ACCORDING TO THE
Pew Research Center, Washington, D.C., our nation has more than 100 million houses and close to 5 million commercial buildings that collectively use about 40 percent of the total energy in the United States. To a building owner, energy represents the single largest controllable operating expense. As a result, the current energy crunch has made conservation measures within these structures more important than ever.
Despite efforts to raise the awareness of energy conservation, less than half of the homes in this country are well insulated and less than one-third of commercial buildings have roof insulation. The Building Owners and Managers Association International, New York, estimates a 30 percent reduction in energy consumption, amounting to $7.2 billion, is readily achievable by improving building operating standards. However, studies suggest although significant improvements in the energy efficiency of buildings appear to be cost effective they are not likely to occur without extensive policy changes. A point that supports this is the fact that reflective, or cool, roofing materials still comprise less than 10 percent of the roofing market.
However, cool metal roofing is gaining in popularity because of its ability to reduce cooling and heating energy usage. Utility companies have become interested in cool roofing because it can reduce the peak demand in electricity during the afternoon hours of summer months, preventing power disruptions. From an environmental point of view, cool roofing can mitigate a phenomenon known as the urban-heat-island effect in which urban areas experience higher ambient temperatures caused by an abundance of nonreflective construction materials and little vegetation.
THE COOL PHENOMENON
Reducing the amount of heat transfer into the space below a roof is paramount to lowering energy usage, thereby reducing greenhouse-gas emissions from utility power plants. Other means can help reduce heat flow, such as insulation and radiant barriers, but none is as effective as reducing the amount of solar energy transferred through the roof.
The cool-roofing phenomenon is described using two primary terms: solar reflectance and thermal emittance. Solar reflectance is the percentage of all solar radiation that is immediately reflected from a surface. Any energy that is not reflected from a surface is absorbed by the material. Some of that absorbed energy is transferred to heat, which can be removed by convective transfer from air flow over the surface. Some of the heat also can be conducted through the surface. More importantly, some of the heat can be re-emitted to the night sky in the form of infrared wavelength energy. Thermal emittance refers to a material's ability to release absorbed heat. The combination of solar reflectance and thermal emittance properties determine the surface temperature of a roof and its ability to limit the heat flow into the space below a roof.
Typically, dark colors absorb most of the visible light striking the surface while lighter colors reflect most of that solar energy. This would suggest that color is an indication of how much visible solar energy will be reflected. The amount of infrared energy that is reflected often is a function of the color, as well, but recently new pigment technology has been introduced to the paint industry that changes that assumption. New infrared reflective pigments incorporated into exterior paint systems allow for darker colors to actually reflect total solar energy. Hence, the solar reflectance value is higher with a corresponding lower surface temperature. More importantly, these cool pigments do not affect the color of the product, meaning traditional dark colors can be matched with identical colors of the new paint systems with higher total solar reflectance values.
Metal roofing has a wide range of solar reflectance and thermal emittance values. In an unpainted condition, a metallic surface has a very low thermal emittance, typically 0.08 to 0.1, but a relatively high total solar reflectance, 0.6 to 0.8. When a paint system is applied to the surface the thermal emittance increases to more than 0.8, in most cases, regardless of the color. However, the total solar reflectance can vary from 0.1 to values greater than 0.75 depending on the color and/or pigmentation used.
Technological improvements in cool metal roofing are continuing in the area of enhanced developments of infrared reflective pigments and paint systems. New research includes night cooling with metal roofing and capitalizing on underside ventilation with certain types of cool metal roofing products.
A research project at the Cocoa-based Florida State Energy Center is focused on harnessing the radiative properties of metal roofing to provide nighttime cooling advantages. Using a prepainted metal roof installed over trusses with no plywood decking in a sealed attic, FSEC has shown the ability to cool a dwelling in the evening hours by utilizing the reflectance and emittance properties of a prepainted cool metal roof.
The metal roof serves as a large-area, low-mass, highly conductive radiator. During daytime hours, the living space of the dwelling is de-coupled from the roof and heat gain to the attic space is minimized by the light-colored cool reflective paint system on the metal roof. During that time of day, the building can be cooled using a conventional down-sized air conditioner. In the evening hours, as the temperature of the interior surface of the metal roof falls below the desired thermostat setting in the living space, the return air for the air conditioner is channeled through the attic space by way of louvers and a variable-speed fan. The warm air from the interior is then moved to the attic and warms the interior side of the metal roof, which then radiates the heat away to the night sky.
In Florida homes, a typical cooling load averages 33 kilowatt hours per day from June to September with roughly 28 percent of that energy being used between the hours of 9 p.m. and 7 a.m. During those hours, night sky radiation can be used to substantially lower a home's cooling needs. Additional research and testing is planned.
Above Sheathing Ventilation
Oak Ridge National Laboratory, Oak Ridge, Tenn., is simulating tile and shake products with stone-coated metal roofing and revealing promising benefits from the air flow between the raised profiles of a metal roof and the roof-deck sheathing.
The project involves modular metal-roofing products installed using conventional batten- and counter-framing systems. The use of this construction technique and the raised profile of the modules create a gap in which natural convective forces move heated air directly below the metal roof. This is referred to as above sheathing ventilation, or ASV. By measuring these metal-roof products and comparing them with gray direct-to-deck asphalt shingles, ORNL has found a cool-metal-roof surface can show a 15 percent reduction in heat gain. The benefit of the ASV is an additional 30 percent reduction in heat gain when compared to similar roof products of the same total solar reflectance but installed direct to the deck. Hence, a cool metal roof of this type combined with ASV can theoretically reduce heat gain into a living space by up to 45 percent compared to non-cool products installed direct to deck. A second phase of research is being planned to test this concept using commercial standing-seam metal roofing where an air gap can be introduced above the deck.
Cool-color Roofing Materials
The Sacramento-based California Energy Commission's Public Interest Energy Research project about cool-color roofing materials was completed in August 2006. The research was conducted by Lawrence Berkeley national Laboratory, Berkeley, Calif., and ORNL. The goal was to develop residential roofing products that at least meet the Energy Star-qualifying solar reflectance of 0.25, utilizing novel cool pigment technology.
Prepainted metal roofing was part of this study and showed a solar reflectance increase of 0.17 to 0.25 compared with conventional dark colors using cool-pigment technology. The report also showed a new annual cooling energy savings of up to 650 kWh for the cool metal roof at practically no cost premium. Demonstration homes in Fair Oaks, Calif., were constructed with conventional and cool roofs as part of this research. Results showed the application of cool coatings lowered the surface temperature, which in turn reduced the average daytime heat flows through the roof deck, by 32 percent for prepainted metal shake compared to 30 percent for asphalt shingles and 20 percent for tile. A second phase of the PIER project has begun to accelerate further development of these products and deployment of cool-color roofing technology.
A COOL CHOICE
In light of rising oil, gas and electricity costs, cool metal roofing is even more attractive. Whether it is federal, state or local green-building standards, incentives, codes or rebates, the use of cool roofing continues to grow as a way to reduce overall energy consumption in our country's buildings. Metal roofing's range of solar-reflectance and thermal-emittance properties allows it to be engineered to optimize the energy efficiency of a roof system depending on the climate and type of application.
An unpainted metal roof, with lower thermal emittance but high reflectance is desirable in cooler climates where heating loads dominate. On the other hand, a prepainted cool metal roof with high solar reflectance and high thermal emittance would be desirable in warmer climates where cooling loads dominate. In addition to the energy efficiency of a metal roof, the ecological benefits of recycled content, recyclability, low weight, durability and product safety become important in its selection as a sustainable building product. Design flexibility, combined with the sustainability and low life-cycle cost make cool metal roofing a wise choice amidst the winds of change.
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