Solar Load Based on Paint Color - Cool Roofs Are Greener
Roof Color & Energy Consumption
On a hot sunny day, the sun can easily overheat a room with unshaded windows. Likewise, it can make air conditioners work harder than they should need to just by heating a building’s roof. The color and material of a roof are important factors in how much sunlight they reflect or absorb. In general, dark colored roofs absorb more solar energy than light colored roofs, while light colored roofs reflect more solar energy back to the sky. Some materials are also better at reflecting solar rays than others.
A roof that absorbs a large amount of solar energy transfers some of the absorbed heat to the building’s interior. In cold climates, this may actually help reduce heating energy cost, but in warm climates, it is a concern because it can contribute significantly to the building’s cooling load. Conversely, a roof that reflects a large portion of the solar energy that strikes it stores a trivial amount of solar energy in its mass and very little heat can transfer to the building’s interior.
Solar load is greater in summer, when the sun is higher in the sky and the days are longer. Therefore, in most climates that use at least some air conditioning, the negative effects of dark colored roofs in summer are greater than their positive effects in winter. The problem amplifies when air intakes for air conditioners, and in some cases the air conditioners themselves, are located on dark rooftops. Air conditioners then have to work extra hard to cool this exceptionally hot air.
Urban Heat Island
Not all of the heat absorbed by roofs transfers to the building’s interior. Much of it radiates back into the air. This is not the same as reflected sunlight, which bounces back into the sky. Once roofing material absorbs energy from the sunlight, it is stored as heat energy and can transfer to the building interior and the surrounding air via the three methods of heat transfer: conduction, convection and radiation.
When there are many dark colored roofs in a city or neighborhood, all of this heat contributes to the urban heat island effect, which is the increase in temperature in cities compared to the vegetated land just outside cities.
Roofs are not the only culprits. Asphalt, concrete, stone and gravel all absorb more heat than vegetation does. Consider a bird’s eye view of a dense city. Most of the horizontal surfaces are rooftops. What is not a roof is likely a street or parking lot. Urban heat island is an environmental concern because of increased energy costs, increased smog, human health concerns associated with heat, and over all decreased comfort.
Addressing the roofs of buildings citywide is an important part of urban heat island mitigation. Adding vegetation to cities, both on the ground and on roofs is part of the solution. Transpiration in plants helps cool the air and plants provide shade, which lowers the temperature. For these reasons and others, green roofs are becoming popular. However, green roofs are also more expensive and require extra maintenance, and they are not always practical, either because of the cost, need for upkeep, or building structure limitations. An inexpensive and highly effective alternative is a cool roof.
Standard roofing materials like asphalt and shingles heat up quickly, but there are materials and coatings available that make roofs remain cooler in the heat of summer. Cool roofs have high solar reflectance (albedo) and high emissivity (the ability to radiate heat energy). A material’s albedo is the ratio of light it reflects light to light it absorbs. The lower the albedo, the more light it reflects, and the cooler it stays in the sun. So, most of the solar energy striking a cool roof reflects back into the air, rather than being absorbed by the roof.
Cool roof materials are typically coatings or thin materials that cover the actual roofing material. Concrete and clay tiles made with special reflective pigments are also available for pitched roofs. Most cool roof materials and paints are light in color, but some new options come in darker colors with additives that increase albedo.
At the building scale, cool roofs reduce energy consumption and cooling costs for buildings. They are particularly beneficial for tiered utility rate plans because they reduce peak demand. They also improve comfort of the indoor environment, with or without the presence of air conditioning.
At the city scale, multiple cool roofs help reduce urban heat island and smog which forms more readily at high temperatures, as well as pollution and greenhouse gas emissions due to reduced energy consumption. They also benefit human healthy by improving city air quality due to less pollution and smog and lower temperatures.
California Energy Commission, Cool Roofs https://www.consumerenergycenter.org/coolroof/
Cool Roof Rating Council https://www.coolroofs.org/
Roof Savings Calculator (RSC), Beta Release v0.92, Oak Ridge National Laboratory & Lawrence Berkeley National Laboratory https://www.roofcalc.com/
Image: Courtesy of DOE/NREL, PIX number 03510, Credit - Craig Miller Production