In today’s tough economic climate, the decision about which roofing material to use can hinge as much on the material’s sustainable attributes—such as energy efficiency—as on its appearance and cost. Because a roof can have a significant impact on the energy use in a building, the material selection is an important decision. Increasingly, building designers, engineers and owners are turning to cool metal roofing products to meet their demands for durability, energy efficiency and cost-effectiveness as they look for ways to enhance their investment.
Cool roofs have radiative properties that reduce surface temperature, minimize the amount of heat transferred into the living space below and reduce the heat released from the surface to the ambient air. The energy efficiency benefits of a cool roof are determined primarily by two properties: total solar reflectance and infrared emittance. Cool metal roofing products comprise unpainted metal, prepainted metal and granular-coated metal for steep-slope and low-slope applications. They are part of an interdependent system of exterior roofing surfaces, substrates, underlayments, configurations, ventilation and insulation.
In terms of durability, metal roofs are resistant to wind, fire, hail and snow, and are less affected by the hot-cold and wet-dry cycles that can destroy other materials. Metal roofs inherently are non-combustible, thereby reducing the spread of fire in and among buildings. They are made of inert, safe materials that do not pose a health risk. Commercial metal roofs have been in service for 30 years or more, and painted roofs are credibly warranted up to 30 years. This durability translates into lower life-cycle costs, making metal roofing the choice of many school, government, commercial, industrial and institutional building owners.
Energy Efficiency
A key consideration in the selection of a cool roof is the product’s energy efficiency. In 2004, the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) Buildings Technology Center completed a three-year comparison study to evaluate the energy efficiency and service life of metal roofing systems. The study compared various metal roofing materials placed side-by-side in steep-slope and low-slope applications, including painted and unpainted galvanized steel, painted and unpainted Galvalume-coated steel and painted aluminum roofing. The materials were tested at ORNL in Oak Ridge, Tenn., and at test fence exposure sites in Florida, Nova Scotia and Pennsylvania. Additional test fence data from 10, 15 and more than 30 years of exposure also were evaluated.
The evaluations included a comparison of solar reflectance and emittance data for the materials. Reflected solar energy allows the roof surface to remain cooler, ensuring that less heat is transferred into the building. Unpainted metal roofs, as well as metal roofs painted with cool coatings, reflect much of the solar radiation that strikes the roof, keeping the building cooler.
The infrared emittance of a roof is a measure of absorbed solar radiation that is re-emitted from the roof surface to the sky (see Figure 1). The emittance of metal roofing varies with the surface finish; painted or granular-coated metal roofing can be as high as 90 percent.
The ORNL study demonstrated that:
- Metal panels maintain high levels of reflectance even after continued exposure to the elements throughout many years (see Figure 2).
- The pre-painted metal roofing retained 95 percent of its initial solar reflectance over the testing period. The panels also maintained high levels of emittance, which increased slightly in some cases.
- The Washington, D.C.-based Environmental Protection Agency’s Energy Star program required field testing at three different building sites. The results for painted metal showed the reflectance to be very similar whether exposed in Florida, Nova Scotia or Pennsylvania.
- Long-term weathering does not significantly affect the solar reflectance or the infrared emittance of painted metal roof panels.
- Painted and unpainted metal panels maintained their energy efficiency better over time than any of the other roofing systems studied.
Choosing the Right Roof for Your Climate
Where annual cooling loads dominate, a highly reflective and highly emissive painted or granular-coated metal roof is optimal for reducing energy consumption and can actually re-emit up to 90 percent of absorbed solar radiation. In areas where annual heating loads dominate, an unpainted metal roof may be more desirable because of its low infrared emittance, which allows more solar energy to come into the building envelope in winter. The emittance of painted or granular-coated metal roofing can be as high as 90 percent.
When choosing and installing a cool roofing system, material specifiers and building owners also should know that:
- Buildings consume one-third of all energy and two-thirds of all electricity generated. Cool roofs help reduce energy consumption by lowering cooling loads.
- Cool metal roofing helps mitigate the urban heat island effect because highly emissive roofs can lower urban air temperatures, benefiting the environment by reducing smog formation.
- Because it has a lighter weight per unit area than heavier non-metal roofing alternatives, metal roofing can yield structural savings in certain buildings.
- In some applications, energy savings from a painted metal roof, when compared to a traditional built-up roof, can translate to significant cost savings, allowing the roof to pay for itself in as little as nine years. In other applications, roof payout is well within its projected 30- to 50-year lifespan.
- Subject to local codes, most metal roofing, including standing seam, can be installed directly onto an existing asphalt roof. This saves removal and disposal costs, reduces landfill waste, and promotes quick installation. It also provides decades of environmentally sound, maintenance-free strength and beauty.
- Metal roofing contains at least 25 percent recycled content, and is 100 percent recyclable when removed as part of a building renovation or demolition. Unlike many other construction products, most metals can be easily recycled back into new products, without the need to downcycle the material.
- Metal roofs contribute to the Washington-based U.S. Green Building Council LEED rating system because of their recycled content and contribution to reducing the urban heat island effect.
Ease of Maintenance
Metallic-coated or prepainted metal roofing is inherently a very low-maintenance product that will last 30 to 40 years or more. The ORNL study proved that painted and unpainted metal roofs show less effect from weather extremes over time, maintaining surface properties and resisting soiling.
Metal roofing has been proven to retain its solar reflective properties far better than competing cool roof products, without any regular cleaning or maintenance. In the ORNL study, the painted metal roofs maintained their original manufactured appearance. Rains with a measured pH of 4.3 did not etch the metal finish after three years.
When selecting a roofing product for commercial projects, cool metal roofs make good business sense, providing long-lasting, sustainable benefits for a strong return on investment.
Mark A. Thimons, P.E., is director of construction sustainability for the Steel Market Development Institute, a business unit of the American Iron and Steel Institute. He is located in Pittsburgh. Mark can be contacted at mthimons@steel.org.