**4.4 The effect of shading**

440 Solar Radiation

their colour and thermo physical properties. For facade materials, the orientation determines the angle of incidence of the solar rays, the duration of insolation, as well as the hours of the

In the case of northern facades, the solar radiation, which is received throughout the summer period, is limited. Consequently, the surface temperatures of north-facing materials, as the in-situ measurements show, are lower than or equal to the air temperatures

The insolation of southern facades is largely affected by the high sun altitude angles (74,4o for Athens, at noon on June the 21st). As a result, most south-facing materials do not record

Eastern and western facades receive large sums of solar radiation at angles which are nearly normal to their surfaces. Western facades may record higher surface temperatures than eastern ones due to the fact that during the afternoon, the large sum of solar radiation,

The material property that defines its behaviour under the effect of solar radiation is reflectivity, which depends mainly on its colour (Givoni, 1998). Light-coloured materials record lower surface temperatures than dark-coloured ones. This fact has been largely documented by many researchers (Givoni, 1998; Akbari, et al., 1992; Doulos, et al., 2004) and

However, an important concern regarding light-coloured paving materials, is the effect of soiling and weathering on colour and, therefore, on their thermal behaviour. This fact is evident from the comparison of the maximum and the mean surface temperatures of the insitu and the experimental measurements carried out with light-coloured materials in this

Even though orientation (see 4.1) and colour (see 4.2) affect the surface temperatures of the materials exposed to solar radiation, so also their thermo physical properties. This fact is evident from the maximum surface temperatures and the daily temperature variation of certain groups of materials, such as loose, earthen materials, light-weight concrete and

The high surface temperatures (Tables 1 and 2) of loose earthen materials directly depend on their thermo physical properties. Dry earth has low thermal conductivity (0.25 to 0.30 W/mK) and density (300 to 1600 kg/m3) (Oke, 1995) values, depending on the composition of the soil (Oke, 1995). The values for wood are similar (about 0.09 W/mK for softwood and 0.19 W/mK for hardwood) (Oke, 1995). Consequently, the ability of these materials to diffuse heat through their mass (Oke, 1995; Givoni, 1998) and their ability to store heat (Oke, 1995; Szokolay, 1980), which are expressed by the theoretical properties of diffusivity and admittance, respectively, are relatively low. As a result, earthen materials and wood have a rather contradictory thermal behaviour. They heat up considerably until noon, but cool

which impinge on western facades, coincide with the highest daily air temperatures.

very high absolute maximum surface temperatures in summer.

was also evident from the results of this work shown above.

**4.3 The effect of thermo physical properties** 

day when it occurs.

throughout the summer days.

**4.2 The effect of colour** 

study.

wood.

The measurements in the in-situ study clearly demonstrate that the effect of shading is of utmost importance in the case of dark-coloured materials (e.g. asphalt, dark-coloured stone, etc.). The differences in the surface temperatures of exposed and shaded dark-coloured materials are very large, ranging from 19 to 28 oC for the absolute maximum temperatures, and from about 13 to about 21 oC for the mean temperatures. In comparison to the mean ambient temperatures, the mean surface temperatures of the exposed materials are approximately 10 to 18 oC higher, whereas shaded materials are, most of the time, cooler than the air temperature. Finally, around sunset, exposed materials are 5 to 9 oC warmer than the air temperature, while shaded materials are considerably cooler. A detailed report on the effect of shading on the surface temperatures of materials is given by Bougiatioti (2005).
