A new thermal performance index for dwelling roofs in the warm humid tropics

This research aims to find a scientific rating scheme for roof system for the warm humid tropics. The overall air-to-air thermal transmittance (U-value) or thermal resistance (R-value) of roof are most widely used properties, but these are based on steady-state heat conduction and do not respond to changing conditions of climate, design and comfort needs. The paper presents a new thermal performance index, which is based on thermal comfort and actual thermal performance of a roof design option. It provides a basis for identifying the optimum roof design for unconditioned and acclimatised regimes. A review of the acceptable limits of ceiling surface temperature is presented. The mathematical modelling for predicting performance of roof based on admittance procedure and derivation of thermal performance index is described. In the proposed scheme the galvanized iron roof is given 0% rating and the roof satisfying the comfort needs for the given climate is rated as 100%. Illustrative examples of built roofs or design options are taken from India and Australia. It clearly shows that the roofs with the highest thermal resistance are not necessarily optimum solutions.