In this study, the optimum insulation thickness was calculated for the heating season for external walls in the different directions of a building. For this reason, a building used for housing in Istanbul, Turkey was taken as model. The indoor and outdoor temperatures, along with the interior and exterior surface temperatures of the building’s external walls, were continuously measured using thermocouples and recorded in four different directions throughout the year. The effects of solar radiation, which vary based on the direction, were assessed for the heat transfer through the external walls. The results of this study indicate that the optimum insulation thickness for the north, south, west, and east facing walls should be 6.47, 2.87, 6.97, and 6.98 cm, respectively, based on the differences in the amount of solar radiation exposure of the walls in the different directions. The optimum insulation thickness of the building’s external wall was calculated as 5.25 cm, regardless of its direction. An economic analysis of the thermal insulation cost was conducted using the P1-P2 method, and then the payback periods were calculated. The heating energy consumption of the building designed using the optimum insulation thicknesses, as identified separately based on the direction, decreased by 17%, compared to the present building with 3 cm of thermal insulation.