Four main environmental variables make up the environment. These are the dry-bulb temperature, the mean radiant temperature, the vapor pressure, and the air velocity. In any combination of these variables man's physiological reactions are also influenced by his activity level (internal heat production), and the type of clothing he wears. Three basic parameters characterize the clothing. These are the dry insulation value expressed in clo units, the resistance to water vapor transfer as it affects the efficiency of cooling by regulatory sweating, and the emissivity of the surface of the clothing. The second parameter is expressed either by the "Permeability Index" introduced by Woodcock , or by the "Permeability Efficiency Factor" introduced by Nishi and Ibamoto. For ordinary clothing, the third parameter, namely the surface emissivity, has always been assumed to be similar to that of the skin, close to unity, and as a result was disregarded as a factor. As new fabrics are developed, particularly those with low emissivity and therefore having the capability of reducing radiation loads, the surface emissivity of the clothing should be considered in characterizing the clothing.

The insulation value of any clothing ensemble can be determined by the use of the copper manikin. Sprague and Munson3 reported such measurements for a great variety of individual men and women's garments as well as clothing ensembles. They also developed linear regression equations for predicting the insulation values of men and women's clothing ensembles from the individua1 insulation values of the garments of each ensemble. In a recent paper by Nishi et a1. direct colorimetry was used to measure the insulation value and moisture transfer characteristic of clothing with live subjects inside a climatic chamber. The objective of the present paper is to present a third alternative method for estimating the thermal insulation value of a clothing ensemble, when the use of a copper manikin, or a climatic chamber with all necessary instrumentation, is not readily available. The method is based on a simple heat transfer model which predicts the thermal insulation values of single layer garments from the physical data of their fabrics. It is then recommended that the theoretically predicted thermal-insulation values of garments be used to predict the insulation values of clothing ensembles from the linear regression equations developed by Sprague and Munson.