1) High Solar Reflectance: Reflectance measures how much sunlight - visible, infrared, and ultraviolet - is reflected. The higher the solar reflective value, the more efficient the cool roof is in reflecting sunlight and heat away from the building, reducing the roof temperature.
A cool roof is one that strongly reflects sunlight and also cools itself by efficiently emitting radiation to its surroundings. The roof stays cooler and reduces the amount of heat conducted to the building. Because the roof is exposed to the sun all day long, the whole assembly quickly gets saturated with heat and continues to transfer heat inside, even during the night. Heat gain through the roof is the largest single load that the heating and air conditioning (HVAC) system has to counteract.
Terms to remember
Two key terms to remember when considering cool roofs are solar reflectance and emissivity. Always aim for a high solar reflectance and a low emissivity for the materials you use for your cool roof. A further explanation of these terms can be found below.
2) Low Emissivity: All materials absorb and release heat - the amount of heat that is released is measured as emissivity. Emissivity is measured as a coefficient between 0 and 1, or as a percentage of heat that is absorbed by a material and not released.
You can get a good idea of the emissivity of a material by gauging the temperature when it has been left in the sun. Compare the black and chrome surfaces of your car when it is left in the sun. The black surfaces get hot in the sun; however, they do not stay hot to the touch as long - they absorb a lot of heat but do not release it. This means that black surfaces have a high emissivity. In contrast, chrome surfaces also get hot in the sun, but they stay hot much longer because they release a lot of heat. Because chrome releases more heat, it has a lower emissivity.
What can you do about it?
Add cool roof coating.
Cool roof coatings are white or special reflective pigments that reflect sunlight. These coatings are like very thick paints that can protect the roof surface from ultra-violet (UV) light and chemical damage, and some offer water protection and restorative features. Products are available for most roof types.