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Based on a physical model, in which a human is depicted as a collection of appropriately sized cylinders, clothing insulation and vapour resistance are calculated for standing persons in stilI air, when the clothing ensemble thickness, total fabric thickness, number of clothing layers and number of trapped air layers are specified for each cylinder. Specific knowledge of the clothing material is not required, except when coatings of films are involved. The resulting reference values for clothing insulation and vapour resistance are accurate to a standard deviation of 0-01 1 m2K/W and 1-8 mm of air equivalent, respectively, compared to thermal manikin measurements. The reference values are modified for sitting, walking, and cycling at various rates, and for the combined effect with wind. The formulas are regression equations on a database of literature. The resulting total insulation and vapour resistance are accurate to 0.022 m2K/W and 3.6 mm of air equivalent, respectively. The physical model, which is available as software, is a challenge to existing methods for the determination of insulation and vapour resistance with respect to simpleness and accuracy.

A condensation theory is presented that enables the calculation of the rate of vapour transfer with its associated effects on temperature and total heat transfer inside a clothing ensemble consisting of underclothing, enclosed air, and outer garment. The model is experimentally tested by three experiments: (1) impermeable garments worn by subjects with and without plastic wrap around the skin, blocking sweat evaporation underneath the clothing; (2) comparison of heat loss in impermeable and semi-permeable garments and the associated discomfort and strain; (3) subjects working in impermeable garments in cool and warm environments at two work rates, until tolerance. The measured heat exchange and temperatures are calculated with satisfying accuracy by the model (mean error = 11, SD = 10 Wm-2 for heat flows and 0.3 and 0.9 degree C for temperatures, respectively). A numerical analysis shows that for total heat loss the major determinants are vapour permeability of the outer garment, skin vapour concentration and air temperature. In the cold the condensation mechanism may completely compensate for the lack of permeability of the clothing as far as heat dissipation is concerned, but in the heat impermeable clothing is more stressful.

A method is described for measuring the water vapor resistance of textiles under variable conditions of relative humidity (RH). It consists basically of varying the position of the sample in an air gap between a wet and a dry surface while keeping all other conditions constant. The resistance is determined by the rate of water loss from the apparatus and the temperature of the water. The results for microporous PTFE and polyurethane films show little variation with RH, but fabrics and films with hydrophilic coatings added show strong variations, the resistance increasing substantially with decreasing RH. 1 Current addres: Mustang Ind. Inc., Richmond, BC, Canada. . The water vapor resistance of textiles can be measured by a variety of techniques, but these rarely give the same results. This may be in part because of inaccuracies inherent in any such method, but it is probably largely due to the fact that the different methods actually measure different things.The concept of water vapor resistance is derived from an expression such as Equation 1:Here M is the rate of diffusion of the mass of water vapor per unit area across the sample ( kg J m 2s ), C, and C2 are the concentrations of water vapor in the air on either side of the sample ( kg/ m 3 ), and R is the resistance of the sample ( s / m ) . To obtain a physical feel for the quantity, it is often convenient to express the water vapor resistance as the thickness of a layer of still air that would present the same resistance to the diffusion of vapor as does the sample ( d~ ). This equivalent thickness is related to the actual resistance through the diffusion constant D for water vapor into air, D is about 2.5 X 10-5 m 2 / s at room temperature.Typi~lly, uncbated textiles have a resistance of the order of 100 s/m, equivalent to a few millimeters of still air.The value of R in Equation 1 is the resistance of everything between the two planes where C, and C2 are determined. Often, in application as well as in measurement, this includes air layers on one or both sides of the textile layer, as well as the textile itself. Since these air layers are usually several millimeters thick, their influence can be significant, especially in apparatus originally developed to measure low permeability materials. Different sorts of apparatus with air layers of different thicknesses can give different apparent resistances unless those air layers are carefully controlled and their resistance subtracted from the total.Another possible source of discrepancy is that the textile fabric can experience different conditions of temperature and humidity in the various sorts of apparatus, and thus the actual resistance of the sample may genuinely be different in the different measurements. This is particularly likely for textiles coated with vapor-permeable hydrophilic films. The water vapor resistance of films from cellophane [ 2 ] , keratin [ 3 ] , and nylon [ 6 is known to increase with decreasing RH, whereas the diffusion rate in polyethylene [6] ] decreases with decreasing RH. Lotens [...