Performance evaluation of mean radiant temperature calculated from inner surface temperatures of envelope with various emissivities

Mean radiant temperature is a predominant parameter for thermal comfort prediction. The inner surface temperature based method, as an indirect method for obtaining the mean radiant temperature, is recommended by the international standard and used as the benchmark to calibrate the mean radiant temperature obtained from the direct methods. Given the energy-saving potential of low-emissive coatings on the inner surfaces of the envelope, this study evaluates the applicability of the inner surface temperature based method to indoor environments with various emissivities. Theoretical analyses show that the inner surface temperature based method does not strictly conform to the definition of the mean radiant temperature by simplifying the calculation of the radiant heat exchanger between the human body and its surroundings, which could fail in low-emissive indoor environments. Furthermore, case studies show that under both winter and summer scenarios, the inner surface temperature based method cannot accurately capture the radiant heat exchange between the human body and its surroundings. This results in that the error in thermal comfort prediction based on the inner surface temperature based method increases with the decreasing emissivity of the inner surfaces of the envelope, and exceeds the acceptable range. The critical emissivity reveals that the inapplicability of the inner surface temperature based method to low-emissive indoor environments would impede the spread of the low-emissive coatings for energy saving.