The aim of this study is to determine the optimum heat insulation thickness for the glasswool and rockwool insulation material. Since natural gas is mostly used for heating in Turkey, it has been selected as fuel for the calculation. In order to calculate the optimum thickness of the insulation, the number of the degree-day and total environmental factor have been used. For the optimum insulation thickness, the decrease in exergy loss, CO2 emission and the fuel consumption were 75%, 73%, and 71% for the glasswool, respectively. On the other hand, for the rockwool, they were 35%, 18%, and 43%, respectively. Optimum thickness was calculated as 0.40 m for glasswool, and 0.18 for the rockwool. [ABSTRACT FROM AUTHOR]