Did you know that the color of your roof can affect your household energy usage? In fact, the color of your roof impacts much more than just that, like the overall temperature in your area and even global climate change to a degree.
How Is This Possible?
Have you ever noticed that most roofing, especially in urban areas, is black? When the sun shines on these black surfaces they absorb heat, slowly heating up the buildings themselves. In urban areas, most everything else is black, too; roads, sidewalks, parking lots, bridges, and garages. All these black surfaces absorb and trap heat, causing the temperature in cities to rise dramatically. In fact, urban areas tend to be 15 to 20 degrees hotter than their suburban neighbors. This rise in temperature has lasting consequences, such as stress on the power grid, increased pollution, and greater risk of breathing problems — the rates of asthma in cities are almost double that of suburban areas.
Within particular cities are areas called “hot spots.” These are places where the temperatures reach a dangerous level due to industrialization, lack of vegetation, and high population. Manhattan's Lower East Side, for example, has been targeted by climate change activists as one such hotspot.
Thankfully, there is something we can do to help mitigate this growing problem. Recent studies have shown that by painting your roof with a solar reflective white coating, your infrastructure will reflect away the sun's rays instead of grounding and trapping them.
What Do White Roofs Do?
We all learned in school that the color white reflects heat and the color black absorbs it. Ever remember your coach telling you not to wear black to practice during the summer? Or do you know from experience how black cars are like ovens on a hot day? Well, this same principle applies to the color of your roof. A black tar roof on a 90-degree day can reach 180 degrees Fahrenheit, which is double the temperature of the outside air! You can quite literally cook eggs on it.
Scientists studying climate change rate the reflectivity of a surface based on the solar reflective index, or SRI scale. A surface's SRI is a measure of its ability to reflect solar heat and to emit heat that has already been absorbed. A black roof has almost no reflectivity and no emittance. A white roof, however, has a reflectivity of over 90 percent and emits 100 percent of its absorbed thermal energy. A home with a white roof maintains a temperature cooler than that of the outside air.
So, What Does This Mean for You?
When your black roof is absorbing heat, your home is also absorbing heat, especially during the summer when we are depending on air conditioning to cool down our homes. But instead of simply cooling us down from the summer heat and humidity, our air conditioners are working extra hard trying to cool us down from the thermal energy our roofs are absorbing as well. Without this extra heat source, our homes would be naturally cooler and our air conditioners would be expending much less energy, which translates to lower energy costs overall. Painting your roof with white solar-reflective paint can lower energy use by 10 to 40 percent. And, using less energy means you are cutting down substantially on your own carbon footprint.
Now, this may beg the question, if white roofs keep my home cool in the summer, won't they also keep it cool in the winter, thus increasing my heating costs? The short answer is no. First, during the winter, daylight hours are much shorter and the angle of the sun is indirect, meaning that not as much sun is absorbed as heat into your home during the winter, to begin with. Second, since heat rises, any thermal heat that is absorbed by your home during the winter will rise, probably into your attic, not lending much warmth to you and your family. Does it snow in your area? If so, any small warming benefits your black roof may provide will be covered by a layer of white snow in any case.
As well as reducing your energy costs, white roofing requires less care and maintenance. Black, non-reflective roofing commonly cracks and warps due to the heat extremes during the summer. White roofing has a much longer lifespan, commonly lasting up to twice as long as black roofing. Did you know that roofing replacement generates 8 to 10 million tons of roofing waste? 95 percent of this waste ends up in our landfills.
In addition to benefiting your wallet, painting your roof white can have global effects. By eliminating black heat-absorbing roofs and surfaces in urban areas, especially hot spots, cities will begin to cool down. Currently, Americans spend about $40 billion a year only on air conditioning (1/6 of all electricity generated in the country!). And, 5 to 10 percent of summer electricity is used to mitigate excess heat in urban areas. If only 1/8 of the black surfaces in a heavily impacted city were painted white, summer temperatures would decrease by 2 degrees, reducing the stress on energy grids enough to prevent blackouts and brownouts.
White roofing can also help curb climate control, according to the Department of Energy. Their studies have shown that “Replacing nonreflective, dark roofing with white ones (every 1,000 square feet) would result in an equivalent CO2 offset of 10 metric tons, about $250 annually.” The study concludes by stating that the rate of smog formation increases exponentially with temperature and that making the switch to white roofing would reduce smog by 10 percent in urban areas, which is the equivalent of removing three to five million cars from the roads.
Black roofing is also responsible for heating our waterways. When summer rain falls on a sizzling 180-degree black tar roof, it actually absorbs this heat. Rainwater that falls on hot black surfaces becomes 30 degrees hotter than normal rainwater! And, during a summer storm, this water drains directly into our rivers and streams, raising their temperature well above safe levels for fish and wildlife. Rainwater that falls on a white roof, however, absorbs no heat.
Making the Change
Painting your roof white can be as easy as painting any surface. If, however, you have a slanted roof or have never worked on a rooftop before, you may want to consider contracting the help of a professional painter.
First, find a good paint. You want one specifically designed to be reflective, and one that will easily adhere to the material you have below. For a tar or rubber roof, be sure and use a silicone-based product instead of an elastomeric roof paint.
Next, find a time when you will have sunny skies and temperatures above 50 degrees Fahrenheit for at least two days. Rain or cooler temperatures will interfere with the paint drying properly. Before you begin, wash your roof so you are starting with a clean surface.
You will need paint rollers, brushes, gloves, and enough paint for two thick coats. Mix the paints thoroughly in the cans, or pour the paint into mixing trays. When working with a large space, many painters find it helpful to grid the roof into 100 square-foot blocks using a measuring tape and a can of spray paint. Then, use a 2 to 5-inch brush to paint around the perimeter of the roof, painting up to one foot on the parapets, if you have them.
Next, starting at the far end of the roof from the exit, use your rollers to paint each block of the grid, moving towards the exit. Paint the area near the exit last. Then, allow to dry overnight and repeat the process with a second (thicker) coat again the next day.
All done! You now have a lasting and durable rooftop that will not only reduce your energy bill but your carbon footprint as well.