In the United States, buildings are responsible for almost two-thirds of national electricity consumption and more than one-third of total primary energy use.
Cool Metal Roofing
As a result, the current energy crunch has made conservation measures within these structures more important than ever. Heating and cooling costs are generally the major expenses associated with the operation of a building. Therefore, any reduction in these costs with energy-efficient building envelope component makes sense.
The roof can be the least energy-efficient component of the building envelope. This is one of the forces driving the trend toward cool roofs, which is gaining popularity due to its ability to reduce cooling and heating energy use. Utility companies are also interested in cool roofing since it can help reduce electricity use during peak demand times (i.e. the afternoon hours of summer months), preventing power disruptions. From an environmental point of view, cool roofing can also help mitigate the heat island effect.
Coming to terms with cool roofing
Cool roofing design is based on the premise of minimizing heat gain through the roof surface. The solar energy spectrum shows ultraviolet (UV) energy in lower wavelengths amounts to only three percent of the total energy striking the earth’s surface. Visible light energy is 40 percent and infrared (IR) energy accounts for the largest percentage of the spectrum at 57 percent. Whether in short or longer wavelengths, IR energy striking the Earth’s surface is felt as heat.
Cool roofing is described by two main terms: solar reflectance and thermal emittance. Total solar reflectance (TSR) 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 this is transferred to heat which can be removed by convective transfer from air flow over the surface. Some of the heat 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. The latter phenomenon is known as thermal emittance (TE). The combination of solar reflectance and thermal emittance properties of a material determine the surface temperature of a roof and its ability to act cool.
Some typical radiative properties of common roofing materials are shown in Table 1. Metal roofing has a wide range of solar reflectance and thermal emittance values. In the unpainted condition, a metallic surface has a very low TE but a relatively high TSR. When a paint system is applied to the surface, the TE is very high regardless of the color. However, the TSR can vary depending on the color and/or pigmentation used.
Variety of Voluntary and Mandatory Programs
In addition to the obvious energy saving benefits of cool roofing to a building owner, its use is also influenced by codes, standards, rebate/incentives, or marketing programs. And, in this regard, the landscape is bursting with an ever-increasing list of cool roofing programs and initiatives.
These policies can be national and local programs, each with their own unique criteria and definitions of cool roofing. Most of the programs are voluntary, but some have mandatory criteria. They often pertain to both low and steep slope roofing. Figure 2 shows some of these policies and indicates the requirements for compliance.
EPA estimates that an Energy Star labeled roof can lower a roof temperature by as much as 100° F. The lower surface temperature translates into lower heat flow into the attic space below the roof and reduces the load on the air cooling system. To date, more than 60% of the products listed on the Energy Star labeled directory are metal roofing products or coatings used specifically in the metal roofing industry.
The Cool Roof Rating Council (CRRC) was established in 1998 as a non-profit organization to develop a methodology for evaluating and labeling all types of roofing products. Its program was launched in 2002. This organization is comprised of roofing manufacturers, distributors, suppliers, trade associations, contractors, consultants, government agencies, educational institutions, code bodies, energy suppliers and independent laboratories. The CRRC is now recognized by the CEC as the sole entity responsible for labeling roofing products that are allowed in the California Energy Code Title 24. It is important to note that the CRRC does not establish criteria or definitions for cool roofing.
California is leading the way with an energy code as part of its overall building code. The latest version of its state building and energy code, Title 24, became effective in October 2005. For the first time, this version of the energy code contains language specific to cool roofing requirements.
Compliance in Title 24 is based on meeting prescriptive requirements of 0.70 TSR, 0.75 TE and being a CRRC-listed product. Compliance can also be achieved using a method of building component tradeoffs and energy budgets. The current code pertains only to non-residential conditioned buildings with a low slope roof. The impact on metal roofing is that unpainted metal such as Galvalume sheet can not comply with the prescriptive criteria. However, compliance using the tradeoff calculations is possible, but costly.
The Energy Policy Act of 2005 includes tax incentives for commercial construction. An eligible commercial contractor can receive a tax deduction equivalent to $1.80 for every square foot of the building, if the building is designed to conserve energy. The use of energy efficient building envelope components is required, and a cool metal roof can be used to lower the cooling energy use in the building.
Metal Roofing Can Contribute to LEED Credits
Another program gaining in popularity in the construction industry is the US Green Building Council’s Leadership in Environmental and Energy Design (LEED) program started in 2000. This is a whole-building design program that encourages an integrated design and construction process whereby points are awarded for the use of sustainable products or building practices.
The flagship for LEED has been its New Construction (NC) program. Other programs include Existing Buildings, Commercial Interiors, Core and Shell Projects, Neighborhood Development, Homes and Schools. Four levels of LEED certification are possible based on a graduated system of points awarded. An increasing number of LEED certification requirements for federal, state and local public building construction projects are being mandated.
Metal roofing can contribute to the LEED-NC certification point system in several categories. Its high recycled content can be used to raise the overall building’s average recycled content in order to receive up to 2 points. Supplying a metal panel from a location within 500 miles of a jobsite can contribute to a point for regional manufacturer. The fact that metal is 100% recyclable helps with the waste management section of LEED. One credit can be awarded for metal in the Heat Island Roof section where cool roofing is defined.
Metal Roofing a “Cool” Choice for Any Building
As the cool roof movement continues to grow in the wake of rising oil, gas and electricity costs, metal roofing is poised to become the product of choice. With its wide range of TSR/TE properties, it can be engineered to optimize the energy efficiency of a roof system depending on the climate and application.
For cooler climates where heating dominates, a lower TE may be desirable which can be met with an unpainted metal roof. In contrast, in warmer climates where cooling loads dominate, a high TSR and TE is desirable which can be met with a prepainted metal surface using lighter colors and/or IR reflective pigmentation. This design flexibility, combined with the sustainability, strength, durability, and low life cycle cost makes metal roofing a cool choice for any building.
Scott Kriner is technical consultant to The Metal Initiative, an industry-wide program designed to educate building owners, architects and contractors about the use and selection of metal roofs and walls in commercial, industrial and institutional buildings. For more information, visit www.themetalinitiative.com.
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