Cool Roofs

What are Cool Roofs?

Cool roofs reflect (solar reflectance) and radiate heat (thermal emittance) away from the building and reduce roof temperature (see Figure 1). The Solar Reflectance Index (SRI) measures roof reflectance and emissivity on a scale of 0 to 100, and the higher the value, the “cooler” the roof.^[1] White, light-colored, and reflective colored surfaces have a higher SRI than dark-colored roofs.

Figure 1: Reflectance and Emittance of cool roofs (Source: Cool Roof Rating Council)

Conventional dark colored roofs absorb sunlight, generating heat and increasing the cooling load of the building. The heat generated by conventional roofs also contributes to the heat island effect in urban and dense suburban areas. Heat island effect takes place when built up or densely developed areas experience an increase in temperature due to larger areas of dark pavement and roofs that absorb heat from the sun. Rising temperatures result in increased energy demands, which can lead to higher air conditioning costs, increased greenhouse gas emissions, pollution, and increased risk of heat-related illnesses.^[2] Urbanized and densely populated suburban regions are more vulnerable to the heat island effect, and conditions tend to be worse during the summer months.

Many building codes and green building programs have a cool roof and urban heat island mitigation requirements that refer to ENERGY STAR and the Cool Roof Rating Council (CRRC) voluntary labeling programs.

ENERGY STAR labels roof products that meet the following reflectance requirements and provides information about emissivity.^[3]

(Source: Energy Star)

The Cool Roof Rating Council (CRRC) does not set minimum requirements but uses independent and accredited testing laboratories and a random testing program to identify products on its online Rated Products Directory that meet the ANSI/CRRC S100 accredited standard.^[4]

Cool roofs options are available for most standard roofing materials and come in common roofing colors (e.g., gray, red).^[5] There are two main types of cool roofs: low-sloped and steep-sloped (the majority of commercial buildings have low-sloped roofs).

The following table lists roofing coverings for low-slope and steep-slope roofs.^[6]

(source: National Institute of Building Sciences, Whole Building Design Guide)

How to Incorporate Cool Roofs

The decision to incorporate cool roofs depends on the local climate, a building’s heating and cooling loads, peak energy demands, and shading provided by surrounding trees or structures.^[9] Incorporating cool roofs requires proper selection and installation of the roof product or roof coating. Refer to the Energy Star and CRRC products list. Cool roof design combined with appropriate roof insulation, shade tree placement, and properly sized HVAC equipment maximizes energy and cost savings (See Insulation, Properly-Sized HVAC Equipment, and Tree Protection and Placement).^[10] Complementary strategies include vegetated roofs and roof-top photovoltaic systems (see Vegetated Roofs, Photovoltaic Systems).

Benefits

Cool roofs offer several potential benefits, including:

• Energy cost savings by reducing cooling loads.^[11]
• Lowered peak electricity demand which decreases the likelihood of power outages.^[12]
• Reduced roof temperatures which may reduce roof maintenance and replacement expenses by extending roof life.^[13]
• Increased indoor thermal comfort in summer, helping to prevent heat-related illness and deaths.^[14]
• Mitigated heat island effect in cities and suburbs.^[15]
• Reduced air pollution and smog formation.
• Water reduction savings for urban irrigation.^[16]
• Minimized heat loss in winter in cold climates when combined with appropriate insulation levels.^[17]

Example

A study by the NYC Department of Design & Construction (DDC) found that installing cool roofs throughout the New York Metropolitan region could reduce the effects of urban heat island effect by 2-3°F.^[18]

Figure 2 – The light-colored roof of the EDA Tech Center in Camden, NJ reduces the urban heat island effect and lowers summer cooling loads. (Source: Clint Andrews, Rutgers Center for Green Building).

Costs

A roof’s life cycle costs include upfront installation (materials and labor) and ongoing savings and maintenance costs (repair, recoating, and cleaning). The installed costs vary depending on the type and size of the roof, complexity of the roof system, and building location. In many cases, cool roofs cost about the same as conventional dark roofs with the cost premium ranging from $0.05 to $1.50/SF over conventional roofing products.^[19] These costs can be recovered in the relatively short-term (1 year or less) through decreased energy consumption.^[20]

For a list of price premiums for cool roofs, see the Global Cool Cities Alliance’s Cool Roof and Pavements Toolkit. For up-to-date information on available incentives, please visit the New Jersey Clean Energy website.

Resiliency

Cool roofs contribute to resiliency by providing a durable roofing material and through energy efficiency.  For example, cool roofs enhance roofing durability by reducing thermal flux and increasing the roof’s resiliency to changing climate conditions, heat and UV aging.  Cool roofs also contribute to decreased air-conditioning loads, thereby extending the life of AC units. Regarding energy efficiency, cool roofs, combined with proper insulation levels, can reduce heating and cooling loads, reducing stress on the grid, and help to regulate thermal comfort in the event of a power outage.

^[1] Cool Roof Rating Council (CRRC) http://coolroofs.org (accessed March 16, 2018).

^[2] Solecki, William D. et al., Urban Heat Island and Climate Change: An Assessment of Interacting and Possible Adaptations in the Camden, New Jersey Region, New Jersey Department of Environmental Protection, Division of Science, Research, and Technology (2004). http://www.state.nj.us/dep/dsr/research/urbanheat.pdf  (accessed March 16, 2018).

^[3] ENERGY STAR. “Roof Products Key Product Criteria.” http://www.energystar.gov/index.cfm?c=roof_prods.pr_crit_roof_products (accessed March 16, 2018).

^[4] Cool Roof Rating Council. 2016. “Policy Makers and Code Official: Learn about Cool Roofs.”

http://coolroofs.org/documents/Policy.pdf (accessed June 22, 2018).

^[5] Global Cool Cities Alliance. 2012. “A Practical Guide to Cool Roofs and Cool Pavements: Primer.” Page 13. https://www.coolrooftoolkit.org/wp-content/pdfs/CoolRoofToolkit_Primer.pdf (accessed June 22, 2018).

^[6] Whole Building Design Guide – Roofing Systems. https://www.wbdg.org/guides-specifications/building-envelope-design-guide/roofing-systems (accessed May 31, 2018).

^[7] Slope = rise/run. Low slope means that for every 12 horizontal inches, the roof’s rise is three vertical inches or less.

^[8] Slope = rise/run. Steep slope means that for every 12 horizontal inches, the roof’s rise is three vertical inches or greater.

^[9] Global Cool Cities Alliance. 2012. “A Practical Guide to Cool Roofs and Cool Pavements: Primer.” Page 21. https://www.coolrooftoolkit.org/wp-content/pdfs/CoolRoofToolkit_Primer.pdf (accessed June 22, 2018).

^[10] Global Cool Cities Alliance. 2012. “A Practical Guide to Cool Roofs and Cool Pavements: Primer.” Page 9. https://www.coolrooftoolkit.org/wp-content/pdfs/CoolRoofToolkit_Primer.pdf (accessed June 22, 2018).

^[11] US EPA. Using Cool Roofs to Reduce Heat Islands. https://www.epa.gov/heat-islands/using-cool-roofs-reduce-heat-islands (accessed March 16, 2018).

^[12] CRRC. “Resources for Code Officials and Policy Makers” http://coolroofs.org/documents/Policy.pdf (accessed March 16, 2018).

^[13] Bryan Urban and Kurt Roth, Ph.D. 2010. “Guidelines for Selection Cool Roofs.” V. 1.2. Prepared by the Fraunhofer Center for Sustainable Energy Systems for the U.S. Department of Energy Building Technologies Program and Oak Ridge National Laboratory. https://www.energy.gov/sites/prod/files/2013/10/f3/coolroofguide.pdf (accessed June 25, 2018).

^[14] US EPA. Using Cool Roofs to Reduce Heat Islands. https://www.epa.gov/heat-islands/using-cool-roofs-reduce-heat-islands (accessed March 16, 2018).

^[15] The “heat island effect” refers to developed areas that experience an increase in temperature that has the effect of increasing energy demands, air conditioning costs, greenhouse gas (GHG) emissions, and pollution while increasing the risk of heat-related illnesses.

^[16] Vahmani, Pouya. (2017) “Water Conservation Benefits of Urban Heat Mitigation.” Nature Communication 8, Article: 1072, http://newscenter.lbl.gov/2017/10/20/cool-roofs-water-saving-benefits/ (accessed March 16. 2018).

^[17] Ramamurthy, P. et al. “The joint influence of albedo and insulation on roof performance: An observational study,” 2015, Energy and Buildings, Volume 93, pages 249-258. https://www.osti.gov/pages/biblio/1254734-joint-influence-albedo-insulation-roof-performance-observational-study (accessed March 18, 2018).

^[18] New York City Department of Design and Construction. DDC Cool & Green Roofing Manual. http://www.nyc.gov/html/ddc/downloads/pdf/cool_green_roof_man.pdf (accessed March 16, 2018).

^[19] Bryan Urban and Kurt Roth, Ph.D. 2010. “Guidelines for Selection Cool Roofs.” V. 1.2. Prepared by the Fraunhofer Center for Sustainable Energy Systems for the U.S. Department of Energy Building Technologies Program and Oak Ridge National Laboratory. Page 12, Table 5. https://www.energy.gov/sites/prod/files/2013/10/f3/coolroofguide.pdf (accessed June 25, 2018).

^[20] Global Cool Cities Alliance. 2012. “A Practical Guide to Cool Roofs and Cool Pavements: Primer.” Page 35. https://www.coolrooftoolkit.org/wp-content/pdfs/CoolRoofToolkit_Primer.pdf (accessed June 22, 2018).