By: Robert J. Dulovics, RRC, RRO
Why does the color of your roof affect how fast you can drive your car?
Well, it doesn’t at this point in time. But, in the near future it may. Houston and other large cities that struggle with air quality issues are evaluating factors that contribute to the problem and developing creative solutions for reducing pollution.
You may remember the hubbub surrounding one of the earliest solutions implemented in Houston. Speed limits were reduced in an effort to help mitigate the contribution of automobile tailpipe exhaust to air quality. Negative public reaction to the change quickly forced speed limits back up, but that wasn’t the end of the environmental effort. In fact, other methods for reducing air pollution in the Houston area are now being explored, including some that may ultimately impact the roofing industry as well as building owners and managers.
Temperature Affects Air Quality
The Houston-Galveston Area Council (HGAC), in association with the Houston Advanced Research Center (HARC) have put together the “Cool Houston Plan” based on the notion that reducing the overall temperatures in the city will help alleviate the air quality problem.
Large cities are generally 6º to 8º F hotter than surrounding suburban and rural areas. This phenomenon has been dubbed the “urban heat island effect.” Major causes of the effect are dark roofs and dark pavement on roadways, parking lots, and sidewalks. These surfaces act as giant solar collectors that absorb and retain heat from the sun during the day, releasing it later in the day as the air temperature begins to cool.
One effect of this phenomenon in cities such as Houston is diminished air quality. Reason being is that ozone and smog form more readily at higher temperatures. Cool paving technologies, reflective and green roofs, and increased vegetation are all ways to mitigate against the urban heat island effect; each is a part of the Cool Houston Plan.
The surfaces of traditional roof systems (i.e. built-up roofs, modified bitumen, etc.) can reach temperatures of 150º F to 180º F during Houston’s summer months. These extreme temperatures can have many adverse effects for building owners, the roof itself and the region where they are located. Therefore, one of the goals of the Cool Houston Plan is to achieve the widespread use of cool roofing in the metro Houston area over the next 10 years.
What Exactly is Cool Roofing?
The color of a roofing surface has a profound effect on the temperature it will reach while exposed to solar radiation. Studies have shown that a cool roof system can reduce rooftop temperatures by 50º to 60º F during peak summer months. Solar energy reaches the earth as ultraviolet rays (3%), visible light (40%), and infrared energy (57%). Solar reflectanceis defined as the percentage of solar energy that is reflected by a surface. This value is often referred to as Solar Reflectance Index (SRI). The “coolest” materials have a high reflectance across the entire solar spectrum, primarily for the visible and infrared wavelengths. Solar energy that is not reflected is absorbed by the roofing material and converted to heat. The ability of a material to give up the heat that it does absorb is called emissivity. For a roofing material to function well as Cool Roofing, it must be both highly reflective and highly emissive. There are a variety of cool roof solutions that meet both these requirements. The most common applications include liquid-applied coatings that can be applied to existing roofing, single-ply manufactured membranes with high reflectance values, and metal panel and specialty roof systems finished specifically to be reflective. Green roofs are another alternative. These systems incorporate vegetation and a growing medium as part of the roof assembly, which promote cooler temperatures through shading and the cooling effects of water in the plant life.
The Benefits of Being Cool
Cool roofing not only reduces temperatures that contribute to air pollution, but it can also pay for itself over time through energy savings. Cool roofs actually reduce electricity consumption during hours of peak demand. This also helps to avoid building excess power generation capacity, saving consumers money and avoiding new air pollution sources. Peak power reductions for the Houston area are estimated to be 1.3 percent of peak energy use from cool roofing on office and retail space alone.
Furthermore, solar radiation and high temperatures are the enemies of most roofing materials. Reducing the temperature of the roof surface through the use of Cool Roofing will extend the life of the material (when compared to similar, hotter materials). This increased life is especially true for green roofs where the membrane is almost completely isolated from the weathering elements and exposure to UV radiation.
Putting the Cooling into Effect
The Cool Houston Plan puts forward several potential mechanisms for accomplishing its goals. One mechanism is to provide economic incentives to the end user. This program would offer qualifying participants a “rebate/reward” based on the amount of installed roof area. A similar program, implemented by the State of California, offers incentives that range from 15¢ to 25¢ per square foot of roofing for the installation of reflective roofing materials.
Other programs might include required use of cool roofing in municipal code and proactive education of public agencies such as school districts, local governments, community colleges, public universities, and hospitals encouraging the use of cool roofs.
So, amongst other benefits, cool roofs can help allow us to continue to drive fast in Houston. Undoubtedly this roofing trend, along with its environmental and financial benefits, will become more common in the near future.
Mr. Dulovics has as Associates Degree in Architectural and Construction Engineering Technology. He has more than 20 years experience in roof consulting and is a Registered Roof Consultant and Registered Roof Observer (Roof Consultants Institute).