Urban Heat Island

Brochure on the Use of Cool Pavements to Reduce the Urban Heat Island Effect, prepared by the Town of Gilbert

Background


The Town of Gilbert recognizes the importance of cool pavements and other measures in helping reduce the impact of the Urban Heat Island and has included policies in its planning documents promoting the use of materials and techniques where suitable, in the planning and development process. With references to cool pavements, cool roofs and shading of paved areas to reduce heat island impacts in its comprehensive planning documents, planning staff seek to promote, encourage and educate applicants for development projects of the benefits of using these mitigating applications in appropriate situations. Citizens, architects, project designers and contractors can make a significant difference in environmental quality by the use of cool pavements.

Urban Heat Island Effect Defined 

Urban Heat Island Effect -means the occurrence of higher air and surface temperatures occurring in medium and large sized urban centers due to the retention and emittance of mainly solar heat from roads, buildings and other structures, than in surrounding rural areas. The heat stored in pavements and buildings has the effect of maintaining higher temperatures in urban centers than in surrounding rural areas. Rural areas cool faster after sunset and at night than urban areas because of this stored heat. Some other sources of man made heating such as heat and exhausts from vehicles, air conditioners, engines and machinery also contribute to the urban heating effect. 

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The higher temperatures in urban centers can cause the following adverse impacts:

  • accelerate the chemical reactions that produce ground level ozone and smog that potentially threatens public health and effects comfort of residents, 
  • higher urban temperatures can amplify extreme weather events such as heat waves that impact urban residents and may cause heat stroke, especially in the elderly,
  • result in the increased cooling costs and the associated use and generation of electricity. 

Cool Pavements Defined 

Cool Pavements-mean materials and construction techniques that are used in roads, driveways, parking lots, sidewalks, pedestrian ways and other hard surfaces, which act to reduce the absorption, retention and emittance of solar heat, a factor contributing to the urban heat island. Cool pavements utilize coloration, materials, porosity and processes that increase solar reflectivity to reduce surface heating and also to promote cooling through increased air filtration and evaporation.

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The landscape shading of paved and hardscape surfaces and use of high reflective and porous materials can significantly reduce the heat gain of pavements by the sun. Light colored surfaces are more reflective than darker. Pervious and open grid materials such as pavers, stone, blocks and interlocking concrete pavements with high-albedo reflective material reduce heat absorption from the sun and result in lower emitted heat. The Solar Reflectance Index (SRI) is a method used to measure a constructed surface’s ability to reflect solar heat, as shown by a smaller temperature rise in the surface of the material. Black surfaces have a very minimal reflectance level with only about 5% of sunlight reflected and a SRI value of 0; while white surfaces have a very high reflectance with up to 90% of sunlight reflected and a SRI value of 100. Lighter color surfaces reflect more of the suns energy reducing the absorption of heat. An albedo of 29 or higher is necessary for LEED, Leadership in Energy and Environmental Design qualification. 

As a rule of thumb, energy use in medium and large cities due to the increased demand for cooling from air conditioning increases by approximately 1.5-2% for each 1 degree (F) increase in summertime temperatures. Therefore, a City’s peak utility load may increase by 7.5 to 10% as a result of a 5 degree urban heat island effect. 

The use of cool paving materials helps to reduce the heating of roads, sidewalks and parking lots as a result of reduced heat absorption. Cool pavement can help to limit the impact of the heat island by reducing absorption, retention and emitting of heat. The EPA, Environmental Protection Agency provides a heat island reduction initiative with information available at their website (www.EPA.gov/heatisland). The combination of high albedo and pervious pavements are especially well suited for relatively light traffic flow areas such as driveways and parking lots while helping to mitigating the heat island effect and allowing storm water to pass through and permeate into the sub-base and ground.

Research has found that cool pavements can help to reduce the summertime heat island effect sufficiently to result in lower air temperatures, improved air quality and improved quality of life of residents. Large parking areas, terminals, air fields, urban roadways and large paved areas are especially suitable for cool pavement. Solar reflecants and permeable materials result in cooler pavement surfaces in the summer. Cool pavements containing porous/permeable paving promotes cooling and evaporation and due to infiltration of rainfall, reducing runoff and the need for stormwater retention.

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Use of Cool Pavements and White Roofs to reduce the Urban Heat Island Effect

An effort to decrease the urban heat island effect can help to produce the benefits of improving air quality, reducing air-conditioning costs and promoting the health and comfort of city residents. Pavements, roads, driveways, parking lots, sidewalks and other hard surfaces may comprise 33-35% of the land area of large urban centers. Efforts to reduce the urban heat island effect primarily involve:

  • use of urban landscape and vegetation to reduce direct sunlight on buildings and hard-scape surfaces to provide shading and reduce heating, 
  • use of white roofs for commercial, industrial, institutional and some residential buildings, where the use of a white surface on primarily flat or low-sloped roofs reflects light and reduces heat gain, and 
  • use of cool pavements which also act to increase surface reflectants and promote porosity to reduce absorption and retention of heat. 

Cool pavement can act to maintain a lower surface temperature and have lower heat absorption, heat retention and later heat emittance allowing for lower daytime and overnight temperatures. This reduces the requirement for air-conditioning. Temperature increases in summer time are typically 2-3 or more degrees (F) higher and range from 7 to 12 degrees (F) higher as overnight low temperatures with comparison to surrounding rural areas, unaffected by the urban heat island. The studies indicate that the urban heat island effect often extends upward 3000 to 5000 feet above the urban center. Other benefits of cool pavements can be through the use of increased permeability which allows pavements to be cooled through evaporation and air filtration as well as opportunities for noise reduction primarily through the use of rubberized asphalt surfaces. 

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Promoting greater reflectance of pavement can be achieved by:

  1. Roller compacted concrete
  2. Concrete over asphalt (white topping and ultra thin white topping)
  3. Use of light colored aggregate in asphalt
  4. Asphalt, concrete and pavers with modified colors
  5. Porous and open grade pavements permitting water and air to pass through to permit cooling by the air movement and evaporation.
  6. Use of set in place material such as brick, stone, pavers, interlocking concrete pavement, compacted decomposed granite (DG) and grass-crete and gravel-crete or alternate high density high-polyethylene base grid for grass and gravel.
Surface temperature comparison at the Town of Gilbert indicate black asphalt parking lot temperatures are significantly higher than those of standard concrete surfaces, decomposed granite (DG) areas and landscape areas during clear summer days. Studies at Arizona State University indicate that surface temperatures of asphalt may reach as high as 150 degrees (F) during peak sunlight conditions. Studies in Atlanta indicate daytime parking lot surface temperatures may be as much as 50 degrees (F) higher than surrounding forested areas. An additional adverse effect of standard pavement is that the heated surfaces continue to emit heat gained during the day, at night. This results in the slower cooling of these paved surfaces and maintains higher overnight temperatures than surrounding areas as the daytime heat is slowly released.

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Cool pavements have lower heating levels and emit their stored heat more quickly during evening and early nighttime hours than standard pavement. The rapid cooling helps to reduce the nighttime heat island effect. Cool pavements can be achieved with placement of a layer of rubberized asphalt over the concrete road surface. Rubberized pavements have been found to be cooler at night than adjacent concrete surfaces. Rubberized pavement is both cooler in the day and at night than standard asphalt. 

Use of Porous Pavement and Stone Recharge Beds for Stormwater Retention

 

Porous pavement surfaces have the benefits of being cool pavements that also provide for the absorption of rainfall and stormwater management to reduce the need for stormwater retention facilities. An example is cited in the Land Development Today magazine, April, 2006 issue, for a 12 acre development project in Maryland with a 7 acre parking lot which initially was required to provide a 1.5 acre stormwater retention basin. Through the use of pervious concrete to absorb rainfall and manage stormwater, the need for the stormwater retention basin was completely eliminated saving the developer $400,000 relating to the cost of land and construction of the retention basin. 

This type of cool pavement and water retention system utilizes porous concrete or asphalt pavements with a stone recharge bed. During storms, water drains through the open graded asphalt or concrete mix surface into the sub-grade stone infiltration bed, providing stormwater storage volumes similar to retention basins. These paved surfaces are designed to have infiltration rates of about ½ inch of water per hour and can reduce total peak volume of runoff and stormwater retention needs. Water passes through the porous pavement surface, and is temporarily stored in the stormwater recharge bed that acts as a retention basin until it is absorbed into the sub-soil. Puddles, surface ponding and runoff from storms on paved surfaces are also reduced or eliminated. 

A porous asphalt pavement consists of 2-4 inch open graded (uniform size of aggregate) surface and an underlying deep stone recharge bed with a bottom filter fabric placed over natural un-compacted soil. The recharged bed contains 1½” to 2 ½” uniform graded, clean washed, crushed Highway/Transportation standard No. 3 stone, which acts like an underground retention basin during a storm event. The 40 percent void spaces between stones provides area for the stormwater storage volume with the stormwater storage designed to drain away within 24-72 hours allowing underlying soils to dry completely between storms. The natural underlying soil base must be level or terraced and provides for water infiltration while helping to eliminate pollution by removing suspended solids from stormwater. The non-woven geo-textile fabric liner maintains a separation between the stone recharge bed and natural soil’s base. 

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At the top of the recharge bed, a 1-2” layer of clean, 1/2” Highway/Transportation standard No. 57 stone is placed to provide a firm paving base for the porous asphalt surface. The porous asphalt pavement surface consists of an open graded asphalt mix with 20 percent voids and use of stiffer binders than conventional asphalts. Polymer-modified asphalt, rubberized asphalt and fibrous additives are also used. Guidance on these mixes is available from the National Asphalt Pavement Association (NAPA) publication IS-115; Design, Construction and Maintenance of Open Graded Asphalt Friction Courses. 

Stormwater runoff from other adjoining impervious surfaces such as roads, driveways and roofs may also be conveyed directly to the parking lots stone recharge bed through perforated piping. The depth of the recharge bed is generally 4-8 inches or more depending on the amount of stormwater to be retained. A 12 inch deep stone recharge bed can retain a 5 inch rainfall event, or the equivalent amount of stormwater from other impervious surfaces. 

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Costs of Porous Pavements

The cost of pervious asphalts and stone recharge beds may be higher than standard dense-grade asphalt surfaces due mainly to the amount of materials required for the stone recharge bed. This cost difference however, is offset by the savings in the area of land required by surface stormwater retention basins or underground stormwater containment systems. The Town of Gilbert will provide credits for parking lots containing porous pavements and associated stone recharge beds toward the requirements for the sites stormwater retention. Such systems must be designed and construction certified by a professional civil engineers and subject to approval by the Town engineer. An additional benefit is that pavements are cooler, with a lower surface temperature promoting pedestrian comfort and reducing the urban heat island effect. 

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Permeable Interlocking Concrete Pavers

Permeable Interlocking Concrete Pavers (PICP) are a type of cool pavement that reduces runoff, promotes cooling through reflection of sunlight and the movement of air permitting evaporation to occur around paver materials. This occurs through the use individual 3 1/8th" thick concrete paver blocks with openings and joints between individual pavers that are wide enough to allow air and water infiltration. These openings are maintained by filling with sand or small sized, open graded crushed stone and built on an open graded stone base and sub-base. PICP can achieve infiltration rates of up to 3” of rainfall per hour and when combined with stone recharge beds, can also be used for stormwater retention. These are well suited for walkways and parking lots. 

Conclusion

The use of cool pavements along with white roofs and shaded parking lots can help reduce the impact of the urban heat island effect. Higher temperatures in urban areas due to the urban heat island effect support the formation of ground level ozone and smog, result in higher air conditioning costs and affect the health and comfort of residents. Even small efforts to reduce the emmittance of heat from pavements, roofs and the shading of parking areas, have a valuable benefit. The Town of Gilbert encourages applicants for development for suitable commercial, industrial office, public/ institutional and high density residential projects to consider the use of measures to help to mitigate the urban heat island effect including the use of cool pavements. The Town also provides credits for stormwater retention requirements for parking lots utilizing porous pavements and stone recharge beds. Individual citizens and developers can make a difference in using measures to mitigate the urban heat island effect by the construction of cool roof buildings, cool pavements and the use of landscaping to shade paved surfaces. For more information please contact the Town of Gilbert Planning Department at 480-503-6700.

Town of Gilbert Planning Dept. June 15, 2006 



Town of Gilbert General Plan and Commercial and Industrial/Employment Design Guidelines on cool pavements and mitigation of the Urban Heat Island

The Town of Gilbert encourages the use of cool pavements, white roofs and urban forestry within its planning documents as a means to help reduce the Urban Heat Island effect. The following goals and policies concerning cool pavements are contained in the Gilbert General Plan and Commercial and Industrial/Employment Design Guidelines.

General Plan, Goal 5.0 Promote mitigation of the Urban Heat Island (UHI) effect

Policy 5.1 Develop criteria that will identify which projects must include an evaluation of UHI mitigation techniques. Criteria will identify particular projects that are of a nature that may contribute to the UHI effect and will require an evaluation of mitigation techniques.

Policy 5.2 Seek sponsors to partner with to promote UHI awareness among landowners, developers, engineers, and architects. This may include educational institutions, utility companies, government entities, and others.

Policy 5.3 Promote design concepts utilizing engineered green space to maximize shading of surfaces that tend to heat up, promote education and awareness of "cool roof" materials and construction techniques, and promote "cool pavement" technologies in parking areas.

Town of Gilbert Commercial and Industrial/Employment Design Guidelines, adopted as part of the Land Development Code

Section B, Site Design and Planning, Vehicular Circulation and Parking:

d) Disperse parking into smaller fields instead of large paved areas and consider cooler paving materials. 
h) Use of canopy trees in parking lots to break up the scale of large parking lots, provide additional shading and reduce “heat island” impacts.

Section D, Environmental Sensitivity:
  • Mitigate the urban heat island effects with the use of cool roofing materials, shade trees and cool paving materials and in the Commercial Design Guidelines, Mitigate the UHI. 
These policies have been adopted to recognize the importance of cool pavements to reduce the impacts of the urban heat island. With references to cool roofs, cool pavements and shading of paved areas in the planning ordinances, planning staff is able to promote, encourage and educate applicants for development proposals to utilize these applications in appropriate situations. 

Research and Review of Materials and of Policies of other Agencies on Urban Heat Island Effect 

The following organizations and agencies recognize cool and pervious pavements as an important aspect in reducing the Urban Heat Island effect. Information from these organizations and agencies has been utilized in the background research for this report, 

APA American Planning Association 

Policy Guides support the use of cool and pervious pavement. 

APA Policy Guide for Sustainability (APA April 17, 2000).

Appendix A -Planning Actions towards Sustainability
Section III. Housing and Building Actions towards Sustainability. Reduction of Activities that encroach upon nature through: reduction or elimination of impervious paving materials.

APA Policy Guide for Smart Growth

Section III. Specific Policy Position
Environmental Protection and land conservation. 
Comprehensive Water Supply, Distribution, Treatment and Storm water Planning to protect water supplies, preserve water quality and prevent flooding.
Minimize paving and impervious services that inhibit natural water drainage and groundwater recharge.

EPA Environmental Protection Agency 

Clean Energy Environment Guide to Action 
  • reduce peak electrical loads 
  • lower emissions of greenhouse gases and improve air quality.
  • save money by lowering energy demand and supply cost.
EPA Cool Pavements Study- Task 5

LEED - Leadership in Energy and Environmental Design 

LEED, is a national voluntary standard for green building who manage and maintain building and sites. US Green Building Council establishes LEED as green building format and structure sustainable sites heat island reduction storm water management.

Green Building rating system for new construction and major renovation.
5.2 Site developments to maximize open space.

Option 3 - Where zoning ordinances exist, provide vegetated open space of 20% of the project site area. 

6.1 Storm water design quality control - limit disruption of natural water hydrology by reducing impervious cover, increasing on site infiltration, reducing or eliminating pollution from storm water run-off and elimination of contaminants.

Potential technologies and strategies:
- Design the site to maintain natural storm water flows by promoting infiltration.
- Specify pervious paving and other measures to minimize impervious services.

6.2 Quality Control - implement a storm water management plan that reduces impervious cover, promotes infiltration and captures and treats storm water runoff from 90% of the average annual rainfall using acceptable best management practices (BMP). Use alternative surfaces, pervious pavements, air grid pavers, to reduce imperviousness and promote infiltration thereby reducing pollutant loadings.

1.1 Heat Island Effect…Non-roof areas.

Reduce heat islands (thermal gradient differences between developed and undeveloped areas to minimize impact on microclimate and human and wildlife habitat). 
- Provide paving materials with a Solar Reflectants Index (SRI-see definition below) of at least 29 with the use of an open grid pavement system.


Websites on porous pavements:

EPA has prepared MIST - Mitigation Impact Screening Tool, an interactive website for 240 municipalities nation wide providing modeling and showing the effect of increasing or decreasing albedo (a measure of a surfaces reflection level of solar energy) and the impact on Urban Heat Island for those municipalities. With this information, it was determined that Phoenix with it’s sunny skies and low humidity would be one of or “the best,” benefactors in the nation, for reducing solar heat gain by efforts to increase albedo or the amount of solar energy reflected off of light colored surfaces. 

When the MIST tool applied to Phoenix to increase the albedo from a present assumed neutral (0.0) position to a maximized (+0.5) position could have the affect of:
  • Decreasing summertime UHI effect temperatures by about 5º to 9º, 
  • Reduce cooling degree days by 384 to 425 from the current 3,815 annually level
  • Reduce modern (constructed post 1980) buildings, electrical cooling costs by 10-22%, and
  • Reduce ozone levels by 6 to 9 PPB from typical 1 hour maximum concentrations of 110 PPB or potential 8.7% reductions.
Although these potential reductions for Phoenix may not realistically be expected to be achieved, they do show significant benefits may be achieved by increasing the albedo. 

Article from Stormwater Magazine about pervious concrete and the flood control project in Milwaukee, including results of Ecocreto Pavement. 

Ecocreto of Texas, Inc. provides a viable pervious concrete pavement surface which is strong, durable, and suitable for stormwater retention, has a reasonably high albedo and is claimed to be 15º (F) cooler than standard asphalt surfaces. Ecocreto contains an exclusive additive to concrete to increase strength and aid bonding.