Incorporating radiative cooling photonic structures into the cooling systems of buildings presents a novel strategy to mitigate global warming and boost global carbon neutrality. Photonic structures with excellent solar reflection and thermal emission can be obtained by a rational combination of different materials. The current preparation strategies of radiative cooling materials are dominated by doping inorganic micro-nano particles into polymers, which usually possess insufficient solar reflectance. Here, a porous polymer metafoam was prepared with polycarbonate (PC) and polydimethylsiloxane (PDMS) using a simple thermally induced phase separation method. The metafoam exhibits strong solar reflectivity (97%), superior thermal emissivity (91%), and low thermal conductivity (46 mW m-1 K-1) due to the controllable morphology of the randomly dispersed light-scattering air voids. Cooling tests demonstrate that the metafoam could reduce the average temperature by 5.2 °C and 10.2 °C during the daytime and nighttime, respectively. In addition, the simulation of a cooling energy system of buildings indicates that the metafoam can save 3.2-26.7 MJ m-2 per year in different cities, which is an energy-saving percentage of 14.7-41%. The excellent comprehensive performances, including the passive cooling property, thermal insulation and self-cleaning of the metafoam makes it appropriate for practical outdoor applications, exhibiting its great potential as an energy-saving building cooling material.