If you’re reading this article in the peak of summer, you are likely experiencing some fairly hot weather. You may even be in the midst of a heat wave. Roofers are no strangers to working in the heat; the extremely high temperatures on the rooftop can be brutal. The transfer of the sun’s heat into the building through the roof can also be brutal for those living or working inside, especially in the absence of efficient insulation and air conditioning.
Hot weather affects people inside air-conditioned buildings, too. Air conditioning is expensive to run, particularly during peak times of the day. The sharp increase in energy used to cool buildings during a heat wave can also lead to a disruption to the electrical grid. Anyone who has experienced a power failure during an extreme heat event intimately knows how unbearable it quickly becomes inside the building. Further, if you’re among the estimated 83% of the U.S. population living in an urbanized area, as noted by the University of Michigan’s Center for Sustainable Systems, relief outdoors may not be easy to come by.
The negative impacts of extreme heat are intensified by the urban heat island (UHI) effect, a phenomenon in which cities are hotter than surrounding rural and suburban areas. According to the U.S. Environmental Protection Agency (EPA), daytime temperatures in urban areas are about 1 °F to 7 °F (0.5–3.4 °C) higher than temperatures in outlying areas, with nighttime temperatures about 2 °F to 5 °F (1.1–2.8 °C) higher. This temperature difference is a result of heat from the sun being retained in areas with a high concentration of buildings, parking lots, and roads, and a lack of trees and green space. Tall buildings that block or slow air movement, along with waste heat released by vehicles and air-conditioning units, contribute to the formation of UHIs.
Cool roof coatings help provide passive cooling during extreme heat events, particularly during power outages and in poorly insulated or non-air-conditioned buildings, and can help mitigate the impacts of the UHI effect.
A cool roof coating highly reflects sunlight (solar radiation) away from the roof surface during the day, which reduces building solar heat gain, while releasing any absorbed heat at night. As the name suggests, the surface temperature of the roof is cooler than a less reflective or darker roof. A cool roof helps reduce the need for air conditioning, decrease peak electrical demand, and improve occupant comfort. Collectively, cool roofs lower outdoor temperatures, which also slows the formation of ground-level ozone, a key ingredient in smog.
The “coolness” of a roof material is determined by two basic properties:
1. solar reflectance (sometimes called albedo),
2. thermal emittance.
Solar reflectance is the fraction of solar radiation that is reflected away from the roof, while thermal emittance is the relative ability of the roof’s surface to reradiate any heat that was absorbed. The values of both properties range from 0 to 1.
In addition to those two metrics, the “coolness” of a roof may also be represented by the Solar Reflectance Index, or SRI, a calculated value that combines solar reflectance and thermal emittance into one number. SRI values are usually between 0 and 100, with particularly cool materials exceeding 100.
Credits and Codes
Cool roofs were first introduced into building codes and standards more than 20 years ago, around the time the ENERGY STAR Roofing Program was launched, to drive market adoption of cool roofing materials. Several of the earlier codes offered compliance credits for the installation of cool roofing materials on commercial buildings with low-sloped roofs, such as California (2001) and Florida’s (2001) statewide building standards and American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1-1999. Today, several building codes include prescriptive requirements for cool roofs, including the widely adopted International Energy Conservation Code (see https://coolroofs.org/resources/codes-programs-standards for a comprehensive listing).
California currently has the most stringent cool roof requirements in the United States, which apply to nonresidential, single family, and multifamily buildings. Other states, including Alabama, Florida, Georgia, Hawaii, and Texas, also prescribe cool roofs, particularly for low-sloped roofs.
Due to the rise in heat waves, many U.S. cities have adopted policies and programs that require or encourage the installation of cool roofs on new and existing buildings. Chicago paved the way for prescriptive requirements as the first city in the nation to adopt the use of reflective roofing materials after the devastatingly fatal heat wave of 1995. Los Angeles, Denver, Philadelphia, New York City, and other metropolises followed suit. New York City, for example, has made significant strides to increase cool roof installations on low-sloped roofs through the NYC CoolRoofs initiative, and more recently, through Local Laws 97 and 94, which passed as part of the Climate Mobilization Act.
To help more cities identify which areas would benefit most from cool roofs, Google announced plans to release an extreme heat tool that maps out the solar reflectivity of cities.i
Several federal agencies also promote the use of cool roofs for reducing building energy use and mitigating UHIs. For example, the U.S. EPA provides education about cool roofs as a heat island reduction strategy,ii the U.S. Department of Energy has a cool roof guide for contractorsiii on the Building America Solution Center website, and the U.S. Department of Housing and Urban Development (HUD) lists cool roofs as one of six climate resilience strategies.iv HUD also provides funding for cool roofs to municipalities through Community Development Block Grant funds and for affordable housing projects funded through the HOME Investment Partnerships Program (HOME).
Not surprisingly, green building rating systems also promote cool roofs as heat mitigation strategies. The Leadership in Energy and Environmental Design (LEED) rating system offers up to two points for the installation of a cool roof under the Sustainable Sites (SS) Credit for heat island reduction.v The credit is the most widely used of all of the LEED SS credits.
The LEED heat island reduction credit is also synergistic with the SITES Rating System.vi A construction project could achieve points through both rating systems for the installation of a cool roof material that meets the specified minimum SRI value.
Points are also awarded for cool roof installations through Green Globes,vii 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 cool roof that meets the specified minimum SRI value.
Resource for Coatings Professionals
It can be a daunting task to search for roofing products that meet a building owner’s needs and comply with local building code requirements, voluntary green building standards, or rebate programs that specify the use of cool roofing materials.
The Cool Roof Rating Council (CRRC) maintains the Rated Roof Products Directory,viii a free publicly available online resource. The CRRC Directory lists more than 3,000 roofing products (nearly 800 are roof coatings) that have undergone testing, weathering, and rating in accordance with the CRRC’s strict protocols. Important note: A CRRC product rating is not a certification, ranking, or approval of a product; it describes the radiative performance of the roofing material and includes products with a wide range of solar reflectance, thermal emittance, and SRI values.
Though it may be a hot summer, these tools can hopefully keep rooftop coatings contractors cool.
Editor’s note: This column first appeared in the July 2023 print issue of CoatingsPro Magazine. Reprinted with permission.
i Brandt, Kate (2023, March 29). How we’re helping people and cities adapt to extreme heat. The Keyword, https://blog.google/outreach-initiatives/sustainability/extreme-heat-support/