Heat Island Effect
A heat island is an urban area which is significantly warmer than its surrounding rural areas. The main cause of the urban heat island is simply the modification of the land surface by urban development. Heat islands can be caused by buildings blocking surface heat from radiating into the relatively cold night sky, the lack of sufficient wind, changes in the thermal properties of surface materials and a lack of evapotranspiration in urban areas. Materials commonly used in urban areas, such as concrete and
asphalt, have significantly different properties that store and release heat than surrounding rural areas.
These temperature differentials are generally greater at night than during the day. Heat island effects are also major contributors to smog in urban areas. LEED defines heat islands as temperatures 2 to 10 degrees greater than those of surrounding urban areas. LEED addresses heat island elements related to roofs and nonroof components.
Pavement and roofing materials can be huge contributors to heat islands due to their thermal properties, acting as heat sinks that collect and store heat. Conventional paving and roofing materials that are darker in color exacerbate this problem by absorbing more of the sun’s energy.
Emissivity is the ability of a material to emit heat by radiation, solar reflectance (albedo) is the measure of a material’s ability to reflect sunlight and Solar Reflectance Ratio (SRI) is the measure of a material’s ability to reject solar heat. The ideal relationship is lower emissivity and higher albedo and SRI. Pavement and roofing materials that exhibit these qualities are often referred to as cool pavements and cool roofs.
asphalt, have significantly different properties that store and release heat than surrounding rural areas.
These temperature differentials are generally greater at night than during the day. Heat island effects are also major contributors to smog in urban areas. LEED defines heat islands as temperatures 2 to 10 degrees greater than those of surrounding urban areas. LEED addresses heat island elements related to roofs and nonroof components.
Pavement and roofing materials can be huge contributors to heat islands due to their thermal properties, acting as heat sinks that collect and store heat. Conventional paving and roofing materials that are darker in color exacerbate this problem by absorbing more of the sun’s energy.
Emissivity is the ability of a material to emit heat by radiation, solar reflectance (albedo) is the measure of a material’s ability to reflect sunlight and Solar Reflectance Ratio (SRI) is the measure of a material’s ability to reject solar heat. The ideal relationship is lower emissivity and higher albedo and SRI. Pavement and roofing materials that exhibit these qualities are often referred to as cool pavements and cool roofs.
Terms
Solar Reflectance is the fraction of the solar energy that is reflected by a surface, such as a roof or pavement, expressed as a number between zero and one. The higher the value, the better the roof reflects solar energy. For example, a white reflective coating or membrane has a reflectance value of 0.85 (reflects 85% of solar energy hitting it and absorbs the remaining 15%), while asphalt has a value of 0.09 (reflects 9%)
• Emittance is the amount of absorbed heat that is radiated from a surface, expressed as a number between zero and one. The higher the value, the better the surface radiates heat
• Solar Reflectance Index (SRI) indicates the ability of a surface to reject solar heat, and is the combined value of reflectivity and emittance. It is defined so that a standard black is zero (reflectance 0.05, emittance 0.90) and a standard white is 100 (reflectance 0.80, emittance 0.90). Because of the way SRI is defined, very hot materials can have slightly negative SRI values, and very cool materials can have SRI values exceeding 100
Green roofs are becoming popular in the U.S. in larger metropolitan areas, such as Chicago.
In addition to assisting stormwater management by reducing quantity of flow and increasing quality of release, green roofs reduce the building’s energy consumption, provides vegetated open space and refuge for habitat. Green roofs also reduce the heat island effect of absorbing the sun’s energy by providing natural vegetation and soils which also contributes to the cooling effects of evapotranspiration, the release of water from plants into the atmosphere. Green roofs are a great sustainable strategy for a project.
Strategy considerations for reducing heat island effects on the microclimate and human and wildlife habitat:
● Nonroofs
• Reduce area of hardscapes including roads, parking lots/parking structures, walks or courtyards
• Shading: combination of any of the following strategies for 50% of the total hardscape area
• Shade from existing tree canopy or, for new trees, within 5 years of installation
• Shade from structures covered by solar panels that produce renewable energy
• Shade from architectural features that have a SRI of at least 29
• Cool pavements: Use hardscape materials with a SRI of at least 29
• Open grid pavement systems which are at least 50% pervious
• Place a minimum of 50% of parking spaces under cover
• Roofs used to shade or cover parking must have a SRI of at least 29
● Roofs
• Cool Roofs: Use roofing materials for 75% of the roof surface with controlled SRI values
• 78 SRI for low sloped roofs less than or equal to 2:12 slope
• 29 SRI for steep sloped roofs greater than 2:12 slope
• Green Roofs: Install a vegetated roof that covers at least 50% of the roof area
• Install high albedo (SRI) cool roof and vegetated roof surfaces that, in combination, meet the following:
• (area of roof meeting minimum SRI / 0.75) + (area of vegetated roof/0.5) = Total Roof Area
• Emittance is the amount of absorbed heat that is radiated from a surface, expressed as a number between zero and one. The higher the value, the better the surface radiates heat
• Solar Reflectance Index (SRI) indicates the ability of a surface to reject solar heat, and is the combined value of reflectivity and emittance. It is defined so that a standard black is zero (reflectance 0.05, emittance 0.90) and a standard white is 100 (reflectance 0.80, emittance 0.90). Because of the way SRI is defined, very hot materials can have slightly negative SRI values, and very cool materials can have SRI values exceeding 100
Green roofs are becoming popular in the U.S. in larger metropolitan areas, such as Chicago.
In addition to assisting stormwater management by reducing quantity of flow and increasing quality of release, green roofs reduce the building’s energy consumption, provides vegetated open space and refuge for habitat. Green roofs also reduce the heat island effect of absorbing the sun’s energy by providing natural vegetation and soils which also contributes to the cooling effects of evapotranspiration, the release of water from plants into the atmosphere. Green roofs are a great sustainable strategy for a project.
Strategy considerations for reducing heat island effects on the microclimate and human and wildlife habitat:
● Nonroofs
• Reduce area of hardscapes including roads, parking lots/parking structures, walks or courtyards
• Shading: combination of any of the following strategies for 50% of the total hardscape area
• Shade from existing tree canopy or, for new trees, within 5 years of installation
• Shade from structures covered by solar panels that produce renewable energy
• Shade from architectural features that have a SRI of at least 29
• Cool pavements: Use hardscape materials with a SRI of at least 29
• Open grid pavement systems which are at least 50% pervious
• Place a minimum of 50% of parking spaces under cover
• Roofs used to shade or cover parking must have a SRI of at least 29
● Roofs
• Cool Roofs: Use roofing materials for 75% of the roof surface with controlled SRI values
• 78 SRI for low sloped roofs less than or equal to 2:12 slope
• 29 SRI for steep sloped roofs greater than 2:12 slope
• Green Roofs: Install a vegetated roof that covers at least 50% of the roof area
• Install high albedo (SRI) cool roof and vegetated roof surfaces that, in combination, meet the following:
• (area of roof meeting minimum SRI / 0.75) + (area of vegetated roof/0.5) = Total Roof Area
