The manufacturer of the sleeping bag controls the comfort range of the sleeping bag by their selection of insulating material (for the coefficient of thermal conductivity) and the thickness of that insulating material. Thus, the lower temperature limit of the bag is limited by the type and thickness of the insulating material surrounding the body.
This insulation allows the body to maintain warmth (by conserving heat which is internally generated by the body’s metabolic process) by blocking the loss of heat by thermal conductivity through the walls of the sleeping bag. However, in addition to conduction, heat may be also be lost by the other means of heat transfer which include convection, radiation and evaporation. This discussion addresses methods by which heat loss may be reduced by the addition of separate and/or combined moisture/vapor and radiation barriers.
For a liquid to undergo a phase transition from the liquid state to the vapor state a significant amount of heat is required. In the case of water, while it takes only one calorie of heat to raise one gram of water one degree C, it takes 540 calories to evaporate the same one gram of water (note one dietary calorie is equal to 1000 calories). That is, by supplying 540 calories of heat energy one gram of water will be transformed from a liquid to a vapor without an increase in temperature. This number is called the latent heat of vaporization for water. Measurements have shown that the body looses somewhere between 0.5 L to 1.0 L (note that one milliliter of water has a mass of one gram) of water while sleeping outside, in a sleeping bag on a long winter night at below freezing temperatures. This moisture is lost through respiration and insensible perspiration.
Winter air at -18 degrees C (zero degrees F) that contains a moisture level of 50% relative humidity when heated by a sleeper’s body inside the sleeping bag to 25 degrees C (75 degrees F) has a relative humidity of 3.4% (this can be seen by referencing a psychrometric chart). This moisture level is below that encountered in the driest dessert conditions. For comparison the Sahara Desert’s average relative humidity is reported to be 25% and the typical summer afternoon relative humidity on the Mojave Desert is reported to be approximately 10%. In this low humidity environment, the body loses moisture from its surface in the form of insensible perspiration. That is, perspiration that evaporates before it is perceived as moisture on the skin. This means that the sleeper is losing moisture without sweating and without knowing it. Still, every gram of water that is evaporated wither through respiration or insensible perspiration, takes 540 calories of heat from the sleeper’s body. So to evaporate a liter of water, 540,000 calories (or 540 dietary calories) would be extracted from the person sleeping. This energy would have to be supplied by the sleeper’s body converting food into heat energy.
As this moisture in the form of water vapor moves outward from the body through the inner lining of the sleeping bag and then through the insulation it will be cooled. Once it reaches the dew point temperature it will condense from a vapor to a liquid. This condensation will take place in the insulation between the inner lining and outer shell of the sleeping bag. If there is additional cooling of the moisture, it could possibly freeze. Condensation of moisture in the insulating material has a deleterious effect on the insulation ranging from increasing its thermal conductivity to causing a thickness decrease, thus having a negative effect on the temperature range of the sleeping bag. It should be noted that the accumulation of moisture in the insulating material also adds unwanted weight to the bag.
To block this moisture loss from insensible perspiration a vapor barrier liner can be used inside the sleeping bag. In this case the person sleeping surrounds himself with a material that is impervious to moisture in either it’s liquid or vapor form. This can be anything from a garbage bag to a special sleeping bag lining. Note that the air inside the vapor barrier will have a low humidity at first, however, as the body gives off moisture the humidity inside the vapor barrier will rise until it is very high, approaching 100% relative humidity, at which point evaporation from the body ceases, and thus heat loss by evaporation ceases.
Heat loss by radiation automatically occurs between a warmer body and a colder body through electromagnetic radiation. No transfer medium is required for heat transfer by radiation. Radiation heat transfer occurs in the vacuum of space. The warming of the earth by the sun is an everyday example of heat transfer by radiation. This type of heat transfer is described by the Stefan-Boltzmann equation
Q = εσAT4
Q is the amount of heat transferred per unit time
ε is the emissivity of the surface of the body loosing heat energy
is the Stefan-Boltzmann Constant
A is the surface area radiating
T is the temperature of the radiating body
If a hot object is radiating to cooler surroundings the heat loss rate can be expressed:
Q = εσATh4- Tl4
Th4 is the temperature of the warmer body
Tl4 is the temperature of the cooler body
The heat loss is driven to the differences in temperature to the fourth power but is controlled by the emissivity of the surface. Thus, if the emissivity, ε, can be reduced significantly, then the heat transferred by radiation can be minimized. Since dark, rough surfaces have a high value of emissivity (.94 for flat black paint), and shiny, silvered surfaces have low values of emissivity, (0.03 for aluminum foil), then by using an aluminized (or other low emissivity coating) inner liner, heat loss by radiation can be greatly reduced. For example, if the nylon lining of a sleeping bag had an emissivity of 0.85, then replacing it with an aluminized surface having an emissivity of 0.03 would reduce heat loss by radiation by 96%.