In the old days, it was common to bring blankets for the expeditions. They protected some degree of the cold, but it was clear that the efficiency was low and so it was necessary to carry a large volume of blankets and warm clothing. At the time, wool and feathers / goose feathers were the best (in fact, goose or duck feathers are still considered very efficient, but also very, very expensive!). What made wool or feathers better than other fibers? Its ability to retain air, a great thermal insulation, that made it possible to minimize the heat loss of our body. In other words, these fibers do not “heat up” our body, but they prevent the heat from the body from dissipating in the environment.
From this concept came the hollow synthetic fibers. Through an industrial process it was possible to imitate wool (poorly comparing, it is a kind of thin tube, in which the air is encapsulated in its interior. This encapsulated air reduces the heat exchange and therefore makes the protection against the cold more efficient) .
Another way to improve thermal insulation is to prevent the loss of heat by evaporating moisture from the body. However, this causes a problem: during the night, our body can eliminate up to half a liter of water, which if it can not pass through the insulation layers will condense close to the body, causing discomfort and reducing thermal protection, since water is a conductor of heat much larger than air. To address this, manufacturers sacrifice a portion of the insulation capacity to allow this moisture to dissipate. To get an idea, if you are in an emergency situation where your sleeping bag is not providing enough protection against the cold and has an emergency aluminized blanket available (for example, Blanket ref. 8235 or ref. 9815, both of Coghlan’s), you can wrap your sleeping bag with it, getting up to 5 degrees Celsius more in thermal protection. However, you will almost certainly wake up with damp clothing, especially in the region of the trunk, or with the outside of the sleeping bag wet due to the condensation of the perspiration itself.
The thermal efficiency of a sleeping bag can be evaluated by its insulating capacity in relation to its weight, ie if two models offer the same thermal protection, the lighter one will be the most efficient (of course, since both manufacturers have used the same protocol of evaluation, which is quite controversial, as we will see later). One of the most common features to improve efficiency is to adapt the design of the sleeping bag to the contour of the body.
Square drawing is less efficient than the mummy shape – narrower in the feet and wider in the chest region – since the latter do not leave as much volume of free air in the inner part, which wastes heat from the body. Another feature in more elaborate bags is to avoid the exchange of indoor / outdoor air masses by placing a thermal collar in the chest region along the zipper and, in particular, by providing a hood to minimize head loss of heat ( a region of the body where the greatest loss of heat occurs).
THE THERMAL INSULATOR
An important point to note is that when we lie down we will be compressing the fibers of the lower part of the sleeping bag, greatly reducing its insulation capacity. For this reason, and not only for the sake of comfort, the use of a thermal insulation, or inflatable mattress, significantly improves the thermal comfort in overnight stays in nature. The thermal insulation is the inseparable companion of the sleeping bag!
All sleeping bag manufacturers report a degree of thermal protection on their products. This degree of protection is a very controversial scale, as we will hardly find the ideal conditions under which the tests are performed. The degree of thermal protection assigned to a particular sleeping bag generally represents the lowest temperature at which an average person could comfortably sleep in it. The parameters used in this test assume a young person, in good health, well fed and hydrated, in a shelter where there is no wind, with the sleeping bag dry, using a thermal insulation underneath, with fat and standard body pressure. Obviously, this results in the fact that a particular sleeping bag can provide comfort for one person and be a refrigerator for another.
HOW IS THERMAL PROTECTION MEASURED?
Each manufacturer adopts different methods to assign the degree of thermal protection to their products, which makes these numbers just a reference, not being an exact science until everyone starts to use the same method. One of the most common protocols is to use a human dummy placed inside the sleeping bag being evaluated in a laboratory environment, with controlled temperature and humidity, when measuring the manikin’s temperature drop over time. But there are those who measure the temperature difference between the side with a hot source and the cold side for a certain period of time, then estimating how much it could represent to the human body. All of these tests are standardized, but may offer different results, in addition to the fact that each person reacts differently under the same conditions.
SYNTHETIC OR NATURAL FILLING FIBERS?
And what would be the best filler to use in a sleeping bag? For those who wish – and can – have a lighter, more compressible and very efficient sleeping bag, but do not go to places with high humidity or possibility of soaking the sleeping bag (and do not have budget problems), the answer is the goose feather. When well cared for, ie kept clean and out of the box when not in use, they will maintain their insulation capacity and breathability for 10 years or more. But note that if it gets wet it will lose much of its characteristics, being able to form agglomerates when drying, and there went its investment. On the other hand, synthetic fibers, even those of better quality, tend to lose their original capacity in 2 or 3 years of normal use, or less if used intensely.
The most common and economical fiber is a tangle of solid yarn made of polyester fiber, often called Acrilon. Provides reasonable thermal insulation and durability. It tends to settle quickly with time, losing its initial characteristics. For the first 20 nights of use, you may lose about 1/3 of your initial volume, and with that too your initial insulation capacity (these are the entry models, almost always sold in supermarkets). Above it we have the hollow fibers, which can have one or more holes along the wires. These are much more durable and offer superior thermal protection at the same volume. Examples of such hollow fibers have trade names such as Qualofill or Polarguard. A newer version, and that has achieved quite good results, is a tangle of solid polyester yarns, but extremely thin (a fraction of the thickness of a hair) that can hold a large volume of air between them and are also more moisture tolerant. This last type is known by trade names like MicroTec, among others, being an interesting evolution of the synthetic fibers used in the last years. One of the most interesting characteristics, besides its high insulating capacity, is the fact that it retains much of its characteristics even when wet.
TYPES OF SEWING
In building a sleeping bag, it is still possible to improve its insulation capacity by paying attention to the way seams are made. To protect the wearer at positive temperatures, say above 5 degrees Celsius, a seam that crosses the insulation layers to form the grooves does not represent significant loss of heat from the body through the holes of the sewing needle. But in sleeping bags for lower temperatures, this will make an increasing difference as the outside temperatures are lower. Sleeping bags for negative temperatures use “hot seams”: instead of sewing the sleeping bag’s knobs in only one layer, the construction provides two insulation layers on each side, with the seams of each layer not matching the adjacent layer . Thus, even if part of the body’s heat exits through the seam holes of the layer close to the body, it will be retained by the second layer. Another way is to use the “loose-shell” system, where the insulation fiber layer is sewn only by the sides, avoiding heat loss through the holes of the sewing needles.
DEGREE OF PROTECTION
Many manufacturers, such as Tracks and Routes, report degrees of protection in 3 temperature ranges: Comfort, Tolerance and Extreme. Comfort temperatures are those in which you would be well protected entering your sleeping bag in light clothing. In Tolerance, you will probably feel better if you also wear a sweater you took to walk at night. At the Extreme, you will need to wear gloves, cap, socks and the warmth you took to sleep in comfort. As we have said above, each person has a feeling of comfort depending on their personal metabolism, food, degree of tiredness, etc. Therefore, if you are of the type chilly, buy a sleeping bag with a degree of protection superior to the conditions that you want to find.
In choosing your sleeping bag, you should keep in mind what will be your most frequent use. If you are going to spend the night only in mountains in Brazil, you will rarely find negative temperatures, but if you plan to take it to colder places, invest in a model with a higher degree of protection. Also remember to keep your sleeping bag always clean and out of the box when not in use, which will prolong the life of your investment.