Effect of an Air Layer on Thermal Insulation of Clothing Materials

In order to study the thermal property of a combined system of fabrics and air layer, we measured heat loss from a heat-source plate covered by fabric and an air layer of different thicknesses up to the atmosphere, for various kinds of materials. The experiment was performed under a wind of such a low speed that natural convection may dominate the heat transfer in the atmosphere. For four typical fabrics, the results were compared with a model describing thermal property of fabric/atmosphere and air layer/fabric/atmosphere systems, and an attempt was made to separate the contribution of each part. The main conclusions are summarized as follows : (1) The thermal insulation by clothing materials is greatly affected by the underlying air layer except for very thick fabrics of low thermal conductance. The overall thermal transmittance decreases greatly with the increasing thickness, da, of an air layer up to da_??_ 0.5 cm, and then, reaches and remains at a nearly constant level (_??_ 5 Wm-2K-1) since the transmittance of the air layer becomes dominated by radiative transfer and indifferent to its thickness. (2) The heat transfer rate to the atmosphere is larger from the fabric surface than from a smooth plate of the same surface temperature because of the ruggedness or hairy structure of fabric surface. (3) The thermal contact resistance between the fabric and plate is found to be an order of0.02 W-1m2K, which, for thin materials, is even larger than the thermal resistance of fabrics themselves. (4) The heat transfer rate attended with air passage through the fabrics is found to be of the same order as that due to conduction under condition that the natural convection dominates over the heat transfer in the atmosphere.