This study focuses on the optimization of the thermal insulation thickness applied to external walls of a building. Different from other studies in the literature, taking into account the incident solar radiation on a wall, the variation of optimum thermal insulation thickness in respect to the wall orientation has been investigated. By using the measured average air temperatures and the calculated solar radiation flux, the optimum insulation thicknesses have been determined for different orientations of a building wall (east, west, north, south). The life cycle cost analysis, which considers lifetime, discount and inflation rates, and based on the degree-days (DD) has been used for the optimization. As a result, while the total DDs based on only the outside air temperature data is 1827, the total DDs considering solar radiation are 1633, 1628, 1719 and 1535 for east-, west-, north- and south-facing vertical surfaces, respectively. Accordingly, the optimum insulation thicknesses for these surfaces have been calculated as 4.8 cm, 4.8 cm, 5.2 cm and 4.3 cm respectively.