Assessment of Insulation Thickness and Window Thermal Transmittance Effects on Peak Heating and Cooling Loads in Residential Buildings

Enhancement of building envelope components is a primary strategy for mitigating thermal loads; however, the relative contribution of insulation thickness and window thermal transmittance to peak heating and cooling demand in hot climates has not been sufficiently quantified. This study investigates the sensitivity of peak heating and cooling loads to variations in wall insulation thickness and window U-value for a reference residential building located in southeastern Türkiye. A parametric framework was established by considering five insulation thickness levels (0–10 cm) and three window thermal transmittance values, generating fifteen simulation scenarios under identical occupancy schedules and thermostat setpoints. Peak heating and cooling loads were determined for each configuration to evaluate the influence of envelope modifications on maximum heating and cooling system capacity requirements. The results indicate a substantial reduction in peak heating load, reaching approximately 46% between the baseline (uninsulated, high U-value) case and the best-performing configuration. In contrast, peak cooling load exhibited a comparatively moderate reduction of about 20% under the same conditions. Furthermore, the incremental benefit of increasing insulation thickness diminished beyond 8 cm, particularly for cooling demand. Although reductions in window thermal transmittance contributed to overall performance improvement, their effect was more pronounced when combined with higher insulation levels. The findings demonstrate that, under hot climate conditions, envelope improvements exert a significantly greater impact on peak heating demand than on peak cooling demand, offering quantitative guidance for residential heating and cooling system sizing.