In recent years, insulation is an important issue for reduction of energy consumption, in particular residential sectors. Present study deals with thermal performance and optimum insulation thickness of building including external walls, ceiling, floor and widows with various insulations under climatic conditions of Tehran, Iran. Rock wool, polystyrene and autoclaved aerated concrete (AAC) blocks as insulation material were selected. In addition, BCS19 simulation software, which designed by administrators of Iranian building codes, was used for determination of declining in energy consumption. Results showed that reduction in energy consumption for rock wool, polystyrene and AAC blocks insulations were 47, 46 and 35%, respectively. Also, optimum thicknesses of rock wool and polystyrene insulations were determined 60 and 30 mm for walls, 80 and 40 mm for roof and 40 and 30 mm for floor, respectively.