The current study used degree-day method to determine the optimum insulation thickness for different insulation materials. Some of the commonly used insulation materials available in the market are considered in the study. Materials used in the study are, fiberglass rigid, urethane rigid, fiberglass urethane, perlite, and extruded polystyrene. The cooling degree days were calculated using the base temperature varies from 18ºC to 26ºC for the four major cities like Mumbai, New Delhi, Kolkata, and Chennai of India. The study aims to analyze the effect of the number of cooling degree days and base temperature on insulation thickness and also determine the variation of annual cooling cost with the insulation thickness. The result shows that the optimum insulation thickness varies with the cooling degree days and is also influenced by the electricity rate and the cost of insulation material. 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 result shows best suitable insulation materials for Delhi, Mumbai, Kolkata, and Chennai are urethane rigid, fiberglass urethane, urethane rigid, and fiberglass rigid respectively. The optimum insulation thicknesses vary between 1 and 12 cm for the different base temperatures and cities. Annual cooling cost per m2 is calculated for the base temperature 21 ºC and 24 ºC, and the result shows that fiberglass urethane has the lowest annual cooling cost for different cities. Energy-saving varies with the thickness of the insulating materials apply on the wall. In addition with the help of insulation, energy can be saved up to 80% and achieve energy-efficient buildings.