A large part of energy cost on cooling applications is attributable to heat income through external walls. The best way to save energy is to use the optimum insulation thickness. In this study, the optimum insulation thicknesses, insulation-energy costs, and savings resulting from the use of these insulations is calculated in cities selected from four different climate zones in Turkey for four various insulation materials such as foamboard, extruded polystyrene (XPS), rock wool, and expanded polystyrene (EPS) in cold storage applications. Calculations show that rock wool insulation material yields the highest savings. Optimum insulation thicknesses vary between 0.074 and 0.159 m, energy savings vary between 30.59 and 80.61 $/m(2), and payback periods for investment cost vary between 3.15 and 3.73 years for the cooling. DOI: 10.1061/(ASCE)EY.1943-7897.0000085. (C) 2013 American Society of Civil Engineers.