In the present work, degree day's method is used to get the optimum insulation thickness for different insulation materials. Some of the commonly used insulation materials available in the market are considered in the study. The study analyses the effect of CDD and base temperature on insulation thickness. The CDD was calculated from the base temperature that varies from 18°C to 26°C for the four cities of India. Based on the result, it is found that optimum insulation thickness is affected by the thermal conductivity of the material, base temperature, CDD, material cost, and fuel cost. The optimum insulation thicknesses vary between 1 and 12 centimetres for the different base temperatures. Annual cooling cost per m2 is found minimum at 24°C base temperature, and energy-saving varies with the thickness of the insulating materials apply on the wall.