**1. Introduction**

418 Solar Radiation

Yamada, M., (May, 1998) "International Corroboration Research on Solar Energy Utilizing

The materials, which are used for the pavements of urban spaces and the external renderings of vertical (facades) and horizontal (flat roofs) surfaces of buildings, constitute, the "skin" of a city. In Greece, the selection of these materials is most of the times, based on economic and aesthetic criteria. While the importance of these selection parameters is clear and irrefutable, it should be noted that paving and facade materials play a decisive role on the heat transfer processes, which take place between the city and the climatic environment.

Greece has a warm Mediterranean climate, where the long hours of sunshine and the intensity of solar radiation during the summer result in elevated temperatures of the horizontal and vertical city surfaces in different periods of the day. These high temperatures largely affect the deterioration of the urban heat island (Oke, 1995; Akbari, 1992; Santamouris, 2001a) and thermal comfort conditions in urban open spaces. For the city of Athens, the development of the urban heat island during the summer has grave consequences on the energy consumed for cooling (Santamouris, 2001), and negatively influences many aspects of the citizens' everyday life.

The thermal behaviour of paving materials has been documented by various researchers, both in-situ in urban open spaces (Givoni, 1998; Marques de Almeida, 2002; Cook, et al., 2003; Labaki, et al., 2003) and experimentally on samples building materials (Cook, et al., 2003; Doulos, et al., 2004; Atturo and Fiumi, 2005, Synnefa, et al., 2006). The thermal behaviour of the materials, which form the outer surfaces of building facades, has been investigated in studies. The studies, which were concerned with the effect of shading, and the surface temperatures of various façade materials were carried out by such workers as Hoyano, 1988; Cadima, 1998; Papadakis, et al., 2001; Cantuaria, 2002; Boon Lay, et al., 2000, as well as in a previous study by the author Bougiatioti, et al.( 2009).

The study presented in this chapter attempts to combine the two different approaches. It examines the thermal behaviour of paving and façade materials during the summer period, both in-situ in urban open spaces and buildings in Athens, Greece, and samples of building materials placed on a flat roof and exposed to solar radiation during the summer period. The aim of the research is to provide a large set of experimental data (surface temperatures), which can help better in understanding the summer thermal behaviour of the most commonly used surfacing materials in Greece. This experimental data can be qualitatively incorporated as input to the total "image" of each material, in such a way as to be easily evaluated and understood by architects and planners.

The study is divided into two parts: the first part presents the results that concern paving materials and the second presents those involving the facade materials. In each, both the insitu and the experimental measurements are presented in a cumulative way, in order to draw conclusions on the thermal behaviour of the various building materials during the summer period and the parameters that affect and determine it.

For paving materials, in-situ measurements were conducted in a number of selected urban open spaces in Athens, while experimental measurements involved samples of building materials placed on a flat roof. The materials, which were measured during both the in-situ and the experimental study, were classified into the following general categories: Loose, earthen materials, Natural stone products (slabs of marble, granite), Cement products (gavel concrete, slabs and blocks,), Ceramic products (tiles and blocks), Wooden products (boards), Asphalt products (asphalt concrete), Vegetal surfaces (in-situ measurements) and Water surfaces (in-situ measurements).

For façade materials, in-situ measurements were conducted on the facades of a number of selected buildings, while experimental measurements involved samples of building materials placed on an experimental setting facing, first towards the West and then towards the South. The materials of which temperatures were measured during both the in-situ and the experimental study, were classified into the following general categories: Natural stone products (slabs of marble, granite), Cement products (lime-cement mortar on selected building facades), Ceramic products (tiles and panels), Wooden products (composite panels), Metal products (metal sheets and composite panels), Vegetal surfaces (in-situ measurements of wall covered with climbing plants) and Photovoltaic (PV) panels (in-situ measurements).

The in-situ surface temperatures of paving, as well as of facade materials, are influenced by their contact with the substrate, which, in turn, influences thermal storage and time lag. As a result, the surface temperatures, which are measured on placed materials, can be different from those measured on samples of building materials (Cook, et al., 2003). On the other hand, conducting measurements on samples of building materials exposed to the same environmental conditions provides the ability to simultaneously measure the surface temperatures of a large number of materials. While the measurements on samples of building materials might not reflect their real thermal behaviour, they provide comparative information on the fluctuation of their surface temperatures. Either way, due to the large number of parameters, which influence the surface temperature of building materials in the urban context, it goes without saying that the experimental measurements presented in this chapter should be considered as indicative and not absolute.

The results of the in-situ measurements are presented in Table 1 for paving materials and Table 3 for facade materials, while those of the experimental surface measurements are presented in Table 2 for paving materials and Tables 4 and 5 for facade materials with west and south orientations, respectively.

For all the materials, the differences from the respective air temperature values were calculated in order to obtain a more accurate understanding of the results of the measurements. Furthermore, the air temperature around sunset (T19:30) is also mentioned, as it is considered to be indicative of the effect that the materials have on the development of the urban heat island and thermal comfort conditions in the evening.
