Late last year, the Department of Energy (DOE) passed legislation that requires gas furnaces and other appliances to operate with greater efficiency. This rule follows a long line of efforts to reduce the environmental impact of both commercial buildings and personal residences. With more emphasis being placed on sustainability, it is important to note that increasing a building’s energy efficiency goes beyond installing certain appliances. It also includes considering how adjacent building systems, like a roof, support these efforts.
In terms of creating an energy-efficient structure that provides a comfortable interior temperature, roofs play an integral role. And while the design of a roof can impact thermal performance, material choice has a greater effect. In the past, metal roofs have been cited as making buildings and homes hotter, but is this the truth? Performance data says otherwise.
Metal roofs can directly and indirectly contribute to more comfortable interiors. Knowing how can help owners make more informed decisions on what systems they want overhead.
One way metal roofs support building efficiency is by reducing heat absorption through cool roof ratings. In general, a cool roof reflects more sunlight and absorbs less solar energy than a conventional roof. As a result, homes and buildings provide occupants with more comfortable interior temperatures, put less strain on air conditioning systems during peak operating hours, and help reduce energy costs. The key to these benefits often comes down to material color.
While other roofing materials are commonly limited to darker hues, metal roofs can be finished in lighter and more reflective colors to achieve cool roof ratings and contribute to a more comfortable interior environment. Choosing a cool roof does not have to mean compromising the design aesthetic of the building envelope. Drexel Metals offers a wide range of durable and fade-resistant color options that reflect and re-emit solar radiation by up to 90 percent. These colors can be added to almost any substrate and profile type, giving designers and project owners several options for meeting design intent and sustainability targets.
While cool-roof-rated systems support ecologically conscious building practices, metal roofs also contribute to cooler interiors through their high thermal emittance rate.
Heat absorption is only part of the efficiency equation. The other part is how quickly a material emits absorbed energy. This is especially important in climates where temperatures fluctuate greatly between the hottest and coolest parts of the day. Roofing materials with a high thermal mass and a low thermal emittance rate tend to stay hotter longer. This prolongs the number of hours HVAC systems must operate at a higher capacity to maintain comfortable interior temperatures.
In contrast, metal roofs have a lower thermal mass and higher thermal emittance rate than other materials. As such, they shed heat efficiently—quickly reverting to the same temperature as the surrounding air once the brightest hours of the day are over. This minimizes the peak operating hours required of HVAC systems to contribute to cooler temperatures and more energy-efficient buildings.
Although heat absorption and emittance are the two most direct ways metal roofs keep interiors cool, there is a third, indirect way metal roofs contribute to cooler interiors: reducing the heat island effect.
Heat islands occur when several buildings absorb and store heat and so raise the temperature of an entire area. In addition to reducing energy efficiency, this phenomenon has also been linked to adverse health conditions and impaired water quality according to the Environmental Protection Agency (EPA). Because cool roof rated metal roofs absorb less heat, reflect more solar energy and shed heat more efficiently, they can help reduce the heat island effect.
On the one hand, this contributes to healthier and more comfortable environments. On the other hand, it reduces the amount of energy needed to cool a building within a heat island. Because reducing heat islands also reduces the temperatures around individual buildings, HVAC systems do not need to work as hard to maintain a comfortable interior temperature.
It seems the myth that metal roofs make interiors hotter is just that—a myth. From their ability to sport cool roof ratings to their high thermal emittance rate, metal roofs contribute to cooler, more comfortable and more sustainable interiors. In fact, few assemblies combine aesthetic appeal, functionality and sustainability in the way metal roofing systems do.
Drexel Metals boasts a proven track record of providing premier metal roof systems that meet design and performance goals across a wide range of applications. From curved patterns and one-of-a-kind cornices to a large color palette with many cool roof rated colors, Drexel Metals offers fully customizable metal roofing solutions that can meet the demands of nearly any project.
