Chapter 6 Infrared Thermographic Analysis of the Urban Environment and its Individual
6.8 Strategies to reduce UHI effects in relation to buildings
An effective, cheap and simple way of reducing UHI effect is to use high-albedo materials on urban surfaces such as roofs, roads, car parks and footpaths (Taha et al, 1992).
The terms cool roofs and cool pavements are popular terms used to describe the implementation of high albedo materials on these surfaces. Cool surfaces have become an increasingly popular method of cooling due to its low-cost, efficiency, and ease of use (Rosenfeld et al, 1995). The two major factors that contribute to heating a surface are its level of solar reflectance (its albedo) and its infrared emittance. Increasing solar reflectance and infrared emittance will decrease the surface temperature of the surface and in turn reduce ambient air temperature as the intensity of heat convection (sensible heat flux density) from a cooler surface is lower (Synnefa et al, 2008). Implementing cool surfaces can be an easy process: by simply painting a surface white, the solar reflectance is increased dramatically. Taha et al (1992) measured the albedo and surface temperatures of a variety of surfaces in the field and found that white coatings with an albedo of 0.72 were 45°C cooler than black coatings (with an albedo of 0.08). Painted building surfaces are generally painted every 10 years; in the next painting cycle, they can be painted with high solar
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reflectance and high emittance paint as part of the normal upkeep of the building.
Incorporating cool surfaces into building codes is an easy way to ensure that new buildings or any new works use high-albedo materials (Taha et al, 1992). Using high albedo surfaces is an especially effective way of mitigating the UHI in locations where weather conditions or water availability restrict the ability for vegetation to act as a climate moderator (Taha et al, 1992). Building X was designed to be a cool building and uses high-albedo surfaces and efficient cooling systems. It is an example of how a large building can have a low mean temperature (roof temperature 12.3°C, table 6.1) by incorporating cool surfaces in its construction.
There is significant potential for using high-albedo surfaces in urban areas. For example, Berdahl and Bretz et al (1997) found that Sacramento (USA) had surface components which were 28% rooftop, 16% streets, and 14% other impervious surfaces, of the city area. High-albedo surfaces could be applied to many of these surfaces to reduce urban temperatures. However, there are some issues that are associated with implementing high-albedo surfaces. Most notably, glare problems could arise from the highly reflective surfaces, which may lead to hazards, discomfort and radiation to surrounding surfaces. Also, colour preference could limit the selection of colour/hue being used on homes and buildings; white or other high albedo colours may not suit a particular design of a building (Taha et al, 1992). Another issue is the dirt attracting qualities of the surfaces treated with these colours. Albedos change over time as the surface weathers and wears. Building surfaces are exposed to solar radiation, dust, rain and humidity for instance; other urban surfaces like roads and footpaths are exposed to vehicle and pedestrian traffic (Taha et al, 1992).
The Government of South Australia released a discussion paper on cool roofs regulation in South Australia in December 2010. Public comment was sought on the proposal for a cool roof requirement for certain categories of commercial building.
Specifically sought were views relating to the additional costs of supplying the specialist coating required in order to reflect heat. Public consultation closed on 11 February 2011 followed by a review of the responses. The review concluded that introducing cool roofs in commercial buildings was justified and as a result, a South Australian state variation was
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included in the Building Code of Australia for air-conditioned commercial buildings from 1 May 2012.
An alternative to a high albedo roof (cool roof) is a green roof, a roof that has a vegetation layer on it. Green roofs offer a unique opportunity to introduce vegetation into a dense, urban area where ground vegetation is often not an option. The surface of a green roof can be cooler than ambient air temperatures, but by contrast, conventional rooftop surfaces can exceed ambient air temperatures by up to 50˚C (EPA, 2008). High moisture levels in vegetation play a role in reducing daytime surface temperatures through evaporation and plant transpiration, because the conversion from liquid water to vapour in these processes consume large amounts of latent heat, leaving less for heating (Rosenzweig et al, 2006). Green roofs also reduce temperatures from shading by vegetation. The greater the foliage density of plants, the more the conductive heat flux through a roof decreases and the more that surface temperatures drop (Clark et al, 2008). A lower surface temperature reduces the amount of heat transmitted into the building and re-emitted into the atmosphere. Surface temperatures have been recorded at being 11-25˚C less due to the shading of trees (Akbari et al, 1997). Green roofs may offer benefits in savings of energy used for heating and cooling. Reducing surface and ambient air temperatures reduces the amount of cooling needed, while in winter the green roofs act as insulation to reduce heating and associated costs (EPA, 2008).
Trees and vegetation also help in mitigating the effects of UHIs. Shading from trees reduces the amount of solar radiation that reaches the canopy below a tree or plant (Akbari, 2001). The shade from the tree can cover buildings or other urban surfaces, reducing the amount of heat that is transferred into the building or transmitted back into the atmosphere. Also, trees cool the air through evapotranspiration. Ideally, a scheme to combat the effects of UHIs would include the incorporation of cool surfaces and urban vegetation. But the emphasis should be on cool surfaces first, because they are more effective than trees, and cost little if high-albedo surfaces are incorporated into routine maintenance programmes. Also, the results from cool surfaces are seen immediately, whereas trees may require many years to grow large enough to produce significant energy savings (Rosenfeld et al, 1995).
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