Urban Heat Island Effect: Urban Surface Materials and Cool Roofs

1. Introduction

With changing urbanization and consumption patterns, deteriorating urban climate exhibits climatic behaviors that exceed human comfort conditions, together with the effects of global climate change [1]. Cities are the areas where climate change is felt most acutely. In cities, the reduction of vegetation cover and evaporating surfaces, the increase in structural surfaces such as concrete and asphalt, alter the topography, ecological structure, and atmospheric properties of nature, creating a different ecology and atmosphere. Within the scope of this study, it was first aimed to explain the urban heat island effect, the most important climate event that distinguishes cities from surrounding areas, and to focus on the properties of surface materials and cool roof solutions in the urban heat island effect.

2. Urban Heat Island

The surface characteristics of urban areas differ from surrounding rural areas due to land cover, surface characteristics, and human activity [2]. Such differences affect heat generation and transfer that can lead to different surface and air temperatures in urban and rural areas. The presence of concentrated heat-generating sources in cities, the storage of daytime solar heat by urban surface materials and its release at night, and the effects of air-conditioning devices used in summer and winter months have resulted in the formation of dust domes over cities (Figure 2.1). As a result of human activities, cities become areas with their own unique climates by causing atmospheric temperatures different from natural environments, thereby causing regional warming.

Figure 2.1. Formation of Urban Heat Island.

One of the factors influencing different climate events between urban and rural areas is urban surface materials and surface properties. In rural areas, solar energy is not obstructed by any obstacle but is used in the evaporation of water in vegetation and soil. In cities, due to the scarcity of vegetation cover and natural land cover, part of the incoming solar radiation is absorbed by elements affecting urban geometry such as high-rise buildings and street widths, causing delays in re-emission. Radiation absorbed throughout the day by structures and asphalt roads in the city is later converted to heat and released back into the environment, raising air temperature in the city. At night, structures and asphalt roads in the city slowly release the solar energy they absorbed throughout the day into the atmosphere.

Figure 2.2. Rural Area and Urban Area

Therefore, while the atmosphere cools quickly in rural areas, it occurs slowly in cities, causing noticeable temperature variations. Greater cooling occurs at night in open areas compared to locations in the city center. Temperature differences between nighttime conditions in areas surrounded by buildings in the city center and open areas have been observed to reach as high as 4°C [3]. As a result, significant temperature variations occur in cities and rural areas due to climate change. This temperature phenomenon, which causes urban areas to be warmer than surrounding natural areas, is defined as the "Urban Heat Island" [4].

3. Urban Surface Materials

As cities have developed, changes in land use have led to the creation of new urban surfaces. The specific heat capacities and colors of urban surface materials are factors that influence urban heating and cooling. The color of a surface can determine its properties of reflecting and absorbing incoming solar rays. The thermal properties of materials used in buildings or materials covering urban surfaces in urban areas differ greatly from materials naturally found in rural areas. For example, the specific heat capacity of moist soil is approximately 50% higher than that of asphalt and concrete [5]. The specific heat capacities of objects also affect temperature distribution. When the same energy is supplied in unit time and unit volume to objects with different specific heat capacities, the temperature of the material with lower specific heat increases more. Similarly, during heat loss periods, materials with lower specific heat lose more heat and cool down quickly. Based on this property, two adjacent surfaces on the ground can be observed to have different temperature conditions [10]. In rural areas, soil and stone surfaces have low specific heat capacity, resulting in rapid heating and cooling. In cities, materials such as asphalt, brick, and concrete have high specific heat capacity, which is why temperature rises and falls slowly in these areas. For this reason, urban surface materials, which also define the framework of this study, are important. On a hot, sunny summer day, urban surfaces such as roofs and sidewalks are 27-50°C, while rural areas are typically cooler due to their shadier and more humid surfaces [8]. In areas with dense construction, materials covering building surfaces should be selected with higher reflectivity coefficients and materials suitable for absorbing solar energy in order to reduce the urban heat island effect. In Figure 3.1, the thermal effect of the surface temperature is shown through thermal measurement by applying white strips to part of a brick wall on a building facade. White strips on the brick wall reduce temperature by approximately 3-5°C (5-10°F) and are cooler compared to the rest of the brick wall [8]. In areas with dense construction, materials covering building surfaces should be selected with higher reflectivity coefficients and materials suitable for absorbing solar energy in order to reduce the urban heat island effect. In Figure 3.1, the thermal effect of the surface temperature is shown through thermal measurement by applying white strips to part of a brick wall on a building facade. White strips on the brick wall reduce temperature by approximately 3-5°C (5-10°F) and are cooler compared to the rest of the brick wall [8].

Figure 3.1. Albedo Effect on Surface - Temperature Effect.

One of the important parameters affecting temperature distribution on Earth's surface is the angle at which solar rays arrive. The more perpendicularly solar rays strike a horizontal surface, the greater the absorption by the surface and consequently the higher the surface temperature [11]. Roofs are the most exposed surfaces to solar radiation among urban surfaces. In this context, the color, specific heat capacity, and reflective properties of materials used on roofs are important in the formation of urban heat islands that result from the conversion of solar energy to thermal energy in the built environment. In a study, it was observed that some areas showing very high surface temperature data were roofs using metal materials belonging to large buildings such as factories [3]. In this context, "cool roofs," one of the methods to reduce the urban heat island effect, will be evaluated.

4. Cool Roofs and Cool Materials

Cool roofs can be defined as a sustainable roof technology made with cool materials that reflect solar rays and, especially in hot climate regions, assist in cooling buildings during summer months, significantly reducing the energy needs of building users [6]. Cool roofs are surfaces with low specific heat capacity and thermal emittance values and high thermal conductivity values. Cool roof materials may have lower heat storage capacity than traditional materials. In conventional roofing materials, surface temperatures on roofs reach 66-85°C, creating conditions for hot surfaces. With cool roof materials, these temperatures can be reduced to 28-33°C [8]. Cool roofs reduce building cooling loads and provide energy savings. They particularly reduce the use of air-conditioning devices in hot climate regions. Average energy savings range from 7% to 15% [7]. The definition of cool-colored material is as follows: Ultraviolet and infrared rays are invisible to the eye and have no effect on colors. However, all light, both visible and invisible, heats the objects they are absorbed by. Objects that absorb more solar energy heat more. Conversely, if an object reflects such rays extensively, it will heat up less. Two objects may appear the same color to the eye, but they may have very different reflective properties in the infrared spectrum. An object that reflects infrared rays will remain cooler than one that absorbs infrared rays. And since infrared light comprises exactly half of sunlight, when heat accumulation is a concern, an object's reflection of infrared light is more important than its color. In other words, an object does not have to be white to remain cool in sunlight [11]. In other words, cool materials are materials with high solar reflectivity and high thermal emittance properties for releasing absorbed/absorbed heat to the environment.

4.1. Cool Roof Coating Materials and Properties

Within the scope of this study, the cool material classification conducted by Çelik (2012) [9] was taken as basis. Accordingly, cool roof coating materials are addressed under 7 headings:

White Coatings and White-Colored Materials:

White is the color with the highest solar reflectivity. However, the reflectivity value of white color varies depending on the size of the pigment giving white color to the material, the thickness of the paint applied to the material, and properties such as specific heat capacity. By using white-colored coating materials with high solar reflectivity on roof surfaces or by painting roof surfaces with acrylic white paint, heat storage on roof surfaces throughout the day can be prevented and roofs can be made cool.

Figure 4.1. Utah Olympic Oval, Cool Roof Coating.

As seen in Figure 4.2, spray coating is being applied to a low-sloped roof. Coatings are the best surface treatments that can be applied on low-sloped roofs. They have a thick paint consistency, contain chemical substances to prevent moss and fungus growth, and are washable in nature with dirt being carried away by rain [8]. Additionally, the use of cool materials and photovoltaic solar panels on roof surfaces provides energy savings (Figure 4.2).

Figure 4.2. Spray Coating Application on Low-Sloped Roof, Installation of Photovoltaic Solar Panels on Cool Roof.

Figure 4.3. Application of Cool Roof Membrane to Existing Roof Surface.

Aluminum-Pigmented Cool Materials:

Aluminum pigments can improve cool roof performance. The solar reflectivity of black asphalt material with aluminum pigments was measured at 50%. In metallic materials, the high infrared emittance (Ie) value is low. Furthermore, there is an inverse relationship between reflectivity (SR) and high infrared emittance (Ie) in metallic materials. As (SR) increases, (Ie) decreases. Therefore, the more aluminum pigments present, the higher the (SR) and the lower the (Ie). These types of materials are more resistant to wear compared to white coatings. [9]

Figure 4.4. Different Roof Surfaces and Reflection of Roof Surface Temperature.

In Figure 4.4, differences in surface temperatures are visible on roofs covered with dark-colored, metallic, and white-colored roof coating materials with different reflection and emittance ratios. On hot, sunny summer days, a dark roof reflects 5% of solar energy and emits more than 90% of the heat, and absorption in the space below the roof can reach 82°C (180°F). A metal roof, on the other hand, reflects a significant amount of solar energy; absorbing around one-quarter of the heat, the temperature in the space below the roof can rise to 71°C (160°F). On a white roof, most solar energy is reflected and emitted; the maximum temperature that can be reached in the space below the roof is 49°C (120°F) [8].

Cool-Colored Coating Materials:

Despite having high reflectivity values, white-colored surfaces on sunny days can cause eye discomfort due to glare and brightness. For this reason, materials such as light-colored aggregate shingles, metal roof sheets, and reflective tiles may be preferred. On the other hand, by applying improved "cool pigments" in place of conventional pigments, high-reflectivity surfaces can be obtained.

Cool Membranes:

White-colored membrane materials have reflectivity values between 75% and 85% [9]. Cool membranes can be applied prefabricated on low-sloped roofs. Materials are generally bonded or mechanically fastened to the entire roof surface or fixed with a heat source [8]. Single-ply cool membranes can be applied to roof surfaces as EPDM, CSPE, and PVC.

Figure 4.5. Cool Single-Ply CSPE Membrane.

Asphalt Shingles: Asphalt shingles can be cool materials. They are produced by embedding coated stone granules into asphalt. White-granule shingles have reflectivity between 20% and 30% [9].

Figure 4.6. Cool Asphalt Shingle, Flat Cool Roof, Cool Metal Roof Examples.

Thermodynamic Coating Materials:

Thermodynamic materials can change their optical properties through climatic or electrical effects (for example, solar rays and temperature). A building envelope made of such materials can alter its behavior according to energy requirements. In winter, it accepts solar rays; in summer, it reflects solar rays to prevent excessive heating of the building. Thermochromic materials in the thermodynamic materials category respond to their surroundings by changing color. As heat increases, they transition from dark to light tones. As heat decreases, the opposite of this process occurs [9].

Phase-Change Materials:

Phase-Change Materials (PCM) store heat and release it back to the environment due to their chemical properties. As temperature rises throughout the day, the material reaching its melting point undergoes a phase change and transforms from solid to liquid state. When ambient temperature drops, it undergoes phase change again and becomes solid. In this way, such materials store energy within themselves throughout the day and have a reducing effect on cooling expenses [12].

Conclusion

In urban heat island formation, parameters such as decreasing vegetation cover, increasing impervious surfaces, factors blocking heat distribution, rising energy demand, and urban surface materials have accelerated the urban heat island process. Today, with the development of cities, an increase in built-up areas is observed. In this case, to provide solution alternatives by intervening in existing structures, afforestation, roof gardens, or selection of materials with high reflective properties on roof surfaces can improve air quality and minimize urban heat island formation. The material properties of surfaces constituting urban areas are another factor that must be considered. Roofs form large surfaces in urban areas and are the areas most directly exposed to solar rays. For this reason, materials with high solar reflectivity values on roofs are effective materials in reflecting solar rays and reducing roof temperature. Additionally, surfaces with high emittance capacity enable the formation of cool surfaces. As is known, white is a color with high reflectivity. In urban areas occupying a large share such as roof surfaces, building facades, or asphalt roads and sidewalks, materials with high reflective properties should be selected. Cool roof applications or roof gardens should be incorporated into existing buildings and new designs. In addition to protecting against solar energy, also considering its utilization, photovoltaic (PV) panels used together with cool membranes will enable the achievement of energy-efficient roof surfaces. The reflectivity property of cool roof materials can be reduced over time due to both atmospheric effects and weather conditions. For this reason, periodic washing or brushing of surfaces to which these materials are applied can extend their service life and improve performance.   İnci Tozam Architect İnci Tozam Architecture    

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