling you decide upon, bear in mind that it needs to be held in place over and around the body. Synthetic insulation is usually supplied in batts and so is easy to sew up in a single layer (for summer use) or in a double, offset layer (for year-round use). Down, on the other hand, is a free-moving product which must be blown into cubes of material. This process is timeconsuming and expensive when compared to the construction of synthetic bags.