Daytime radiative cooling technology can cool objects to sub-ambient temperatures under direct sunlight without energy consumption. The technology relies on high reflectance of solar irradiation and high emissivity in the atmospheric window (infrared emission with 8 to 13 μm wavelengths). We report a bilayer structured coating for passive daytime sub-ambient radiative cooling. The bilayer radiative cooling coating has high solar reflectance (0.94), and high infrared emissivity (0.96) in the atmospheric window. The bilayer coating achieved a sub-ambient temperature of 3.6°C under solar irradiance of 990  W  /  m² at an ambient temperature of 26.6°C, and the averaged sub-ambient cooling temperature of ∼8  °  C during the night. A test with two model rooms shows that the indoor air temperature reached a maximum difference of 9.7°C between the one with the bilayer coating and that with normal white coating.