Clothing evaporative heat resistance--proposal for improved representation in standards and models

Clothing heat and vapour resistance are important inputs for standards and models dealing with thermal comfort, heat- and cold-stress. A vast database of static clothing heat resistance values is available, and this was recently expanded with correction equations to account for effects of movement and wind on the static value of heat resistance in order to obtain the dynamic heat resistance of clothing ensembles. For clothing vapour resistance, few data were available so far. Indices for vapour permeability (im) and reduction factors for vapour transfer (Fpcl) of clothing were used instead, using a relation between heat and vapour resistance to derive the clothing vapour resistance from the value for clothing heat resistance. This paper reviews the two commonly used approaches (im and Fpcl), as well as five alternative approaches to the problem. The different approaches were evaluated for their accuracy and their usability. The present paper shows that the currently used relations are not adequate when the wearer of the clothing starts moving, or is exposed to wind. Alternative approaches are shown to improve the determination of dynamic clothing vapour resistance, though some are thought to be too complex. An empirical description of the relation between the clothing permeability index (im) and the changes in clothing heat resistance due to wind and movement was selected as the most promising method for deriving clothing vapour resistance. For this method the user needs to know the static heat resistance, the static im value of the clothing and the wind- and movement-speed of the wearer. This method results in a predicted maximal decrease in clothing vapour resistance by 78%, when clothing heat resistance is reduced by 50%, which is consistent with theoretical expectations and available data.