ASTM Testing for Cool Roof, Wall, and Pavement Applications

by | Dec 2024 | CoatingsTech

The Cool Roof Rating Council (CRRC) was established in 1998 to develop fair, accurate, and credible methods for evaluating and labeling the surface radiative properties of roofing products, specifically solar reflectance and thermal emittance. In addition, a standard practice for calculating a solar reflectance index (SRI) has been developed for horizontal and sloped surfaces. In recent years, these ratings have also been used to develop “cool walls” and “cool pavements.”

One important part of the CRRC Product Rating Program is the real-time, three-year weathering process that informs product field performance over time, augmented by the Rapid Ratings Program based on ASTM laboratory aging testing. ASTM methods are also used to define the properties of the paints needed for this application.

First, some important definitions[1]:

  • Solar absorption (SA): The fraction of sunlight absorbed by a surface
  • Thermal emittance (TE): The efficiency with which a surface emits thermal radiation; bare metal has low thermal emittance, while nonmetallic surfaces are high
  • Solar reflectance (SR): The fraction of sunlight reflected from a surface; high SR is the most important property of a cool surface
  • Thermal resistance (TR): The measure of a material or system’s ability to prevent heat from flowing through the substrate
  • Solar reflectance index (SRI): A calculated value based on SR and TE that determines a material’s surface value under the conditions outlined in E1980; the higher the value, the cooler the surface in the sun (see Figure 1)

Figure
Figure 1: Schematic for high and low SRI values[2]

Coating Standards

There are two basic types of cool-roof coatings: elastomeric and cementitious. The properties of elastomeric coatings are governed primarily by D6083/D6083M-24 Standard Specification for Liquid-Applied Acrylic Coating Used in Roofing. This specification defines the viscosity, volume, and weight solids required for cool coatings. It also establishes the minimum efficacy standards derived from successful in-service performance throughout the United States.

These performance evaluations were conducted on more than 20 roofs ranging in age from a few months to 20 years old.[4] There are no correlating standards for cementitious cool coatings. Table 1 lists standards for evaluating other physical properties required for acrylic elastomer cool coatings.

Table 1
(Table graphic courtesy of CoatingsTech) 

Cool Roof Material Standards

The primary standard for evaluating SRI is E1980, Standard Test Method for Measuring Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces. The combined effects of solar reflectance and thermal emittance are measured, depicting surface temperature and energy performance.

Table 2 lists standards for measuring the main variables associated with SRI, along with a standard methodology for laboratory simulation of the effects of aging exposure on reflectance and emittance properties.

Table 2
(Table graphic courtesy of CoatingsTech) 

The use of ASTM standards governing cool surfaces provides a systematic universal basis for evaluating a wide variety of substrates that correlate well with actual field exposure for paints that ultimately save energy and cool urban heat islands.

To learn more, reach out to the author at cgosselin@chemquest.com.

References

1. A Practical Guide to Cool Roofs and Cool Pavements, Cool Roof Toolkit, January 2012.

2. “What is SRI or Solar Reflectance Index?,” https://neotechcoatings.com/what-is-sri-or-solar-reflectance-index/ (accessed August 12, 2024).

3. A. Muscio, “The Solar Reflectance Index as a Tool to Forecast the Heat Released to the Urban Environment: Potentiality and Assessment Issues,” Climate, 2018, (6)1, 12.

4. E. Seyfried, “RCMA in Review: ASTM D-6083 Made Easy,” RSI Magazine, March 2000.

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