City-scale rooftop retrofit prioritization: linking thermal performance, energy impacts, urban morphology, and population exposure

Urban rooftops strongly influence heat exchange and cooling demand, yet most studies oversimplify energy fluxes, neglect district morphology, or lack decision-ready metrics. This study develops a city-scale rooftop modeling framework to evaluate six roof types—concrete, asphalt, cool coatings, ceramic tiles, radiative surfaces, and green roofs—across Hong Kong. The framework captures the full surface energy balance, quantifies morphology effects, and translates rooftop physics into citywide outcomes on energy, emissions, costs, and retrofit priorities. High-resolution LiDAR and building footprint data were integrated with ArcGIS solar radiation modeling to resolve district-specific shading and exposure. Results show clear contrasts: asphalt drives persistent overheating, while radiative and cool roofs sustain near-ambient states and achieve passive cooling. If all rooftops adopted radiative cooling, compared with current conditions, citywide cooling electricity demand would fall by 11.8 %, imported electricity by 13.5 %, cooling costs by 11.8 %, and total CO₂ emissions by 1.7 %. The Composite Retrofit Priority Index reveals two priorities: suburban districts with extensive rooftop areas achieve the largest energy and carbon reductions, while dense urban cores deliver the greatest equity benefits by reaching large populations. Together, these insights provide clear guidance for climate-resilient and socially equitable retrofit planning.