Cool roofing: a really hot topic.

For millennia shrouded in history, man has had a roof overhead, be it made of stone, twigs, ice, thatch, slate, wood, asphalt, tin, steel, painted metal, tile, and various and sundry materials. All served, to one degree or another, to keep out the elements and maintain the interior of the dwelling at a livable temperature. For the vantage point of those who are living in the first decade of the Third Millennium A.D., not much has changed--many, perhaps all, of these materials are in use somewhere in the world today, and their basic purpose is essentially unchanged. Even when limited to modern building materials in general use throughout the industrialized world, and with particular emphasis on painted metal, the topic is still huge and interesting--and getting more interesting by the day. In the United States alone, approximately 20 billion square feet of new and replacement roofing are installed every year, and a new dimension is being added to the traditional requirements placed upon roofing: energy conservation. Not just energy conservation for any given building or structure, but energy conservation on a local, regional, and national scale.

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It is easy to imagine standing on the roof of a house or building on a sunny summer day and feeling the waves of heat rising from the roof. During the hot summer months, the surface temperature of a roof can be considerably hotter than the surrounding air temperature, perhaps by as much as 40[degrees]F or more. Almost anyone in the construction industry can affirm that a roof surface--especially a dark-colored one--is absorbing a certain amount of radiant energy from the sun. The roofing material cannot dissipate the heat completely, and it therefore heats up until it reaches an equilibrium temperature, where the rate of heat loss equals the rate of heat gain. There is more, however. Some of the energy absorbed by the roof will make its way into the building cavity. In the summertime, and in those areas of the country where air conditioning is prevalent, the greater the amount of energy transferred into the building, the greater is the amount of cooling required. More cooling equates to greater demand for electricity, and--as this demand increases--more electrical power generating facilities are required. This "vicious circle" is exacerbated by the fact that electricity cannot be stored. Enough electricity needs to be available for the greatest peak demand period, or shortages will be experienced. Since electricity is principally created by burning something (oil, natural gas, coal), more electricity equates to more pollution. Is it any wonder, then, that both the Department of Energy (DOE) and the Environmental Protection Agency (EPA) are interested in ways to slow the constantly increasing need for electricity, which should, in turn, lead to a resultant improvement in air quality?

The problem with this "hot roof" phenomenon, however, doesn't stop there. A "hot roof" creates the need, not just for more "cooling" in any given building, but for greater cooling capacity, on a regional basis. To reduce the heat flow into the building cavity, additional insulation will help to reduce cooling demand. As the roof heats up and reaches its equilibrium temperature, however, it is releasing some of this thermal energy to the surrounding environment. This creates localized heating, and is commonly called the "heat island effect." Leave the wooded park in August to pick up some items at the local "big box" and you will feel the heat island effect as you enter the urban area, with a paucity of trees and preponderance of concrete. This heat island effect leads to elevated air conditioning use, increased air pollution (due to more consumption of fossil fuels to meet this increased energy demand), and the formation of smog, which increases with higher temperatures. The heat island effect may have devastating consequences. In July 1995, it is estimated that between 500 and 700 people died in the city of Chicago during a heat wave. (1,2) Certainly the daytime--and nighttime--temperatures during those few July days were high, but the heat island effect added many degrees to the urban Chicago area. It is natural, therefore, that the EPA and National Laboratories (3) are interested in addressing heat island issues from the perspective of improving air quality.

No single approach or technology can be expected to overcome these issues, but "cool roofing" is rapidly becoming recognized for the benefits it provides to all players in the roofing and construction industry. Research by the National Laboratories, as well as other institutions, points to the importance of cool roofs in lowering the energy demand of buildings and mitigating urban heat island effects. Cities and states across the country are taking note of this development through the incorporation of cool roof requirements into their building codes or the initiation of cool roof incentive programs. As a result, the concept of cool roofing is currently one of the hottest topics in the building and construction industries, as well as in those related industries, such as paints and coatings, which supply them with products and services. What, however, is "cool roofing," why is it of such interest, and how can coatings contribute to making regular roofs into "cool roofs"?

WHAT IS COOL ROOFING?

In a nutshell, cool roofing is any roofing solution that reduces energy demand and mitigates the heat island effect. Cool roofing may be achieved in a number of ways. Simply altering the color of the roofing material to a lighter color will lower the roof temperature. Asphalt shingles can be formulated in light colors, flat roofs that may utilize a layer of black tar can be coated with a number of field-applied coatings which are bright white, and--while not always practical or even possible--a roof may be "vegetated," where grass, ivy, and other low-maintenance ground cover is grown on the roof surface. The most important aspect of cool roofing, regardless of what kind of roofing system is being considered, is that the radiative properties of the uppermost layer of the roof control all of the radiative properties of the entire roofing system. This is where coatings technology enters. A one-mil factory-applied coating exerts as much influence on the radiative properties of a metal (or any other) roof as a 20-mil field-applied roof coating, or a 100-mil roof membrane. When it comes to cool roofing, the "coating is king."

Benefits of cool roofing are clear and virtually universal in hot, sunny climates, particularly with regard to the use of air conditioning, which can be decreased, if cool roofing is installed. One rule of thumb suggests that cool roofing is generally cost-effective in the U.S. at locations found in Zones 1, 2, and 3, shown in Figure 1. In colder climates, the same roofing performance characteristics that reduce solar heat gain (thus cooling load) are also at work during the heating season, meaning that additional energy is required to heat a "cool roof" building. The emphasis on cool roofing, however, is placed on cooling issues, rather than heating issues, because it is estimated that, in the U.S., we consume three times as much electricity for cooling as we do for heating. An easy-to-use calculator is available on the Oak Ridge National Laboratory website (4) that can be used to evaluate the savings potential for an individual building, on a "year-round" basis, using calculations based upon its location, type of roof, air conditioning system, and local (sometimes seasonal) power rates.

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For existing buildings in air conditioning-dominated climates, application of cool roofing offers three important benefits affecting overall building performance:

* Reduction in air conditioning energy costs

* Improvement of the building occupants' comfort

* Possible extension of the life of the roofing materials

For new buildings, cool roofs can reduce the air conditioning tonnage in which the owner must invest.

From the perspective of the power grid, utility and power supply planners are chronically concerned about peak power demand--a demand driven substantially by air conditioning loads in commercial and residential buildings in nearly every region of the country (not just the Sunbelt). Most people quickly recognize that reduced peak power demand means fewer power plants, and the substantial investments that they require. What is sometimes underestimated--at least until the massive outage event in August, 2003, in the northeastern quadrant of the U.S. and southern regions of Canada near Niagara Falls and as far north as Hamilton, Ontario--is the fragile nature of this grid and the need to minimize peak demands. In some states, cash rebates in return for the use of cool roofing are used as a promotional strategy both by state agencies and utility companies to help create greater awareness of cool roofing benefits and encourage building owners to install cool roofs. Many utility groups charge premium rates during peak demand periods, so the ability to reduce demand for electricity during peak periods is beneficial for virtually everyone.

KEY PARAMETERS: SOLAR REFLECTANCE, THERMAL EMITTANCE, AND AGING

Light-colored surfaces, in general, are fairly good reflectors of solar radiation, regardless of composition. So are shiny metallic surfaces. No surface, however, is a "perfect" reflector--and certainly not the real-world surfaces that we commonly find on roofs. When sunlight strikes a roof, the laws of physics dictate that whatever solar energy is not reflected by the surface must be absorbed by it (or pass through it, a case that will be ignored in this discussion, since roofing materials are virtually always opaque). The absorbed energy presents a problem, because it has to go somewhere. There are three possibilities for heat flow:

* Conductive heat loss is the transference of heat into the building envelope via surface-to-surface contact.

* Convective heat loss occurs as a liquid or gas (moving air and wind in the case of roofing) absorbs some of the heat energy.

* Radiative heat loss occurs as the roofing material itself radiates some of its energy back into the atmosphere.

Most roofing materials have reasonably similar temperature-dependent conductive and convective heat transfer properties, and the latter variable is more a function of wind speed than anything else. The key properties for roofing materials, therefore, are their reflective properties (which affect the roof temperature) and their ability to re-radiate the heat to the sky.

In the roofing industry, the Total Solar Reflectance (TSR) is the key value that describes the extent to which a material reflects all wavelengths of energy, not just the infrared region of the electromagnetic spectrum. The ability of a material to emit energy by a radiative process is known as its Thermal Emittance (TE). Whereas TSR can be rather intuitively related to the color of a material, perhaps based on our life experience with car interiors and dark-colored roofs (e.g., dark colors are good absorbers, and light colors are good reflectors), TE follows no such simple rule. TE is a basic material property, and, in the roofing market, most materials in commercial use have TE values ranging from 0.75-0.90. For example, a TE value of 0.80 means that the surface being measured radiates 80% as effectively as a perfect emitter, which would have a value of 1.0 (100%). As a general rule, TE is not color dependent. Black painted metal has about the same TE as white painted metal, as long as the two coatings have the same resin chemistry, are applied at the same thickness, and are coated onto the same substrate. The exception to this rule is bare, unpainted metal roofing, which typically exhibits TE values in the range of 0.10-0.30 (10-30%). Materials with high TE values emit more heat than shiny metal materials at the same temperature. If a material cannot radiate heat as effectively, it will be hotter when compared to materials that are good radiators of heat. Although mathematically related under certain conditions, it makes greater sense, for practical purposes involving building materials technology, to consider these two radiative properties--total solar reflectance and thermal emittance--to be separate and independent of each other.

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Specifications for cool roofing will dictate TSR and/or TE values. At this point in the evolution of cool roofing, most of the emphasis is being placed on flat roofs, where the TSR requirements are between 0.65 and 0.70, minimum (depending upon the code authority or municipality). These values essentially limit the color of coatings to white, off-white, and some very light colors. This restriction, of course, makes sense. Light colors reflect a great deal of solar energy, whereas dark colors absorb a great deal of the energy. Cool roofing specifications usually require a thermal emittance minimum as well, and this will be discussed in more detail later in this article.

ASTM methods well suited for determining solar reflectance on roofing materials include:

* E 1918 Standard Test Method for Measuring Solar Reflectance of Horizontal and Low-Sloped Surfaces in the Field

* C 1549 Standard Test Method for Determining Solar Reflectance Near Ambient Temperature Using a Portable Solar Reflectometer 

* E 903 Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres is appropriate for laboratory measurements of small samples

* C 1371 Standard Test Method for Determination of Emittance of Materials Near Room Temperature Using Portable Emissometers

* E 408 Standard Test Methods for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques

* ASTM E 1980 Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces represents a new term, Solar Reflectance Index (SRI), which combines both TSR and TE into one value

Over time, the performance characteristics of most materials and systems degrade through normal wear and tear. Radiative properties of roof surfaces are no exception. For cool roofs, dirt accumulation and biological growth can cause performance degradation, as can simple wear due to exposure to the elements, including UV radiation, rain, high humidity, film erosion due to wind-borne abrasive materials, and so forth. Generally, researchers have found that most performance drop-off involving radiative performance occurs in the first several years, after which time the performance stabilizes. It is imperative, therefore, that manufacturers of roofing materials understand not only the initial radiative properties of their materials, but also these same properties after several years of exposure has taken place.

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SLOPE MATTERS

In general, roofing and roofing materials are characterized as being "low-slope" or "steep-slope." Low-slope is generally considered in the industry as a roof with a "rise-over-run" of two feet in 12 ("2:12") or less. (Flat roofs are de facto considered to be in the "low-slope" category). Steep-slope would be any roof with a rise-over-run relationship of greater than 2:12 (>10[degrees]). In general, steep-slope roofs are far more common in residential applications, whereas low-slope roofs are more common in commercial applications, although this is certainly not a hard-and-fast rule. Another significant difference between low-slope and steep-slope roofing has to do with aesthetic considerations. Generally, low-slope roofing cannot be seen from ground level, so the appearance of such roofing is not a major factor. Steep-slope roofing, however, is easily visible--and "looks count." As a result, manufacturers of steep-slope roofing products invest considerable resources addressing aesthetic issues. Asphalt shingles are now commonly supplied with dimensional characteristics and variegated color schemes. Painted metal roofing comes in thousands of different colors, shapes, sizes, and textures. Concrete tile, clay tile, and cedar shake roofing are all designed with performance and aesthetics in mind. Because of the different requirements for steep-slope vs low-slope roofing--i.e., steep-slope roofs are typically designed as water-shedding systems, and low-slope roofs are designed to form a seal against water penetration--different materials are typically used in their composition and construction.

NATIONAL INITIATIVES PROMOTING COOL ROOFING

The environmental community, and those concerned with "sustainability," recognize the significant benefits of cool roofing. In the U.S. Green Building Council's Leadership in Energy and Environmental Design Rating System[TM] (LEED), (5) cool roofing comes into play in two different ways:

* Qualified cool roofing materials receive a direct credit under the "Sustainable Sites" category, in recognition of the benefits of cool roofing in mitigating heat island effects.

* Cool roofing can help the buildings to attain energy efficiency levels needed to obtain points in the "Energy and Atmosphere" section. This credit recognizes the environmental benefits of reduced electric power consumption.

LEED represents an incentive-based initiative that promotes energy-efficient design in all aspects of building construction. Roofing is just one of the components of a building that can be used to maximize the effective use of energy.

The U.S. EPA's Energy Star[R] program was begun in 1992 to provide guidance to consumers on a wide array of products. A key goal of the Energy Star Program is to assist consumers in identifying specific products, within any general class of products, which are more energy-efficient than the others. For any given product, Energy Star chooses an appropriate rating system (with substantial input from its manufacturing and other partners) and the level of performance Energy Star performer. Consumers thus know that products bearing the Energy Star label, whether computers, kitchen appliances, air conditioning units, or roofing materials, are efficient performers.

For roofing materials, the Energy Star program (6) has been available since early 1999. Energy Star focuses exclusively on the solar reflectance of materials, including both initial values and values obtained following three years of weathering. Manufacturers self-report reflectance values, although they are required to use an appropriate ASTM test method for determining reflectivity, as previously described. Three-year aged values are determined by field measurement of actual roofs (chosen by the manufacturers) that have been in place for at least three years. The numerical average of multiple samples from different regions of the country determines the final three-year value. Energy Star also allows the use of "test farm" samples (coupons), provided that the samples are [greater than or equal to]24 sq. in. in area and exposed at a weathering facility which is in compliance with the requirements of ISO/IEC 17025: 1999 General Requirements for the Competence of Testing and Calibration Laboratories. Energy Star permits washing of samples prior to testing. Again, these aged reflectance values are self-reported by the participants. For low-slope materials, Energy Star qualification requires an initial total solar reflectance of at least 0.65 ("65%") and a three-year value of at least 0.50 ("50%"). Note: A solar reflectance value of 0.65 is basically a white material. For steep-slope materials, initial and "aged" values of 0.25 and 0.15, respectively, qualify. The allowance of substantially lower TSR values for steep-slope roofing recognizes the broad appeal of--and desire for--colored roofing (i.e., not a white material) in applications where the roof is visible to the casual observer. As of January 28, 2004, there were 625 products listed as Energy Star qualified roof products.

These different slope characteristics, combined with the wide range of materials used in roofing--from cedar shakes to asphalt shingles; from tile to painted metal shingles; from built-up roofs to various singleply systems and many others--make "one-size-fits all" measurement and rating systems tricky at a practical level, but, after years of effort, the extant methods seem to handle most situations quite well.

NEW BUILDING CODE REQUIREMENTS FOR COOL ROOFING IN CALIFORNIA

With an eye toward increasing the energy efficiency of buildings, state and municipal code bodies are increasingly requiring more effective roof radiative performance, and California has taken the lead. The California Energy Commission (7) (CEC) recently adopted the 2005 language for the state's building energy efficiency standard, Title 24. This update will have a very significant impact with regard to the use of cool roofing on low-slope, non-residential buildings. In essence, this change in the building code requires cool roofing for all significant roofing jobs for low-slope, non-residential buildings, including "greenfield" new construction and re-roofing jobs. (It is estimated that there are three times as many re-roofing jobs as there are new installations.) In other words, in the State of California, any job involving roofing, and requiring a building permit, falls within the scope of this change. The final effective date for the 2005 update is not yet determined, but a date sometime in the latter half of 2005 is a reasonable estimate for now. This language can be found on the California Energy Commission's Title 24 website. (8)

Before discussing California any further, it is sensible to step back and ask if genuine benefit comes from these energy efficiency initiatives. The best quantitative answer to this question comes from a study done by the Natural Resources Defense Council (NRDC). In their work, Energy Efficiency Leadership in California: Preventing the Next Crisis, the graph shown as Figure 2 was developed to demonstrate the total effect of the overall efforts that have taken place in California, over the past few decades, to minimize energy consumption.

This figure demonstrates a number of interesting things. The first is that, in 1977, the per capita consumption of electricity in California was close to the per capita consumption of the other 49 states. The 1974 energy crisis was a wake-up call for California. The State launched a large number of initiatives to slow the growth in electrical power demand, and the graph clearly shows a lower slope for California after 1977 than for the rest of the U.S. Roofing, of course, is only part of the equation. To achieve the results shown in Figure 2, greater utilization of high-tech window technology, enhanced levels of insulation, development of energy-efficient appliances, and air conditioners, etc., was necessary.

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California has had much experience with the concept of a State Energy Code. Specifications for windows and air conditioning have been codified for some time. In the case of roofing materials, the CEC had no desire to create its own specified department to collect and disseminate radiative data relevant to materials proposed for use within the state to comply with the Energy Code. Instead, it provided the seed money to create the Cool Roof Rating Council (CRRC, see below), and has designated it as the sole supervisory entity for the purpose of maintaining a credible rating system for radiative properties of roofing materials.

Per Title 24, alternate code compliance methods are available for those situations where a cool roof is not feasible for some reason. Under one compliance method, it is possible to "trade off" other (improved) building efficiency measures against roofing materials that fall short of the solar reflectance and thermal emittance requirements for cool roofing. For example, assuming that compliant roofing is not possible, more insulation or less window area may be accepted in exchange. There are other issues associated with Title 24 compliance, but which fall outside of the scope of this article. Readers interested in learning more are advised to visit the CEC website for the details.

In future years, California is likely to consider cool roofing requirements for residential low-slope and steep-slope roofs as well as for nonresidential steep-slope roofs. As is the case with Energy Star, it is likely that the cool roofing definition in such cases will be different from the current nonresidential, low-slope requirements.

COOL ROOF RATING COUNCIL (CRRC)

Formed in 1998, the CRRC (9) was established with a commission to set up a fair, accurate, and credible rating system for the radiative properties of roofing materials. It requires that values be established for both solar reflectance (TSR) and thermal emittance (TE). As with Energy Star, the CRRC requires initial and three-year aged values. Unlike Energy Star, the CRRC system does not define "cool" in terms of any particular performance level; it simply establishes a system to collect credible radiative data (both initial and aged), and maintains a directory of rated products with their rated properties. The CRRC rating program began on September 1, 2002; as of February 2004, the CRRC Directory of Rated Products has listed 143 products.

In another significant departure from the Energy Star program, the CRRC protocols require the use of independent, third-party, CRRC-accredited testing laboratories to establish rated values, utilizing appropriate ASTM test methods. Manufacturers cannot "self-report" performance values. The CRRC system also requires that a portion of rated products be subjected to random testing each year, to assure system integrity.

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Under the CRRC requirements, "aged" values are determined by examining exposure data generated at three designated "test farms" representing hot/humid, hot/dry, and "Midwestern" exposure conditions. All rated products are exposed at all three test farm sites in exactly the same manner and then tested, after three years, using the same ASTM test methods that were used to establish the initial values. The CRRC protocol, unlike the Energy Star protocol, does not permit washing of samples prior to the aged test measurements. The CRRC aged testing program component was scheduled to begin in March 2004.

The Energy Star program for cool roofs is well designed to reinforce its primary mission of encouraging consumers to choose high-performance roofing products for their buildings, on a voluntary basis. The CRRC system, on the other hand, is more focused on maintaining a technically rigorous rating system for use by various parties who may wish to use the CRRC system to assure conformance to performance levels that they choose. The two organizations maintain a close and productive working relationship. U.S. EPA and DOE (sponsoring organizations of Energy Star) assisted in the establishment of the CRRC, and are well-represented on its Board of Directors.

ASHRAE AND OTHER STATES

Currently, the American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) (10) has established two energy efficiency standards--90.1 for commercial buildings and 90.2 for low rise residential buildings. The current ASHRAE standard requires a minimum total solar reflectance of 0.70 and a minimum thermal emittance of 0.75 when tested in accordance to ASHRAE's listed test methods. ASHRAE 90.1 and 90.2 are referenced by 37 states in their energy codes and are also included in the 2003 version of the International Energy Conservation Code (IECC). Other jurisdictions, notably the states of Arizona, Florida, Georgia, and the City of Chicago have either adopted or are considering adoption of cool roofing standards for inclusion in their building codes.

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COOL ROOFING--SOUND CONCEPT, EXCELLENT TIMING

The concept of cool roofing is based upon the sound premise that whenever energy can be saved, power generation will either be contained or decreased, and the environment will subsequently be improved. The roofing market is a $10+ billion market, and many opportunities exist for coatings to contribute to the success of this market, both aesthetically and with regard to energy conservation. There are new challenges facing today's coatings chemists, but there are also new formulating options available to them as well, and responsible and productive use of all of the available tools at our disposal can have profound effects on the use of energy and the subsequent improvement of the environment. Can reflectance be increased on an entire customer's standard product line of a rainbow of colors? New pigment technology now exists that helps to answer this question. Can a thin-film coating take advantage of a metal substrate's inherently high solar reflectance, while at the same time maximizing the thermal emittance, which is an area where bare metal suffers in this cool roof environment? How about "smart" coatings that change from dark, heat-absorbing colors when the air temperature is cold (i.e., during the winter months), to a white or light heat-reflecting color during the hot summer weather? Such coatings would have heat-absorbing properties when heating is required, and heat-reflecting properties when cooling during the summer is required. The thin film or coating on a roof--presumed by so many people to provide only aesthetic value--controls all of the radiative properties of the entire roof. This is truly a new era for coatings technology, where a newly-recognized set of material properties--solar reflectance and thermal emittance--provides opportunities for all of us to improve the environment and minimize energy demands.

References

(1) http://www.sws.uiuc.edu/atmos/statecli/General/1995Chicago.htm.

(2) Klinenberg, Eric. "Heat Wave: A Social Autopsy of Disaster in Chicago," 2002.

(3) http://eetd.lbl.gov/Heatlsland/.

(4) http://www.ornl.gov/sci/roofs+walls/facts/CoolCalcEnergy.htm.

(5) http://www.usgbc.org/leed/leed_main.asp.

(6) http://www.energystar.gov/index.cfm?c=roof_prods.pr_roof_products.

(7) http://www.energy.ca.gov/.

(8) http://www.energy.ca.gov/2005_standards/rulemaking/documents/index.html.

(9) http://www.coolroofs.org/.

(10) http://www.ashrae.org/.

by David A. Cocuzzi and George R. Pilcher Akzo Nobel Coatings Inc.*

*P.O. Box 489, Columbus, OH 43216-0489.