Buildings consume more than a third of the world's energy demand, and painstaking adjusting the insulation thickness of external walls is an effective way to save energy. Thermal insulation plays an important role by reducing the heat transfer rate to save energy, and it is important to determine the thickness of the appropriate insulation material in the walls. In this research, considering only the heating factor, four different building materials (Pumice, Hollow concrete block - HCB, Hollow red clay block - HRCB and Reinforced concrete - RC) and eight different insulation materials (Extruded polystyrene - XPS, Expanded polystyrene - EPS, Expanded polyurethane - PUR, Wood fiber - WF, Hemp fiber - HF, Linen fiber - LF, Fiber glass - FG and Sheep wool - SW) are investigated on the exterior of a building in Sivas province of Türkiye. To reach more accurate results, life cycle cost analysis is taken as a basis, and the optimal insulation thicknesses, payback periods and energy savings of 32 different wall + insulation combinations with solar radiation (SR) are estimated using heating degree-hours. The heating requirement of a building covers more than three quarters of the year in Sivas, depending on different facing walls. These rates are calculated as 85 % (without SR), 80 % (North), 78 % (West), 77 % (East) and 75 % (South). Results also showed that the total minimum heating cost, the total maximum energy saving cost and the shorthest payback period are obtained HRCB + FG + South with 14.19 €/m2, RC + FG + solar radiation free with 72.76 €/m2 and RC + LF + solar radiation free with 1.60 year.