We all know that drinking ice-cold lemonade in the cool shade of a tree is a summertime treat. It provides a tasty escape from the heat of direct sun exposure. Wouldn’t it be great if we could feel that same relief in our homes that the shade tree provides? According to the U.S. government, approximately 1/6th of all electricity produced is used for air conditioning. In other words, we spend about $40 billion per year to air condition our homes and work places.
Our roofs take the brunt of the sun’s energy that heats our homes during hot summer months. Therefore, a huge benefit to our pocket books is to break the link between the sun’s radiant heat and the temperature inside our homes. Just like the appliances in our homes are Energy Star rated so are the roofing materials over our heads. A key metric used by the EPA to qualify a roof for Energy Star is its Solar Reflectivity Index (SRI), which is quantified between 0 (least efficient) to 1 (most efficient).
Carl G. Cash, PE, and Edward G. Lyon, PE determined that roof temperature is most greatly influenced by geographic location, roof aspect, color (solar reflectance), and ventilation between the roof and living space. We can’t control the effect of geography on a roof and if you already own a home you can’t change the direction your roof faces. However, you can change your roof’s solar reflectance and ventilation.
According to research conducted at Oak Ridge National Laboratory, the most impactful roof attribute for a cool roof is its ability to reflect the sun’s heat. Typical 25-year, gray asphalt shingle roofs, which most of us have on our roofs, have a reflectivity of approximately 0.09. Increasing solar reflectance cools the roof and reduces summertime cooling demand. Another factor is roof material emissivity, the measure of how quickly a roof cools after the sun has stopped shinning on it. Higher emissivity roofs cool quickly while lower emissivity roofs act like a thermal mass and continue to heat a building after the sun has stopped shinning on it. In practical terms, the more heat your roof holds, the more your air conditioner will need to run, the less heat your roof holds, the less your air conditioner will need to run.
David Kozlowski notes that roofing materials have improved to the degree you can get the benefits of a white roof, but the aesthetics of a color roof and the benefits of a quickly cooling roof.
Solar Reflective Index is a measurement of how efficient a roof reflects solar energy. A value of 1.0 is ideal, so an SRI of 0.87 is more efficient than an SRI value of 0.01. According to a study at Oak Ridge National Laboratory, the most impactful technologies in affecting SRI values is paint. An Oak Ridge National Lab study found colored asphalt shingle SRI values ranged from 0 for black shingles to 0.40 for white shingles. High performance fluoropolymer paint used to coat standing seam metal roof systems have SRI values that range from 0.27 for black to 0.85 for white. Note that metal coated with fluoropolymer paint out performs shingles when similar colors are compared and fluoropolymer coated metal outperforms singles in terms of SRI, longevity, durability, recyclability, recycled content, dependence on foreign oil, maintenance and metal roofing weighs less than shingles.
Many metal roof systems are Energy Star compliant; therefore they enjoy additional benefits such as a tax credit. Metal roofing is also much more than mass produced corrugated metal. Standing seam metal roofing is an architectural feature; it can be curved or tapered, made from coated steel, copper, zinc and other metals.