Spectrally resolved radiative properties of roofing materials for urban heat mitigation and building energy assessment

Urban heat mitigation strategies increasingly rely on building materials with favorable radiative properties. This study investigates the spectral radiative behavior of common roofing materials over the 0.25–20 μm wavelength range and assesses its implications for urban cooling and climate resilience. Spectral hemispherical reflectance and emissivity were experimentally characterized across the UV–Vis–NIR and mid-infrared regions. The results reveal a pronounced aging-induced transition in radiative performance. While new zinc exhibits high solar reflectance and very low mid-infrared emissivity, aged zinc develops high emissivity accompanied by reduced solar reflectance. Measured emissivity increases from 0.03 for new zinc to 0.92 for 150-year-old zinc. Radiative cooling power calculations highlight a clear performance trade-off: aging enhances nighttime radiative cooling through increased emission within the atmospheric window (8–13 μm), while simultaneously increasing daytime solar heat gains. The strong nocturnal radiative cooling potential of aged zinc suggests a capacity to mitigate nighttime heat accumulation at the building scale. Building energy simulations of a warehouse building in Paris further demonstrate that conventional constant-emissivity assumptions introduce substantial material-dependent errors. Compared with simulations using measured spectral properties, cooling demand is overestimated for aged zinc and underestimated for new zinc by up to 25.8% and 86.1%, respectively. The results highlight the critical role of material aging in shaping the long-term effectiveness of passive radiative cooling strategies and demonstrate the need for spectrally resolved radiative properties in building energy and urban climate assessments.