Which property combination best describes cooler roof surfaces under the same conditions?

• A. Higher emissivity and higher reflectivity
• B. Higher emissivity and lower reflectivity
• C. Lower emissivity and higher reflectivity
• D. Lower emissivity and lower reflectivity

Answer: (C)

Cool roof performance hinges on energy balance: how much solar energy is absorbed and how much heat the surface can radiate away. Higher reflectivity means the roof reflects more of the sun’s rays, so it absorbs less energy and stays cooler under the same conditions. Higher emissivity means the roof can emit infrared radiation more efficiently, letting heat escape to the sky, which also helps keep the surface cooler. Put together, a surface that is both highly reflective and highly emissive minimizes heat gain and maximizes heat loss, leading to the coolest roof. If either property is reduced—lower reflectivity, more solar energy absorbed, or lower emissivity, less heat radiated—the roof tends to become warmer under the same conditions.