The thermal performance of any structure depends on the thermal characteristics of the structural material. Thus, it is crucial to study the behavior of that material for thermal resistance and thermal insulation. The performance of that material is governed by its thermal conductivity, which describes the ability of the material to allow the flow of heat across it in the presence of differential temperature. Thermal variations are highly hazardous from the point of view of durability of the structure and needs more attention to improve the structural properties. It has been proved that thermocol (Polystyrene foam) has good insulating properties and also has a low thermal conductivity and therefore a good thermal insulating material. In this study, experimental tests are conducted on concrete slab with and without using thermocol for flexure and as an insulating material in R.C. slab for studying the behavior of slab for thermal insulation. Concluding from the results of thermal conductivity it is proved that the thermocol is a good insulating material and the slab is good in thermal insulation compared to RCC slab. © 2022, The Author(s), under exclusive license to Springer Nature Switzerland AG.