What Do You Get When You Cross Solar Reflectance With Emissivity?
You get The Solar Reflectance Index for your roofing materials, and by reducing that number you can take a giant step for man toward a cooler roof and a greener solution to putting a roof over your head!
To obtain this value you begin by testing the Solar Reflectance. Solar Reflectance is a test performed on the surface properties of a roof. This reflectance of the solar energy from the roof is the fraction of the radiation produced by the sun and reflected off of the surface of the roof. The more reflective the roof is the “cooler” the roof remains. This means that less of the heat is absorbed by the roof. These reflective properties help reduce the amount of air conditioning the building needs to keep it cool. It also helps alleviate the Urban Heat Island effect, thus diminishing power consumption.
Alleviating the Urban Heat Island effect produces several benefits for the environment. It reduces peak electricity demand. This helps prevent blackouts and over consumption of electricity saving it for fun things like television and those addictive video games. It lowers electricity bills, and we all can appreciate that. It also lowers air pollution from the power plants in the form of sulfur dioxide and carbon dioxide. These atmospheric pollutants create acid rain that destroys forests, and automobiles to name a few. This also improves the air quality by keeping the pollutants out of the air and lowering the ambient air temperature.
So, how do you test for Solar Reflectance? The first thing that you need is a similar source of light to the sun in wavelength. Testing can be accomplished indoors and out.
If you perform emissivity testing outdoors, the testing must be performed with no cloud cover during the time of the day when the sun is at its highest point. This occurs between 10:00 a.m. and 2:00 pm and is performed with an instrument called a Pyranometer.
If testing is done inside this piece of equipment referred to as a solar reflectometer which measures the wavelength of the electromagnetic waves that are reflected back after being bounced from the material being tested. This device consists of a conical, aluminum encased, light source that has a 1 inch opening in the top. Then it has an opposing flat black top that fits down over the cone shaped light source. Why black you ask? It is flat black because all surfaces emit temperature dependent radiation. The black top will absorb all of the light emitted so that the device can be zeroed before use. Once zeroed the instrument measures the amount of light from the sun source that is reflected back when bounced off of the surface of the material being tested.
Once you have your solar reflectance you then need to test for Emissivity. Emissivity is considered to be the ability of a material to emit energy in the form of radiation that has been absorbed by a material. These measurements are taken for wavelengths in the infrared range.
A high value means that your material will absorb energy from the sun and radiate it back into the atmosphere more readily than a material with a low Emittance. (A non-adsorbing surface would have an emittance of zero and a surface that adsorbed all of the energy shined on it would have an emittance of 1.0.) A high emissivity surface aids your home in staying cooler and reducing your power bills.
The testing is relatively easy. You use an Emisometers and sandwich the sample between the 2.25 inch aperture and a heat sink. The instrument does the rest by reporting out the Emissivity of the material.
Once you have these two values you can plop the numbers into a formula and obtain the solar reflectance index of the material. This value is used extensively by Energy Star in providing a reference value for roofing materials. This system is also used by the Cool Roof Rating Council, Green Globes, and LEED to apply green credits to roofing materials.