Ashwin Lewis
Sleeping bags are designed to keep us warm when we sleep outdoors. They work by trapping the heat produced by our bodies inside the bag, preventing it from escaping into the cold air outside. This is achieved through the use of insulating materials such as down feathers or synthetic fibres, which create a layer of trapped warm air around the body. The effectiveness of a sleeping bag depends on various factors, including the quality and type of insulating material, the construction of the bag, and the fit of the bag relative to the user's body.
| Characteristics | Values |
|---|---|
| Purpose | Insulating the body heat produced to keep the user warm |
| Mass | Depends on the quality of materials used; the lightest bags are made of sturdy yet soft materials |
| Filling | Down (duck/goose feathers) or synthetic (e.g. polyester) |
| Downsides of down filling | Expensive, loses insulating properties when wet, takes long to dry, ethical concerns |
| Downsides of synthetic filling | Heavier and bulkier |
| Shape | Mummy, semi-rectangular, rectangular, relaxed mummy |
| Fit | Should minimise 'dead space' inside the bag; smaller spaces warm up faster and retain heat more efficiently |
| Performance | Depends on insulation and construction; bags with good insulation and poor construction are less effective |
| Breathability | Measured using the Ret test; lower the Ret number, the higher the breathability |
| Waterproofing | Necessary to protect from outer moisture; higher water resistance means lower breathability |
| Tips for warmth | Use a sleeping bag liner, do light exercises before use, consume hot food/drinks, use a hot water bottle |
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What You'll Learn
Insulation and thickness
Sleeping bags are designed to trap the heat produced by your body and prevent heat loss, rather than actively generating warmth. The insulation in the bag, along with the construction method, provides the airspace or loft required to reduce heat loss from your body as you sleep. The thickness of a sleeping bag gives a reasonable idea of how much air it traps and, consequently, how warm it will be.
The insulating material in most sleeping bags is easily compressed, and the air can be pushed out from under the sleeper. Therefore, the thickness of the sleeping bag is crucial to maintaining the trapped air that provides insulation. A sleeping bag for very cold temperatures may have around 3 inches (76 mm) of insulation, while the average sleeping bag may have less than 1 inch (25 mm) of insulation.
Sleeping bags with down insulation are considered excellent insulators that can be packed down small and light. Down is derived from the fluffy plumes that serve as an undercoat for geese and ducks. It is warmer than synthetic insulation by weight, but it loses its ability to provide warmth when wet and takes a long time to dry. Down insulation is also more expensive and raises ethical concerns about the living conditions of the animals it is sourced from.
On the other hand, synthetic insulation is more affordable and maintains its insulating properties when wet. However, synthetic sleeping bags tend to be heavier and bulkier than down bags. Some sleeping bags employ a hybrid insulation technology, combining synthetic and down insulation to achieve the best performance characteristics of both types.
The shape of the sleeping bag also affects its insulation properties. Most backpacking and mountaineering bags have a mummy shape, which minimises "dead space" inside the bag, allowing smaller spaces to warm up faster and retain heat more efficiently. Some people may find these restrictive and prefer a "relaxed mummy" or rectangular bag, which offers more room but may not retain heat as effectively. Women's bags typically have added insulation in the upper body and foot box and feature a shorter length, narrower shoulders, and wider hips compared to men's bags.
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Filling type
Sleeping bags are designed to trap the heat that your body radiates outwards, preventing the exchange of warm and cold air. The filling used in a sleeping bag is the most important factor in determining the degree of insulation it provides. There are two main types of filling: man-made (synthetic) and natural (down).
Synthetic fillings are made from polyester fibres, and are generally heavier and bulkier than down bags of comparable warmth. Synthetic bags are easier to clean, retain their insulating properties when wet, and are less expensive than down. However, they may have a shorter lifespan. Synthetic bags are a good choice for those who are tentative about the care required to maintain a down bag.
Down fillings are made from the fluffy plumes that serve as an undercoat for geese and ducks. Down bags are excellent insulators that pack down small and light, but they lose their insulating properties when they become saturated with water. Down bags are typically more expensive than synthetic bags, and they require more care. Down bags should be stored loose, either hung up or in a large stuff sack that does not compress the bag.
The quality of the down in a sleeping bag makes a significant difference to its performance. The fill power of a down bag refers to the amount of loft or 'fluffiness' it has, which impacts its insulating power. The higher the fill power, the less down is needed to achieve the required loft for a given comfort range. The composition of down fill is described by the ratio of down to feather, with a higher ratio of down indicating better quality.
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Fit and shape
The fit and shape of a sleeping bag are important factors in determining its performance characteristics and insulation efficiency. A sleeping bag should be long and wide enough to fit you comfortably. However, minimising "dead space" inside the bag is crucial, as smaller spaces warm up faster and retain heat more effectively. This principle is utilised in the design of performance sleeping bags, which typically feature a closely contoured, tapered mummy shape. The mummy shape is prevalent among backpacking and mountaineering bags, but some users may find these restrictive. For those who prefer more freedom of movement, semi-rectangular or "relaxed mummy" shapes are available. These bags are more spacious, allowing users to spread out or roll over more easily.
The construction of the bag, including the management of insulation between the outer shell and inner lining, also plays a significant role in the bag's performance. Insulation technology has evolved to include hybrid systems that combine synthetic and down insulation to optimise performance. The arrangement of insulation is crucial, with techniques such as Insotect Flow utilising vertical baffles and special down management gates to achieve a more efficient construction that fits the sleeper better. This design reduces the number of baffles required, resulting in a lighter bag with less fabric.
The shape of a sleeping bag can also influence its size, weight, and insulation properties. Down sleeping bags, made from duck or goose feathers, excel at insulation while remaining lightweight and compact. Synthetic bags, on the other hand, tend to be heavier and bulkier, impacting their portability and storage convenience. The fill weight and type of insulation are important considerations when choosing a sleeping bag, as they determine its suitability for different seasons. For instance, a lightweight down bag is ideal for summer use, while a synthetic bag designed for winter will be larger and heavier.
Additionally, it is worth noting that the fit of a sleeping bag can be influenced by the use of a sleeping bag liner. A liner prevents the bag from direct contact with the sleeper's body, helping to keep the bag clean. It is also important to consider the storage of the sleeping bag when not in use. Long-term compression of the insulation can impact its ability to loft, so it is recommended to store the bag in a larger mesh or cloth sack when not in use.
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Moisture management
A good sleeping bag should be able to absorb a certain amount of physical moisture without becoming damp and uncomfortable. This is measured in grams and refers to the amount of water that can be stored in the filling's microstructure without the user experiencing any moisture or cold.
To prevent moisture from building up, a sleeping bag liner can be used. This wicks moisture from the skin and spreads it out over a broad area where it can dissipate easily. Liners can be washed easily and dried quickly, and they also act as a protective barrier, preserving the cleanliness of the sleeping bag and extending its overall lifespan.
To prevent moisture from entering the bag, the shell fabric must be water-resistant. The higher the water resistance, the lower the breathability, so it is important to choose a material that balances these two parameters. A thicker, less porous shell material might also help to keep body vapour from moving out to the exterior of the bag, where it can then condense.
Another way to manage moisture is to avoid sleeping "hot". Light exercise before bed can help to raise your heart rate so that you feel warm, but be careful not to work so hard that you start to sweat.
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Temperature rating
The temperature rating of a sleeping bag is a key factor when choosing one. The standardised ISO test provides three temperature thresholds to help guide users: a comfort rating, a limit rating, and an extreme rating. The comfort rating indicates the temperature at which a cold sleeper might feel comfortable. This is the temperature rating used for women's bags. The limit rating, or lower-limit rating, is always lower than the comfort rating and indicates the temperature at which a warm sleeper might still be comfortable. This is the temperature rating used for men's bags.
It's important to note that temperature ratings are estimates and not definitive guarantees of warmth. Individual factors such as body shape, size, age, metabolism, and personal preferences can affect how warm or cold one feels when using a sleeping bag. Additionally, external factors such as weather conditions, food intake, clothing, and sleeping location can also impact the effectiveness of the sleeping bag. Therefore, it is recommended to give yourself a buffer when choosing a sleeping bag, especially if you tend to feel cold easily.
The EN rating system, used by some manufacturers, also takes into account the physiological differences between traditional genders, with women generally requiring a higher temperature to feel comfortable compared to men due to differences in metabolic rate and muscle mass. Older people may also require higher temperature ratings as they tend to lose muscle mass with age and feel colder.
Seasonal ratings are another way to categorise sleeping bags. For example, a one-season bag is designed for summer use, while a three-season bag can be used throughout spring, summer, and autumn. Four-season bags are designed for winter use when temperatures drop below freezing.
When choosing a sleeping bag based on temperature ratings, consider the lowest temperature you are likely to encounter and your personal tolerance for cold. If you often feel cold, opt for a bag with a comfort rating that is significantly lower than the expected coldest temperature. If you tend to sleep warm, you may rely on the lower-limit rating, but it is still recommended to have a buffer of a few degrees Celsius.
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Frequently asked questions
Sleeping bags work by trapping the heat produced by your body, preventing it from escaping. This is why it's important to do some light exercises before climbing into your sleeping bag—to raise your body temperature.
The two main types of filling that sleeping bags are made from are man-made (e.g. polyester) and natural materials (e.g. duck/goose down feathers).
Firstly, consider the weather conditions you'll be using it in. If you're camping in cold conditions, choose a bag with good insulation and a high fill power. Down filling provides better insulation than synthetic, but it's more expensive and doesn't perform well when wet. Synthetic bags are more affordable and can maintain their insulation properties when wet.
The mummy shape is the most popular for backpacking and mountaineering as it minimises 'dead space', allowing smaller spaces to warm up faster and retain heat more efficiently. However, some people find these restrictive and prefer a semi-rectangular or rectangular bag.
To get the most out of your sleeping bag, consider using a sleeping bag liner to prevent it from getting dirty. Using a waterproof stuff sack will also ensure it doesn't get damp in your rucksack. When camping, set up your sleeping system at least 15 minutes before bed to give your bag time to loft.
