##It's a system
It's a system
##What's a "clo"?
What's a "clo"?
#How much heat?
How much heat?
#What's the target measure?
What's the target measure?
##More insulation is more
No.
It's a system
Consider the clothes and the sleeping bag to comprise a "sleeping system"...
The functional demand on a sleeping system is to enable the user to obtain a specified period of sleep in a given thermal environment. The required insulation bears a linear relationship to the environmental temperature ( ~ 4 clo/20 °C decrease in environmental temperature).
What's a "clo"?
The insulation of clothes are often measured in the unit "clo", where
- clo = 0 - corresponds to a naked person
- clo = 1 - corresponds to the insulating value of clothing needed to maintain a person in comfort sitting at rest in a room at 21 ℃ (70 ℉) with air movement of 0.1 m/s and humidity less than 50% - typically a person wearing a business suit. i.e., 0.155 m2K/W.
Furthermore,
One clo unit of insulation can be defined as that insulation which not only allows, but requires the transfer of 10 kcal/hr for an average man (1.8 m2) for each °C difference between his mean skin temperature (Ts, which can be defined for comfort at 32 °C) and ambient temperature. Therefore, at an ambient air temperature of 0 °C the non-evaporative heat loss will be 320 kcal/hr with a 1 clo sleeping system; it will be 160 kcal/hr with a 2 clo sleeping system, etc.
How much heat?
Consider a soldier of average size; i.e., weight of 70 kg and height of 173 cm, which corresponds to a total body surface area of 1.8 m2. Such an individual produces about 72 kcal/hr while sleeping (i.e., 0.8 MET) and loses about 25% of this amount from the body by respiration and evaporation of the body water diffusing through the skin, which typically maintains a minimum 6% wettedness; i.e., a skin relative humidity of .06. This leaves 54 kcal/hr to be lost by the body by non-evaporative heat loss through the sleeping system to the ambient environment if body heat content is to remain unchanged.
What's the target measure?
Two definitions can be used for "adequacy" of a cold-weather sleeping system: (1) "comfort", which implies that the sleeping soldier loses just the 54 kcal/hr by non-evaporative avenues, i.e., can maintain heat balance without shivering or sweating; or (2) "six hours of restful sleep", which allows him to incur a total body heat debt of 80 kcal during a six-hour sleeping period and, therefore, to lose 67.3 kcal/hr, i.e., 54 + 80/6.
More insulation is more
[T]he highest insulation values obtained to date for a sleeping system, ca. 8 clo (for the U.S. Army's Extreme Cold LINCLOE, sleeping system when used by a man sleeping in clean long underwear and socks with the insulated air mattress), can be extended by adding additional clothing items to achieve a 9 clo level. It was possible to approach a 10 clo level of sleeping system insulation when the most sensitive and heat loss prone areas of the body, i.e, the hands and feet, were provided with supplementary warm sleeping gloves and bootees. Conversely, much lower values of insulation were obtained when just the long underwear and socks were worn and the extreme cold LINCLOE sleeping bag was used on bare ground, without the insulated air mattress which is part of the sleeping system; on a bare cement floor, the insulation dropped from about 8 clo to less than 6 clo.
- Source: Chapter 6b, Biomedical Effects of Sleeping Systems in Goldman, R., & Kampmann, B. (2007). The Handbook on Clothing. Biomedical Effects of Military Clothing and Equipment Systems.