Evolution of Cool-Roof Standards in the US

Abstract

Evolution of cool roof standards in the United States submitted to Advances in Building Energy Research, 2007-06-15 Accepted for publication, August 2007. Evolution of cool-roof standards in the United States Hashem Akbari and Ronnen Levinson Heat Island Group Lawrence Berkeley National Laboratory Berkeley, CA 94720 USA H_Akbari@LBL.gov Abstract Roofs that have high solar reflectance and high thermal emittance stay cool in the sun. A roof with lower thermal emittance but exceptionally high solar reflectance can also stay cool in the sun. Substituting a cool roof for a noncool roof decreases cooling-electricity use, cooling-power demand, and cooling-equipment capacity requirements, while slightly increasing heating-energy consumption. Cool roofs can also lower citywide ambient air temperature in summer, slowing ozone formation and increasing human comfort. Provisions for cool roofs in energy-efficiency standards can promote the building- and climate- appropriate use of cool roofing technologies. Cool-roof requirements are designed to reduce building energy use, while energy-neutral cool-roof credits permit the use of less energy-efficient components (e.g., larger windows) in a building that has energy-saving cool roofs. Both types of measures can reduce the life-cycle cost of a building (initial cost plus lifetime energy cost). Since 1999, several widely used building energy-efficiency standards, including ASHRAE 90.1, ASHRAE 90.2, the International Energy Conservation Code, and California’s Title 24 have adopted cool-roof credits or requirements. This paper reviews the technical development of cool- roof provisions in the ASHRAE 90.1, ASHRAE 90.2, and California Title 24 standards, and discusses the treatment of cool roofs in other standards and energy-efficiency programs. The techniques used to develop the ASHRAE and Title 24 cool-roof provisions can be used as models to address cool roofs in building energy-efficiency standards worldwide. Introduction Roofs that have high solar reflectance (high ability to reflect sunlight, spectrum 0.3 – 2.5 µm) and high thermal emittance (high ability to emit thermal radiation, spectrum 4 – 80 µm) stay cool in the sun. The same is true of roofs with lower thermal emittance but exceptionally high solar reflectance. Roofs that stay cool in the sun by minimizing solar absorption and maximizing thermal emission are hereafter denoted “cool roofs.” Benefits of cool roofs Low roof temperatures lessen the flow of heat from the roof into the building, reducing the need for space-cooling electricity in conditioned buildings. Since building heat gain through the roof peaks in mid-to-late afternoon, when summer electricity use is highest, cool roofs can also reduce peak electricity demand. Prior research has indicated that savings are greatest for buildings located in climates with long cooling seasons and short heating seasons, particularly