Roof Retrofit Scenarios for Thermally Vulnerable Homes: A CFD-Based Study of Indoor Thermal Performance in San Antonio, Texas

In the hot and humid climate of San Antonio, Texas, residential buildings consume approximately 43 percent of the city's electricity. Yet older, heat-vulnerable homes, particularly in lower-income neighborhoods, remain largely underserved in energy efficiency efforts, as local government weatherization programs mainly provide standardized solutions that require the building envelope to be in good condition. Roofs are the most exposed elements in the envelope and strongly influence heat gain and indoor conditions. This study assesses the thermal performance of roof retrofit scenarios in an older, detached single-family home representative of this underserved housing stock. We employed a transient Computational Fluid Dynamics (CFD) model developed in ANSYS Fluent to simulate unsteady indoor airflow using the Stress-Blended Eddy Simulation (SBES) turbulence model and the P1 radiation approach for radiative heat exchange. Four roof-albedo scenarios (a = 0.20, 0.40, 0.60, 0.80) were evaluated under typical summer conditions representative of passive ventilation periods. Model validation against on-site sensor data achieved coefficients of determination of 0.90 for air temperature and 0.89 for velocity, confirming strong predictive accuracy. Increasing roof albedo from 0.20 to 0.80 reduced roof heat absorption by approximately 75%, decreased conductive heat transfer by 60%, and lowered indoor peak temperature by 0.8oC, shortening exposure above 32oC by roughly 20%. Complementary drone thermal imagery collected using DJI Matrice 350 RTK corroborated substantial surface-temperature reductions between dark and reflective roofs transitions funded by the City of San Antonio's Cool Roof Program. The findings demonstrate that high-albedo cool roofs represent a cost-effective and scalable retrofit strategy to mitigate indoor overheating and energy burden in ASHRAE Climate Zone 2A, advancing equitable climate adaptation and housing resilience.