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FIELD TECHNIQUE: Reflectivity and Emissivity

   Author: MetalMag

THERE is a big focus today on energy and going green. We all hear about things like energy savings, cool roofing and recyclability. You can see it everywhere--from your local grocery store to large manufacturing companies.

With the price of gas going through the roof, everybody is looking for ways to cut costs. Turn up the thermostat in the summer and lower it in the winter. Eat out less and don't take weekend road trips. Another idea is to replace the existing roof on your house with a thermally efficient roof system. A thermally efficient roof must have high reflectivity and emissivity values. This month I would like to discuss these energy-related terms--reflectivity and emissivity.

I thought I should get the dictionary and look up the definitions of reflectivity and emissivity. After about an hour of surfing the Web, I found out the Cambridge Dictionary doesn't have a definition for either term, and the definition was not helpful. So I went to the Washington, D.C.-based U.S. Environmental Protection Agency's Web found a more user-friendly glossary of terms.

To be thermally efficient requires that the roof have high reflectivity and emissivity values.

Solar reflectance, also known as albedo, is a measure of a material's ability to reflect sunlight--including the visible, infrared and ultraviolet wavelengths--on a scale of 0 to 1.0. An albedo value of 0.0 indicates that the surface absorbs all solar radiation and an albedo value of 1.0 represents total reflectivity. EPA's Energy Star rating specifies an albedo of 0.65 or higher for low-slope roof applications and a minimum 0.25 for steep slopes.

Emittance of a material refers to its ability to release absorbed heat. West Conshohocken, Pa.-based ASTM International's Standards C1317 and E408 use numbers between 0 and 1.0, or 0 and 100 percent, to express emittance. With the exception of some materials, most roofing materials have emittance levels above 0.85, or 85 percent. To help illustrate, a mirror may reflect 98 percent of the energy that strikes it while absorbing 2 percent of that energy. A blackbody surface will reverse the ratio, absorbing 98 percent of the energy and reflecting only 2 percent.

The words albedo--or solar reflectance--and emittance are words that relate to energy efficiency in a roof system. Most roof manufacturers have total-reflectance and thermal-emittance values listed for their various roofing colors and finishes. These values usually are listed in decimals with 0.99 being the best and 0.000 being the worst. ASTM Standards E903 and E1918 are used to measure the Solar Reflectance values. Paint suppliers furnish the metal panel manufacturers with the tested values. Metal has a distinct advantage because the Washington-based U.S. Department of energy, Energy Star and Sacramento-based California Energy Commission are requiring aged reflectance values and, for the most part, painted metal values drop very little as a roof ages. In fact, in some cases the values improve. This is not the case with other roofing products.

Figure 3 shows the energy balance on a roof surface. All things in nature must be in balance; therefore, the energy generated from the sun on a roof surface must also be in balance. As you can see in this figure, the energy is either repelled into the atmosphere or absorbed into the roof system. Much of the energy is repelled by reflectance and emittance, along with wind or convection. The remaining energy is absorbed by the roof system and is called the net heat flux. The higher the net heat flux, the more an air conditioner will run.

The Pittsburgh-based Cool Metal Roofing Coalition has published a case study that verifies that a roof's reflectivity and emissivity influence heating and cooling bills. This case study was conducted on two identical schools in Georgia. The only difference between the schools was that one of them had higher reflectivity/emissivity values than the other. The results were eye opening; th e school with the higher reflectivity/emissivity values had an annual savings of more than $14,000 in its gas and electric bills. This report is available on CMRC's Web site,

That's enough of the heavy stuff. A simple way of showing someone how this works is to take two panel samples, one that is painted white and one that is painted dark brown or black. Put them outside in a parking lot on a sunny day and let them sit for about an hour. Then go and pick them up. You will notice that the dark-colored panel is hotter than the light-colored panel. This is because of the panel's emissivity value. Darker colors generally have lower emissivity values and will retain more of the heat generated by the sun. This simple demonstration is a powerful sales tool when trying to sell someone on a thermally efficient roof system.

Paint manufacturers now have developed dark colors that have improved reflectivity values. They are using "cool pigmentation" to accomplish this. The cool pigmentation improves reflectivity values, but it also lightens the colors in some cases. By introducing cool pigmentation, a wider rainbow of Energy Star-compliant colors are available.

Everyone is going green, recycling and looking for energy efficiency. This trend is not going away. We need to become familiar with reflectance and emittance so we can relay the message to customers. Metal roof systems are the way to go for the energy-concerned future, so go ahead and jump on the green bandwagon.

Interesting Facts

> Typical steep- and low-slope metal roofing contains at least 25 percent recycled content. At the end of their useful lives, these roofs are 100 percent recyclable.

> Steep- and low-slope metal roof systems typically weight 40 to 135 pounds (18 to 61 kg) per 100 square feet (9 m2), making them among the lightest roofing products.

> The most common thickness of steel roofing used in commercial construction is 24 gauge.

> The most common thickness for aluminum roofing is 0.032 inches (0.8mm).

> On a 1 to 5 scale with 5 being most desirable, owners and property managers rated metal roofing at 4.75 in a survey by Ducker International. Asphalt achieved 4.3 and single-ply rated 3.91.

> A metal roof system can have a 50-year-plus life span with few repairs.

> Installation of reflective metal roofing can save up to 40 percent in summer-cooling energy costs while highly emissive metal roofs can reduce urban air temperatures by as much as 12 F (7 C).

> Metal roofing can add as much as $1.45 per square foot to the value of a home.

> Roofs have three different load zones: Zone 1 is the field, or middle area, of the roof; Zone 2 is the perimeter, or the eave, rake and ridge; and Zone 3 is the corner, or eave/rake and rake/ridge intersections.

> Ridge and rake/ridge corners are only on roofs with a slope of 2:12 (9 degrees) or greater.

> At the end of their useful life, aluminum roofing and siding panels can be repeatedly recycled into roofing and siding products without loss of quality.

> Buildings consume one-third of all energy and two-thirds of all electricity generated in the U.S.

> Steel is made to exact specifications so there is very little on-site waste.

> The North American steel industry has been recycling steel scrap for more than 150 years through 1,800 scrap processors and about 12,000 auto dismantlers.

Dave Fulton is vice president of research and development for a metal-building and -component manufacturer. He is on the board of directors for the Pittsburgh-based Cool Metal Roofing Coalition and holds several U.S. patents related to the metal industry.

Comments: FIELD TECHNIQUE: Reflectivity and Emissivity

This was a very informative article. Thank you.
By Shelly Sandhu
This page would be an awesome place to post a table showing the various reflectivity values for different roof colors. . .
By J.Cooper
Post a comment > -

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