Moisture and clothing layers: effect of ambient temperature on heat loss and insulation

During the latest years the research on the effects of moisture on clothing system has been boosted. New information has revealed phenomena, e.g. “heat pipe” effect with its condensation-evaporation cycle(s) that has not been considered earlier in prediction of physiological reactions or evaluating clothing properties. Considering the material properties, e.g. the evaporative resistance measurements, the tests at homogenous conditions with registration of mass loss would be probably the correct approach. On the other hand, until there is no clear picture where condensation occurs, role of wicking and the probability of re-evaporation in multilayer clothing at different environmental conditions measuring the real heat losses in order to evaluate human thermal responses in realistic test conditions is important. An example of such need is the selection of proper means for protection against cold in prehospital care at accident sites. In this study thermal manikin was tested with wet underwear and wrapped in 1, 2 or 7 layers of woollen rescue blankets at -15 and +10 °C. This paper discusses the issues related to possibility to improve predictions for the cases when other situations, materials or exposure temperatures are involved. A method to quantify “heat pipe” effect was proposed, and for control the calculation of dry insulation from wet tests was applied. The measured apparent insulation, i.e. insulation based on total heat loss in wet conditions, was higher at -15 °C than at + 10 °C. That could be related to higher condensation rate in materials or suppressed evaporation. However, the measured weight loss rate (higher at 15 °C) and accumulation in layers (lower at 15 °C) did not support this conclusion. Effect could partly be related to the lower water pressure gradient between wet clothing at manikin surface and ambient air at +10 (2.5 kPa) than -15 °C (3.0 kPa). In the case of 7 layers the highest accumulation occurred in the layers near body and in the outermost layer while only minimal accumulation of moisture was observed in the middle layers. The total accumulation was divided into ratios for each layer, and expected condensation heat to environment was based on insulation (7 layers, 1/7 of the first and 7/7 of the outer layer leaves the system). When this correction was applied to “heat pipe” effect then the corrected heat loss did lead to insulation values similar to dry tests. The method worked also for 1 and 2 layer systems with highest difference for 1 layer system. The method could be tested more accurately on a sweating cylinder/torso, where layers may be separated in order to avoid wicking or vice versa set to allow it. Using different number of layers, layer thickness and less hygroscopic materials than wool may improve estimation of the “heat pipe” effects