Innovative optimization of insulation thickness: Integrating solar‐air temperature and advanced heating demand analysis for sustainable building design

Abstract This study aims to optimize the insulation thickness of external walls for energy‐efficient and cost‐effective building design under the climatic conditions of Muğla, Türkiye. The proposed approach integrates heating degree hours (HDH), solar‐air temperature, and life‐cycle cost analysis to provide a more realistic assessment of heating demand than conventional ambient‐temperature‐based methods. Unlike traditional formulations, the method distinguishes free heating potential (HDH free ) from residual mechanical heating demand (HDH mech ) by accounting for solar radiation and radiative cooling effects. The results indicate optimal insulation thicknesses of 2.8, 3.4, and 3.9 cm, depending on the heating scenario, with heating cost reductions of 15%–20% and a maximum annual saving of 3.24 $/m 2 . HDH mech is highlighted as the most accurate method for determining insulation needs in regions with high mechanical heating demands, while HDH free is ideal for leveraging passive solar heating in high‐irradiance areas, reducing mechanical heating requirements by 30%. The findings further show that increasing radiative cooling differential (ΔT) intensifies mechanical heating demand while reducing the contribution of passive solar gains. The minimum payback period was found to be 3.4 years under the mechanical‐heating‐based case. As a result, the proposed framework improves both thermal and economic decision‐making for sustainable insulation design in climates with variable solar and radiative conditions.