Cool roofs combine high solar reflectance with high infrared emittance, thereby simultaneously reducing heat gain from the sun and increasing heat removal through radiation. Radiative cooling thus offers potential for passive cooling for residential and commercial buildings. Traditional building surfaces, such as paint coatings, brick and concrete have high emittances of up to 0.96. They radiate heat into the sky to passively cool buildings at night. If made sufficiently reflective to sunlight, these materials can also achieve radiative cooling during the day.