Energy Efficiency Through Roofing
June 1, 2006
Environmental regulations and constraints are changing the way that building components will be designed and constructed throughout the 21st century. The current and future regulations will have a significant impact on roof systems. The most prominent environmental issue involving roofing is energy performance. The continual increase in energy costs, coupled with the depletion of natural resources, has propelled these issues to the forefront of the roof design debate. States and municipalities are increasingly requiring energy-efficient roof designs that meet cool roof or reflective standards.
Radiation and Roof Systems
Radiation has a significant impact on roof systems. Ultraviolet radiation has a negative effect on all types of roof systems. In built-up roof systems, ultraviolet radiation degrades exposed bitumen through a chemical process known as photo-oxidation in asphalt-based systems and the evaporation of volatiles from coal tar-based systems. In photo-oxidation, the number of high-molecular weight hydrocarbons and water-soluble products in bitumen are increased.
This combination of heat and ultraviolet radiation manifests itself in a migration of oily constituents to the surface and hardening of the bitumen. The bitumen, in its hardened form, cracks in a process known as “alligatoring.” Mineral aggregate surfacing is applied on built-up roof systems to protect the bitumen flood coat from life-shortening ultraviolet radiation.
Ultraviolet radiation has a devastating effect on unreinforced polyvinyl chloride (PVC) membranes. It has been widely reported throughout the industry that a defection of the plasticizer in the PVC compound migrates out through exposure to ultraviolet radiation. Since the plasticizer is applied in the PVC compound to provide flexibility to the membrane, the diffusion results in an embrittled, hardened membrane, which is susceptible to shrinkage, cracking and shattering.
Ultraviolet radiation contributes to the loss of oils from EPDM membranes, causing shrinkage of the sheets. Most elastomeric membranes achieve ultraviolet resistance from a compound additive called carbon black. Carbon black prevents the ultraviolet radiation from absorbing the chemicals included in the polymer mixture.
Modified bitumen membranes incur deterioration from the same photo-oxidation that attacks asphalt-based built-up roof systems. This is due to the asphalt content in these membranes. To protect against ultraviolet deterioration, the modified bitumen membranes utilize a variety of shielding surfaces, such as factory-embedded mineral granules, metal foils, and field-applied reflective coatings. Significant premature aging failures have provided evidence that atactic polypropylene (APP) membranes require protective surface coating during installation for long-term ultraviolet protection.
The Heat Island Effect
The single largest contributing factor to electricity blackouts is the sun. The light and heat of the sun correlate with the increase of electrical demand charges and average occupancy levels of commercial, industrial and institutional buildings. The heat of the sun is the most intense in the summer at midday.
This is also the time when there is the highest electrical output at occupied businesses from air conditioning, office equipment and lighting. It is reported that every one-degree increase in temperature increases air conditioning use by an average of two percent.
The intensity of the sun also contributes to a phenomenon known as “urban heat islands.” Scientists have indicated that this phenomenon exists in most metropolitan areas of the United States. The urban heat island effect is the pocket of hot air that settles over urban areas. Recent studies have indicated that urban areas are on average five degrees hotter in the summertime than the surrounding suburbs. The major reason for this temperature change is attributed to plants and trees that are prominent in suburban areas and largely nonexistent in most major urban areas. Trees provide cooling shade and the water that evaporates from the leaves cools the air. Black roofs, asphalt roads and parking lots add to the heat island phenomenon.
The urban heat island effect also contributes to the formation of smog as a result of the chemical reactions in the air. A one-degree rise in temperature in a major urban area such as Los Angeles could increase the smog risk by as much as three percent. Smog can cause irritation of the eyes and induce asthma attacks; it can also contribute to permanent lung damage.
In order to provide a better understanding of the heat island effect and provide solutions to these issues, the Department of Energy (DOE) and the Environmental Protection Agency (EPA) have funded The Heat Island Program at the Lawrence Berkeley National Laboratory (LBNL). Dr. Hashem Akbari, who heads up the Heat Island Group, states, “Heat island research is conducted to find, analyze, and implement solutions to the summer warming trends occurring in urban areas, the so called ‘heat island’ effect.”
He notes that the group “currently concentrates on the study and development of more reflective surfaces for roadways and buildings.”
The group’s research in the roofing field has centered on providing reliable measurements of current roofing materials for solar resistance and infrared emittance and establishing a database of roofing materials that can be used for construction and improvements in these areas. The group also collaborates with roofing material manufacturers and assists them in the development and fabrication of cool roofing materials.
In the last decade, there has been a concentrated effort to determine the most efficient means to reduce the urban heat island effect. Universities, organizations and governmental agencies - including the EPA and NASA - have conducted numerous studies that correlate the effects of this phenomenon.
The results of these studies have all provided similar recommendations to sufficiently cool these areas. The recommendations include replacing the dark city streets and roofs with lighter, more reflective surfaces.
The studies also recommend planting more greenery in urban areas. A theory exists that if these recommendations were followed on a wide scale, there would be a corresponding decrease in the summertime temperature of major urban areas. There is caution that such temperature decreases can only be realized with widespread changes to the affected urban areas; selected plants, cool roofs and light-colored roads will have little cumulative effect if they are not completed throughout a majority of the urban area.
Changes to a single roof system can reduce the individual building owner’s energy cost, and citywide changes could also decrease the urban heat island effect. It is possible to reduce the solar heat gain of a roof by applying reflective and emissive materials that reflect the sun’s radiant energy back toward the sky. Proper reflective surfacing can cool roofs even in cooler climates, and studies have indicated that white roofs are better at cooling the building than black roofs, even in colder climates. This is due to the fact that dark materials absorb more heat from the sun than lighter colored surfaces.
Dr. Akbari indicates that black surfaces in the sun can become up to 70°F (40°C) hotter than most reflective white surfaces. “If the dark surfaces are roofs, some of the heat collected by the roof is transferred inside the building,” he says. The roof heat transfer contributes to higher indoor and surrounding outdoor temperatures. The roof surface temperature is determined by three types of external energy heat flows at the outside surface:
solar absorption, radiative cooling and convective cooling. Of the external energy flows, convective cooling is the least precisely known. The solar and infrared radiative cooling can be readily calculated if the solar reflectance and infrared emittance is known. Once the roof temperature is known, the heat flow leaking into the interior can be computed.
Roofs with reflective surfaces can reduce surface temperature by as much as 30 percent and can extend the service life of some membrane systems by reducing expansion and contraction. The reduction of the interior temperature lessens the need for air conditioning, which results in lower utility costs and less-intensive HVAC maintenance over the life of the equipment. Reflective roofs in conjunction with other specialized roof components can also increase interior ambient light, reducing yearly electricity costs.
Studies conducted by the EPA and the Heat Island Group comparing light-colored roofs and dark-colored roofs have indicated that lighter colors can reduce the heat island effect and save energy. The study analyzed the energy saving potentials of light-colored roofs in 11 metropolitan areas throughout the United States. Ten residential and commercial building prototypes were developed in each area. The group considered both the savings in cooling and penalties in heating as a means of determination.
“We estimated saving potentials of about $175 million per year for the eleven cities,” notes Dr. Akbari. “The extrapolated national energy savings were about $750 million per year.” The Heat Island Group conducted a similar test in Sacramento, Calif., which monitored light-colored, more reflective roofs. Dr. Akbari states that the study found that light colored roofs “used up to 40 percent less energy for cooling than buildings with darker roofs.” This was consistent with a separate study conducted by the Florida Solar Energy Center, which also found cooling energy costs to be 40 percent lower for light-colored roofs.
The Heat Island Group has been working in conjunction with low-slope and steep-slope material manufacturers in an effort to provide advanced cool roof technology. On low-slope roof systems, cooling can be achieved through the use of reflective coatings. Typically, commercial coatings are colored with conventional pigments that absorb near infrared radiation (NIR) that provides more than half the power of sunlight.
The Heat Island Group has found that by replacing conventional pigments with pigments that absorb less NIR radiation, manufacturers can yield similarly colored coatings that reflect a greater proportion of solar energy. In the words of Dr. Akbari, “These cool coatings lower roof surface temperatures, reducing the need for cooling energy in conditioned buildings and making unconditioned buildings more comfortable.”
LBNL has recently begun collaboration with Oak Ridge National Laboratories (ORNL) and a consortium of 16 residential roofing manufacturers under a commission established by the California Energy Commission. The goal of the commission is to create dark shingles with a solar reflectance of 0.25 and other non-white roofing products, such as tile and metal, with a solar reflectance of no less than 0.45. The focus in shingles is on the development of pigments that provide greater solar resistance for the granules. Solar resistance of colored tiles can be achieved by using clay or cement with low concentrations of light-absorbing impurities.
Cool, non-white pigments can be applied to metal for higher reflectivity. Through extensive research and development the commission has achieved great success and predicts that cool, non-white roofing materials will be available in the market within the next three to five years.
A current industry debate focuses on the merits of cool roofs in the colder northern regions of the country. Although Dr. Akbari indicates that warmer climates (such as those in the sunbelt regions) benefit more from cool roofs, he is quick to add that “cool roofs save energy in all buildings that need air conditioning cooling in all regions of the United States.”
Another way to minimize urban heat island effect and reduce interior costs is by applying a green or garden surfacing. The application of garden roofs can provide an urban area with required greenery to reduce the urban heat island effect. Garden roofs can also serve as protective surfacing to improve the energy efficiency of the building, improve the quality of air that we breathe and improve storm water runoff - a major advantage in older urban areas with limited or worn infrastructure.
It should be pointed out while many cities place percentage requirements on new buildings that have garden roofs, garden roofs must also be installed on a percentage of existing buildings for temperature changes to be realized.
The increased use of insulation can also prove to be a source of energy efficiency. Some states, such as California, and many municipalities throughout the country have initiated tax rebates and paybacks to building owners who increase the thermal value of the insulation that is used on their roof systems. Proper research should be conducted prior to the installation of insulation on the roof system. Insulation has become one of the most highly scrutinized of all roof system components. The energy crisis of the 1970s promulgated the initiation of a number of energy codes and programs, with variable success rates.
Thermal R-values were established as a parameter of acceptable design matrix for insulation materials in roof systems. The theory behind the use of higher Rvalue insulations was that they were used to help reduce energy demand; this in turn would help reduce environmental risks. Unfortunately, the insulations that have provided the highest R-values are determined to be high environmental risks. These are insulations that are damaging to the ozone layer or are produced with chemicals that are suspected of contributing to global warming.
The LEED Program
The research conducted by LBNL and other groups has contributed to energy programs that are increasingly being implemented as codes in the United States. Two programs that are having a significant impact on the roofing industry are the LEED program and Energy Star.
The LEED Program (Leadership in Energy and Environmental Design) was recently developed by the U.S. Green Building Council. The USGBC is a nonprofit coalition of building professionals representing all segments of the industry.
These members are working together to promote the design and development of environmentally and economically responsible buildings. Through the USGBC, LEED has been developed as a voluntary national standard for the development of high-performance, sustainable buildings. The program covers new construction (commercial and residential) and major renovation projects.
LEED was created to provide a definition of “green building” and establish a common standard of measurement for the process. The USGBC also hopes that this program will promote integrated design practices that encompass the whole building and minimize the impact on the environment. This can be achieved by raising consumers’ awareness of the benefits associated with green building practices.
The main objective of the LEED program is to decrease the energy consumption and the environmental impact of buildings. The LEED program is structured to provide the proper assessment of a building’s performance to ensure that it meets sustainability goals. The sustainability goals are established from scientific standards, and they can be met by implementing state-of-the-art strategies throughout the whole building process.
Energy savings can be realized through sustainable site development, water savings, energy efficiency, materials selection and indoor environmental quality. In addition to promotion of green building practices, the USGBC also provides expertise in these areas by offering training, resources, project certifications and professional accreditation.
In the LEED program, a building based is issued a rating based on its sustainability and energy efficiency. The ratings are based on a set of performance standards for the sustainable operation of existing buildings. The LEED-EB criteria cover building operations and systems upgrades in existing buildings where the majority of interior or exterior surfaces remain unchanged.
The rating is based on points accumulated through a number of sustainable or energy efficiency categories. A building can achieve one of four levels of certification based on the accumulated points. The Platinum level is 52 points or more, the Gold level is 39 to 51 points, the Silver level is 33 to 38 points, and the LEED Certified level is 26 to 32 points. Even though this is currently a voluntary program, some states and municipalities have adapted LEED as part of the building code, and LEED certification has become a requirement on most federal buildings.
In some states, the local energy companies are providing rebates and discounts to owners whose buildings reach certain LEED levels. This trend is expected to continue, particularly at the state and local levels.
Higher LEED levels indicate higher energy efficiency, which also translates into greater energy savings (and thus lower utility costs) for the building owner over the life of the building. There are seven LEED categories from which points are allocated. They are:
Sustainable sites . . . . . .14 points
Water efficiency . . . . . . . .5 points
Energy and atmosphere .17 points
Materials and resources .13 points
Indoor environment . . . .15 points
Design process and innovation . . . . . . . . . . . .4 points
LEED Accredited Professional . . . . . . . . . . . .1 point
It is the responsibility of the building owner to register the project with the USGBC during the design phase, select qualified/certified products and submit documentation at or near building occupancy.
The Energy Star Program
The LEED program utilizes Energy Star requirements for reflectivity and emissivity. Energy Star is a government backed program designed to help businesses and individuals protect the environment through superior energy efficiency.
The program was established by the EPA in 1992 as a voluntary labeling program designed to identify and promote energy efficient products to reduce greenhouse gas emissions. Emissions from greenhouse gases are believed to be collecting in the atmosphere and contributing to a trend of global warming.
Computer hardware and monitors were among the first products to be labeled with the Energy Star rating. By the mid-90s, the labeled products expanded to office equipment and residential heating and cooling equipment.
In 1996, the EPA partnered with the United States Department of Energy, and the result was an expanded base of participating industries and product categories. Today there are over forty product categories and thousands of product models participating in the Energy Star program. The labeling program now includes new homes, as well as commercial and industrial buildings.
Recent data reveals that Energy Star now partners with over 8,000 public and private sector entities. This list is growing on a continual basis. It is also estimated that the use of Energy Star-labeled products results in an annual savings of approximately $9 billion for businesses, consumers and organizations in the United States. Energy Star labels have now become prominent in construction materials, particularly roofing materials.
There are two primary reasons for Energy Star’s inception. The first reason is a carryover from the energy crises experienced in the 1970s, and it is emblematic of the country’s reluctance to have a repeat occurrence. The second reason centers on the North American electrical grid, which was constructed in 1950s and 1960s. The grid was state of the art at the time; however, it was not developed to handle the capacity necessary for today’s technology. The current electrical grid is required to provide capacity for a plethora of appliances and equipment that it was not designed to handle at its inception. We now have multiple televisions, telephones, air conditioning and appliances in most households in the country. It is estimated that approximately 80 percent of all American households currently have computer systems; this technology was not available in the 1960s, when the current grid was established. The recent energy crisis in California and the blackout in parts of the Midwest and East Coast in the summer of 2003 is a further indication of current concerns.
LEED and Roofing
Roofing materials can have an impact in three LEED categories: sustainable sites, energy and atmosphere and materials and resources. Roof systems can have an impact through thermal capacity, reflectivity and emissivity. Thermal capacity is achieved through higher Rvalues, predominately from insulation.
High thermal value insulation will reduce heat loss in the winter months, which decreases heating costs and energy capacity. There are currently no national code regulations regarding minimum R-values for low-slope roof systems.
Some states and municipalities have enacted their own requirements, which are typically R-19 or higher. The Energy Star Program develops reflectivity and emissivity rates used in the LEED program. Reflectivity ratings are based on the roof surface’s ability to reflect ultraviolet rays from the sun.
Studies have indicated that reflective surfaces will keep the building cooler in the summer, decreasing the use of air conditioning. The Energy Star Program establishes the reflectivity rate of a roof surface for low-slope (less than 2:12) roof systems at 0.65 for the first three years and 0.50 after three years. LEED provides points based on this program.
Emissivity is the ratio of radiation emitted by a blackbody or a surface and the theoretical radiation predicted by Planck’s law. Blackbody emissivity is frequently referred to as a single number. A material’s surface emissivity is a measure of the energy emitted when a surface is directly viewed. The LEED emissivity rating for roofing is a minimum of 0.90 based on ASTM E 408.
Due to these regulations, there has been a concentrated effort by roofing manufacturers to produce products that meet Energy Star requirements. The use of these products contributes to LEED points for green buildings. LEED points can also be achieved if any of the roof system materials are manufactured within 500 miles of the building site.
Additional points can be accumulated if a member of the project team is a LEED accredited professional and for exceptional performance in innovation of application or design. Recyclable materials or materials manufactured from recycled products (including some perlite and fiberboard insulations, for example) can also provide LEED points on a project.
At this time, these types of programs are voluntary standards that have not been adopted by national codes. However, some states - such as California and Nevada - and some cities - such as Chicago - have begun enacting them (or variations of them) as local codes in an effort to pro- mote green and cool buildings. California has passed Title 24, which is a regulation that sets energy efficiency design and construction standards for residential and
nonresidential buildings. The intent of this legislation is to reduce the peak energy demands that created power blackouts in 2000 and 2001. Title 24 is the latest revision of the Warren Alquist Act that was passed in 1978 and is updated every three years by statute. The latest revision took effect in October of 2005 and stipulates all roofs must meet minimum standards established by the Cool Roof Rating Council (CRRC), which are initial emissivity rating of 0.75 and initial reflectivity rating of 0.70.
For further information regarding the LEED program, visit the U. S. Green Building Council’s Web site, www.usgbc.com.