A 'Cooling Balm' for Humid Cities: Cement-Based Integrated Cooling Paint for Passive Radiative and Evaporative Cooling
Passive radiative cooling dissipates heat through the atmospheric transparency window (8–13 μm) into cold outer space, offering energy-free building cooling. However, its performance degrades substantially in humid environments; for instance, in Singapore, where average relative humidity is ~80%, achievable cooling power can be as low as ~20 W/m², far below the theoretical maximum of ~150 W/m² under dry conditions. Restricted sky view factor on building facades further curtails efficiency. Evaporative cooling, leveraging water's high latent heat of vaporization (~2256 J/g), provides an omnidirectional heat dissipation pathway but porous materials like hydrogels suffer from swelling, poor adhesion, and structural degradation. Here, we report a cement-based integrated cooling paint (CCP) that synergistically combines radiative and evaporative cooling. The paint utilizes a calcium silicate hydrate (C-S-H) porous network matrix with barium sulfate nanoparticles, polyvinyl alcohol (PVA), and lithium chloride (LiCl). The optimized formulation (CCP-30) achieves high solar reflectance of ~93% in the dry state and maintains ~89% reflectance when wetted. Its high emissivity (~95%) within the atmospheric window ensures efficient radiative heat dissipation. PVA and LiCl inhibit plastic shrinkage and promote continued hydration, yielding a denser, robust microstructure. The interconnected porous structure and hygroscopic components enable passive water capture from rainfall and ambient moisture, driving sustained evaporative cooling. Field tests in Singapore showed a ~5°C lower surface temperature on CCP-coated facades compared to commercial radiative cooling paint, and an ~8°C reduction on a proximate black absorber, indicating mitigation of local heat island effects. Building energy simulations indicated 30–40% more savings in air conditioning electricity consumption. The paint is prepared via a simple one-pot method compatible with standard production, indicating excellent commercialization potential.