As extreme weather conditions become more common, cool roofs offer even more benefits in terms of energy savings and resilience.
By Sarah Schneider, Author
Chilled roofs were first included in building codes and standards more than 20 years ago, around the time the Energy Star Roofing Program was launched to drive market acceptance of chilled roofing materials. Several of the previous state regulations and national standards provided compliance points for the installation of cool roofing materials on commercial buildings with low pitched roofs, such as For example, the Statewide Building Standards of California (2001) and Florida (2001) and ASHRAE 90.1 Standard (1999). In 2001, Chicago paved the way for mandatory requirements as the first city in the country to introduce the use of reflective roofing materials. In subsequent years, mandatory requirements for chilled roofs were introduced in many building energy codes, including the International Energy Conservation Code.
Today, numerous U.S. jurisdictions mandate or encourage cool roofs for new and existing commercial and residential buildings (both pitched and flat roofs) to reduce cooling energy savings, reduce peak energy demand, and improve local air quality and reducing urban heat islands.
In recent years, federal efforts to increase the energy efficiency of existing buildings, the adoption of building performance standards at various levels of government, and the local implementation of policies and programs to increase climate resilience have contributed to greater adoption and promotion of chilled roofs across the country. New York City has made significant strides in increasing the installation of cool roofs through the NYC CoolRoofs Initiative and local Laws 97 and 94, passed as part of the Climate Mobilization Act (also called NYC’s New Green Deal).
Cool roofs required
To meet climate and energy goals and to address the increasing intensity and frequency of heat waves, many cities, such as Denver, Philadelphia, and Scottsdale, Arizona, are requiring cooling roofs to be installed on new and existing buildings.
Some cities are also promoting cool roofs through voluntary green building programs, income-matched or affordable housing programs, and financial incentives. For example, Austin, Los Angeles, Louisville, and Orlando offer cool roof discounts, and Boston, New York, and San Antonio have programs that install cool roofs on homes and businesses.
The introduction of cooling roofs can also be observed at state level. California currently has the most stringent chilled roof requirements in the country, which apply to non-residential, single-family, and multi-family buildings. Other states such as Alabama, Florida, Georgia, Hawaii and Texas also mandate cool roofs. Hawaii is currently considering aligning the state’s requirements for cool, low-pitch roofs with California building standards.
In many major US cities, heat waves kill more people than hurricanes, tornadoes, floods, lightning, and snowstorms. Extreme heat also causes heat illness and increased respiratory and cardiovascular problems that can strain health services, as well as disrupting critical infrastructure such as power grids and water supplies.
The negative effects of extreme heat are compounded by the Urban Heat Island (UHI) effect, a phenomenon in which cities are hotter than rural and suburban areas. According to the U.S. Environmental Protection Agency (EPA), daytime temperatures in urban areas are about 1-7 degrees Fahrenheit higher than temperatures in remote areas, with nighttime temperatures being about 2-5 degrees higher. This temperature difference arises because the sun’s heat is retained in areas with a high concentration of buildings, parking lots, and streets and a lack of trees and green spaces. Tall buildings that block or slow air movement, along with waste heat released from vehicles and air conditioning, contribute to the formation of UHIs.
Cool roofs contribute to passive cooling during extreme heat events, particularly during power outages and in poorly insulated or unair-conditioned buildings, and can help mitigate the effects of the UHI effect. Several federal agencies recognize the benefits of cool roofs in mitigating UHIs in addition to reducing building energy use. For example, the EPA offers cool roof education as a strategy to reduce heat islands, and the US Department of Housing and Urban Development lists cool roofs as one of six climate resilience strategies. The agency also provides funding to communities through Community Development Block grant funds and for affordable housing projects funded through the HOME program for cooling roof installation.
Green building rating systems also promote cool roofs as heat mitigation strategies. The most notable rating system is Leadership in Energy and Environmental Design (LEED), which offers up to 2 credits for installing a chilled roof under the Sustainable Sites Credit for reducing heat islands. The credit is the highest performing of all LEED credits for sustainable locations.
The LEED heat island reduction credit is also synergistic with the SITES rating system. Through either rating system, a building project could earn points for installing a cool roofing material that meets the specified minimum Solar Reflectance Index (SRI) value.
Points are also awarded for cooling roof installations through Green Globes, an online assessment protocol and rating system for the design, operation and management of commercial buildings. Up to six points are awarded for the installation of a cooling roof that meets the specified minimum SRI value.
To help cities and urban planners identify which areas would benefit most from cool roofs, Google announced plans to release an extreme heat tool that maps cities’ solar reflectance.
During the day, a cool roof strongly reflects sunlight (solar radiation) away from the roof surface, reducing the building’s solar heat gain, while at night it releases absorbed heat. As the name suggests, the roof’s surface temperature is cooler than a less reflective or darker roof. A cool roof helps reduce the need for air conditioning, lower peak power demand, and improve occupant comfort. Overall, cool roofs improve outdoor air quality by slowing down the formation of ground-level ozone.
Cooling roofs are available in a variety of product types. In the past, gently sloping roofs were converted into cool roofs by applying a white coating. Today there are “Cool Color” products on the market that use infrared reflecting (IR) pigments. The photo below shows a comparison of the measured reflectance values of “cold” colors (top row) with colors without IR pigments (bottom row). The “R” stands for reflectance, which is measured on a scale of 0 to 1 (or 0 – 100 percent reflective). The higher the number, the more reflective the product.
The “coolness” of a roofing material is determined by two fundamental properties: solar reflectance (sometimes referred to as albedo) and thermal emittance. Solar reflectance is the fraction of solar radiation that is reflected off the roof, while thermal emissivity is the relative ability of the roof surface to re-radiate absorbed heat. The values of both properties range from 0 to 1.
In addition to these two metrics, a roof’s “coolness” can also be represented by SRI, a calculated value that combines solar reflection and heat emission. SRI values typically range from 0 to 100, with particularly cool materials exceeding 100.
The Cool Roof Rating Council (CRRC) maintains the Rated Roof Products Directory, a free resource that lists roofing products based on radiant output. The CRRC Directory contains over 3,000 roofing products that have been tested, weathered and rated according to the CRRC’s rigorous protocols. A CRRC product assessment is not a certification, rating or approval of a product; It describes the radiant performance of the roofing material and includes products with a wide range of solar reflectance, thermal emittance and SRI.
Sarah Schneider is Associate Director of the Cool Roof Rating Council (CRRC), a 501(c)(3) not-for-profit organization that develops science-backed methods for rating and characterizing the radiant properties of roofing and exterior wall products.