WHAT is the Heat Island Effect?
Cities and urban areas are 3 to 8 °F (2 to 4°C) warmer than surrounding areas due to the heat island effect as shown in Fig. 1. This temperature difference is attributed to more buildings and pavements that have taken the place of trees and vegetation. Vegetation physically shades the ground, reducing surface temperatures. In addition, vegetation and trees cool the air and reduce ambient temperatures through transpiration (evaporating water through leaves). Research has shown the average temperature of Los Angeles has risen steadily over the past half century, and is now 6 to 7°F (3 to 4°C) warmer than 50 years ago(1).
|Fig. 2. Composition of horizontal surfaces in Sacramento, California(2)|
HOW Does Concrete Help?
Concrete provides reflective surfaces that minimize the urban heat island effect. Urban 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. Where paved surfaces are required, using 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.
In cities such as Sacramento, California, paved surfaces comprise 50% of all horizontal surfaces as shown in Fig. 2. Great potential exists for lessening the effect of the urban heat islands by using concrete rather than asphalt.
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. It is estimated that 3 to 8 percent of the electricity demand in cities with populations greater than 100,000 is used to offset the heat from the heat island effect. In addition, studies indicate people will often avoid using air-conditioning at night (they will not turn it on) if temperatures are lower than 77°F (25°C). Mitigating the urban heat island effect to keep temperatures in cities lower than 77°F (25°C) at night has the potential to save large amounts of energy.
Smog levels have also been correlated with temperature rise (3). 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% with every degree F (5% with every degree C) of temperature rise. Studies for Los Angeles and 13 cities in Texas have found that there are almost never any smog episodes when the temperature is below 70°F (21°C). The probability of episodes begins at about 73°F (23°C) and, for Los Angeles, exceeds 50 percent by 90°F (32°C). Reducing the daily high in Los Angeles by 7°F (4°C), the heat island effect, is estimated to eliminate two-thirds of smog episodes.
Smog and air pollution are the main reasons for the EPA creating and mandating expensive clean fuels for vehicles and reduced particulate emissions from industrial facilities such as cement plants and asphalt production plants. The EPA now recognizes that air temperature is as much a contributor to smog as NOx and VOCs, the contributing emissions(4). The effort to reduce particulates in the industrial sector alone costs billions of dollars per year. Installing high albedo roofs, walls, and pavements at the time of their construction or replacement is a cost-effective way to reduce smog.
WHAT is Albedo?
Albedo, which in this case is synonymous with solar reflectance, is the ratio of the amount of solar radiation reflected from a surface to the total amount reaching that surface. The solar radiation reaching an object on earth includes visible and ultraviolet light and infrared radiation. Solar reflectance is measured on a scale of 0.0 to 1.0, from not reflective at all to 100% reflective. Generally, materials that appear to be light-colored in the visible spectrum have high solar reflectance and those that appear dark-colored have low solar reflectance. Because reflectivity in the solar radiation spectrum determines albedo, color in the visible spectrum is not always a reliable indicator of solar reflectance.
Surfaces with lower albedos absorb more solar radiation. The ability to reflect infrared radiation is of great importance because infrared radiation is most responsible for heating. To illustrate this point, surface temperatures were measured on various materials on a bright clear November day in central California. During the time of the experiment, the ambient air temperature was 55°F (13°C). Darker materials such as black acrylic paint attained a maximum temperature of 142°F (61°C), while white colors such as white acrylic paint had a temperature of 74°F (23°C)(5). The color and composition of the materials greatly affects the surface temperature and the amount of absorbed solar radiation. This temperature rise can be estimated using a measure called the solar reflective index (SRI).
What is SRI?
A composite index called the solar reflectance index (SRI) is used by the U.S. Green Building Council and others to estimate how hot a surface will get when exposed to full sun. The temperature of a surface depends on the surface’s reflectance and emittance, as well as solar radiation. 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 standard white surface to zero for a standard black surface. The SRI is calculated using ASTM E1980, “Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces.” Materials with the highest SRI are the coolest and the most appropriate choice for mitigating the heat island effect.
Emittance, also known as emissivity of a surface, is a measure of how well a surface emits or releases heat. It is a value between 0 and 1. Highly polished aluminum has an emittance less than 0.1. A black non-metallic surface, on the other hand, has an emittance greater then 0.9. However, most opaque non-metallic materials encountered in the built environment (such as concrete, masonry, and wood) have an emittance between 0.85 and 0.95, and a value of 0.90 is usually assumed(6). Further, for these materials, SRI is mostly a function of solar reflectance. In other words, a building material with a high solar reflectance will probably also have a high SRI.
Table 1 shows the reflectance, emittance and SRI of some common building materials.
The effect of albedo and solar radiation on surface temperatures is referred to as the sol-air temperature and can be calculated.
One method to reduce the "urban heat island effect" is to change the albedo of the urban area. This is accomplished by simply replacing low albedo surfaces with higher albedo materials. In one study, a computer model was used to predict the effects of changing the albedo of Los Angeles. Aerial photographs were used to identify areas where the albedo could be increased. Areas identified for albedo modification were roofs and pavements, with equal areas available for modification. Through simulation, roofs were lightened through the use of lighter colored shingles and white coatings, and roads were changed from asphalt to concrete. The study indicated that lightening the city’s roofs and pavements would decrease average mid-afternoon temperature by 4°F (2°C), thus eliminating a significant amount of smog and energy usage (7).
Measured Albedo (Solar Reflectance)
Ordinary portland cement concrete generally has an albedo or solar reflectance of approximately 0.35 to 0.45 although values can vary. Solar reflectance of the material increases as the surface reflectance characteristics of the concrete’s sand and cementitious materials increase. Surface finishing techniques also have an effect because smoother surfaces generally have a higher solar reflectance. For “white” portland cement, values are reported in the range of 0.7 to 0.8 (8). New asphalt concrete generally has a reflectance of approximately 0.05 and asphalt concrete five or more years old has a reflectance of approximately 0.10 to 0.15. Asphalt is initially dark but becomes lighter with time as the asphalt binder wears off and aggregate is exposed. The asphalt binder also becomes lighter with time due to oxidation. Solar reflectances (albedo) of select materials are shown in Table 1.
Albedo is most commonly measured using a solar reflectometer (ASTM C1549) or a pyranometer (ASTM E1918). The solar reflectometer, ASTM C1549, “Standard Test Method for Determination of Solar Reflectance Near Ambient Temperature Using a Portable Solar Reflectometer”, is usually done in a laboratory using a sample in the range of 2 to 5 in. sq (50 to 120 mm sq). Measuring solar reflectance using a pyranometer, ASTM E1918, “Standard Test Method for Measuring Solar Reflectance of Horizontal and Low-Sloped Surfaces in the Field,” is performed in the field on surfaces at least 13 ft (4m) in diameter. Measurements are done on a sunny day when the angle of the sun to the earth’s surface is greater than 45 degrees. For northern cities like Chicago, the best time to take measurements is April through August.
The change to higher albedo surfaces is most cost effective when done in the initial design or during renovation or replacement due to other needs. Ideas for lightening pavements include using traditional concrete or white topping. A more expensive option is to use white cement in the concrete rather than ordinary portland cement.
Table 1. Solar reflectance (albedo), Emittance, and Solar Reflective Index (SRI) of select material surfaces (9,10,11,12)
|Black acrylic paint
|“ White” asphalt shingle
||0.2 to 0.3
||19 to 32|
|New concrete (ordinary)
||0.35 to 0.45
38 to 52
|New white portland cement concrete
||0.7 to 0.8
||86 to 100|
|White acrylic paint
*See also the Section on LEED Below
Effects of Weathering on Albedo
One area of concern for lightened pavements is the long-term color stability. As light colored pavements age, they become darker. Oil drips, pavement scrapes, tire marks, debris, and dirt decrease the albedo of concrete from the range of 0.35 to 0.45 when new to the range of 0.2 to 0.35 when aged.
In contrast to portland cement concrete surfaces, the albedo of asphalt surfaces increases with age due to the asphalt binder on the surface wearing away revealing the aggregate and through oxidation of the binder. In general, the degree of lightening of the pavement depends on the color of the aggregate, and the rate of lightening depends on the amount of traffic, the total hours of solar radiation, and other factors. The asphalt surface generally has an initial albedo of 0.05, increasing to 0.1 over time.
Moisture and Albedo
Moisture in concrete helps to cool the surface by evaporation. Concrete when placed has a moisture content of 100% relative humidity (saturated). The concrete surface gradually dries over a period of one to two years to reach equilibrium with its surroundings. Concrete surfaces exposed to rain and snow will continue to be wetted and dry. This moisture in the concrete surface will help to cool the concrete by evaporation whenever the vapor pressure of the moisture in the surface is greater than that of the air. In simpler terms, when the temperature and relative humidity of the air are greater than that just beneath the concrete surface, the concrete will dry and hence cool somewhat by evaporation.
The albedo of concrete decreases when the surface is wet. Consequently albedo is lower when concrete is relatively new and the surface has not yet dried, and when the concrete becomes wet due to rain or other sources of moisture. The albedo of new concrete generally stabilizes within six weeks to 3 months.
High Albedo and Winter Heating Costs
Because light colored surfaces reflect solar radiation, high albedo surfaces may increase winter heating bills. However, the effect of light colored horizontal surfaces in winter is estimated to be one-tenth that of those in summer due to less solar radiation, shorter days, and the increased possibility of overcast skies. Therefore, the beneficial effects of cool materials in the summer are not necessarily a large disadvantage in the winter in U.S. climates.
Dark colored pavement surfaces may provide some benefits in cold weather climates. Although no quantifiable research was identified, pavement color may affect the quantity or frequency of deicer chemical applications. In certain situations, dark pavements are warmer and appear to shed snow and ice more quickly than light colored pavements.
How Else does Concrete Affect Heat Islands?
Thermal Mass and Nocturnal Effects
The thermal mass of concrete delays the time it takes for a surface to heat up, but also delays the time to cool off. For example, a white roof will get warm faster than concrete during the day, but will also cool off faster at night. Concrete surfaces are often warmer than the surrounding air in the evening. Concrete’s albedo and thermal mass will help mitigate heat island effects during the day, but contributes to the nocturnal heat island effect. Inhabitants can usually endure the heat during the day as long as their space cools to 77°F (25°C) at night when they sleep. The moisture absorbed by concrete helps reduce the daytime and nocturnal heat island effect when it evaporates. The challenge for the concrete industry is to use it to mitigate heat islands while keeping night time temperatures as cool as possible.
Vegetation and Shade
Open grid concrete pavers also help reduce temperatures due to the earth and vegetation in the opening. Planting trees for shade near buildings also helps mitigate the urban heat island effect. Shade directly reduces the air-conditioning load on buildings. Using deciduous trees shades the buildings in the summer and allows the sun to reach the buildings in the winter.
HOW does a Project Obtain LEED® Credit for Reducing Temperature in Heat Islands?
Concrete surfaces can earn a LEED for New Construction and Major Renovation (LEED-NC version 2.2) credit 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% of parking spaces undercover (such as underground, under deck, under roof, and under building); using an open-grid pavement system with more than 50% perviousness; or provide shade within 5 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% 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% 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.
A study on the influence of pavement albedo on air temperature in Los Angeles predicted that increasing the albedo of 1250 km2 (490 sq miles) of pavement by 0.25 would save cooling energy of $15,000,000 per year and reduce smog-related medical and lost-work expenses by $76,000,000 per year (13). Concrete surfaces often have higher albedo than surfaces constructed of other materials, and can be made even higher by judicious selection of constituent materials.
Reducing the urban heat island effect through cool surfaces is being promoted by organizations such as the U.S. EPA, Energy Star, LEED, and energy codes.
1. Akbari, H., Fishman, B., and Frohnsdorff, G, Proceedings of the Workshop on Cool Building Materials, National Institute of Standards and Technology, Gaithersburg, Maryland, February 1994.
2. Rosenfeld, A.H., “Urban Heat Islands” presentation at the Toronto Urban Heat Island Summit, May 2, 2002, average of under the canopy values for Sacramento.
3. Cooling Our Communities - A Guidebook on Tree Planting and Light-Colored Surfaces, Document No. 22P-2001, US EPA, Washington DC, 1992.
4. Akbari, H., Rosenfeld, A. H., and Taha, H., "Cool Construction Materials Offer Energy Savings and Help Reduce Smog", ASTM Standardization News, pp. 32-37, November 1995.
5. Berdahl, P. and Bretz, S, "Spectra Solar Reflectance of Various Roof Materials", Cool Building and Paving Materials Workshop, Gaithersburg, Maryland, July 1994.
6. ASHRAE 2005 Handbook of Fundamentals
, IP edition, American Society of Heating, Refrigerating, and Air-Conditioning Engineer, Atlanta, Chapter 25. http://www.ashrae.org/
7. "Heat Islands -- and How to Cool Them", Center for Building Science News, Lawrence Berkeley Laboratory, Vol. 1, No. 2. Spring 1994.
8. Levinson, R. and Akbari, H., “Effects of Composition and Exposure on the Solar Reflectance of Portland Cement Concrete,” Lawrence Berkeley National Laboratory, Publication No. LBNL-48334, December 2001, 39 pages.
9. 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.
10. 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.
11. Berdahl, P. and Bretz, S, "Spectral Solar Reflectance of Various Roof Materials", Cool Building and Paving Materials Workshop, Gaithersburg, Maryland, July 1994 14 pages.
12. 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.
13. 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.