Personal factors in thermal comfort assessment: clothing properties and metabolic heat production

In the assessment of thermal comfort in buildings, the use of the Predicted Mean Vote (PMV) model is very popular. For this model, data on the climate, on clothing and on metabolic heat production are required. This paper discusses the representation and measurement of clothing parameters and metabolic rate in the PMV context. Several problems are identified and for some of these solutions are provided. For clothing insulation it was shown that effects of body motion and air movement are so big that they must be accounted for in comfort prediction models to be physically accurate. However, effects on dry heat exchange are small for stationary, light work at low air movement. Also algorithms for convective heat exchange in prediction models should be reconsidered. For evaporative heat resistance of the clothing worn, which is currently not an input factor in the PMV model, it was shown that in cases where special clothing with high vapour resistance is worn (e.g. clean-room clothing), comfort may be limited by the clothing as it will induce a high skin wettedness. Thus, for such cases clothing vapour resistance should not be neglected in the calculation of comfort using the PMV model, or the induced skin wettedness should be calculated separately. The effects on thermal comfort of reductions in vapour resistance due to air and body movements are also shown to have a substantial impact on the comfort limits in terms of skin wettedness and cannot be neglected either. For metabolic heat production it was concluded that for precise comfort assessment a precise measure of metabolic rate is needed. In order to improve metabolic rate estimation based on ISO 8996, more data and detail is needed for activities with a metabolic rate below 2 MET. Finally, it was shown that the methods for determining metabolic rate provided in ISO 8996 (typically used in comfort assessment and evaluations) do not provide sufficient accuracy to allow determination of comfort (expressed as PMV) in sufficient precision to classify buildings to within 0.3 PMV units as proposed in the upcoming revision of ISO 7730.