In recent decades, the increasing frequency and intensity of heatwaves have posed serious challenges to climate-responsive architectural design and building energy management, particularly in arid and semi-arid regions of northwestern Iran. In this context, the present study introduces a novel composite index, the Composite Cooling Degree-Day (CCDI), aimed at providing a more accurate assessment of building cooling demand under extreme thermal stress and offering a practical tool for architectural and climate-informed decision-making. The CCDI is developed by integrating multi-variable climatic data, including daily mean temperature, relative humidity, solar radiation, and wind speed, extracted from the ECMWF database for the period 1985–2025. The index is computed using a weighting scheme based on Principal Component Analysis (PCA). Heatwaves were identified using the 90th percentile of daily temperature, and their intensity and duration were incorporated into the index formulation. Unlike the conventional Cooling Degree Days (CDD) index, which relies solely on temperature, CCDI captures the combined influence of key climatic variables affecting thermal comfort and building energy performance. Spatial analysis results indicate that the highest cooling demand is concentrated in the southwestern and southeastern parts of the study area. In some regions, the difference between CCDI and CDD exceeds 5,000 degree-days, highlighting the greater sensitivity of the proposed index to extreme thermal conditions. Furthermore, SHAP-based feature importance analysis revealed that daily temperature and heatwave intensity are the dominant drivers of cooling demand. This finding has important implications for building envelope design, orientation strategies, natural ventilation optimization, and the selection of climate-adaptive construction materials.

Overall, the CCDI provides a robust and practical framework for sustainable architectural design, urban planning, and building energy management, contributing to enhanced resilience against heatwaves and improved climate adaptation strategies.