A Sustainability-Supporting Cool Metal Roof

A roof receives an incredible amount of sunlight exposure day after day. A cool roof is defined as a roof that reflects more sunlight and absorbs less heat than a standard roof. Consider that a flat roof in the midday sun receives about 1,000 watts of sunlight per square meter. A dark roof will absorb most of this energy, heating the roof, the interior space contained underneath it, and even causing the temperature of the local air outside of the structure to rise, contributing to the urban heat island effect. A cool roof can be engineered to reflect most of the visible light as well as light in the infrared spectrum that hits it, which can dramatically reduce the temperature the surface reaches, even underneath the blazing sun.

According to the Department of Energy (DOE), standard or dark roofs can reach temperatures of 150 degrees Fahrenheit or more in the summer. A cool roof, under the same conditions, could stay more than 50 degrees cooler. This lower temperature delivers important benefits. A cool roof will reduce the heat gain that occurs inside the home, which decreases the load on the air-conditioning unit, ultimately reducing the amount of energy and money that must be consumed to keep the interior cool and comfortable. It also reduces the air temperatures reached above the cool roof, combatting the urban heat island effect that further exacerbates hot times and summer in a city.

In the article, “Cool roofs: beating the midday sun with a slap of white paint,” written by Dyani Lewis and published in The Guardian, sustainable construction expert Chris Jensen from the University of Melbourne explains, “On a black tile roof, you could fry an egg. On a cool roof, you could walk on it in your bare feet. That’s the magnitude of the difference.”

Cool roofs can be constructed from highly reflective shingles or roofing materials coated in a highly reflective paint. “From a sustainability perspective, we always want a roof to be cool instead of hot,” Kipnis explains. “A cool roof dramatically impacts the way that a home manages the sun’s heat and the amount of heat that transfers into the interior of the home and then must be cooled.”

The way that a cool metal roof manages the light and heat-causing infrared radiation in direct sunlight is described by three properties: solar reflectance, thermal emittance, and a solar-reflectance index value that considers both the solar reflectance and thermal emittance in concert. Solar reflectance indicates the percentage of sunlight that is reflected off the roof. Determined on a scale of 0 to 1, lower solar-reflectance values indicate that more energy is absorbed than reflected. For example, a reflectance value of 0.05 means that the surface reflects only 5 percent of the solar energy that strikes it and absorbs 95 percent. Thermal emittance characterizes a surface’s ability to re-radiate or re-emit previously absorbed heat away from itself. Also rated on a scale between 0 and 1, the higher the thermal emittance of an object, the better it is at emitting heat it has absorbed and remaining cool, even when direct sunlight is shining upon it.

The third metric, the solar-reflectance index (SRI) measures a constructed surface’s ability to stay cool in the sun by reflecting solar radiation and emitting thermal radiation. As the USGBC explains, “It is defined such that a standard black surface (initial solar reflectance 0.05, initial thermal emittance 0.90) has an initial SRI of 0, and a standard white surface (initial solar reflectance 0.80, initial thermal emittance 0.90) has an initial SRI of 100.” The SRI can be used as an indicator of how hot a surface is likely to become when solar radiation is incident upon it. The lower the SRI, the hotter the material is likely to become when exposed to the sun’s rays.

Kipnis designed this large, farmhouse-style house with a silo-like turret and an elaborate standing-seam metal roof painted with a cool-roof technology. The roof specified onto the Chicago residence was comprised of 8,500-square-foot panels of 24-gauge steel. The weathered zinc finish had a solar-reflectivity rating of 0.25, an emissivity rating of 0.82, and an SRI value of 22. With these solar-management metrics, the roof will stay cool during the hot summer months and protect the home against the formation of damaging ice dams during those formidable winters in the Second City.

“Hot roofs can create problems even when it’s cold outside,” Kipnis explains. “If there is snowfall on a roof and the roof captures a large amount of the solar energy beating upon it, the roof will warm up and melt the snow before the conditions for snowmelt exist, so the melted snow refreezes when it hits the overhang and an ice dam forms.”

An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow from draining off the roof. Ice dams can be incredibly destructive to the house that hosts them, tearing off gutters, loosening shingles, and even allowing water to enter the interior, which then creates a myriad of problems from sagging ceilings, warped floors, and soggy insulation, which can then breed mold and mildew.

“Cool roofs prevent that premature thaw and simply maintain the snow cover until the seasonal conditions shift and naturally melt the snow. Then it drains away from the house just as it is designed to do,” Kipnis says. “Metal roofs also look great and support many other sustainability criteria far better than the asphalt, wood, or slate materials often used.”

Enhance Solar-Panel Performance

The standing-seam metal roof also provides an ideal surface to host solar arrays that further reduce the carbon footprint of a project. “If we are trying to put solar panels on an asphalt roof, there are a number of problems that can arise. Some installation methods will penetrate the roof in several areas in order to mount the solar panels. This obviously isn’t ideal,” Kipnis adds. “With a standing-seam metal roof, the solar panels can be clamped directly to the standing seams with S-5 clamps, without penetrating the surface of the roof. That’s a huge advantage.”

Photo: A.J. Brown

A cool standing-seam metal roof is an ideal surface to host solar arrays because the panels can be clamped directly to the seams without penetration and the panels remain cooler, which improves their performance.

Using a cool metal roof as a platform for the solar panels instead of a standard or dark roof can also improve the functionality of the solar panels. When the solar arrays are clamped to the standing-seam roof, the panels are raised up off of the metal surface, which allows air to circulate underneath them and helps to cool them off. As previously mentioned, the cool roof also stays cooler than a typical roof material. This is important because solar panels work best in cooler environments. Work by Jensen and his colleagues has shown that solar panels installed on a cool roof produce almost 7 percent more electricity on a typical day than those installed on a conventional roof.

Shed Cleaner Rainwater for Harvesting

The term rainwater harvesting refers to collecting the runoff from a roof and storing it for later use. Traditionally, rainwater runs off the roof and will collect in gutters that channel the water into downspouts and some type of storage vessel. Standing-seam metal roofs offer one of the best solutions for rainwater harvesting, as their collection efficiency is one of the highest of all popular roofing materials. Studies by the University of Texas and the Texas Water Development Board found that a standing-seam metal roof delivers a collection efficiency of up to 95 percent. In fact, less than a single inch of rain falling on a 10-foot by 10-foot standing-seam metal surface can produce 60 gallons of rainwater reserve. For context, it is estimated that clay tiles offer roughly an 85 percent collection efficiency because the tiles are porous and absorb a portion of the rain that strikes them.

Beyond the efficiency of the runoff that a roof can provide, the roofing material also impacts the quality of the water. For example, some roofing materials contain pollutants or other ingredients that could leach into the rainwater and contaminate it. A lot of corrugated material is galvanized, which will leach some zinc into the rainwater. Wood shingles are often treated with fire retardants, which probably isn’t a good additive for rainwater that will be reused. Copper is an herbicide, so if the end goal is to use the rainwater runoff for garden irrigation, a copper roof should be avoided. Many asphalt-shingle roofs, also called composition-shingle roofs, are installed using adhesives that can off-gas for up to three years after installation. For that reason, homeowners may want to avoid using rainwater harvested off of a new asphalt roof to irrigate anything that will be consumed for three years. Asphalt shingles can also release bits of gravel into the runoff, but those can be managed with an inlet screen on the collection tank. Standing-seam metal roofs are often recognized as a superior choice for creating efficient and high-quality rainwater-harvesting systems.