Also known as solar reflectance, a measure of a material’s ability to reflect sunlight (including the visible, infrared and ultraviolet wavelengths), expressed either as a decimal fraction or a percentage. A value of 0 indicates that the surface absorbs all solar radiation, and a value of 1 represents total reflectivity.
Built-Up Roof (BUR)
A class of low-slope roof that consists of layers of reinforcing felt between either asphalt or coal tar bitumen. The reflectivity of BURs depends on the color of the surface layer.
A roof system with high solar reflectance, reducing heat transfer to the building envelope and enhancing roof durability, and a high thermal emittance, releasing a large percentage of the solar energy absorbed. Cool roofs reduce both building cooling loads and the urban heat island effect.
Cool Roof Rating Council
Sponsor of a third-party testing and rating program that provides credible reflectance and emissivity data on roof surfaces for building code bodies, energy service providers, architects and specifiers, property owners and community planners. CRRC maintains a database providing roofing manufacturers the opportunity to label roof products with radiative property values rated under a strict CRRC-administered program.
The ability of a material to release absorbed (non-reflected) heat, expressed either as a decimal fraction between 0 and 1, or a percentage.
As well as being energy efficient, green roofs also absorb storm water and curb runoff. Typically consists of the following components: an insulation layer, a waterproof membrane; a root barrier; a drainage layer, usually made of lightweight gravel, clay, or plastic; a geotextile or filter mat that allows water to soak through but prevents erosion of fine soil particles; a growing medium; plants; and, sometimes, a wind blanket. There are two basic types of green roofs: intensive and extensive. Intensive green roofs require a minimum of one foot of soil depth to create a more traditional rooftop garden, with large trees, shrubs and other manicured landscapes. They are multi-layer constructions with elaborate irrigation and drainage systems. Intensive green roofs add considerable load to a structure and require intensive maintenance. In contrast, extensive green roofs range from as little as 1 to 5 inches in soil depth, adding less load to a building. Extensive green roof systems also generally require less maintenance than intensive systems. Some green roof designs incorporate both intensive and extensive elements.
Heat Island Effect
A measurable increase in ambient urban air temperatures resulting primarily from the substitution of vegetation with buildings, road and parking surfaces and other heat-absorbing infrastructure. The difference between urban areas and the surrounding suburban or rural areas can be as much as 10 degrees. Nearly 40 percent of that increase is due to the prevalence of dark roofs, with the balance coming from dark pavement and the declining presence of vegetation. Relative to remedying the other sources of the problem, replacing dark roofing requires the least amount of investment for the most immediate return.
Peak Energy Demand
The maximum energy load (in megawatts) an electric utility experiences to supply customers instantaneously, generally experienced in summer late afternoons as businesses and residences turn up their air conditioners.
A class of commercial roof comprised of flexible sheets of compounded synthetic materials that are manufactured in a factory to strict quality control requirements. This is in contrast to built-up roofing, which utilizes hot asphalt and other hazardous components as the roof is constructed in place. Consistent product quality and versatility in attachment methods are two of the inherent attributes of prefabricated sheets. Single-ply roof systems provide strength, flexibility and durability. The two major categories of single-ply membranes are thermoplastics and thermosets.
Solar Reflectance Index (SRI)
A value that incorporates both solar reflectance and emittance in a single value to represent a material’s temperature in the sun. SRI quantifies how hot a surface would get relative to standard black and standard white surfaces. It is defined such that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100.
The solar reflectance value of roofing after it has been installed and subjected to actual weather conditions for at least three years. Different rating programs have individual specific conditions defining weathered reflectance. Natural weathering and soiling can affect a roof’s ability to maintain its high reflectance values. Depending on such variables as geographic location and climate; urban, agricultural or industrial setting; the amount and type of discharge from the building and adjacent structures; and roof slope, particles and pollutants of all kinds can accumulate and diminish the roof surface’s inherent reflectivity. Although there will be some loss of reflectivity from years of exposure, the levels will continue to be significantly higher than that of traditional black materials. Even simple cleaning techniques can restore most if not all of the original reflectivity. Studies have shown that washing a weathered cool roof membrane can result in a practically complete restoration of solar reflectance. Studies have also found that most reductions in reflectivity occurred during the first year, then leveled off, with further reductions negligible by the sixth year.