Reflective Roofs in Green Building

Roofing Materials for Green Homes 

Cool roof performance is a function of two radiative properties: Solar reflectance and infrared emittance. These two properties define the “coolness” a roofing product contributes to green building.

Solar reflectance is a decimal number less than one that represents the fraction of light reflected off the roof.  If a high reflectance material has a value of  0.70 (70%), only 30 percent of the light from the sun is retained by the roof.   

Solar reflectance is the most important characteristic of a roof product in terms of yielding the highest energy savings during warmer months. The higher the solar reflective value, the more efficient the product is in reflecting sunlight and heat away from the building. This reduces the temperature of the roof.

Think of the effects of dark versus light clothing on a sunny day. Because dark colors reflect less solar energy, dark is hot and light is cool. The same principles apply to roof temperatures. Light-colored roof materials make for a cool roof by reflecting more of the sun's rays.

Reflective roof coatings act like mirrors, reflecting the sun's radiant energy back into space. These reflective roof coatings, also known as albedo (a fancy word for reflectivity) coatings, are products applied to roofs to reduce solar heat.

Infrared emittance is a decimal number less than one that represents the fraction of heat that is re-radiated from a material to it’s surroundings.  The emittance of a material refers to its ability to release absorbed heat.  One example is a metal tool left in the sun. It is hot to the touch because it has a low emissivity value.

Emissivity can also contribute to a cool roof in warm, sunny climates like Dallas-Fort Worth. Roofing products with high emissivity can help reduce the cooling load on the building by releasing the remaining heat absorbed from the sun.

For buildings dominated by cooling loads, it makes sense to provide roofing products with high reflectivity and high emissivity. These products help reduce cooling loads because the roof is exposed to the sun for the entire operating day. 

Caveat: Because roof coatings tend to lose their reflectance properties over time due to dust and dew, regular cleanings and even reapplications are needed.