For several years, a trend toward energy-efficient roof technology has been emerging in design and construction circles. Cool roofing evolved as a means to help keep buildings cooler and make them more energy efficient. When embarking upon a roof-resurfacing project for a commercial structure, this technology can be an attractive choice, given its potential savings and positive impact upon the environment.
Cool roof systems are typically white or light-colored and are characterized by levels of reflectance and emittance high enough to help reduce the amount of solar energy retained by a building and potentially transmitted into the structure. Reflectance, or albedo, is the percentage of solar energy reflected by a surface; the higher the percentage, the more energy reflected from the surface. Emittance deals with the energy actually absorbed, and is defined as the percentage of energy a material can radiate away. Materials with low emittance tend to heat up more easily because they collect and trap heat. To stay cool, then, a surface should ideally have both high reflectance and high emittance.
Economic and Environmental Issues
Cool roofing offers a solution for metropolitan areas searching for ways to control the demand, cost and availability of electricity. To support growth, states and metropolitan areas have tried building more power plants, overhauling aging power grids, purchasing energy through interstate markets, and placing controls on demand with some stopgap success. Despite these efforts, there continue to be shortages and negative economic impacts associated with meeting the electricity demands of rapidly growing cities.
Besides economic issues, the same urban areas are struggling to control the environmental impacts of growth. Many urban areas have become “heat islands.” These islands of heat have been studied extensively by Lawrence Berkeley National Laboratory (Berkeley Lab), Oak Ridge National Laboratory (ORNL), and other renowned agencies. Urban heat islands (UHIs) are those urban areas where temperatures will reach as high as 8 to 10 degrees Fahrenheit warmer than surrounding rural areas.
Not only do UHIs tax the local buildings’ air conditioning (A/C) units, they have a significant impact on pollution. Cities with UHIs tend to exhibit higher ozone levels. Failing to control ozone levels not only creates a serious health threat to the general population, but cities will face the loss of federal support for infrastructure improvements.
Three main growth factors can contribute to the creation of heat islands:
1. Reducing urban vegetation (e.g., trees, shrubs, and grasses).
2. Increased dark roads and parking surfaces.
3. Concentration of buildings with dark roof surfaces.
Contrary to popular belief, roads are not the main cause of urban heat. In fact, 56 percent of the blame rests with reduced vegetation, 38 percent with roofing, and only 6 percent with road surfaces. While planting more trees is desirable from both impact and aesthetic perspectives, it is a long-term solution whose success will be constantly challenged by the demand for more space. Replacing roads and parking lots with cooler surfaces is also an option, but this costly alternative will have a negligible effect on the urban heat island. This leaves roofing.
Economic expansion brings new structures and more roofs. However, existing roofs also need to be replaced at the end of their useful lives. Berkeley Labs has found significant progress can be made on heat-island mitigation by changing the way buildings are roofed.
By employing more-reflective roof surfaces, buildings stay cooler. Cooler buildings require less energy for A/C, placing less demand on the power grid and reducing operating costs for owners. A cooler local environment also reduces smog and lowers potentially dangerous emissions. Some studies have suggested that cities may also be able to reduce mortality rates associated with extreme heat events by reducing the heat island effect.
Regulatory Pressures and Initiatives
Federal, state, and local initiatives encourage the use of reflective roofing. The U.S. Environmental Protection Agency (EPA) also encourages the use of cool roofing products and has developed a list of qualifying products through its Energy Star Roof Products Program.
This list includes low-slope roof products with an initial reflectance of at least 65 percent and a reflectance of at least 50 percent after three years of weathering. (Natural weathering can have detrimental effects on a roof’s ability to maintain high reflectance and water tightness.) Cool roofing products are also required to carry warranties similar to, or better than, those offered by the same company for similar non-reflective roof products.
Other agencies have borrowed the Energy Star guidelines for developing their own codes and standards. Through its White Roofing Amendment, adopted in 1995, the state of Georgia became the first state to include cool roofing guidelines in its building code. Since then, other jurisdictions have added similar codes, and in February of 2002 the federal government joined them by signing Executive Order 13123 (“Greening the Government through Efficient Energy Management”) into law.
Now known as Federal Acquisition Regulation Case 1999-011, Executive Order 13123 provides guidance in roof selection for federal buildings. The regulation indicates state office buildings must reduce energy usage 30 percent by 2005 and 35 percent by 2010. Industrial buildings and laboratories must reduce their consumption 20 percent by 2005 and 25 percent by 2010.
The federal government also adopted the American Society of Heating, Refrigerating, and Air-Conditioning Engineers’ (ASHRAE’s) 90.1, “Energy Standard for Buildings Except Low-Rise Residential Buildings.” This standard considers the tradeoffs between building functions and allows reducing insulation when special conditions, such as the use of cool roofing products, are met.
Numerous cities across the country have initiated cool roof programs. While these codes are meeting some resistance in certain municipalities, the changes they bring will ultimately help cities temper their summer spikes in electricity demand, not to mention soaring temperatures.
Economic Incentives Can Go a Long Way
Codes and regulations are a way of forcing change, but other methods can be just as effective at implementing new ideas. Owners and facility managers are always concerned with efficient facility operation. Cool roofing systems can save money, improve occupant comfort, increase a roof’s longevity and reliably protect a building and its contents.
Some utility companies recognize the benefits of cool roofing, offering incentives and rebates for their installation. Excel Power, the fourth largest utility company in the United States, has awarded rebates for cool roofing systems in Minnesota (a state not normally associated with oppressive summer heat). By encouraging customers to control summer demand, the utility avoids constructing more generating plants to satisfy short-term energy requirements.
ORNL has developed an energy savings calculator for projecting the potential savings associated with installing a cool roof instead of a black one. Using the calculator to analyze an example comparing a black roof system with aged reflectivity of 5 percent to a cool roof system having a reflectivity of 86 percent after three years of weathering yields dramatic results.
Surface temperatures on a reflective roof membrane can be over 80 degrees cooler than on a black surface. Inside temperatures can run 15 to 20 degrees cooler under a reflective roof membrane, improving occupant comfort and keeping contents cooler.
Cool Roofing and LEED
Another design consideration for reflective roof products is how they can help obtain Leadership in Energy and Environmental Design (LEED) certification for a building. A highly reflective, polyvinyl chloride (PVC) single-ply roof system can help achieve as many as 22 points toward the minimum LEED certification. An Energy Star-labeled product helps with Sustainable Sites (SS) Credit 7.2, “Heat Island Effect: Roof,” and a product complying with ASHRAE 90.1 will help in meeting a prerequisite in the Energy and Atmosphere (EA) Credit 1, “Optimize Energy Performance.”
Most PVC roofing membranes have a track record of excellent durability and reliability, extending the life of an existing structure, and reducing load on A/C units and a building’s contribution to UHIs. Should the system chosen be custom pre-fabricated and shipped from geographically advantageous locations, it can help satisfy LEED’s Materials and Resources (MR) requirements (Credits 2.1, 2.2, 4.1, 4.2, 5.1, and 5.2).
Cool roofing can also help improve indoor environmental quality (EQ), another LEED category. A cool roofing system with good ventilation properties can keep negative air pressures and condensation in check. Plus, high reflectivity keeps a building cooler, allowing A/C units to operate more effectively (EQ Credits 2, “Ventilation Effectiveness,” and 7.1, “Thermal Comfort: Compliance with ASHRAE 55-1992”).
Selecting a Cool Roofing System
Several effective tools and resources are available when selecting a cool roof system, but one must first consider the application. For example, one should consider a complete cool roofing system instead of a more temporary reflective surface coating when long-term, low-maintenance protection - in addition to energy-saving benefits - is desired. (One should select a cool roofing system with the best test results to obtain the best value for the dollar.)
Energy Star’s Roof Products - Qualifying Products List shows roofing products meeting the minimum requirements for reflectivity, including actual tested reflectance percentages. Among the most reflective of the complete cool roofing systems are thermoplastic single plies, such as white polyvinyl chlorides, thermoplastic polyolefins (TPOs), and copolymer alloys (CPAs). The coolest of these thermoplastic systems typically range from 70 percent to 85 percent solar reflectance. (There are several PVC plastics running at the higher end of the range, with one PVC plastic’s albedo as high as 86 percent after 3 years of weathering.)
A membrane with high reflectance, a good warranty, and a history of performance offers the best protection for a facility, as well as the best opportunity to save money through lower energy consumption for years to come. It is important to consider how much reflectance a system retains over time, not just the initial reflectance.
When specifying a comprehensive cool roof system, it is important to develop the best solution for the facility and its environment. Just as climates vary widely across North America, so will the most suitable cool roofing system. Some areas may require little insulation, if any, and local codes may even allow a reduction in insulation when cool roofing is employed. The greatest energy-savings benefit in other areas may be realized through a combination of insulation and reflective roofing.
The greatest potential for saving energy through cool roofing exists in southern climates, where cooling days are numerous. Those urban areas where local weather characteristics increase the potential for pollution can also benefit from mitigating heat island effects.
While the annualized payback for cool roofing may be less in northern latitudes, there is still the potential for significant reduction in peak demand during the summer months. Extreme northern climates offering little if any cooling savings can still benefit from a membrane offering leak-proof protection, and the energy savings derived from these cool roofing systems are an added bonus.
A facility’s intended use also determines the most effective cool roof system. Some may not stand up to the chemicals, oils, or animal fats exhausted through a roof, while others may not perform well in high-wind areas, or in areas with extreme temperature changes.
Cool Roofing Makes Cents
Maintenance requirements can vary widely among manufacturers and between cool roofing systems. Some may be virtually maintenance-free, simply requiring an annual inspection to check that there has been no damage from storms or third parties, while others may require regular re-coating to maintain performance.
Some cool roofing systems can be cleaned with soap and water to maintain the highest possible reflectivity. Berkeley Labs found albedo decreased the most during the roof’s first year, and subsequent losses were negligible. While the albedos of most roofs can be nearly fully restored to their initial values, the costs of washing and limited energy returns make this an impractical procedure.
When given a choice between a cheap, short-term fix and an option with the lowest life-cycle cost over the long term, the specifier should recommend the best option for his client, which in many applications will be a comprehensive cool roof system. A sensible building design solution, cool roofing systems can offer energy efficient performance, facility protection, occupant comfort, and minimal maintenance.