Advances in the Development of Cool Materials for the Built Environment

Extensive urbanization has resulted to economic, social, energy & environmental challenges. The global population increase led to an increasing demand for housing. Natural land has been replaced with artificial surfaces in most cities around the world with undesirable thermal effects. This, together with industrialization growth, has caused a deterioration of the urban environment. Urban heat island (UHI) phenomenon has become a common problem in many major cities worldwide. Several factors influence the urban heat island phenomenon, such as the continuous reduction of green spaces, the changes of wind velocity due to high buildings’ density, the anthropogenic heat release and the alteration of surfaces’ albedo. The aforementioned factors lead to overheating problems in cities due to the absorption of solar radiation by the various surfaces and buildings. Hence, urban climate is one of the most important elements of urban physical environment, which is often ignored in urban planning. To design a sustainable city, it is necessary to take into account the climatic conditions holistically and strategically during the planning process. Since the 1970s, German researchers have developed the concept of urban climate map (UC-Map) that has a strong focus on applied urban climatology. UC-Map is an appropriate tool for translating climatic phenomena and problems into 2-D images including symbols for land use and spatial information suitable for the urban planner. Therefore this map is a useful tool for urban planners, architects and governors in order for them to understand more accurately and evaluate the effects of urban climatic issues on decision-making and environmental control. At the micro-climate level, several UHI mitigations can be implemented to reduce the UHI severity. First is greenery. The benefits of greenery to the built environment have been widely investigated. Greenery dissipates the incoming solar radiation on the building structures through its effective shading; it reduces longwave radiation exchange between buildings due to the low surface temperatures created by plants’ shading; it reduces the ambient air temperature through evapotranspiration. The role of building’s materials, mainly determined by their optical and thermal characteristics, is crucial in reducing the thermal and solar hear gains, in the urban environment. The so-called ‘cool’ materials, characterized by high reflectivity and high emissivity, can improve the thermal conditions in cities by lowering the surface temperatures that affect the thermal exchanges with the surrounding air. Urban ventilation is another important strategy of UHI mitigation. It is important to understand the nature of air flow regimes within urban canyons in order to make further progress in describing the complex interactions between mesoscale forces and the built environment that create the urban boundary layer.

This chapter is a state of the art report focusing on the aspects that concern cool materials for buildings that are white or light colored. It is structured in four sections; in the first section cool white or light colored materials i.e. materials characterized by high solar reflectance and high infrared emittance are defined. Typical values of the two properties are given according to independent studies, the U.S. Cool Roof Rating Council‘s database and the IEE Cool Roofs project database. The impact of solar reflectance and infrared emittance on the surface temperature is explained and performance examples according to experimental results are compiled. The second section focuses on the types of cool white colored materials that are commercially available per roof type. These include build up roofs, single ply membranes, modified bitumen, tiles, coatings, metal roofs etc. A short description of each technology is given and their main characteristics are reported. The benefits of using cool white colored materials on buildings and the urban environment are analyzed in the third section. The benefits include increased thermal comfort conditions in buildings, reduced cooling energy consumption and peak electricity loads, increased life span of the roof system, reduction of the air pollution and greenhouse gas emissions and mitigation of the heat island effect. Each one of these benefits will be supported by experimental and simulation data available in the bibliography. Finally, the problem of ageing is analyzed. Exposure of cool materials to outdoor conditions has the effect of changing their main properties: solar reflectance and infrared emittance, compromising their performance. The main mechanisms (photo-degradation, thermal stress, deposition of pollutants etc.) that are responsible for causing the ageing effect of the materials are reported. Experimental results that estimate the aging effect of various types of materials and for several outdoor conditions are presented. The case of artificial and natural ageing is discussed as well as any effort to model the ageing effect of cool materials.

This chapter provides a discussion on colored cool materials. The relationship between the solar spectral irradiance and the solar spectrum reflectance for colored cool paints is important for the performance of these paints. The majority of the Japanese paint market is occupied by dark colored paints, which have an average solar reflectance of around 20%. Paints with a solar reflectance of more than 20% probably contribute to the urban heat island effect. However, reduction of solar reflectance by adhesion of atmospheric particles is found to occur immediately after application of the paints, and the solar reflectance of some colored cool paints is found to reduce to 60% of the initial value within one year. The roofs of most detached houses in Japan are covered with tiles; it is therefore important to examine the effect of tiles on the urban heat island measures. The average solar reflectance of areas of detached housing is estimated to be around 16%. Roof materials with a solar reflectance of more than 20% probably contribute to urban heat islands. Solar reflectance of an uneven tiled surface can be estimated from the surface shape and the solar reflectance of a flat surface.

The present research investigates the development of thermochromic and PCM doped infrared reflective coatings and the potential of their application on urban structures in energy savings and improvement of the microclimate.

Thermochromic coatings were developed by using thermochromic pigments into an appropriate binder system. The color-changing temperature was 30°C. The same binder system was used for the production of highly reflective (cool) and common coatings, in order to investigate and compare the thermal and optical characteristics of colormatched thermochromic, cool and common coatings. The results demonstrated that during the experimental period, surface temperatures of thermochromic samples were lower than the temperatures of color-matched cool and common. Issues concerning the photodegradationof thermochromic coatings are also discussed.

The performance of organic PCMs used as latent heat storage materials, when they are incorporated in building coatings is also studied. Comparative testing proved that all PCM doped infrared reflective coatings present lower surface temperatures than cool and common coatings of the same color. Studying the daily temperature differences it was noticed that peak temperature differences occur between PCM and common or cool coatings from 7am-10am.These peak temperature differences are not noticed for common and cool coatings. Moreover investigating the temperature gradient it was shown that for this time period the values for the PCM coatings is much lower compared to cool and common. Coatings containing PCMs store heat in a latent form maintaining constant surface temperatures and discharge with a time delay.

Surface temperature of the urban fabric plays an important role in the microclimate, the energy performance of the buildings and the comfort conditions of city dwellers. Pavements (roads, parking spaces etc.) cover a significant percentage of a city’s surface and their thermal characteristics play a dominant role in the formation of the urban heat island effect, which refers to the temperature increase in urban areas compared to rural settings. This paper reports the technologies related to cool paving materials, their thermal and optical properties and the benefits of their use in the urban structures. It also presents the experimental assessment of cool colored thin layer asphalt and the estimation of the potential to improve the urban microclimate.

This chapter consists of two parts. In the first one a brief historical overview of the development of solar paint coatings is given together with the basic principles that make paint coatings spectrally selective. Distinctive properties of Thickness Sensitive Spectrally Selective (TSSS) and Thickness Insensitive Spectrally Selective (TISS) are outlined and the use of the latter paint coatings as cool paints is proposed and demonstrated. Materials’ aspects of the colored cool TISS paint coatings are given by the description of cool pigments, metallic and metallized flake pigments and polymeric resin binders which are used for the production of solar paint coatings. Second part contains information about the chemistries of the paint production showing how to achieve with the help of dispersant molecules uniform distribution of the finely ground pigment particles in the polymeric resin binder. Intentionally, we focused on organo (silicon) i.e. silane dispersants because they enable the stabilization of many different nanoparticle systems and also commercial pigments. Basics of the sol-gel chemistry of silanes are given and the preparation of polyhedral oligomeric silsesquioxane (POSS) molecules is described and the dispersive effect of the latter on pigments demonstrated. The importance of the POSS molecules representing a new multifunctional nanocomposite materials is revealed by describing paints and lacquers with hydrophobic and oleophobic properties and the possibility to attain anti-soiling properties of cool paints is also discussed.

The scope of the present chapter is to present the cool materials rating techniques as extracted and followed by specific technical standards. The methodology to measure solar reflectance and infrared emmitance are presented in detail.

Optical properties (i.e., reflectance, transmittance, etc.) of light scattering materials can be described using two approaches. In the first approach, the interaction of light with the matter’s particles is modeled (microscopic approach) while in the second, the light fluxes into and out of bulked considered matter is modeled (macroscopic approach). Kubelka-Munk (KM) is the most common theory of macroscopic modeling, for calculating the change of light fluxes (two-flux theory) as a function of scattering, absorption, and distance. Modified KM Models, for calculating the optical properties of rough surfaces, revised KM theories as well as, inversion methods from KM analysis are presented. A KM and Mie microscopic model is used for emmitance calculations of coating layers. Three flux models track two diffuse fluxes and one collimated flux, while four flux models track two diffuse and two collimated fluxes. Maheu- Letoulouzan-Gouesbet (M-L-G), multilayered, and other generalized four-flux models are shown. A number of applications of the discussed models in pigments (such us polymer coatings), light scattering from TiO2 and red particles are also presented.

One of the primary reasons for the application of cool materials is their energy and associated environmental impact on the built environment. Cool materials are usually applied on the roof of buildings to reduce cooling energy demand. The relative benefits of this reduction depend on the construction of the building, external weather conditions and use of the building. Through experimental and computational studies, it has been demonstrated that energy reduction benefits are significant in cooling dominated climates but is also observed in moderate climates. This chapter reviews available literature on this and also presents available simplified toolkits that can be used in feasibility studies to determine whether or not energy benefits are likely to materialize. The toolkits also calculate environmental benefits related to energy use by buildings and related cost benefits to the user. This chapter also presents additional environmental benefits related to the improvement of thermal comfort inside buildings which are not air-conditioned, the improvement of external thermal comfort which results from the mitigation of the urban heat island because of the use of cool materials and the effect on general pollution in the cities and health.

Regulatory bodies around the world are considering technological solutions to reduce energy consumption, mitigate climate change, improve living conditions, and encourage a more environmentally-friendly construction industry – both quickly and inexpensively. Cool materials are a technology that can contribute to these objectives, particularly in countries with higher energy use during summer or with areas that suffer from the “urban heat island phenomenon” (e.g. big cities). To compare policies that incentivize Cool Materials, a landscape assessment of relevant policies was conducted. The scope includes the most important international, regional, national and local initiatives. The intention of the Landscape Assessment was to give an overview of existing and forthcoming policies that do or may affect the accelerated introduction of Cool technologies. This assessment is the first step in answering the question “how far has policy moved to help promote Cool Materials” which is the first step in determining the chances of amending policy in favor of Cool technologies. Cool technologies are not the panacea to global challenges in these areas, but they offer an affordable solution for many cities. The Landscape Assessment, which was carried out during 2008-2009, considered standards, building regulations, financial incentives and alternative solutions.

The construction industry recognizes that there is a growing impetus, spurred by well-respected global organizations, to reduce the energy consumption and carbon footprint of buildings. This shift presents both challenges and opportunities for the buildings industry. A major opportunity concerns the role of cool roofs in reducing cooling loads and yielding other benefits to a wide range of stakeholders with an interest in building programmes. As yet, however, a significant sector transformation for cool roofing has not materialized. In this chapter we aim to understand why this has been the case by investigating factors that we believe to be important influencers of sector development. Questions are posed regarding the definitions and structure of the cool roof arena. Associated opportunities and the sector drivers have been investigated. The chapter also considers how sector demand is being generated and discusses whether all of the stakeholders are fully engaged in this process. Finally, an attempt is made to identify the nature and availability of initial information that industry players might require in order to consider investment in the cool roofing sector. In conjunction with this examination, some recommendations are made that it is believed could encourage sector development for cool roofing.

The chapter presents a set of exemplary case studies where the cool materials were implemented and the impact monitored.