The drive for environmentally-friendly products extends to the commercial construction arena, including roofing systems. In this post, we will discuss some of the energy aspects of roofing: reflectivity, emissivity, and the solar reflectance index.
Environmental Impact – Part B
Rooftop reflectivity has received substantial attention during 2009, perhaps most notably in Energy Secretary Steven Chu’s remarks recommending that the world’s roofs be painted white. The upshot is that a roofing material’s ability to reflect solar energy has environmental implications, including its contribution to (or mitigation of) the urban heat island effect and impact on emerging carbon markets, among others. In fact, a roofing system’s energy efficiency is really determined by a couple of interrelated attributes.
Reflectivity (technically, albedo) is the percentage of solar energy reflected by a surface. The higher the reflectance value, the more solar energy that will be reflected from the surface.
Emissivity is the measure of how effectively a material sheds the heat it has absorbed. Materials with a high emissivity value may absorb energy easily, but they also radiate large amounts of it back into the atmosphere.
Roofing systems that provide both high reflectivity and emissivity are most likely to have a significant impact on a building’s energy cost. Some systems can reduce a building’s energy consumption by up to 40%.
The solar reflectance index (SRI) is a tool that evaluates reflectivity, emissivity, and other factors to determine a roof’s overall ability to reject solar heat. Its calculation is defines by ASTM E 1980-01 and is based on a formula that includes values for solar absorptance, solar flux, emissivity, the Stefan Boltxman constant, and other coefficients.
Standard black (reflectivity 5%, emissivity 90%) has an index of 0, and standard white (reflectivity 80%, emissivity 90%) has an index of 100. Very “hot” materials can actually have negative values and very “cool” materials can have values greater than 100. Materials with the highest SRIs are the coolest choices for roofing.
The Duro-Last roofing system, a PVC membrane, has initial reflectivity and emissivity measurements of 87.5% and 95%, respectively, and a resulting SRI value of 108.
Here are the SRIs of some other roofing materials as measured by Lawrence Berkeley Labs and the Florida Solar Energy Center:
|
Roofing System |
SRI |
Reflectivity |
Emissivity |
|
White EPDM |
84 |
69% |
87% |
|
Kynar-Coated Metal, White |
82 |
67% |
85% |
|
New, Bare Galvanized Steel |
46 |
61% |
4% |
|
Light Gravel On BUR |
37 |
34% |
90% |
|
White Granular Surface Bitumen |
28 |
26% |
92% |
|
Dark Gravel |
9 |
12% |
90% |
|
Black EPDM |
-1 |
6% |
86% |
