Jeff Steuben, Executive Director, and Carolyn Richter, Communications Manager, Cool Roof Rating Council (CRRC)
When the weather gets hot outside, your building’s roof receives the brunt of the heat. Your roof’s ability to reflect and shed absorbed energy can have a large impact on the internal temperature of the building. Cool roofs are a simple and effective approach to lowering the cooling energy loads of buildings, increasing occupant comfort, and reducing the Urban Heat Island (UHI) effect.
This article provides an introduction to what makes a roof “cool”, the benefits of properly installed and maintained cool roofs, and building codes and voluntary programs that reference cool roofs.
A cool roof is a roof that both strongly reflects sunlight and efficiently emits heat to its surroundings. The “coolness” of a roof is determined by two basic characteristics: solar reflectance and thermal emittance. Solar reflectance is the fraction of solar radiation reflected away from the roof, while thermal emittance is the efficiency with which the roof reradiates absorbed heat. The values of both properties range from 0 to 1, with higher values indicating “cooler” products.
In addition to these two metrics, the “coolness” of a roof may also be represented by its Solar Reflectance Index (SRI), a calculated property that combines solar reflectance and thermal emittance into one metric that measures a material’s ability to reject solar energy. SRI values are usually between 0 and 100, with particularly cool materials exceeding 100. Like solar reflectance and thermal emittance, a higher SRI value indicates a cooler roof.
Essentially, these radiative properties describe the roof’s ability to minimize the solar heat gain of a building by first reflecting incoming radiation and then re-emitting the heat absorbed by the roof, and preventing the energy from being transmitted into the building structure. As a result, the roof surface remains at a lower temperature than the surface of a traditional roof of similar construction.
The solar reflectance's of some roofing materials, especially those light in color, tend to decrease as the roof ages due to soiling (deposition of particles such as soot and particulate matter) and weathering (exposure to the elements, including sunlight and temperature fluctuations), which affects performance over time. To account for this performance change, the Cool Roof Rating Council (CRRC) Product Rating Program provides manufacturers with product ratings for measured solar reflectance and thermal emittance, as well as SRI, of each product both when new (‘initial’ properties), and again after long-term outdoor exposure (‘aged’ properties). To understand how the product is anticipated to perform over time in different climates, outdoor exposure is conducted for three years in each of three different climates: 1) Miami, Florida: hot and humid; 2) Phoenix, Arizona: hot and dry; and 3) Cleveland, Ohio: cold/temperate. Results from the three sites are averaged to provide a ‘U.S.’ value.
When properly installed and maintained, cool roofs can provide a variety of benefits that contribute to the community’s health, the building occupants, and the owner’s wallet.
Building occupants can experience improved comfort as compared to a conventional dark roof, as the building’s interior is subject to less thermal flux and stays cooler during warm seasons. Reduced indoor temperatures lead to energy savings from reduced cooling energy loads.
Cool roofs are distinguished among energy conservation measures because of their ability to help mitigate the urban heat island effect, a phenomenon characterized by a measured increase in the ambient air temperature in cities over their surrounding rural areas. This effect is due to roofs and non-reflective surfaces that absorb and trap heat, gradually warming the ambient air temperature and causing cities to be up to 8°F warmer than their surrounding areas.
Cool roofs help improve urban conditions by contributing to cooler ambient temperatures by immediately reflecting solar radiation back into the atmosphere before it can degrade to heat, as well as reemitting a portion of infrared light. Reduced air conditioning loads can also improve grid stability, increasing peak energy savings, reducing the risk of brown outs or black outs, and decreasing pollution production.
Lower ambient air temperatures also contribute to improved air quality by combating the formation of ground-level ozone, a pollutant which forms more readily in hot environments. This effect is magnified by the reduction of pollutants and greenhouse gas emissions from power plants by reducing electricity demands for air conditioning.
Cool roofing products are available in a variety of colors and materials, including but not limited to: built-up roofing cap-sheets, foam roof systems, metal panels, field-applied coatings, tile, and single-ply membranes. While white roofs tend to offer the highest solar reflectance's and thermal emittances, other colors and even patterns can perform as well as some white options.
Ultimately, roofing materials should be carefully selected based on the slope of the roof, energy savings goals, the project location and climate, local code requirement or green building credits, and aesthetic preferences. A useful resource to help locate cool roofs by product type is the CRRC Rated Products Directory.
Green roofs, also known as vegetated roofs, use plants as roof covering and are distinct from cool roofs. While typically not highly reflective (a key feature of a “cool” roof), green roofs can stay cooler than a traditional non-cool roof through evaporation. Green roof installations can be combined with the use of reflective materials such as cool pavers for pathways and cool roofs on non-vegetated areas to maximize benefits of sustainable roofing practices.
Increased recognition of the benefits of cool roofing has led to the adoption of cool roof measures in energy codes and the inclusion of cool roofing as part of green building initiatives across the globe, including American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) energy codes; the International Energy Conservation Code (IECC), the International Green Construction Code (IgCC); ENERGY STAR®; LEED®; and Green GlobesTM. Several building codes, as well as voluntary green building programs, either require or allow you to achieve credits for including cool roofs in a project.
Most energy efficiency codes and programs that incorporate cool roofs require minimum values of initial or aged solar reflectance and thermal emittance, or initial or aged SRI. In California, for example, the Title 24 Energy Efficiency Standards for Residential and Nonresidential Buildings prescribes minimum values for three-year aged solar reflectance, thermal emittance, and SRI. At the international level, the globally-recognized LEED® green building certification program awards points for high-reflectance roof based on
minimum initial and aged SRI values. The CRRC maintains a list of cool roof rebates, codes, and voluntary programs at:
www.coolroofs.org/resources/rebates-and-codes
Roofing product databases list pertinent and easily comparable information that can assist in selecting an appropriate product. While existing rating systems are complementary to one another, they do have slight differences in their requirements.
ENERGY STAR®, for example, aims to capture the most efficient products and set minimum requirements for both initial and aged solar reflectance. In order for a product to be listed by ENERGY STAR, it must meet their minimum requirements (initial solar reflectance of 0.65 and three-year aged value of 0.50 for low-slope products and an initial reflectance of 0.25 and aged value of 0.15 for steep sloped
products).
The Cool Roof Rating Council (CRRC), on the other hand, does not set minimum requirements, but does require that all testing be conducted by a CRRC-approved Accredited Independent Testing Laboratory. Additional CRRC requirements include: standardized and consistent test methods for initial and aged ratings, a strict chain of custody, and participation in random testing to verify product ratings.
Both rating systems include aged testing, where products are exposed to natural weather conditions for a three-year period of time. They use three specific locations representing three key climate zones (hot/dry, hot/humid, and cold/temperate) to determine aged product performance. ENERGY STAR accepts products that have been rated by the CRRC so long as the ratings meet ENERGY STAR’s minimum requirements for both initial and aged reflectance values.
In addition to three-year weathering and aged rating, the CRRC also offers Rapid Ratings, an optional addition to its Product Rating Program that utilizes an accelerated weathering protocol to develop interim laboratory-aged values and accelerate understanding of product performance over time.
With roofing codes and standards implementing performance requirements, cool roofs are one of the most effective ways to obtain energy savings and environmental rewards through building envelope design and re-roofing projects. Cool roofs can be the simplest design measure to implement, in both new construction and existing buildings, without compromising project design or performance. The CRRC Rated Products Directory is available to promote informed and confident cool roof choices to meet specified requirements in
codes and programs while maintaining an aesthetically pleasing roof.
Jeff Steuben is the Executive Director of the Cool Roof Rating Council (CRRC). He oversees the day-to-day operations of the CRRC Product Rating Program and works closely with the Board of Directors. Jeff previously served as the Technical Liaison for the CRRC and conducted analysis of CRRC research projects with the CRRC’s Technical Committee. He received a Bachelor of Science degree from Humboldt State University in Environmental Science and a minor in Geographic Information Systems.
Carolyn Richter is the Communications Manager for the Cool Roof Rating Council (CRRC). She helps manage the Product Rating Program and Rapid Ratings Program in addition to the organization’s website, social media platforms, and industry publications. Carolyn received a dual degree in Conservation and Resource Studies and Professional Forestry from the University of California, Berkeley.
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