An often inflamed topic in outdoor, survival and even military is "My sleeping bag is warmer than XXX" or even "XXX isn't that much good because YYY is much warmer!". Things get heated up, but unfortunately mostly based on subjective feelings instead of facts. That's why we're addressing the question here: What is "warm" anyway - and what's best?
Here, as always, the following applies: "Your pers. Preference/experience may differ, for huge side effects marry your doctor and pretend to be a pharmacist!" - (which is to say, if your experience is subjectively different, so be it - but doesn't change the laws of physics!)
When cold, the most important factor is to maintain the status quo of body temperature from 36.0 to 37.4°C. If the body temperature drops below 36°C we start to shiver, above 35°C the body reacts and produces heat by automated muscle trembling. This is the beginning of the so-called hypothermia. In this process, blood vessels in the limbs contract to reduce blood flow to the outer regions of the body. If the temperature continues to drop, consciousness becomes clouded. This clouding can lead to the so-called Kälteidiotie. At this stage, there is already a marked weakening of reflexes, and shivering ceases. This is often mistakenly seen in conjunction with the clouding as a sign of warming, but in reality the body stops shivering because of hopelessness. When the temperature drops below 28 °C, fainting occurs; below 28 °C, the body falls into a suspended animation-like state. If one is alone, this is usually the end
How does our body handle cold / why do we need to "insulate" ourselves?
In nature, there will always be an "average" temperature. If I tip 100ml of hot water into 100ml of ice-cold water, the temperature will settle very quickly to an average value. A warm body gives off heat to its surroundings. If the environment is warmer, we absorb heat (and have to cool down, by sweating). Our body, through its metabolism, maintains a core temperature of 37.5°. If this rises, we speak of hyperthermia (best known manifestation: fever), if it falls, of hypothermia or hypothermia (we "freeze", which is a warning sign of hypothermia (see above)). Our heat power plants, the mitochondria, in the body burn energy (food) and maintain temperature.
Our body permanently releases heat to the environment, if it gets cooler or if other factors come into play (humidity increases the heat release up to factor x20), wind also x5) it gets really cold (e.g. you freeze to death much faster at +5° and rain, than at -5° and dry! A circumstance, which dozens of hikers in national parks of the USA pay with the life). Our skin has a thermal conductivity coefficient of X (unfortunately, I do not know the value). Means with rate X the body gives off heat, if it is naked. If one pulls something over it (clothes), 2 factors result:
an air cushion is formed between the body and the garment
the fabric of the clothing takes the place of the outermost layer of skin
Now the body heats the interspace to 37.5° and is happy, the clothing also passes on the heat with its coefficient. If the clothing together with the air cushion passes on the heat with a higher value than the skin, we have a cooling effect (damp linen, for example), if the fabric together with the air cushion passes on the heat more slowly, we have insulation, the slower the fabric passes on the heat, the less the body has to reheat. If the fabric together with the air cushion, e.g. by very low outside temperature, gives off the heat faster than we can reheat, we start to shiver!
What kind of clothing warms us - and how?
Now the fact is that different materials, have different thermal conductivity coefficients. The worse they are, the slower they give off heat. By the way, the air around us has one of the worst. Of course, this is invalid as soon as you move or even (imperceptible) wind goes (and there is ALWAYS wind, even if we do not notice it). Therefore, the air must be bound immovably. This already happens when we choose thinnest fabric as clothing, which is at least windproof!
Now let's assume, for our example, our skin would release heat with factor 10, the limit what the mitochondria can do would be at 50. We wear a shirt that has only factor 5. Our body is happy, 50% less work, everything tutti! Suddenly it rains, the factor of 5 becomes the factor of 25. Hu, now it gets cool, the body really bolls in. But then the wind also comes, the water evaporates, the factor 25 possibly becomes 75 - the body can no longer keep up, either we protect ourselves - or freeze to death!
Our conclusion? We need something better. The more barrier of material, which would be slower than our body to dissipate, the less our body must reheat -> the better he comes to his task to keep us at 37.5 ° C core temperature
What materials are suitable here then?
Generally speaking, any material that dissipates heat less efficiently than our skin is good! And the more we bring between us and the "cold" (colder ambient air), the better, right? Wrong! If we overdo it, heat builds up and our body reduces its heating capacity and has to start cooling again - we sweat! In winter this is fatal! We must therefore choose correctly!
In general, dense knitted fabrics, even if they are poor conductors either very thin (woolen cloth, softshell, fleece) and thus the insulation is limited. Denser, solid materials are too bulky and not suitable for clothing (cork, styrofoam, etc).
So, for use in insulating clothing (and sleeping bags), one must resort to material that is compressible when needed and, when uncompressed, creates a thick layer of poorly conducting material. This is achieved by using materials that can bind large amounts of immobile air (hollow fibers, fiber fleece and down/feathers). Their common factor is the bulking power, the loft or simply - how thick the materials can "puff up"!
Which is better?
This question is on the one hand not at all, on the other hand very easy to answer: the higher/thicker the bulking power, the better the insulation. But then there are the other factors, such as compressibility, dead weight, ease of maintenance, moisture behavior, etc. A bulking force of 10cm will bind air to 10cm, regardless of the manufacturer or what material the binding is made of! But how heavy it is then, how compact, etc. that is a point to consider.
As example(s):
Wool is relatively insensitive to light moisture (see this article), but heavy and has a loft of less than 1cm - the more windproof it is, the thinner or heavier it becomes. On the other hand wool is insensitive to sparks
Fleece behaves similarly to wool, is lighter, but is very susceptible to sparks and fire
Synthetic and hollow fibers warm well, a loft up to 10-15cm is no problem, are relatively light and insensitive to moisture. With inexpensive fibers, the loft is often not so very durable, or the fiber is heavy and poorly compressible. However, this is negligible, especially for clothing. The best known representatives here are Primaloft, G-Loft, Thinsulate and Hollowfill
Down has excellent bulking power and compressibility with the lowest packing volume and weight - but also very high maintenance, washing and cleaning - and is sensitive to moisture. But especially at temperatures below -15 ° down is the upper league!
The Bottom Line:
In conclusion, we can gather a few things on which this article is based and which should be taken into account when choosing and preparing!
No material warms by itself! Insulation slows down the cooling process!
It is not the material that warms us, but the air trapped in it!
Humidity and wind let us cool down faster. Therefore, we must protect ourselves from it and not sweat!
With identical bulking power / loft height is usually the approximately (pi x thumb) same insulation given, here is decisive the behavior of the material under the influence of the elements, weight, pack size, maintenance in the context of the planned use - and finally the price!
But this also means, an article X with 50% of the bulking power of article Y can not provide an identical insulation with otherwise identical factors! Check this before you decide!
Please note also:
Warmth is dependent on fatigue, food level (full or hungry), alcohol level, metabolism, body weight and BMI, humidity, etc. The loft ratings were tested under laboratory conditions with a norm woman and norm man and from that the comfort rating was statistically determined. And even with duck vs goose down there is slight deviation of the bulking power to insulation curve
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