KC-2831 -- Comparison of Current Two-Parameter Indices of Vapor Permeation of Clothing–As Factors Governing Thermal Equilibrium and Human Comfort

Five indexes of moisture permeation through clothing are considered: (1) the Woodcock-Goldman permeation efficiency factor, iw; (2) the classical Burton thermal efficiency factor, Fcl; (3) the new Lotens-iL, which is defined as the ratio of the effective Lewis number of the clothing layer to that of an equivalent air layer and is used as a modification of (4) the Nishi permeation efficiency factor (Fpcl). Finally, (5) the factor ia defined as hc/h is a function of ambient air movement V and serves as an index of vapor permeability of the equivalent air layer. All five are rationally related by simple elegant relationships, which demonstrate that (1) the two intrinsic properties of the clothing ensemble are Icl and iL; (2) iw is dependent on Icl and air movement V; and (3) its reciprocal is the average of 1/ia and l/iL by the weighting factor Fcl. Finally (4), the ratio Fpcl/Fcl always equals the ratio iw/ia. Experimental data from three sources are used to test these relationships for evaluating iL: first, by direct laboratory measurement of iL for typical fabrics used in everyday clothing ensembles (Berglund, unpublished); second, by use of iw and Ic1 measurements on a Woodcock-Goldman sweating mannequin (KSU published data); and third, by observations on human subjects with partially wet skin surface while clothed in a humid environment with varying air movement (Berglund and Gonzalez 1977). Values of iL found by all three experimental methods are essentially the same. The ratio Iclo/iL is the true index of the vapor resistance of a clothing ensemble, as was the intrinsic Iclo for thermal resistance. A median value of iL of 0.45 was found for typical fabrics used in everyday clothing. Experimentally derived values of iL, iw, and Fpc1 from observed values of ia and Fcl require rationally derived estimates of the fraction of the human body or mannequin surface wet with perspiration or moisture to be valid for any application to human performance and comfort.