Green Roofs on Historic Buildings: Sustainable Roofing Glossary

Aerial view the Chicago City Hall green roof, on the west portion of the City Hall-County Building.

Courtesy of the City of Chicago

Albedo

Albedo, or solar reflectance, is a measure of a material's ability to reflect sunlight (including the visible, infrared, and ultraviolet wavelengths) on a scale of 0 to 1. An albedo of 0.0 indicates that the surface absorbs all solar radiation, and an albedo of 1.0 represents total reflectivity.

American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)

ASHRAE is an international organization that establishes standards for the uniform testing and rating of heating, ventilation, air conditioning, and refrigeration equipment. It also conducts related research, disseminates publications, and provides continuing education to its members.

Cool Roofs

The term cool roof is used to describe roofing material that has high solar reflectance. This characteristic can reduce heat transfer to the indoors and enhance roof durability. Cool roofs may also be highly emissive, releasing a large percentage of the solar energy they absorb.

Crassulacean Acid Metabolism

A photosynthetic process in which stomata are opened primarily at night, during cooler hours, while keeping stomata closed during the hotter day time hours to minimize water loss through evapotranspiration.

Emittance

The emittance of a material refers to its ability to release absorbed heat. Scientists use a number between 0 and 1, or 0% and 100%, to express emittance. With the exception of metals, most construction materials have emittances above 0.85 (85%).

Evapotranspiration

Plants absorb water through their roots and emit it through their leaves. This movement of water is called transpiration. Evaporation, the conversion of water from a liquid to a gas, also occurs from the soil around vegetation and from trees and vegetation as they intercept rainfall on leaves and other surfaces. Together, these processes are referred to as evapotranspiration, which lowers temperatures by using heat from the air to evaporate water.

Green Roofs

Green roofs are rooftops planted with vegetation. Intensive green roofs have a thick layer of soil (12 inches or more) that can support a broad variety of plant or even tree species. Extensive roofs are simpler green roofs with a soil layer of 6 inches or less to support drought tolerant and ground–covering plants.

R–Value

R–value or "thermal resistance value" is a measure of the resistance of a material to heat flow. The term is typically used to describe the resistance properties of insulation. The higher the R–value, the greater the insulation's resistance to heat flow.

Sedum

Sedum is a plant genus containing several species suited for use in green roof construction. Sedum species are a common choice for rooftop applications because they have high water–retention capability, an ability to filter pollution, and are hearty.

Solar Reflectance

Solar reflectance is a measure of the ability of a surface material to reflect sunlight—including the visible, infrared, and ultraviolet wavelengths—on a scale of 0 to 1. Solar reflectance is also called albedo.

Solar Reflectance Index (SRI)

SRI is a value that incorporates both solar reflectance and emittance in a single value to represent a material's ability to reject solar heat. It is calculated using equations based on previously measured values of solar reflectance and emittance as laid out in the American Society for Testing and Materials Standard E 1980. It is expressed as a number from 0 to 100.

Stomata

A stomata is a tiny pore in a plant leaf that opens and closes and serves as a site for gas exchange in the photosynthesis process.

Urban Heat Island Effect

The urban heat island effect is the tendency for increased ambient urban air temperatures resulting primarily from the replacement of vegetation with buildings, roads, and other heat–absorbing infrastructure. The heat island effect can result in significant temperature differences between rural and urban areas.

Definitions of green roof terminology primarily drawn from the Heat Island Effect resource by the Environmental Protection Agency.

Did you know?

Several cities, including Philadelphia, New York, and Washington, DC, offer grants or tax credits to building owners who install green roofs on their buildings.

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