Characteristics and thermal properties. Types according to seasonality and the area to be used. Learn all about How to Choose Sleeping Bag in this article.
Sleeping bags can trap and retain a layer of air that does not circulate close to the body. With the body temperature, this air warms up and the bag becomes a kind of barrier between the air outside and the air inside. So: it is not the bag that heats, it is you who heats it with your body temperature. The function of sleeping bags is to slow down the loss of body heat, which can occur in four different ways: evaporation, radiation, conduction, and convection.
Evaporation: moisture on the surface of the skin evaporates and produces a sensation of cold.
Radiation: by the natural fact of being alive, the human body constantly radiates heat outwards, the colder the atmosphere surrounding the body, the more so.
Conduction: we mean the transfer of heat between two bodies or objects that are in contact. The warmer object transfers heat to the colder one. We heat everything we touch with our skin with our body heat (that is why it is essential to always use a good insulating mat as a complement to the bag).
Convection: air warmed by the body tends to escape, especially if there is wind. New cold air takes its place and cools the body. Heat loss depends on the temperature and the speed with which the new air arrives.
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How to Choose the Right Sleeping Bag
The minimum temperature at which a bag is capable of insulating depends largely on each person’s metabolism and general resistance to cold. Consequently, each person must look for the bag model that best suits him or her according to his or her thermal sensitivity and the climatic conditions of habitual use that are foreseen.
Not only are there important differences in temperature sensitivity between different people, but the same person may react very differently when using the bag on different days.
This “thermal sensation” depends on the following factors: general physical condition; the conditions of the day: fatigue, food, etc.; personal conditions: one resists the cold better when dry and clean at bedtime than when tired and unable to wash. From the outside temperature, of course, but also the humidity. How well insulated you are from the ground, so a good insulator is essential. From the underwear, socks and hat were worn. And how protected you are from the wind (even a slight breeze makes the bag lose performance).
Temperature of Use
When choosing a sleeping bag, we will have to look at its technical specifications concerning its temperature of use. The definitions we will look for are comfort and extreme temperatures.
The first is the temperature to be taken into account at the time of purchase, in which you can sleep comfortably without getting cold. By extreme temperature, we mean the temperature at which the bag protects against hypothermia, i.e. the temperature at which you can survive about 6 hours of rest in the fetal position without feeling cold.
How to Choose Sleeping Bag Temperature
Sleeping bags are usually categorized according to seasonality. If they are to be used in summer their comfort temperature range is about 39.2/+41°F or more; for 2 seasons, 32°F; 3 seasons: 32°F to 23°F; and winter: 14°F and below. Of course, this is a relative classification, and apart from the season of the year, it is necessary to take into account the specific area of use and its particular weather conditions, such as ambient humidity or characteristic winds.
It is always advisable to choose a sleeping bag with a lower comfort temperature range than the lower temperature you expect to find at your destination. For example, if the area has temperatures close to 32°F, it is recommended to choose a 21.2°F sleeping bag instead of a 33.8°F sleeping bag. In case the temperature remains higher than expected, it is easier to ventilate the bag for better air circulation.
How to Choose Sleeping Bag Size
Manufacturing materials, thermal capacity, and shape are determining factors to be taken into account when choosing a model. We must also consider the weight and volume inside the carrying bag, especially if they are carried in backpacks or bicycle saddlebags.
Sleeping bags can come with natural (bird feather) or synthetic thermal padding. Those with natural filling provide greater insulation but are more expensive and have some disadvantages of use, such as the difficulty of maintenance (washing, drying), and the most important: they lose much of their insulating capacity if they get wet. Most campers choose bags with synthetic insulation because of their good performance and price-quality ratio.
These filings are mainly made of polyester fibers and offer advantages such as easy and fast drying; if they get wet they still maintain acceptable insulation against the cold, they are generally cheaper than goose down and do not cause allergies.
The fabrics that make up the outer and inner lining of sleeping bags should be light, breathable, strong, and as water-resistant as possible. To achieve these characteristics, nylon and polyester taffetas are used. The thermal capacity is mainly determined by the following characteristics: the thickness of the insulating layer and its quality, the type of construction (honeycomb, etc.), and the technical details (collar, hood shape, zipper insulation, etc.). These three factors determine to a large extent the heating capacity of the sleeping bag, i.e. the capacity to effectively accumulate warm air inside.