Heat Island GraphHeat island is the name of a phenomenon given by scientists to describe an elevated temperature in urban areas compared to the surrounding rural areas. Cities can be 9 to 16 degrees Fahrenheit warmer than surrounding areas due to a reduction in vegetation and an increase in dark surfaces from paving and roofing. 

Figure 1  LBNL Website:  http://heatisland.lbl.gov

Up to 60 percent of the horizontal surface area of a city is covered by buildings and paving. These dark surfaces such as black roofs and dark pavements generally have a low solar reflectance, so they absorb heat from the sun and warm the air through convection. Lighter-colored materials (such as concrete) generally have a higher solar reflectance, so they reflect heat from the sun.  Shade, from trees and buildings, and the natural process of evaporation of water from the surface of plants also help keep the air cool.

Why Does This Matter?

The daily temperature rise on hot days results in an increase in the total and peak energy consumption in all major cities due to an increase in the air conditioning load. Research indicates that peak electricity demand increases by about 2 to 4 percent for every 1 degree Fahrenheit. This results in more cost to every consumer and more greenhouse gasses generated to produce the needed power.   

Higher air temperatures also contribute to smog formation. As urban temperatures rise, so does the probability of smog and pollution. Research has shown that the average temperature of Los Angeles, California, has risen steadily over the past half century, and is now 6 to 7 degrees Fahrenheit warmer, with the probability of smog increasing 3 percent with every degree Fahrenheit of temperature rise.

Sketch of a Heat IslandWith 80 percent of Americans now living in urban areas, heat islands can cause serious health issues for millions of people. Heat is the Number 1 weather-related killer in the U.S., particularly for days over 90 degrees Fahrenheit, which are associated with heat strokes, asthma attacks, heart attacks, and other serious health impacts.1



How does Concrete Help?

Concrete provides reflective surfaces that minimize the heat island effect. Heat islands are primarily attributed to horizontal surfaces such as roofs and pavements that absorb solar radiation. In this context, pavements include roads, parking lots, driveways and sidewalks, which can comprise 25 to 40 percent of a city's surface. Where paved surfaces are required, materials with higher solar reflectance (albedo), such as concrete, will reduce the heat island effect, save energy by reducing the demand for air conditioning, and improve air quality. 

Fig 2 - Infra-image Concrete Parking LotFig 2 Concrete Parking Lot

Figure 2 - Infra imaging of concrete parking lot and asphalt pavement. Red = warmest; blue = coolest. Image courtesy of www.pavement4life.org.

The daily temperature rise on hot days results in an increase in the peak energy consumption in all major cities due to an increase in the air conditioning load. Smog levels have also been correlated to temperature rise. Thus, as the temperature of urban areas increases, so does the probability of smog and pollution. In Los Angeles, the probability of smog increases by 3 percent with every degree Fahrentheit of temperature rise.

 

How Is Solar Reflectance Measured?

Solar reflectance (albedo) is the ratio of the amount of solar radiation reflected from a surface to the total amount reaching that surface. This includes the light we can see as well as the ultraviolet light and infrared radiation we can’t see. Ordinary portland cement concrete generally has a solar reflectance of approximately 0.35 to 0.45, although this can diminish as the surfaces become dirty or worn. Surface finishing techniques and drying time also affect solar reflectance. Solar reflectance is most commonly measured using a solar reflectometer (ASTM C1549) or a pyranometer (ASTM E1918).

The temperature of a surface depends on both the surface’s reflectance and emittance. Emittance is a measure of how well a surface emits or releases heat. The Solar Reflectance Index (SRI) is used to determine the effect of the reflectance and emittance on the surface temperature, and varies from 100 for a pure white surface to zero for a standard black surface.

Table 1. Solar reflectance (albedo), Emittance, and Solar Reflective Index (SRI) of select material surfaces

Table 1 Solar Reflectance, Emittance, and SRI of select material surfaces

Heat Island Reduction

Light Colored Surface
Trees and light colored surfaces such as concrete help lessen the heat island effect in cities. PCA No. 67548.

Concrete surfaces can earn a LEED® 2009 credits through Sustainable Sites Credit 7.1: “Heat Island Effect, Non-Roof”. The intent of this credit is to reduce the heat island effect. The intent can be met if materials that stay cool in sunlight are used on at least half of the site’s non-roof impervious surfaces, such as roads, sidewalks, courtyards, and parking lots (hardscape). The material’s solar reflectance index (SRI) must be at least 29. Where paved surfaces are required, using materials with higher SRI will reduce the heat island effect, consequently saving energy by reducing demand for air conditioning, and improve air quality. Concrete and concrete pavers are ideally suited to meet this requirement. Ordinary portland cement concrete has an SRI in the range of 38 to 52, although it can vary. However, unless it is actually measured, LEED allows an SRI of 35 for ordinary portland cement concrete (see the LEED-NC Reference Guide). New concrete made with white portland cement has an SRI of 86 according to the Reference Guide.

Other options include placing a minimum of 50 percent of parking spaces undercover (such as underground, under deck, under roof, and under building); using an open-grid pavement system with more than 50 percent perviousness; or provide shade within five years of occupancy. 

Sustainable Sites Credit 7.2: "Heat Island Effect: Roof" can also be achieved with concrete, specifically white cement tiles, with an SRI of 90 in the Reference Guide. The threshold for the roof credit is 75 percent of the roof with an SRI of 78 or better for low-slope and 29 or better for steep-slope. Other compliance options for the roof credit are 50 percent green roof or a combination of green roof and high SRI roofing materials.  Concrete, particularly if waterproof, is an excellent substrate for a green roof because of its strength and durability.

References

Global Cool Cities Alliance, “Cool Roof Toolkit” Levinson, Ronnen and Akbari, Hashem, “Effects of Composition and Exposure on the Solar Reflectance of Portland Cement Concrete,” Lawrence Berkeley National Laboratory, Publication No. LBNL-48334, 2001, 39 pages.

Pomerantz, M., Pon, B., and Akbari, H., “The Effect of Pavements’ Temperatures on Air Temperatures in Large Cities,” Lawrence Berkeley National Laboratory, Publication No. LBNL-43442, 2000, 20 pages.

Berdahl, P. and Bretz, S, "Spectral Solar Reflectance of Various Roof Materials", Cool Building and Paving Materials Workshop, Gaithersburg, Maryland, July 1994, 14 pages.

Pomerantz, M., Akbari, H., Chang, S.C., Levinson, R., and Pon, B., “Examples of Cooler Reflective Streets for Urban Heat-Island Mitigation: Portland Cement Concrete and Chip Seals,” Lawrence Berkeley National Laboratory, Publication No. LBNL-49283, 2002, 24 pages.

Kenward, A., Yawitz, D., Sanford, T., and Wang, R., Climate Central, Summer in the City, Hot and Getting Hotter, 2014, 29 pages.

Suggested Reading

Influence of Pavement Reflectance on Lighting for Parking Lots, SN2458, (2005), Adrian, W. and Jobanputra, R. 
This investigation has compared the lighting performance of concrete and asphalt surfaces of parking lots. The resulting amount of energy saved for a typical parking lot lighting system was attained for equivalent average surface luminances. Equivalent average surfaces were compared in two ways: by modifying lamp power and by reducing the number of lighting poles. PDF.

Finishing Concrete and Texture, PA124, Kosmatka, S., and Collins, T., 72 pages.
This publication is a basic guide for planning and constructing decorative concrete surfaces on concrete slabs. While intended primarily for concrete contractors, it also will be useful to concrete finishers, concrete finishers apprentices, homebuilders, general contractors, architects, engineers, landscape architects, homeowners, vocational education students, specification writers, inspectors, and many others.

"Environmental and Cost Benefits of High Albedo Concrete", (2008), Ashley, E., Concrete in Focus magazine, 2 pages.
The use of light colored exposed concrete in our urban areas and roadways can aid in the overall energy saving, safety, comfort and ambience of the general public. This is the guiding principal of Low Impact Development (LID).

Concrete: The Choice for Sustainable Design, (2008), 11 pages.