Prediction of clothing thermal insulation and moisture vapour resistance

Abstract

Clothing thermal insulation and moisture vapor resistance are two most important clothing properties with respect to thermal comfort. The accurate determination of these two clothing properties is crucial to the selection of clothing for different end uses, functional clothing design and thermal environmental engineering. Although these two properties can be measured by tests on human subjects of by using sweating manikins, it is highly desirable to predict them not only because of the variability, cost and danger in using human subjects for measurements and the scarceness of sweating manikins, but also because of the fact that it is practically impossible to measure them for endless clothing ensembles under the different body motions and various environmental conditions. For the establishment of prediction models for clothing thermal insulation and moisture vapour resistance, it is necessary to first acquire experimental data under various body motions and environmental conditions. With a newly constructed climate chamber of variable temperature, humidity and wind velocity and an improved sweating fabric manikin-Walter with patent pending innovations in terms of the simulation of "walking" motion and real-time water loss measurement, experiments were conducted for the nude manikin and for the manikin wearing 32 sets of different clothing ensembles under various "walking" speeds and environmental conditions. With the data of surface thermal insulation and moisture vapour resistance of the nude manikin, simultaneously measured for the first time under various conditions, the present study showed that there is no significant difference between the surface thermal insulation measured on the non-sweating manikin and those measured on the sweating manikin, indicating the moisture transfer having little effect on the direct heat transfer through the surface air layer; the surface moisture vapour resistances measured under isothermal conditions tend to be greater than those measured under non-isothermal conditions, likely due to the increase of surface air layer with the absence of the temperature gradients; Lewis relation holds under non-isothermal conditions. The present experimental investigation further showed that the clothing moisture vapour resistance measured under the non-isothermal condition is about 17~32% smaller than that measured under the isothermal condition, possibly caused by the moisture absorption and condensation within clothing and the increased temperature gradients under the non-isothermal condition. Based on improved understanding of the effects of wind and walking motion on the heat and moisture transfer through clothing, the present study proposed two new models, the direct regression model and the quasi-physical model, for the prediction of the dynamic clothing thermal insulation and moisture vapour resistance under windy conditions and walking motion from the static values when a clothed person is standing in "still" air. The models can take into account the effects of clothing characteristics through the model parameters. The direct regression model is very simple and effective, but the quasi-physical model has the advantage of incorporating the fundamental mechanisms of heat and moisture transfer. Comparison of the two new models with the published existing models, by applying the models to fit experimental data from both the present investigation and published sources, showed that the prediction accuracy of the direct regression model is very high in most instances, but the quasi-physical model provided the best prediction accuracy. The present study is significant in providing a novel instrumental technique for the measurement of thermal comfort properties of clothing under simulated "walking" motion; in providing an improved understanding of the effects of wind and body motion on clothing thermal comfort and in establishing accurate models for the prediction of clothing thermal insulation and moisture vapour resistance, which have important applications in thermal environmental engineering, functional clothing design and selection of clothing for different end uses.