Albedo, or solar reflectance, is the ability of a surface material to reflect sunlight on a scale of 0 to 1 (0 - 100%).
Asphalt shingles are a type of roof shingle that consists of overlapping panels coated with asphalt (a mixture of mineral aggregate and bitumen) and granules. It is one of the most widely used roofing materials because of its relatively inexpensive cost and simple installation.
Built-up roofs (BUR) are a continuous, multi-ply roof membrane that consists of layers of fabric reinforcement layers between asphalt/coal tar bitumen. While it is traditionally dark-surfaced, BUR can achieve cool roof effects by installing reflective marble chips or gray slag on the surface or finishing with a mineral surfaced sheet.
California Building Climate Zone
The California Energy Commission established 16 climate zones based on energy use, temperature, weather and other factors, representing a geographic area that has similar climatic characteristics.
Cool roofs are roofs that are designed to maintain a lower roof temperature than traditional roofs while the sun is shining.
Green roofs, also known as garden or vegetated roofs, refer to rooftops planted with vegetation. They reduce storm water runoff by absorbing rain water and provide urban heat island mitigation benefits.
Heat transfer is the movement of heat from a higher-temperature region to a cooler-temperature one until two regions reach the same temperature (thermal equilibrium).
Latitude is the geographic coordinate represented by lines running from east to west that are parallel to the equator.
Longitude is the geographic coordinate represented by lines running from north to south. The line passing through the Royal Observatory, Greenwich in England is defined as zero degrees of longitude, or the Prime Meridian.
A low-slope roof refers to a roof with a slope less than or equal to 2 inches of rise over 12 inches of run.
Metal roofs are available in a variety of shapes, textures, and colors. They can come in natural metallic finishes, shingles, or shakes. Cool colors and coatings can be applied to metal roofs to achieve greater solar reflectance.
The number of layers of roofing (i.e. one-ply, two-ply).
Single-ply membrane is a roofing material that consists of one layer of membrane material rather than multiple layers. It is rolled onto the roof and attached with mechanical fasteners, adhered with chemical adhesives, or held in place with ballast (gravel, stones, or pavers). Single-ply membranes come in many different colors including white and other reflective colors.
The degree of roof incline expressed as the ratio of the rise to the run.
Smog, derived from the words smoke and fog, describes a type of air pollution containing ozone and other reactive chemical compounds formed by the reaction between sunlight and hydrocarbons and nitrogen oxides especially from automobile exhaust.
Solar reflectance, also known as albedo, is the fraction of sunlight that a surface reflects. Sunlight that is not reflected is absorbed as heat. Solar reflectance is measured on a scale of 0 to 1 (0 - 100%), where higher numbers indicate more reflective surfaces.
Solar Reflectance Index
Solar reflectance index (SRI) is an alternative metric for comparing the coolness of roof surfaces. It is a temperature scale calculated from solar reflectance and thermal emittance. The higher the SRI, the cooler the roof will be in the sun. For example, a clean black roof has an SRI of 0, while a clean white roof has an SRI of 100. Many building codes and utility rebates now specify SRI.
A roof with a slope greater than 2 inches of rise over 12 inches of run.
Image source: LBNL – Heat Island Group
Thermal emittance describes how efficiently a surface cools itself by emitting thermal radiation. Thermal emittance is measure on a scale of 0 to 1 (0 – 100%), where a value of 1 indicates a perfectly efficient emitter.
Tile roofs are typically made of concrete, clay, or slate. The color of clay and slate tiles differs depending on the earth's composition but clay tiles can be also glazed to provide waterproofing layers and customized colors. Concrete tiles are colored through the entire tile with a pigment additive during manufacturing or coated with a cementious color coating.
Urban Heat Island
The urban heat island effect describes a regional elevation in air temperature resulting primarily from the replacement of trees and vegetation with buildings, roads, and other heat-absorbing infrastructure. Air temperatures in many urban and suburban areas can be 2° - 8°F warmer than those found in rural surroundings.