The contribution of bioclimatic architecture in the improvement of outdoor urban spaces - Solar Energy Sciences and Engineering Applications - page 627

requiring maintenance and water, blocking part of the solar access during winter,
trapping pollutants in the urban canopy layer and taking a significant period to yield
the desired results (McPherson, 1994).
19.2.1.2 Green roofs and façades
Usually green spaces are found in open public areas. However, the technique of green
roofs, public or private, has gained ground during the last few decades, as they reduce
the energy consumption of the building while improving the microclimate of the wider
urban space in which the building is situated.
The most important advantage, or at least the most popular reason for installing
green roofs, is their contribution to a building's insulation, which usually results in
energy savings both for heating and for cooling. The properties that increase the energy
efficiency are: the shading that the plants provide to the roof; the temperature reduction
from evapotranspiration; and insulation from the plants and the growing medium.
Niachou et al. (2001) found that the contribution of green roofs significantly increases
the insulation properties of a structure. The effect of green roofing on the energy
savings of buildings, both for cooling and for heating, is greater in the case of a non-
insulated building, less significant in moderately insulated buildings and smaller in
well-insulated ones.
Apart from their impact on building energy consumption, planted roofs contribute
to the mitigation of heat islands. Their first advantage is that they increase the total
of a city's water-permeable surface area, helping water to be retained in the soil and
allowing larger quantities to be available for evapotranspiration. At the same time,
planted roofs present much higher albedo values than dark urban roof surfaces, thus
reflecting off a greater proportion of the incident solar radiation and not transforming
it into heat (Getter and Rowe, 2006). According to the United States Environmental
Protection Agency (USEPA), green roofs can be cooler than the ambient air, whereas
conventional roof surfaces can exceed ambient air temperatures by up to 50 ◦ C (EPA3).
Wong et al. (2007) studied the effect of an intensive rooftop garden system. It
was found that surface temperature may be reduced up to 3.1 K and the ambient
temperature at 1 m may reduce by up to 1.5 K.
Other advantages of green spaces, including planted roofs, are improved air quality
and lower greenhouse gas emissions, resulting from the reduced energy consumption
and from the removal of air pollutants, enhanced stormwater management and water
quality, and the improved aesthetic experience of the users of the area.
19.2.2 Cool materials
The physical properties of the materials used for urban structures largely determine the
thermal conditions in the city and the energy consumption of buildings. The albedo of
a material or surface is its solar reflectivity, meaning the ability of the material to reflect
off the incident solar radiation. The ability of the material to emit long-wave radiation
is called emissivity (infrared emittance); high emissivity materials release energy that
has been absorbed as short-wave radiation. Albedo and emissivity are the key factors
which determine the temperature of an urban surface, thus having huge impact on
ambient air temperature.