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LEARNING OBJECTIVES

After reading this article, you should be able to:

• Explain the ill effects of urban heat islands and how to mitigate them.
• Describe the properties of cool roofing products and how they work to decrease rooftop
• temperatures.
• Identify key installation principles for cool roofing systems.
• Describe the benefits of green, planted roofs.

Directly causing elevated urban temperatures, the urban heat island (UHI) effect has not only been identified as increasing peak summertime A/C electrical demand by 5 to 10 percent, but UHI has been documented to increase smog, adversely affect health and compromise water quality.

Defined as densely populated areas where temperatures range between 2 F and 8 F warmer than surrounding less urban areas, UHI is a force to contend with.

“Three factors have been identified as the primary causes of UHIs: the reduction of vegetation, such as trees, shrubs and grass; a concentration of dark, energy-absorbing roof surfaces; and an increase in dark, energy-absorbing surfaces for roads and parking lots,” explains Drew Ballensky, general manager, Duro-Last Roofing, Iowa City.

 Essentially, large areas of rooftop surfaces and pavement compromise the environment’s ability to release the sun’s radiation which is absorbed throughout the day. Ideally, natural surfaces—i.e., vegetation and soil—naturally transform the radiation into water vapor through the process of evapotranspiration and cool the surrounding air. However, in heavily built-up areas, non-reflective and water-resistant construction materials impede this process, thereby absorbing high percentages of radiation which is then released as heat, as opposed to cooling water vapor.

To more clearly establish the extent to which urbanization is compromising a city’s ability to naturally cool itself, the percentage of hard surfaces in most U.S. cities is approximately 60 percent, with roadways making up between 35 percent and 45 percent, and rooftop surfaces accounting for around 16 percent, according to Yves Baudouin, director of graduate studies, Department of Geography, University of Quebec at Montreal.

However, Lawrence Berkeley National Laboratory (LBNL) puts these rooftop numbers even higher at between 20 percent and 25 percent of urban land cover.

 “The urban heat island effect is problematic because it makes the warmest times of the year even hotter. This increases human discomfort, drives air conditioning usage higher, generates more greenhouse gas emissions from increased electricity production, places more strain on the electricity grid during peak electricity periods and worsens air quality as increasing temperatures catalyze the formation of ground-level ozone,” explains Benjamin Mandel, graduate student research assistant, heat island group, environmental energy technologies division, Lawrence Berkeley National Laboratory, Berkeley, Calif.

This rise in temperature also compromises water quality as excess heat from pavement and rooftops is then transferred to the stormwater draining into the streams, rivers, ponds and lakes, says Andrea Denny, local climate and energy program lead, U.S. Environmental Protection Agency (EPA), Washington, D.C.

Of greater concern are the adverse health effects stemming from UHIs, particularly targeting the elderly and the young. These include respiratory problems, heat stroke, exhaustion and heat cramps. In fact, according to the Centers for Disease Control and Prevention, excessive heat exposure contributed to more than 8,000 deaths from 1979 to 2003.

With the price tag and dangers of UHIs well established, designers and building owners are actively taking steps to mitigate these ill effects. As the most effective strategy, “cool roofs” are frequently a go-to solution for today’s building projects. In fact, LBNL estimates that converting 80 percent of the nation’s commercial buildings to cool roofs would reduce 6.23 metric tons of CO2 emissions. Practically speaking, this is the equivalent of annual emissions released by 1.2 million conventional cars.

“Cool roofing products are made of highly reflective and emissive materials that release absorbed heat and can remain approximately 50 F to 60 F cooler than traditional materials during peak summer weather,” says Denny. “A cool roof transfers less heat to the building below, so the building stays cooler and uses less energy for air conditioning.”

Approximating these AC savings as falling between 10 percent to 30 percent, Jessica Clark, LEED AP, marketing liaison, Cool Roof Rating Council, Oakland, Calif., also points out that cool roofing’s ability to limit temperature fluctuation also boosts the system’s longevity and decreases maintenance costs.

How it Works

To better understand how cool roofing products work, as compared to conventional dark roofing, performance is characterized by solar reflectance, also called albedo, and thermal emittance.

To assess solar reflectance, a material’s ability to reflect energy at each solar energy wavelength is measured, and then the weighted average of these values is calculated. Whereas traditional roofing offers a low solar reflectance of 5 percent to 15 percent, which means that they can absorb up to 95 percent of solar energy, cool roofing products offer a high solar reflectance of more than 65 percent. In both cases, radiation is reflected across the entire solar spectrum, but particularly in the visible and infrared wavelengths.

Thermal emittance is defined as the roofing surface’s ability, per unit area at a given temperature, to re-radiate heat that is absorbed.

“When exposed to sunlight, a surface with high emittance will reach thermal equilibrium at a lower temperature than a surface with low emittance because the high-emittance surface gives off its heat more readily,” explains Denny.

Combining these two metrics yields the Solar Reflectance Index, which is assigned a value from 0 to 100, the higher numbers signifying a more energy-efficient roof.

What’s important to understand is the fact that it’s the combination of these two values which will yield the best performing products. For example, according to the EPA’s “Reducing Urban Heat Islands: Compendium of Strategies on Cool Roofs” white paper, standard black asphalt roofs have high thermal emittance but low reflectance, so summertime temperatures can hit 165 F to 185 F. And while metallic roofs have high reflectance, their low thermal emittance will send the temperature to between 150 F and 165 F during the summer months. Meanwhile, cool roofing products, with both high reflectance and high emittance, will keep summertime temperatures between 110 F and 115 F.

Even more impressive is the potential collective effect of cool roofing systems. As estimated by a simulation conducted by researchers at Columbia University and Hunter College, a 50 percent adoption of cool roofs in New York City could result in a city-wide temperature reduction of 0.3 F, peaking at a 0.6 F reduction at 3 p.m.

Another LBNL study, commissioned by the U.S. Department of Energy (DoE) and the EPA, studied the air conditioning and peak electrical demand for a 100,000-square-foot retail facility in Austin, Texas. For one year, data was collected on the building’s original black rubber EPDM roof. The roof was then retrofitted with a white, reflective PVC roofing system, and AC and electrical loads were analyzed for a second year.

In comparing thermal and energy performance for the black roof and reflective roof, LNBL discovered AC savings of $7,200 for the white roof, based upon 2001 utility prices, and an estimated lifetime savings of between $62,000 and $71,000—not including load curtailment and/or utility rebate programs—over the course of 13 years.

Another LBNL initiative involved modeling a propotype building in multiple cities to project the hourly heating and cooling loads for a conventional roof versus a cool roof during a typical year. The study found that states with hot climates—namely Arizona, New Mexico, Nevada—stood to benefit the most from cool roofs with calculated average annual savings of 7.69, 6.92 and 6.86 kWh/m2, respectively per conditioned roof area. In contrast, a cooler state like Minnesota would glean a cooling energy reduction of 4.17 kWh/m2 of conditioned roof area throughout the course of the year.

Choosing the Right Product

To achieve such promising results, there are a number of cool roofing products for building teams to choose from. Essentially, they fall into two main categories: protective paints and coatings, and single-ply roofing systems.

While the former can offer an effective short-term approach, most building owners opt for longer-term solutions. Of the different types of single-ply systems, white polyvinyl chloride (PVC) roofing products offer the longest performance track record, according to Ballensky. Originally introduced in Germany in the 1960s, white PVC gained popularity over the next couple of decades. The technology came to the U.S. in the 1970s and was the first single-ply roofing product to obtain a standard designation from the American Society for Testing and Materials (ASTM).

“More recent cool roofing single-ply developments include the introduction of thermoplastic polyolefins and certain new co-polymer alloys during the late 1980s and 1990s,” adds Ballensky.

But before determining which kind of product is right for a given project, building teams must first figure out whether a cool roof makes sense as effectiveness is largely based on climate. While buildings in the DOE’s warmer climate map zones 1-3 will most likely benefit from cool roofing technology, Shad L. Traylor, AIA, NCARB, CDT, MBA, LEED AP BD+C, senior architect, BRPH, Melbourne, Fla., points out that projects located in climate zones 4-7 might actually benefit from a darker roof material to absorb the heat, thereby reducing winter heating costs.

To assist project teams to make this call, a cool-roof calculation tool offered by the DoE on the Oak Ridge National Laboratory website can be very helpful. In addition, the EPA offers a cool roof calculator.

“The software requires a few simple project inputs of your location, proposed roof, energy costs and equipment efficiencies, and it outputs a detailed comparison between the proposed roof and a black roof to yield the annual energy savings,” explains Traylor.

In addition, designers should consider building type, existing building conditions, roof slope, project budget and maintenance considerations when considering cool roofing solutions. Because some cool roofing products may cost more than standard solution, it may be helpful to determine whether the local utility offers an incentive. To quickly determine this, specifiers can reference the Cool Roofing Rating Council (CRRC) current database of programs.

Another nifty CRRC tool is a searchable database of roof product reflectance ratings, which can be very helpful in determining if a specific product will meet local building codes and standards for solar reflectance. In addition, ENERGY STAR publishes a Qualified Products list. The CRRC’s database lists reflectance values as rated by a third-party laboratory, and ENERGY STAR’s list contains products that meet the EPA’s minimum values.

Offering some valuable design pointers, building teams may also want to check out the DoE’s Guidelines for Selecting Cool Roofs and the Global Cool Cities Alliance’s Cool Roof Toolkit.

But regardless of whether cool roofing is chosen, Clark recommends considering additional environmental factors, including extraction method, location of raw materials, toxicity of materials, embodied energy, recycled content, durability and end-of-life recyclability when selecting a roofing system.

Installation Best Practices

Of course, the best quality, highest performance product will not do its job without proper installation. Whereas a good installation job will enable most roof coatings to last more than 20 years, a poor application can cause cool roof coatings to peel or flake off within a couple of years.

“One of the most important steps in a successful roofing project is choosing a knowledgeable contractor who has experience working with cool roofing and the product type one is interested in using,” confirms Clark.

Clark also points out that because cool roofs stay cooler than traditional roofs, this can lead to moisture and condensation issues if not properly installed.

As with any roofing installation, creating an airtight enclosure is key. And while a properly installed air barrier can help accomplish this, access to the proper location or creating a continuous and structurally supported substrate for the air barrier can be difficult to achieve, cautions Traylor.

“The most vulnerable location for air leaks is the transition point between the wall and roof assembly,” he explains. “Spray foam insulation or fluid-applied barriers are favored in most circumstances.”

With a spray-on application, the product can be applied seamlessly from the wall onto the roof substrate, creating a continuous seal. The foam insulation doesn’t require clips, and some products also offer antimicrobial protection as well.

Vapor barriers are also important, particularly in cold weather climates where the formation of frost or condensation can form on the underside of a mechanically fastened roof membrane, according to David Cook, R.A., principal architect, Structural & Architectural Evaluation, CTLGroup, Skokie, Ill.

“The warm, moist air can be pumped up through the insulation as the membrane flutters between fasteners during periods of increased wind,” he explains. “Lighter roofs remain cooler, reducing their ability to thaw or dry. Once the period of colder weather passes, the moisture on the lower surface of the roof membrane thaws, causing what appears to be water leakage on the interior of the building.”

Bandouin also points out that some glues used as sealants to join the membranes can attract rodents, which can damage the roof, so careful product selection is important.

And while cool roofing is designed to boost a building’s energy efficiency, it is not an all-in-one solution.

“Cool roofing can improve the energy efficiency of buildings, but it is only one strategy of many that can be used in a complementary manner,” explains Mandel. “Because cool roofing does not address the need for heating energy in cold months, appropriate levels of insulation and other weather-proofing, such as window glazing, should be ensured to minimize the effect of outdoor temperatures on the indoor environment year-round.”

Although the need for insulation will vary based upon climate, utility prices, building use, code considerations and preference, it will most likely be required in colder climates and in cases where the roof accounts for at least a fourth of the total building envelope.

Offering some general installation advice, Ballensky notes the following:

• Ensure compatibility between the roofing components and materials.
• Investigate the reliability and track record of the roofing material.
• Hot-air welded seams are the most reliable.
• Less seaming is better, as this reduces the risk of errors.
• Horizontal-to-vertical flashing transitions are the most vulnerable and should be executed with care.
• Double check the product’s warranty. Read the fine print and make sure what’s offered is realistic.

Overall, Ballensky explains, “With all of these factors, it is also important to consider the use of the building and how it might affect condensation, energy usage, occupant comfort and control over contents.”

Moving forward, specifiers can expect more products to hit the shelves as new technologies are constantly being developed and tested. To assist manufacturers, LBNL’s Heat Island Group is currently developing an accelerated aging technique to test and assess how products will perform in different climates zones throughout their lifetime.

Mandel also projects that the market will eventually come forward with cool roofing options for product types where cool roofing currently doesn’t exist, such as cool-colored asphalt shingles.

“By making cool shingles more available and cost-competitive, more building owners will be able to embrace cool roofs without compromising their aesthetic preferences,” he says.

One myth to dispel about cool roofing is the idea that the coatings only come in white. While it’s true that white materials are generally very good solar reflectors, colored roofing materials can also be manufactured to reflect high percentages of sunlight, says Denny. Essentially, more than half of sunlight is invisible to the human eye, but still contributes to solar heat gain. Colored surfaces generally work to reflect most of this invisible sunlight, thereby creating the same cooling affect, but appearing as a colored surface, even dark colored.

“These ‘cool-colored’ products allow consumers to find the cool roofing option that is best suited to the particular design considerations for a project,” says Mandel.

For example, if a facility is surrounded by taller buildings, a white roof could create glare issues, so a colored roof may be the best way to go. Or, in many cases, building owners may have an aesthetic preference for choosing a colored roof.

Green, Vegetated Roofs

“Another alternative to white roofing is garden, or vegetative, roof systems. Garden roofs go back as far as the Hanging Gardens of Babylon, but they are enjoying renewed interest primarily as a means of reducing the effects of UHIs while creating aesthetically appealing, environmentally friendly construction,” reports Ballensky.

Offering benefits beyond UHI mitigation and energy efficiency, green planted rooftops are also known for stormwater retention, rainfall pollutant filtering and evaporative cooling. In fact, the surface temperature of a green roof can actually be cooler than the air temperature on hot summer days.

“Rooftop vegetation can also remove air pollutants and greenhouse gas emissions through dry deposition and carbon sequestration and storage,” adds Denny.

Aesthetically, green roofs are well known as beautiful places of respite, particularly when situated within bustling urban environments. As a habitat for native plants, and potentially animals, a good percentage of garden roofs are accessible and may even offer a small walking trail, trellises or water features.

Putting some numbers behind these ideas, a Chicago study compared the rooftop temperatures on a hot summer day between a green roof and neighboring dark, conventional roof. With temperatures in the 90s, the green roof’s surface temperature ranged from 91 F to 119 F, while the darker roof’s temperature reached 169 F. Meanwhile, the near-surface air temperature above the green roof was approximately 7 F cooler than the conventional roof.

For a similar Florida study, a green roof’s temperature was measured at 86 F while the rooftop on the neighboring building, which happened to be light-colored, registered at 134 F.

In an attempt to quantify the extent to which a widespread adoption of green roofs could potentially reduce temperature over a large area, the National Research Council Canada conducted a modeling study for Toronto and predicted that adding green roofs to 50 percent of the downtown area would cool the entire city by 0.2 F to 1.4 F. By irrigating these roofs as well, temperatures could be bumped down another 3.5 F, and extend a 1 F to 2 F reduction over a larger area.

In the same previously referenced NYC simulation study, researchers also found that a 100 percent conversion of the city’s roofs to green roofs would result in city-wide temperature reduction of 0.4 F, with 3 p.m. temperatures falling 0.8 F.

Despite these statistics, Mandel points out that while green roofs have a higher albedo than conventional dark roofs, they aren’t as effective as cool roofs when it comes to the UHI mitigation. Essentially, vegetated roofs’ cooling property is evapotranspiration, which means that their absorbed solar energy is redirected toward vaporizing water, as opposed to heating the building.

“These effects keep cities cooler directly and shave peak load on the hottest days, which has an indirect UHI mitigation effect,” he explains. “However, because green roofs lead to more evaporation, they will cause more rainfall, and the heat absorbed during evaporation is released upon precipitation. This means that green roofs cannot cool the world nearly as effectively as white roofs.”

In fact, a competition between Chicago City Hall, with its green roof, and Chicago’s Cook County Building—which shares the same structure with City Hall, but with a white, reflective roof—resulted in the Cook County Building saving $65,000 on its electric bill in 2011, whereas City Hall only saved $22,000.

Despite these differences, many local governments are promoting, and in some cases, mandating green roofs as a way to combat UHIs. As listed on the EPA’s website the following cities and states are offering incentives for green roofs as a means of meeting local building codes requirements: California, Chicago, Washington, D.C., Kansas City, Boston, Cincinnati, Portland, Pennsylvania, Austin, Dallas, Houston, Arlington and Seattle.

In addition, LEED recognizes planted green roofs as a way to earn credits for Landscape and Exterior Design to Reduce Heat Islands. And depending on how the green roof is designed and integrated into the building, projects can earn up to 14 credits for stormwater management, recycled content, reduced site disturbance, water-efficient landscaping, local/regional materials and optimized energy performance.

Green roofs have also been documented to reap significant energy savings. For example, a study conducted in central Florida found a green roof’s average rate of heat transfer to be 40 percent less than the adjacent light-colored rooftop, resulting in an estimated summertime energy consumption reduction of 2.0 kWh per day.

Another Chicago study, this one a simulation of City Hall’s green rooftop conducted by ASHRAE, revealed that every one degree Farenheit decrease in temperature would cause a 1.2 percent reduction in cooling energy use. Consequently, if all of Chicago was retrofitted with green roofs, then over a period of 10 years, this would amount to $100 million in annual savings from reduced cooling loads.

Meanwhile, another CRRC modeling study conducted in Toronto found that a 32,000-square-foot green roof on a one-story commercial building could save 6 percent in cooling energy and 10 percent in heating energy usage, amounting to a savings of 21,000 kWh per year.

At the same time, green roofs do require some investment beyond growing a few plants on the rooftop. Rather, they require a waterproofing membrane, root barrier, drainage layer, lightweight soil or engineered growing medium and vegetation.

Building owners also need to determine whether to install an inaccessible extensive, lighter-weight roof with more self-sufficient plant types or a more built-up intensive roof. While there are no studies that official document which green roof type is more effective at lowering rooftop temperatures and UHI mitigation, researchers at the University of Quebec are beginning to look into this question have already gathered data on fine airborne thermal coverage.

However, what is known is that extensive roofs offer better energy savings. Although their installation is more expensive, they require more structural support and need more maintenance over the long term.

State of the Art

With noted roofing technology advancements—both for cool and green roofs—one thing is for sure: building owners are enjoying more durable, lasting and flexible roofing materials.

“More sophisticated engineering of roof assemblies is helping improve roof performance in high wind and hail areas and evolving technologies are providing for better fire, oil and chemical resistance in roofing materials,” notes Ballensky.

And aesthetically, products have broken out way beyond initial offerings of just a few colors with unique colors and patterns currently available for highly visible roofs.

With good quality products and increasing choices, cool and green roofing systems are well positioned to help battle UHIs and reap numerous additional benefits along the way. 

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