Nano-engineered metafilms enable radiative cooling windows for energy-efficient buildings

• A nano-engineered metafilms radiative cooling window is proposed. • The metafilm shows visible transparency over 60% and sky-window emittance over 90%. • The structure can achieve a cooling effect of up to 7.5 °C compared to normal glass. • The method provides an effective solution for building energy efficiency. Radiative cooling technology transfers heat through a transparent window in the atmosphere to cryogenic space without consuming energy. In the building sector, transparent thermo-reflective windows effectively manage solar energy and optimize indoor thermal environments by reflecting near-infrared (NIR) heat and preserving high visible (VIS) transmittance. On this basis, integrating radiative coolers with transparent heat reflective windows in passive radiative cooling (PRC) windows can achieve significant building energy savings. In this study, we designed a metafilm-based PRC window and verified it from both theoretical and experimental perspectives. This metafilm consists of a dielectric/metal/dielectric structure combined with a homogeneous polydimethylsiloxane layer. The structural parameters were optimized using a particle swarm optimization method to determine the optimal configuration for efficient radiative cooling. It achieves over 60 % transparency in the VIS light spectrum, the NIR transmittance is as low as 13 %, and an average emissivity greater than 90 % in the atmospheric window. Experimental results show that under direct sunlight conditions, compared to regular glass, the metafilm PRC window can significantly lower the daytime indoor ambient temperature, with a maximum cooling effect of up to 7.5 °C. This PRC window design provides an effective solution for building energy efficiency.