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A cool roof can reduce energy costs, extend the life of a roof and increase interior comfort, all while helping the environment.

When students at Bemis Elementary School in Rialto, California learned that light colors better reflect the heat of the sun, teacher Jeff Saks and his students presented their project to the Rialto Unified School Board and requested that the school’s roof be painted white to keep the building cool during the hot summer months. The board agreed and the school used a basic white roof coating on its dark roof. In the first year, the school saved 229,159 kWh and over $38,000 through this and other energy saving programs.

Better performance and saved energy costs, along with additional
environmental benefits, make cool roofs a cool alternative.

As Bemis Elementary  School’s experience shows, a “cool” roof can significantly lower energy costs and improve building performance. Roofing manufacturers now make a variety of products designed to be cool. These products more effectively reflect the sun’s radiation and emit heat back to the sky rather than transferring it to the building below. This makes the building cooler, increasing interior comfort and reducing the need for air conditioning. The result is a roof that saves energy, reduces electric bills and tends to last longer, decreasing roof maintenance. Cool roof products have been around for years, but because of the efforts of organizations such as the Cool Roof Rating Council (CRRC) and the U.S. Environmental Protection Agency’s Energy Star ®  Roof Products program, combined with the incorporation of cool roofs into certification programs like Leadership in Energy and Environmental Design (LEED) and California’s Title 24 building energy code, the market has expanded significantly. Cool roof products are now available in a range of roofing product types and variety of colors.

Benefits of Cool Roofs
Cool roofs outperform conventional roofs by doing exactly what their name implies - staying cool, especially during the hot summer months. A cool roof means a cooler building and less need for air conditioning.

The most obvious benefit of this feature is lower energy use and, therefore, reduced utility bills, but cool roofs offer several additional benefits, including:
• Increased life of the roof, thanks to decreased thermal stress on materials,
• Reduced HVAC system size, thanks to the smaller peak cooling loads,
• Increased comfort for building occupants,
• Diminished peak demand effects on the power grid, and
• Reduced urban heat island effect, which in turn helps minimize the formation of greenhouse gases and smog. Once Bemis Elementary School painted its roof white, several   other buildings and houses in the neighborhood picked up on the many benefits of a cool roof and painted their roofs white too.

While providing a number of benefits, cool roofs do not pose a large additional cost when integrated into the design for new construction or renovation. Some cool roof options add no cost, for example choosing a lighter color shingle or tile. Other options, such as adding a cool roof coating over a built-up roof, may cost extra, but these options increase cost often by only 5 to 10 cents per square foot. In many locations these costs are offset quickly by lower utility bills and other benefits.

Products Evolve
For millennia, people in hot climates have used light-colored materials to keep buildings cool. In the United States, cool roofs gained popularity in the 1980s, when utility rebates became available for cool-roofing materials. Unfortunately, a lack of standards for the technology led to a variety of disappointing results.

Thankfully, much has changed since those early programs, when the only real requirement was for materials to be light colored and reflective. Today, voluntary programs and energy codes require measured radiative property data and demonstration of performance over time.

In addition, the roofing industry has developed a much greater range of high-performing cool roof products. Although Bemis Elementary School used a simple white roof coating over the existing roof, cool roofing materials are available for
virtually every type of roof, from modified bitumen and single-ply
membranes to field-applied coatings and painted metal
roofing.

Radiative Properties
To help assess how “cool” a roofing material is, both the CRRC and Energy Star ® maintain respective product directories that rate roofing products according to the products’ radiative properties. The CRRC measures two properties, solar reflectance and thermal emittance, while Energy Star ® currently lists only solar reflectance (and only includes products exceeding a minimum reflectance).

Thinking intuitively, a roof will stay cooler by reflecting the sun’s energy rather than absorbing it, as well as by quickly releasing the heat it does absorb. Solar reflectance and thermal emittance quantify these two properties, respectively.

Solar reflectance measures the fraction of the sun’s energy that is reflected by the surface of the roof. Thermal emittance refers to the relative ease with which the roof radiates away the heat it absorbs. Both properties are measured as a fraction (from 0.00 to 1.00) and the higher the value, the cooler the material. These two measurements can also be mathematically combined into one value called the Solar Reflectance Index or SRI.

As the students at Bemis Elementary School learned, light-colored surfaces tend to have higher reflectance values than dark surfaces. Uncoated shiny metal surfaces are good at reflecting the sun’s energy, however, they are not good at emitting heat gained. Because of this low emittance, they become very hot during the day despite being highly reflective. So both high solar reflectance and high thermal emittance are important for improving a building’s energy performance.

Many elements of a roof’s structure - including insulation and radiant barriers - affect performance of the roof. While these and other components are undoubtedly important, the phrase “cool roof” refers specifically to the top surface of the roof. CRRC and Energy Star ®’s programs address only the radiative performance of a roof’s surface.

Focus on Savings
When comparing a cool roof with a regular roof, a difference in the radiative properties of the roofing material can mean huge differences in surface temperatures. While a dark colored roofing material may measure 60 degrees above the ambient outdoor temperature, a cool roof tends to be as few as 10 degrees above ambient.

A lower temperature means the roof conducts less heat into the building, which translates into a smaller cooling load. This reduced cooling load may allow a smaller HVAC system, leading to capital cost savings in addition to energy benefits. The exact amount of savings varies depending on climate, roof performance, insulation and other aspects of the building, but a reasonable average savings expectation for a typical application of cool roofing is in the range of 10-30 percent of the cooling energy required. This range might equate to a total building electric bill reduction of 3-10 percent. These ranges generally apply to low-rise buildings in regions with significant cooling loads.

Northern Exposures
Although the benefits of cool roofing are clear and virtually universal in hot and sunny climates, some managers are concerned that a cool roof will increase heating costs during winter months, particularly in cold northern states. Typically, this loss is less than generally thought because in winter months the sun is low in the sky, is less intense and shines for fewer hours each day.

Especially for commercial buildings, which typically have higher internal heat gains, summertime air conditioning benefits far outweigh any heat gain lost during wintertime. Further, because reduced air conditioning loads reduce peak grid demand and help prevent urban heat island effects cool roofs have several key advantages despite the potential, minor wintertime losses.

Codes and Programs
The many benefits of cool roofs are leading program managers and code bodies to integrate them into energy programs and building codes.

California’s building energy code, Title 24, which was updated in October 2005 to prescribe cool roofs for many low-slope, non-residential applications, is leading the way. Using a cool roof is a simple and cost-effective way of meeting low-slope commercial new construction requirements and it is the only way to comply in non-residential re-roofing situations.

Under Title 24, to qualify as a “cool roof” low-slope roofing materials must have a rated initial solar reflectance of at least 0.70 and thermal emittance of at least 0.75. Title 24 uses the CRRC as the supervisory entity from which these ratings must be provided. Alternatively, Energy Star ® , referenced by other jurisdictions, currently requires a minimum solar reflectance of 0.65 and accepts manufacturer provided data.

Other jurisdictions outside of California have or are considering adopting cool-roof standards, and for the nearly 30 states that reference current ASHRAE standards 90.1 and 90.2, their codes may consider radiative properties. Also, green building programs, including the national LEED certification program, credit savings for using cool roofing. Many electric utilities are offering rebates for cool roofing materials. Utilities in California, Florida, and Idaho currently have cool roof rebates and more are on the way.

What Lies Ahead
Though cool roofing is rapidly becoming widely recognized for its energy savings benefits, the emphasis so far has been mainly on low- lope roofs. This is, in part, because of the visibility of steep-slope roofs. Most people seem to prefer the aesthetics of a darkly colored roof compared to a white or very light colored roof. As a result of this preference, many more cool roof options are currently available for low-slope roofs than for steep-slope.

Research underway by the Public Interest Energy Research Program (PIER), the Oak Ridge and Lawrence Berkeley National Laboratories and several roofing manufacturers is focused on developing new, more richly colored, cool roof products, especially for steep-slope applications. So far, this research collaboration has developed several colored pigments that are significantly cooler than their traditional counterparts. These cool, colored products greatly increase the number of options for cool, steep-slope roofing.

These research efforts combined with greater integration of cool roof standards in energy codes and programs will facilitate an increase in cool roof applications in the future. Building managers, maintenance personnel and engineers will benefit from a better understanding of cool roofing options and from using these systems on their buildings. Better performance and saved energy costs, along with additional environ-mental benefits, make cool roofs a cool alternative.

Stephanie Stern, Administrative Staff works for the Cool Roof Rating Council and can be reached at 510.482.4420 x229 or you can visit www.coolroofs.org.

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