Lawrence Berkeley National Laboratory

Extreme heat is a critical challenge that negatively impacts human health, energy use, air quality, social equity, and economic prosperity. In a world that continues to grow hotter by the year, cities in particular are suffering from an increase in heat waves and heat-related deaths. Buildings can play an integral role in urban heat island (UHI) mitigation through the use of solar-reflective “cool” surface materials. While cool roofs are one commonly used solution, research has demonstrated that cool exterior walls are also effective at mitigating UHIs and reducing cooling loads. As rising temperatures drive the need for more heat-resilient buildings, building energy codes, standards, and voluntary programs are adopting cool exterior wall provisions. ASTM C1549-16(2022), Standard Test Method for Determination of Solar Reflectance near Ambient Temperature Using a Portable Solar Reflectometer, is the primary standard used to measure the solar reflectance (SR) of building materials, but this standard does not contain details for accurately measuring vertical surfaces. Currently, the CRRC-2 Wall Product Rating Program Manual, a proprietary document developed and maintained by the Cool Roof Rating Council for its Wall Product Rating Program, is the only document that contains scientifically backed instructions for measuring the SR of vertical surfaces using ASTM C1549. Since code and standard developers prefer to reference nonproprietary documents, such as ASTM and ANSI standards, some codes and standards direct users to test methods that do not account for the nuances of vertical surfaces. This paper summarizes the heat-mitigation impacts of cool exterior walls and describes the development and technical foundation of the testing requirements found in the CRRC-2 Wall Product Rating Program Manual. It also discusses opportunities to incorporate procedures for measuring vertical surfaces into ASTM C1549 and other areas for improvement relating to the adoption of cool exterior walls.