© 2017 TIBTD Printed in Turkey.In this study we proposed a new modified New Degree-Day Method (NDDM) for the optimization of insulation thickness of the wall where the radiant panels are mounted (WMRP) in which heat generation inside the wall is considered. The existing Standard Degree-Day Method (SDDM) is not applicable to estimate the optimum insulation thickness for the buildings where the WMRP is mounted. Because SDDM method uses indoor air temperature as a base temperature, hence heat generation through the WMRP cannot be taken into account. In the new method, important parameters were obtained from the series of the CFD analysis for different thermal transmittance coefficient (U) and outdoor air temperature (To) values are used to create an empirical equation for the estimation of Tp (new base temperature) with the multiple polynomial regression method. Then the numerical results were validated with experimental results which were obtained from the real-size test chamber. Using the new method optimum insulation thickness, net energy saving and payback periods for radiant wall heating systems were calculated (for Istanbul climate) and compared with the results which were obtained using the standard degree-day method (SDDM). The results showed that, the SDDM significantly lower (85-95%) estimates the optimum insulation thickness and can't be used for the buildings where the WMRP is used. The new method can be used for radiant wall heating systems where the performance of radiant heating systems is significantly affected by the insulation capabilities and has a great importance in the sizing process of the radiant systems.