In tropical climates like India, solar radiation is the major source of heat gain through roofs as they are exposed to the sun throughout the day. As the ambient conditions are dynamic, it is important to look at periodic variation of heat flow through roofs. In this context, apart from U-values that the Energy Conservation Building Code of India (ECBC-2007) prescribes for envelopes, more specific performance indicators should be considered for designing energy efficient buildings. The study presented in this paper evaluates thermal performance of Cement Tile as roofing material and compares it with the performance of two other typical roof insulation materials monitored in hot and dry climate of India. The results are supported with physical structures using Scanning Electron Microscope (SEM) images and energy simulations. Despite having higher overall U-value (over six times), Model 1 (with Cement Tile) shows better thermal performance and negligible energy savings than Model 2 (with extruded polystyrene as insulation material) and Model 3 (with POP false ceiling). Therefore, the overall U-values calculated from the layer wise thermal conductivity along with Discomfort Degree Hour, Building Index, and Average heat flux would provide a better understanding of thermal performance of the roof as was demonstrated by our experiments.

Existing regulations on the thermal efficiency of building envelope assemblies are based on the steady state thermal properties of substrate materials. Heat transfer mechanisms of passive heat curbing methods such as phase change materials and cool materials, which are dynamic in nature, are currently not being accounted for. The effectiveness of thermo-physical and solar radiation properties of building materials (i.e., solid homogeneous layers without air gap) in reducing the heat gain into a building in a hot climate could be well understood with the equivalent thermal resistance (R_eq) concept. A simple and easy-to-use mathematical derivation (i.e., to estimate the instantaneous heat flux across an envelope assembly) is proposed in this paper to understand the mechanism of equivalent R-value (i.e., reciprocal of thermal transmittance, U-value) due to solar radiation properties of passive substrate materials. The model is validated against field experiments carried out at two apartment units of a residential building. The R_eq due to high outer surface solar radiation properties (i.e., by applying a cool coating) is dynamic as it varies with the weather conditions. The effect of a substrate material’s solar radiation and thermo-physical properties on the overall roof thermal performance is investigated using the R_eq model for four cooling dominated climates around the globe, having different diurnal conditions and sky temperatures. Increasing the outer surface’s solar reflectance (from 10% to 80%) reduces net heat gain through the flat roof during both daytime and nighttime. In contrast, adding only thermal resistance (from 5 mm to 75 mm thick polyurethane) or volumetric heat capacity (by adding 5 mm thick phase change material) to the building envelope brings down heat gain during the day, but not in the night. Thermal insulation is found to be the second effective property, followed by thermal mass irrespective of different diurnal conditions and sky temperatures across the climates.