ZEMCH 2015 - International Conference Proceedings | Page 860

Introduction
In cities or metropolitan areas the presence of Urban Green Infrastructures (UGI), such as urban
forests, street trees, parks, turf-grass, private gardens, green roofs and green walls, play an important role in contributing to a broad range of ecosystem services. UGI also contribute to the mitigation of the urban heat island (UHI) effect, reduce the ambient temperatures, improve human
thermal comfort and decrease energy loads on building (Pérez et al. 2014; Kohler and Poll 2010;
Rowe 2011; Fernandez-Caňero et al. 2013; Cameron et al. 2014; Norton et al. 2015). The UHI refers
to the relative warmth of a urban area compared with surrounding rural areas with differences
of air temperature of about 5-6 °C (Rowe 2011; Kanechi et al. 2014; Karlessi et al. 2011; Berdahl and
Bretz 1997; Bretz and Akbari 1997; Bretz et al. 1998; Gentle et al. 2011; Gladis and Schumann 2011; Jo
et al. 2010; Joudi et al. 2013; Li et al. 2013; Prado and Ferreira 2005; Synnefa et al. 2006; Uemoto et
al. 2010; Zinzi et al. 2012). The UHI causes raises in air conditioning systems use and consequently
an excessive energy consumption, raises in air pollution and greenhouse gases concentration in
atmosphere, negative outdoor comfort conditions and possible threats to human health (Karlessi
et al. 2011; Jaffal et al. 2012; Kalkstein and Davis 1989; Petralli et al. 2006). The main factors contributing to the UHI phenomenon are: the use of non-reflective and water-resistant materials for the
building external surfaces; dense urbanization geometry characterized by wide surfaces that can
absorb incident and reflected solar radiation and that contribute to the radiation trapping and to
the wind speed reduction in the emerging urban canyons; the anthropogenic heat emitted from
activities such as from industrial processes, heating and cooling systems, motorized vehicular
traffic (Santamouris 2012; Ryu and Baik 2012).
Green roofs and green walls, as UGI, are living vegetated horizontal and vertical layers on the
external envelope of the buildings with a focus on energy consumption reduction for air conditioning in summer and on thermal insulation increasing in winter (Berardi et al. 2014; Fernandez-Caňero et al. 2013; Santamouris 2012). Green roofs and green vertical systems can be therefore
employed as a passive sustainable technology for mitigating the UHI effect and also for enhancing the energy efficiency of buildings, in particular in dense urban areas where buildings with a
high wall to roof ratio offer large surface areas available for retrofitting (Raji et al. 2015; Cheng et
al. 2010). The cooling effect of greenery systems is obtained by intercepting and absorbing the
solar radiation and the resulting reduction of the buildings solar heat gain implies less energy
consumption for air cooling in summer all around the world; moreover the evapo-transpirative
effect, generated from the plants and the substrate on their surroundings, leads to a cooler ambient temperature and consequently to reduce the cooling load of the buildings (Wong et al. 2010;
Sunakorn and Yimprayoon 2011; Raji et al. 2015).The vegetation and substrate layer can influence
the thermal performance of the buildings in winter acting as an insulation against wind, depending on the climate of the region and on the characteristics of the greenery system used (Berardi et
al. 2014; Fernandez-Canero et al. 2013; Perez et al. 2011; Jim and He 2011; Perini et al. 2011; Cheng et
al. 2010; Köhler and Poll 2010; Vox et al. 2015).
Regions characterized by hot and dry climates, as the Mediterranean region, are reported as areas
where greenery systems can produce more benefits, due to the highest amount of shading and
evapo-transpiration needed (Raji et al. 2015; Castleton et al. 2010); nevertheless, vegetation must
have low irrigation requirements, be suitable for the exposure conditions of the application, and
for the specific weather conditions, in order to fulfill sustainability aims. Europe and North America are reported as the regions where green roofs can be profitably applied requiring low maintenance (Castleton et al. 2010; Refahi and Talkhabi 2015). The evaluation of the effect of greenery systems on energy savings requires further study for defining suitable plant species and options for
the green roofs and green walls to be used in regions characterized by the Mediterranean climate.

858

ZEMCH 2015 | International Conference | Bari - Lecce, Italy