Heat Island Reduction

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Benefits > Heat Island Reduction

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. Trees provide shade that reduces temperatures at the surface. Trees and vegetation give off water (transpiration) that evaporates and cools their surfaces and the surrounding air. Research has shown that 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.

Urban Heat Island Profile [LBNL website http://eetd.lbl.gov/HeatIsland/]

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, 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.

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% with every degree F (5% with every degree C) of temperature rise.

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. Ordinary portland cement concrete generally has a solar reflectance of approximately 0.35 to 0.45 although values can vary. The solar reflectance of new concrete is greater when the surface reflectance of the sand and cementitious materials in the concrete are greater. 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).

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.

Table 1. Solar reflectance (albedo), Emittance, and Solar Reflective Index (SRI) of select material surfaces^{^[1]}^{,^[2],^[3],^[4]}

Material surface	Solar Reflectance*	Emittance	SRI*
Black acrylic paint	0.05	0.9	0
New asphalt	0.05	0.9	0
Aged asphalt	0.1	0.9	6
“White” asphalt shingle	0.21	0.91	21
Aged concrete	0.2 to 0.3	0.9	19 to 32
New concrete (ordinary)	0.35 to 0.45	0.9	38 to 52
New white portland cement concrete	0.7 to 0.8	0.9	86 to 100
White acrylic paint	0.8	0.9	100

*See also the section on LEED below.

HOW does a Project Obtain LEED® Credit for Reducing Temperature in Heat Islands?

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 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.

[1] 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.

[2] 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.

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

[4]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.

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