Green façades to control wall surface temperature in buildings

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Green façades to control wall surface temperature in buildings Giuliano Vox, Ileana Blanco, Evelia Schettini∗ Department of Agricultural and Environmental Science DISAAT, University of Bari, Bari, Italy

A R T I C L E I N F O

A B S T R A C T

Keywords: Urban agriculture Green walls Air-conditioning Energy savings Microclimate Urban heat island

Green façades can represent a sustainable solution for construction of new buildings and for retrofitting of existing buildings, in order to reduce the energy demands of the cooling systems, to mitigate the urban heat island and to improve the thermal energy performance of buildings. Green façades can allow the physical shading of the building and promote evapotranspiration in summer, and increase the thermal insulation in winter. An experimental test was carried out at the University of Bari (Italy) for two years. Three vertical walls, made with perforated bricks, were tested: two were covered with evergreen plants (Pandorea jasminoides variegated and Rhyncospermum jasminoides) while the third wall was kept uncovered and used as control. Several climatic parameters concerning the walls and the ambient conditions were collected during the experimental test. The daylight temperatures observed on the shielded walls during warm days were lower than the respective temperatures of the uncovered wall up to 9.0 °C. The nighttime temperatures during the cold days for the vegetated walls were higher than the respective temperatures of the control wall up to 3.5 °C. The thermal effects of the facades at daytime was driven by solar radiation, wind velocity and air relative humidity. The highest cooling effect of such parameters occurred with a wind speed of 3–4 ms−1, an air relative humidity within the range 30–60% and a solar radiation higher than 800 Wm−2. The long-term experimental test demonstrated that both Pandorea jasminoides variegated and Rhyncospermum jasminoides are suitable for green façades in the Mediterranean climatic area. The results shown in the present research allow to fill the gap in literature concerning the lack of data for all the seasons of the year, in order to obtain a complete picture of the building thermal performance in the Mediterranean climate region.

1. Introduction In dense urban areas the progressive replacement of green areas with artificial surfaces is one of the major causes of the so called urban heat island (UHI) effect, the phenomenon that generates a temperature difference between cities and surrounding rural or suburban areas. As a city grows, more heat is trapped with a consequential increase of the air temperature in downtown even up to 6–8 °C higher in comparison to the surrounding areas [1–5]. Building surfaces and pavements are made mainly with non-reflective and water-resistant construction materials, consequently accumulating incident solar radiation during daytime and then releasing heat at night [6]. Heat is trapped also because the decrease of green areas in cities induces a reduction of shades and radiation interception, together with the reduction of the infrared radiation emitted towards the atmosphere, the limitation of the circulation of air in urban canyons and the high production of waste heat from cooling systems, motorized vehicular traffic and industrial processes [1,3,7]. UHI negatively influences outdoor comfort conditions as well as induces a more use of air conditioning systems with a

∗

raise of peak electricity demand. Nowadays, energy use in buildings accounts up to 36% of Europe's CO2 emissions [8]. The importance of green infrastructures applied on building roofs or façades in cities has been increased due to their ability to sustainably improve the energy efficiency of buildings [3,9–16] and to mitigate the UHI effect [17–21]. Green vertical systems can be used because the surface area of the building envelopes is generally left bare while surrounding areas at ground level are increasingly occupied by buildings and paved surfaces, and portions of the roofs are occupied by building services [22,23]. Green vertical systems offer several benefits on the façade, as the extension of wall lifetime, the thermal insulation of the wall, and the reduction of solar absorbance. They also offer benefits on the building, as the reduction of the heat load and energy consumption, the improvement of the internal comfort due to a reduction of the surface temperature and the attenuation of temperature fluctuations. The enhancement of the acoustic comfort, the increasing of the property values, the implementation of spaces for recreation and amenity are other advantages. Vertical gardens protect the exterior finishes and masonry

Corresponding author. E-mail address: [email protected] (E. Schettini).

https://doi.org/10.1016/j.buildenv.2017.12.002 Received 5 September 2017; Received in revised form 17 November 2017; Accepted 3 December 2017 0360-1323/ © 2017 Published by Elsevier Ltd.

Please cite this article as: Vox, G., Building and Environment (2017), https://doi.org/10.1016/j.buildenv.2017.12.002

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Fig. 1. The three walls at the experimental field of the University of Bari; the right wall is covered with Rhyncospermum jasminoides, the central wall with Pandorea jasminoides variegated and the left wall is the uncovered control.

Table 1 Thermo-physical properties and geometrical characteristics of the experimental cuboids. Wall width (m)

Wall height (m)

Material

Thickness (m)

Total thickness (m)

λa thermal conductivity (Wm−1K−1)

d density (kg m−3)

Cp heat capacity (Jkg−1K−1)

αs solar absorption coefficient (%)

εLWIR LWIR emissivity coefficient (%)

south exposed vertical walls

1.00

1.55

0.20

0.22

0.282

695

840

42.1

95.3

north, east and west exposed vertical walls and roof

1.00

1.55

masonry (bricks single layer joined with mortar) plaster coating expanded polystyrene

0.02 0.03

0.03

0.55 0.037

15

1000 1404

–

–

a

Ref. [36].

in the ground or in pots at different heights of the façade; the plants climb on the building façade directly through morphological features (such as aerial roots, leaf tendrils and adhesion pads) or indirectly on a structural support (such as wire, mesh, trellis) located to a small distance to the wall. Living walls are classified as continuous or modular: the former is based on lightweight and permeable screens in which plants are inserted individually; the latter is composed of modular elements, such as trays, vessels, planter tiles and flexible bags, which include the growing media where plants can grow [26]. The modular elements are fixed to a wall or freestanding frame with artificial irrigation and fertigation system. The presence of a gap between the building wall and the greening system (generally from 3 cm to 15 cm) acts as a thermal buffer, improving its thermal insulation impact on building [27]. Although the cooling influence of the green walls is well recognized, few authors reported experimental data under Mediterranean climatic conditions and no one considered two years of experimental data. Coma et al. [28] reported the results obtained on experimental houses-like cubicles with a green wall system tested in Catalonia, Spain. In two short winter periods, the green wall registered the highest external wall temperature reductions, equal to 16.5 °C, on the south while in east and west the reductions were 4.5 °C and 6.5 °C, respectively. The results obtained experimentally by Hoelscher et al. [1] in Berlin, Germany, showed the cooling effects through shading and transpiration of three different climbing plants as part of direct and indirect green façades. The additional vegetation layer, compared to bare walls taken as control, reduced the surface wall temperatures, by up to 15.5 °C and

Table 2 Description of the vertical greenery systems in Valenzano (Bari). Experimental wall

Green façade with Rhyncospermum jasminoides Green façade with Pandorea jasminoides variegated

LAI

Average thickness (m) Wall

Air gap

Plants

Total

2–4

0.22

0.10

0.20

0.52

1.5–3.5

0.22

0.11

0.15

0.48

from ultra violet radiation, rain, extreme temperature fluctuations and presence of moisture even if they could damage the wall they are covering. Other benefits can be found at a larger scale: energy consumption reduction (decreasing cooling and heating loads); urban heat island effect decreasing; air pollution mitigation (enhancing urban air quality, reducing dust and heavy metal accumulation in air, filtering airborne particles); sound absorption (sound insulation and noise absorption); water management improving (enhancement of stormwater management, water run-off quality, urban hydrology and use of rainwater). Moreover, vertical planting contributes to improve health and well-being and to preserve urban biodiversity, acting as habitat for colonizing species such as spontaneous plant species, weeds, beetles, bees, ants, spiders and birds. Green vertical systems are classified according to construction techniques and characteristics into green façades and living walls [22,24,25]. Green façades are characterized by climbing plants rooted 2

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Table 3 Climatic data of the experimental field in Valenzano (Bari) (data obtained from weather station at the experimental site). Month

Jan-15 Feb-15 Mar-15 Apr-15 May-15 Jun-15 Jul-15 Aug-15 Sep-15 Oct-15 Nov-15 Dec-15 Jan-16 Feb-16 Mar-16 Apr-16 May-16 Jun-16 Jul-16 Aug-16 Sep-16 Oct-16 Nov-16 Dec-16

External air temperature (°C)

Monthly cumulative solar radiation (MJm-2)

maximum

minimum

on vertical wall

on horizonal plane

20.9 18.3 23.7 27.9 36.7 37.5 40.7 40.2 41.4 30.8 27.0 18.4 21.7 22.6 24.9 31.1 34.3 36.2 38.9 39.2 33.4 27.6 24.5 17.2

−0.3 1.5 3.1 4.1 10.6 15.2 18.6 18.3 13.9 8.1 5.7 4.1 1.7 2.0 3.8 6.9 9.9 13.1 15.5 16.2 11.8 8.3 5.1 0.7

338.3 254.2 290.9 330.2 291.5 269.5 304.1 343.9 356.9 320.8 320.8 331.1 289.2 298.1 275.7 334.6 247.1 232.1 278.3 333.6 331.0 318.5 303.8 333.1

196.5 213.4 339.3 545.7 659.7 748.6 802.5 643.4 462.1 282.0 210.0 178.6 177.0 238.1 342.7 558.2 604.3 657.0 760.3 663.4 438.3 301.8 207.1 181.1

Average external air relative humidity (%)

Average wind speed (ms−1)

76.2 77.6 79.9 63.6 60.1 58.8 48.8 64.2 62.3 86.1 84.8 88.8 76.8 76.0 78.8 67.1 66.4 62.1 52.8 62.5 79.3 81.7 81.7 77.9

2.7 2.9 2.8 2.8 2.4 2.4 2.1 2.0 2.3 2.1 1.8 1.8 2.4 3.0 2.5 2.6 2.6 2.2 2.2 2.2 1.9 2.1 2.3 2.3

whole year. In summer the efficacy of greenery systems has been widely assessed while the performance of the greenery systems in winter requires further investigation for its climatic conditions dependence [27,28]. In this paper, an accurate analysis of the surface temperature behind the canopy of two experimental green façades located in Valenzano (Bari, South Italy) is presented. The aim is to evaluate the thermal effect of the installation of two different climbing plants (Pandorea jasminoides variegated and Rhyncospermum jasminoides) in the Mediterranean region through the evaluation of the external surface temperature of the vertical wall in comparison to an uncovered wall that was kept as control. Section 2 describes the experimental green facades, provides the characteristics of the meteorological site and presents the radiometric laboratory tests and the experimental equipment. Section 3 illustrates the impact of the climatic conditions on the thermal insulation performance of the green façades considering different weather scenarios (sunny and cloudy days) in summer and winter. The climatic parameters concerning the walls and the weather conditions were collected during two consecutive years. The thermal performance of the green layers was assessed by evaluating maximum, minimum and mean values of the surface temperature of the walls. The effect of different levels of solar radiation on the surface temperature of the three walls was evaluated by means of the analysis of variance. A solar radiation threshold value was assessed as necessary to incite relevant cooling effects on the walls behind the green layers during daytime. Then, the analysis of variance was used to evaluate also the effect on the surface temperature decrease, due to the green layers, of wind speed and air relative humidity during daytime. In Section 4 a comparison between the results obtained and those provided by literature are finally discussed. Section 5 discusses the conclusions and the perspectives of this research.

1.7 °C for the exterior and interior walls, respectively. Stenberg et al. [29] under warm temperate climate conditions at Oxford, United Kingdom, found a wall temperature reduction of 9.15 °C generated by a 45 cm thick ivy cover on a south exposed wall. Chen et al. [30] reported for a living wall system a reduction of the exterior wall temperature by a maximum of 20.8 °C and of the interior wall by 7.7 °C comparing to the bare wall situation. The test was carried out from July to September 2012 in Wuhan, China, characterized by a hot and humid climate. Wong et al. [16] analysed the thermal performance of eight different vertical greenery systems installed in 2008 in Singapore, characterized by tropical rainforest climate. Maximum reductions of the wall surface temperature were of 11.58 °C and were observed in clear days during daytime hours of April on a living wall made of plant panels. The panels were embedded within stainless steel mesh panels inserted into fitting frames, on a vertical interface, using small plants. Available literature reports the thermal behaviour of the green façades as vegetative cooling in summer daytime and the ability to act as a thermal screen with a vegetative warming in winter nighttime. However, some studies reported the maximum temperature differences while other researchers reported the average temperature differences [31]. Furthermore the magnitude of thermal performance strongly depends on the wall orientation, size, characteristics of the building on which the experimental wall is positioned, apart from plant species, substrate type and foliage layer thickness. This wide variation in data collection together with the short time of gathering make the comparison of the green façades performance even more difficult. The parameter most commonly reported for assessing the thermal performance of green vertical systems is the wall external surface temperature of the building or prototype, even if comparisons between surface temperature studies do not allow an adequate assessment of thermal performance, due to differences in building construction characteristics [32]. However, the analysis of this parameter permits comparing the potential effects related to the change of one parameter, such as the plant species, the distance of the supporting structure from the building for the indirect green façades, the irrigation regimes. Several authors presented experimental data at real scale concerning short summer periods [27,30,33,34]; more studies are necessary to analyse the thermal performance of a building throughout the

2. Materials and methods From June 2014 to December 2016, an experimental research was carried out at the University of Bari in Valenzano (Bari, Italy) at latitude 41° 05′ N, longitude 16° 53′ E, altitude 85 m ASL. This area is characterized by a Mediterranean climate classified as Csa, according to the 3

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Fig. 2. External air temperature, surface temperature of the external plaster of the three walls exposed to solar radiation, solar radiation on the vertical wall facing south and on horizontal plane: (a) a hot summer sunny day, 17 September 2015; (b) a hot summer cloudy day, 15 August 2015.

solar wavelength band from 200 to 2500 nm were carried out by means of a double beam UV-VIS-NIR spectrophotometer (Lambda 950, Perkin Elmer Instruments, Norwalk, CT, USA), in steps of 10 nm using radiation with a direct perpendicular incidence. An integrating sphere (diameter 60 mm) was used as the receiver of the spectrophotometer to measure the fraction of diffuse radiation reflected from the sample. Tests in the LWIR range, between 2500 and 25000 nm, were carried out by an FT-IR spectrophotometer (1760 X, Perkin Elmer Instruments, Norwalk, CT, USA) in steps of 4 cm−1; near normal reflectivity was measured, i.e., with a radius of incidence on the sample forming an angle of 10° with the normal to the same. Emissivity was calculated from the reflectivity by Kirchhoff's law [6]:

Kopper-Geiger climate classification [35]. It is characterized by warm temperate climate with calm, dry and hot summer and by a notably variation of solar radiation intensity with season; the winter months are much rainier than the summer months and the average annual temperature is 16.1 °C. Three walls were built outdoors facing south following the typical Mediterranean building solutions, i.e. a single skin of perforated bricks joined with mortar (Fig. 1). The walls were designed to simulate common building walls. The side covered with plants simulates the external side of the building envelope; the other side simulates the internal one. In order to shade and insulate the internal side of the walls, which otherwise would be directly exposed to the solar radiation, and to evaluate the influence of the vegetation layer on the wall, a sealed structure was made on the backside of the wall with sheets of expanded polystyrene. A shading net was also positioned onto the structures in order to reduce the effect of the incident solar radiation on the sealed structure. The radiometric properies of the wall surface were evaluated by laboratory tests. The reflectivity of the wall surface samples was measured in the solar range (200–2500 nm) and in the long wave infrared radiation (LWIR) range (2500–25000 nm). The measurements in the

ελ = 100 − ρλ where ρλ and ελ are the spectral reflectivity and the spectral emissivity at wavelength λ, respectively. The radiometric coefficients of the materials were calculated as average values of the spectral values over different wavelength bands: the solar wavelength range (200–2500 nm) and the LWIR range (7500–12500 nm). The thermo-physical properties and the geometrical characteristics 4

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Fig. 3. External air temperature, surface temperature of the external plaster of the three walls exposed to solar radiation, solar radiation on the vertical wall facing south and on horizontal plane: (a) a warm summer sunny day, 3 August 2016; (b) a warm summer cloudy day, 16 August 2015.

measured with an AccuPAR PAR/LAI Ceptometer (model LP-80, Decagon Devices Inc., Pullman, WA, USA); it varies throughout the year in a range of maximum and minimum values (Table 2) because, despite the selected plants are evergreen, in winter they lose some leaves. The external air temperature and relative humidity, the wind speed, the surface temperature of the wall on the external plaster exposed to the solar radiation, the solar radiation on a horizontal plane and the solar radiation incident on the vertical surface were measured during the test. The value of solar radiation on a horizontal plane is a reference radiation value useful for the comparison of different climatic zones. The solar radiation on the vertical wall represents the fraction of solar radiation incident on the south facing green façades. The external air temperature was measured by a Hygroclip-S3 sensor (Rotronic, Zurich, Switzerland); it was adequately shielded from solar radiation. The temperature of the external plaster surfaces exposed to the solar radiation was measured using thermistors (Tecno.EL s.r.l. Formello, Rome, Italy). Both the solar radiation on a horizontal plane and the solar radiation normal to the wall were measured by means of pyranometers (model 8–48, Eppley Laboratory, Newport, RI, USA) in the wavelength range 300–3000 nm. The parameters were measured with a frequency of 60 s, averaged every 15 min and recorded

of the material layers comprising the building envelope of the experimental cuboids are reported in Table 1. Created cuboids do not intend to simulate the behaviour of a smallscale building room, thus functionality and occupancy features are not provided. The cuboids construction intends to avoid the direct action of solar radiation on the inner façade of the three walls and, thus, any influence on the surface temperature of the south facing external side of the walls. The planted species are evergreen climber type, in particular Pandorea jasminoides variegated and Rhyncospermum jasminoides. These plants were selected due to their ease to climb the wall and their well adaptation to the climatic conditions of the experimental area. Pandorea jasminoides variegated and Rhyncospermum jasminoides were transplanted on June 18, 2014. A third wall was kept uncovered for control. As plant supporting structure, an iron net was placed at a distance of 15 cm from the wall. The green systems stratigraphies are shown in Table 2. The plants were irrigated with the drip system. Throughout the period of measurement, the maintenance schedule consisted in the eradication of weeds from the culture substrate and in the arrangement of branches along the support net in order to provide the most compact and uniform canopy layer as possible. Plant Leaf Area Index (LAI) was 5

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Fig. 4. External air temperature, surface temperature of the external plaster of the three walls exposed to solar radiation, solar radiation on the vertical wall facing south and on horizontal plane: (a) a cool summer sunny day, less warm than average, 23 August 2016; (b) a cool summer cloudy day, 16 July 2016.

3. Results

on a data logger (CR10X, Campbell, Logan, USA) throughout the experimental test. Analysis of variance (ANOVA) was carried out with the CoStat software (CoHort Software, Monterey, CA, USA) to identify significant differences between the walls surface temperature and to assess the influence of solar radiation, wind speed and air relative humidity on the cooling effect of the façades. The monthly values of the climatic parameters measured at the experimental field in Valenzano are reported in Table 3. In the period from January 2015 to December 2016, the experimental field was characterized by values of the external air temperature ranging from −0.3 °C to 41.4 °C in 2015 and from 0.7 °C to 39.2 °C in 2016. The yearly cumulative solar radiation on a horizontal plane was equal to 5282 MJ m−2 and 5129 MJ m−2 in 2015 and 2016, respectively. The monthly value of cumulative solar radiation ranged from 177 MJ m−2, recorded in January 2016, to 802 MJ m−2, recorded in July 2015. The annual cumulative solar radiation on the vertical wall was equal to 3752 MJ m−2 and 3575 MJ m−2 in 2015 and 2016, respectively. The monthly value of cumulative solar radiation on the vertical wall ranged from 232 MJ m−2 (June 2016) to 357 MJ m−2 (September 2015).

3.1. Thermal performance in summer The effect of the climatic conditions on the thermal insulation performance of the green façades was analysed by considering different weather scenarios (sunny and cloudy days) in summer and winter. One representative day of each weather scenario was selected and the results are presented and discussed. The two summers were characterized by external air temperatures ranging from 15.7 °C to 41.4 °C, with an average value of 27.6 °C, during 2015, and from 15.6 °C to 39.2 °C, with an average value of 25.4 °C, during 2016. Therefore, both for a sunny day and for a cloudy day, “a hot day” (Fig. 2), “a warm day” (Fig. 3) and “a cool day” (Fig. 4), characterized by temperature respectively over the average, on average and below the average were analysed. In summer daytime, the maximum value of the wall surface temperature of the green façade was always recorded at least 1 h late in sunny days and less than 1 h late in cloudy days in comparison to the maximum value of the wall surface temperature of the control wall. It is due to the phase shift of the thermal wave due to the green façades 6

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Fig. 5. External air temperature, surface temperature of the external plaster of the three walls exposed to solar radiation, solar radiation on the vertical wall facing south and on horizontal plane: (a) a cold winter sunny day, 1 January 2015; (b) a cold winter cloudy day, 18 January 2016.

façades were higher than the temperatures on the control wall up to 2 °C. A hot summer partly cloudy day is shown in Fig. 2b: the presence of clouds, with a reduction of solar radiation intensity, reduced the differences between the curve of the external wall surface temperatures of the two green façades and of the control wall. Throughout the hours from early evening to early morning, the external wall temperatures of the two green façades were higher of about 1–2 °C than the external wall temperature of the control. Fig. 3a shows the external air temperature, the wall surface temperature of the three walls exposed to solar radiation, and the curves of the solar radiation both on a vertical wall facing south and on a horizontal plane for a warm summer sunny day, characterized by the average external air temperature of the day (equal to 25.9 °C) similar to the average value of 2016 (equal to 25.4 °C). During the hot hours of the day, the vegetative cooling was kept up to maximum 6 °C while at nighttime the external wall temperatures of the green façades were higher than the temperature of the control wall up to 2 °C. A warm summer cloudy day is shown in Fig. 3b: the external wall

highlighted in presence of the sun. The day characterized by the highest maximum external air temperature, equal to 41.4 °C, was chosen to analyse a hot summer sunny day during the experimental test period (Fig. 2a). During hot sunny days, the differences between the curve of the surface temperatures of the two green façades on the external plaster exposed to the solar radiation and of the control wall without greening cladding were wide particularly in the hot hours of the day, as shown in Fig. 2a, while a reduction of solar radiation intensity during afternoon promptly compressed these differences. During the daytime, the external wall temperature of the control wall was always higher than the external wall temperature of the green façades. The presence of the vegetation layer mitigated the temperature of the external plaster of the walls, and the amount of vegetative cooling was kept within a range of 6–7 °C. The maximum reduction of temperature between the control wall without greening and the covered ones was equal to 9.0 °C and was recorded on 31/08/2015 at 13.00 h for the wall protected with Pandorea jasminoides variegated. At nighttime, the temperatures on the external wall of the green 7

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Fig. 6. External air temperature, surface temperature of the external plaster of the three walls exposed to solar radiation, solar radiation on the vertical wall facing south and on horizontal plane: (a) a cool winter sunny day, 27 January 2016; (b) a cool winter cloudy day, 20 February 2016.

3.2. Thermal performance in winter

temperatures of the two green façades closely followed the external wall temperature of the wall without plants. In the afternoon and at nighttime the external wall temperatures of the two green façades were higher than the external wall temperature of the control; the amount of vegetative warming was kept within a narrow range of 1–2 °C. On a cool summer sunny day (Fig. 4a), the external wall temperature of the control wall rose in the morning being synchronized with the solar radiation more than the external wall temperature of the wall covered with plants. During daytime the differences between the external wall temperatures were about 3–4 °C while a reduction in solar radiation intensity promptly reduced these differences. A cool summer cloudy day is shown in Fig. 4b: the external wall temperatures of the two green façades closely followed the external wall temperature of the wall without plants. After both a sunny and a cloudy day, the green façades acted as thermal screens during night with a higher external wall temperature of the two green façades; this behaviour in summer is not desirable.

The winter periods were characterized by temperatures ranging from −0.3 °C to 20.9 °C, with an average value of 9.6 °C, during 2015 and from 1.7 °C to 22.6 °C, with an average value of 10.9 °C, during 2016. Two different weather scenarios were analysed through “a particularly cold day” (Fig. 5), “a cool day” (Fig. 6) and “a warm day” (Fig. 7), characterized by temperature below the average, on average and over the average, respectively. Regardless of the different weather conditions, in winter the vegetation layer increased the insulation performance of the green façades from sunset to early morning: the external wall temperature of the green façades was always higher, within a range of 1–2 °C, in comparison to the external wall temperature of the control wall without greening cladding (Figs. 5–7). During a cold wave, the highest increase of temperature of the external covered surface in comparison with the control wall without greening was equal to 3.5 °C and it was recorded on 06/01/2016 at 19.45 h for the Pandorea jasminoides variegated. The green façades act as a thermal screen with a vegetative warming. This behaviour is desirable 8

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Fig. 7. External air temperature, surface temperature of the external plaster of the three walls exposed to solar radiation, solar radiation on the vertical wall facing south and on horizontal plane: (a) a warm winter sunny day, 2 February 2016; (b) a warm winter cloudy day, 5 February 2015.

performed for each month of the two years test period in order to compare the mean values of the daily external air temperature and surface temperature of the external plaster of the three walls exposed to solar radiation, with reference to the maximum, minimum and mean daily values. Statistically significant differences at P < 0.05 were found, thus Duncan's test was applied with a significance level of 0.05. The differences among the monthly mean values of the maximum, minimum and mean daily external air and external surface temperature of three walls were evaluated (Tables 4–6). The monthly mean of the daily maximum surface temperatures recorded on the control wall were mainly statistically higher than the values recorded on the green façades during all the seasons (Table 4), demonstrating the cooling effect of this green technology during daytime. The cooling behaviour is desirable in warm period. The mitigation of the wall surface temperature due to the plants was observed throughout the year; no significant difference was pointed out between the two plants. The differences between the mean values of the maximum daily temperatures recorded for the control and for the walls covered with the plants ranged between 0.3 °C and 5.7 °C for Rhyncospermum jasminoides, and between 0.9 °C and 5.1 °C for Pandorea

in wintertime contributing to reduce the heating energy cost. Winter sunny days were characterized during the daytime by an external wall temperature of the control wall higher of 3–4 °C on a cold day (Fig. 5a), of 5–6 °C on a cool day (Fig. 6a), of 4–5 °C on a warm day (Fig. 7a), in comparison with the external wall temperature of the green façades. Cloudy days were characterized by air temperature regimes connected with the feeble solar radiation environment: the diurnal temperature ranges have been reduced and small peaks were displayed as responses to the presence of solar radiation rays. During the daytime of winter cloudy days, the wall temperature curves cluster together as shown in Figs. 5b-6b-7b.

3.3. Surface mean temperatures Surface temperature of the external plaster of the three walls exposed to solar radiation gathered during the experimental period from January 2015 to December 2016 fluctuated over the course of the day and over the varying of the seasons. One-way ANOVA analysis at a 95% probability level were 9

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Table 4 Monthly mean values of the maximum daily external air temperature and surface temperature of the external plaster of the three walls exposed to solar radiation. Month

Monthly mean of the daily maximum temperatures (°C) Rhyncospermum jasminoides

Jan-15 Feb-15 Mar-15 Apr-15 May-15 Jun-15 Jul-15 Aug-15 Sep-15 Oct-15 Nov-15 Dec-15 Jan-16 Feb-16 Mar-16 Apr-16 May-16 Jun-16 Jul-16 Aug-16 Sep-16 Oct-16 Nov-16 Dec-16

Table 6 Monthly mean values of the external air temperature and surface temperature of the external plaster of the three walls exposed to solar radiation.

ab

15.1 13.6ab 15.3a 19.3a 24.3b 27.2b 32.6c 30.6c 27.4b 20.7b 17.1b 14.5c 13.2b 15.7c 14.2b 20.5b 20.9b 26.5b 30.5c 28.0c 22.6b 20.0b 16.5c 12.4c

Pandorea jasminoides b

13.6 12.5b 14.9a 19.4a 25.2b 28.1b 32.9c 30.6c 27.7b 21.2b 17.2b 14.8bc 13.2b 16.0bc 15.2b 21.2b 21.8b 26.9b 31.2c 28.9c 23.2b 20.6b 17.1bc 13.2bc

Month

Control a

15.4 14.1a 15.8a 21.0a 27.1a 30.8a 37.1a 35.6a 32.2a 24.2a 20.5a 18.1a 15.9a 18.8a 17.2a 24.7a 24.1a 31.3a 35.9a 33.7a 27.7a 24.3a 20.9a 16.9a

external

Monthly mean temperatures (°C) Rhyncospermum jasminoides

ab

14.2 14.1a 16.5a 21.3a 27.7a 30.5a 35.8b 33.8b 30.6a 23.6a 19.1ab 15.9b 14.7ab 17.6ab 17.1a 24.2a 24.7a 30.3a 33.3b 30.8b 26.6a 22.9a 18.7b 14.0b

Jan-15 Feb-15 Mar-15 Apr-15 May-15 Jun-15 Jul-15 Aug-15 Sep-15 Oct-15 Nov-15 Dec-15 Jan-16 Feb-16 Mar-16 Apr-16 May-16 Jun-16 Jul-16 Aug-16 Sep-16 Oct-16 Nov-16 Dec-16

a

Pandorea jasminoides a

a

8.9 8.6ab 10.8ab 14.3a 20.1b 23.1b 27.8b 26.2b 22.8a 17.4a 12.9a 10.0a 9.3a 12.3a 11.1a 16.1a 17.4a 22.4b 26.1a 24.3a 19.7b 17.1a 13.6a 8.5a

9.1 8.7ab 10.8ab 14.2a 19.9b 22.9b 28.0b 26.4b 22.9a 17.3a 12.9a 10.0a 9.4a 12.2a 10.8a 15.9a 17.3a 22.5b 26.1a 24.2a 19.5b 16.9a 13.4a 8.5a

Control 8.4 7.9b 10.2b 13.8a 19.7b 23.0b 28.4ab 27.1ab 23.4a 17.4a 12.9a 10.0a 9.1a 12.1a 10.8a 16.2a 17.4a 23.1ab 26.9a 25.0a 20.3ab 17.3a 13.7a 8.5a

external 9.2a 9.2a 11.7a 15.4a 21.5a 24.7a 29.5a 27.8a 24.1a 18.4a 13.8a 10.6a 10.1a 12.5a 11.6a 17.0a 18.9a 24.0a 27.4a 25.1a 21.0a 17.7a 14.0a 8.6a

a-c

Monthly mean values of the temperature in a row with a different superscript letter statistically differ at P < 0.05 using Duncan's test.

a-b

Table 5 Monthly mean values of the minimum daily external air temperature and surface temperature of the external plaster of the three walls exposed to solar radiation.

Table 7 Mean temperatures of the external plaster of the three walls exposed to solar radiation at Valenzano (Bari), in 2015–2016, as a function of the solar radiation intensity levels (I).

Month

Jan-15 Feb-15 Mar-15 Apr-15 May-15 Jun-15 Jul-15 Aug-15 Sep-15 Oct-15 Nov-15 Dec-15 Jan-16 Feb-16 Mar-16 Apr-16 May-16 Jun-16 Jul-16 Aug-16 Sep-16 Oct-16 Nov-16 Dec-16

Monthly mean values of the temperature in a row with a different superscript letter statistically differ at P < 0.05 using Duncan's test.

Monthly mean of the daily minimum temperatures (°C)

Solar radiation I on a horizontal wall (W m−2)

Rhyncospermum jasminoides

Pandorea jasminoides

Control

external

5.4a 5.1a 7.2ab 9.7a 15.3a 18.2a 22.6a 22.3a 18.6a 14.3a 9.7a 7.2a 6.4a 8.8a 7.7a 11.6a 13.6a 18.2a 21.4a 20.0a 16.6ab 14.0a 10.8a 5.5a

5.6a 5.3a 7.3ab 9.8a 15.2a 18.3a 22.2a 22.0ab 18.3a 14.2a 9.7a 7.1a 6.3a 8.8a 7.8a 11.6a 13.2ab 17.7a 21.0ab 19.7a 16.5ab 13.9a 10.8a 5.2a

4.0a 3.7b 5.9b 8.4a 13.8b 16.8b 21.1b 21.1b 17.4a 13.1a 8.2b 5.6b 4.8a 7.3a 6.5b 10.1b 12.1b 16.8a 20.0b 18.8b 15.6a 12.6a 9.5a 3.6b

5.4a 5.4a 7.4a 9.8a 15.3a 18.4a 22.5a 22.4a 18.9a 14.7a 10.0a 7.1a 6.3a 8.1a 7.4ab 11.1ab 13.2ab 17.7a 21.0ab 19.7a 16.9a 13.5a 10.3a 4.8a

I=0 0 < I ≤ 200 200 < I ≤ 400 400 < I ≤ 600 600 < I ≤ 800 I > 800

Mean temperatures (°C) Rhyncospermum jasminoides

Pandorea jasminoides

Control

14.46a 15.89a 17.92b 20.51b 24.38b 26.44b

14.58a 16.02a 17.87b 20.34b 24.23b 26.38b

13.37b 15.93a 19.35a 22.82a 27.53a 30.47a

a-b

Mean temperatures in a row (i.e. for a specific solar radiation range) with a different superscript letter statistically differ at P < 0.05 using Duncan's test.

Table 8 ANOVA results concerning the climatic conditions influence on the temperature reduction of the wall surface behind the Rhyncospermum jasminoides. Sourcea Main Effects I W RH Interaction I*W I * RH W * RH I * W * RH Error

a-b

Monthly mean values of the temperature in a row with a different superscript letter statistically differ at P < 0.05 using Duncan's test.

df

MS

F

P

3 5 7

5521.32 356.22 80.93

1523.23 98.27 22.33

*** *** ***

15 20 34 75 20288

48.74 11.90 38.21 5.84 3.62

13.45 3.28 10.54 1.61

*** *** *** ***

***P ≤ 0.001. a I: solar radiation intensity; W: wind speed; RH: air relative humidity.

jasminoides variegated, the higher differences being recorded from July to September each year (Table 4). The monthly mean of the daily minimum surface temperatures recorded on the wall not covered with plants were statistically often lower than the values recorded on the walls covered with climbing

plants during all the seasons (Table 5), demonstrating the heating effect of this green technology during nighttime. The heating behaviour is desirable in cold period. No significant temperature difference was 10

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factors and of their interactions at daytime (Tables 8 and 9). The climatic parameters (factors) considered were the solar radiation intensity (I > 200 Wm−2), the wind speed (W) and the relative humidity (RH) of the outdoor air. The statistical analysis carried out by ANOVA revealed significant differences. The variability determined by the solar radiation intensity is greater than those of the other factors at P < 0.001 for both the two green façades (Tables 8 and 9). TukeyKramer's test was applied to investigate the effects of these three factors (Tables 10–12). The comparison of the mean values carried out by Tukey-Kramer's test confirmed an almost direct dependence of the temperature reduction on the solar radiation intensity for both the two green façades (Table 10). The wind influenced the cooling performance of the green façades with an increasing trend for speed values falling in the range 0–4 ms−1, reaching the maximum effect in the range 3–4 ms−1; once exceeded the value of 4 ms-1, the cooling effect of both plants starts decreasing for both the two green façades (Table 11). The air relative humidity influenced the cooling performance of both green façades with an almost decreasing trend, reaching the maximum effect for percentages falling in the range 30–60% (Table 12). The temperature reductions related to relative humidity < 50% are steady, for both green façades; the same behaviour occurs for percentages > 80%.

Table 9 ANOVA results concerning the climatic conditions influence on the temperature reduction of the wall surface behind the Pandorea jasminoides. Sourcea Main Effects I W RH Interaction I*W I * RH W * RH I * W * RH Error

df

MS

F

P

3 5 7

5758.87 155.72 85.04

1892.37 51.17 27.94

*** *** ***

15 20 34 75 20288

28.24 22.44 22.00 4.47 3.04

9.28 7.37 7.23 1.47

*** *** *** **

***P ≤ 0.001. **P ≤ 0.01. a I: solar radiation intensity; W: wind speed; RH: air relative humidity.

recorded between the surfaces covered with the two plants. The differences between the lowest mean temperatures recorded for the wall shielded with plants and the control ranged from 1.0 °C to 1.9 °C for Rhyncospermum jasminoides, and from 0.9 °C to 1.7 °C for Pandorea jasminoides variegated (Table 5). The minimum surface temperature of the external plaster protected with the two green walls closely followed the daily minimum external air temperature. Significant differences were highlighted between the monthly mean daily surface temperatures recorded on the control wall and on the green façades during all the seasons only in a limited number of cases (Table 6). In order to deepen the study of the thermal behaviour of the green façades, a regression analysis was conducted to assess the dependence of the temperature of the walls external side on the level of intensity of solar radiation (I). The regression analysis confirmed the statistically relevant dependence at P < 0.05 for both green systems; a direct dependence of the surface temperature on the level of intensity of solar radiation was recorded. Subsequently, a one-way ANOVA analysis at a 95% probability level was performed to evaluate the external surface temperature as a function of the different levels of solar radiation (I = 0; 0 < I ≤ 200; 200 < I ≤ 400; 400 < I ≤ 600; 600 < I ≤ 800; I > 800 Wm−2). Statistically significant differences at P < 0.001 were found in all the ranges except for 0 Wm−2 < I ≤ 200 Wm−2. Duncan's test was applied with a significance level of 0.05 for evaluating the differences among the mean values of the external surface temperature of the three walls (Table 7). At nighttime (I = 0 Wm−2) the temperatures on the green facades were statistically higher than that on the control wall. Solar radiation has to reach a minimum level of 200 Wm−2in order to activate an effective cooling performance of the experimental green façades. Once exceeded this limit, cooling behaviours of Rhyncospermum jasminoides and Pandorea jasminoides were statistically the same. Finally, in order to determine whether and which climatic conditions mostly influence during daytime the cooling effect of the two green façades (i.e. external surface temperature reduction in comparison to the control wall), an analysis of the overall variance was performed. A three-way ANOVA analysis at 95% probability level was applied for evaluating the significance of the influence of the single

4. Discussion The field test of the present research was carried out to overcome a lack of literature on experimental data on green façades for a long period [37,24,27,28,31,37]. Simulation models often were not validated with real data [27,32,38]. The experimental daylight temperatures observed on the south oriented green façades during warm days were lower than the respective temperatures of the uncovered wall up to 9.0 °C. In one summer month with Thessaloniki's climatic conditions, characterized by warm temperate humid climate, Eumorfopoulou and Kontoleon [39] reported a temperature reduction of the maximum values in the exterior surface of a plant-covered east wall equal in average to 5.7 °C, varying from 1.9 °C to 8.3 °C. Pérez et al. [27] reported the following reduction of the external building surface temperature: from 1.7 °C to 13 °C in warm temperate climate region in the case of a wall covered with traditional green façades during summertime. Susorova et al. [33] reported an average decrease of the façade surface temperatures due to the presence of vegetation on the façade from 1.0 °C to 9.0 °C during summer on brick infills external surface; a south façade of a building covered with plants was monitored for four days, from August 29 until September 1, 2012, at Illinois Institute of Technology, characterized by a hot humid continental climate. Results of the present study showed that the nighttime temperatures during the cold days for the vegetated walls were higher than the respective temperatures of the control wall up to 3.5 °C. Also Cameron et al. [10] studied the thermoregulative performance of wall shrubs and climbing plants on brick walls in controlled environments. Test was carried out during 2010 from September to December. Among all tested plants, wall surface temperatures increases up to 3 °C have been registered by experimentally testing a Hedera helix layer in comparison with control. The results of the present research concerning the cooling and

Table 10 Mean temperatures of the external plaster of the green façades, during daytime, as a function of the solar radiation intensity levels, analysed with Tukey-Kramer's test. Solar radiation intensity (I) on a horizontal surface (W m−2) 200 < I ≤ 400 Mean temperature reduction (°C)

a-b-c-d

Rhyncospermum jasminoides Pandorea jasminoides

d

1.43 1.48d

400 < I ≤ 600 c

2.31 2.48c

Mean values of the temperature in a row with a different superscript letter statistically differ at P < 0.05 using Tukey-Kramer's test.

11

600 < I ≤ 800 b

3.15 3.30b

I > 800 4.03a 4.09a

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Table 11 Mean temperatures of the external plaster of the green façades, during daytime, as a function of the wind speed levels, analysed with Tukey-Kramer's test. Wind speed W (m s−1)

Mean temperature reduction (°C)

a-b-c-d-e-f

Rhyncospermum jasminoides Pandorea jasminoides

W≤2

2 < W≤3

3 < W≤4

4 < W≤5

5
W>6

1.73c 2.15d

2.71b 2.74b

2.96a 2.88a

2.57b 2.49c

1.93c 1.92e

1.19d 1.26f

Mean values of the temperature in a row with a different superscript letter statistically differ at P < 0.05 using Tukey-Kramer's test.

Table 12 Mean temperatures of the external plaster of the green façades, during daytime, as a function of the air relative humidity levels, analysed with Tukey-Kramer's test. Air relative humidity RH (%) RH ≤ 30 Mean temperature reductions (°C)

a-b-c-d-e-f

Rhyncospermum jasminoides Pandorea jasminoides

2.93

a

3.02a

30 < RH ≤ 40 2.83

a

2.89a

40 < RH ≤ 50 2.73

a

50 < RH ≤ 60 2.41

2.75b

b

2.54c

60 < RH ≤ 70 1.95

c

2.17d

70 < RH ≤ 80 1.33

d

1.52e

80 < RH ≤ 90

RH > 90

e

1.02e

1.16f

1.20f

0.97

Mean values of the temperature in a row with a different superscript letter statistically differ at P < 0.05 using Tukey-Kramer's test.

Rhyncospermum jasminoides and Pandorea jasminoides variegated, was observed over the course of the day and over the varying of the seasons. Environmental conditions and surface temperature of the external plaster of the three walls exposed to solar radiation were gathered throughout the two years of experimental test. The external surface temperature of the building wall was chosen as parameter useful for assessing the thermal performance of the green vertical systems. The two years of data gathered in the present research were useful to evaluate under different weather conditions the thermal behaviour of the green façades in the Mediterranean climate region. Similar analyses using data recorded during a short period of time, from few days to a month or to at least a season, can be unreliable. The results shown in the present research allow to fill the gap in literature concerning the lack of data for all the seasons of the year. In the present research the façades showed a good capacity of cooling in the warm periods; the daylight temperatures observed on the south oriented green façades were lower than the respective temperatures of the uncovered wall up to 9.0 °C. The façades also acted as thermal screen during the cold days; the nighttime temperatures for the vegetated walls were higher than the respective temperatures of the control wall up to 3.5 °C. The thermal effects of the façades at daytime was driven by solar radiation, wind velocity and air relative humidity. The highest cooling effect of such parameters occurred with a wind speed of 3–4 ms−1, an air relative humidity within the range 30–60% and a solar radiation higher than 800 Wm−2. The long-term experimental test demonstrated that both Pandorea jasminoides variegated and Rhyncospermum jasminoides are suitable for green façades in the Mediterranean climatic area. Some energetic aspects should be investigated in future research, one concerns the question if the heating effect behind green façades during the evening/night period of warm days can reduce the advantage in cooling energy demand reduction obtained during the warmest hours of the day. Similarly, the reduction of heating due to the shading effect at daytime during the cold days should be evaluated in relation with the thermal screen effect obtained at nighttime.

heating magnitude fall within the ranges shown in the literature for temperate climates [1,27,28,40]. The differences highlighted between the monthly mean surface temperatures recorded on the control wall and on the green façades during all the seasons were more evident in relation to the maximum and minimum values than for the mean values. No significant differences emerged in the behaviour of the two selected plant species. The application of the green layer significantly reduced wall surface temperatures during warm days; it also resulted advantageous during cold weather scenarios by retaining warmth around the building, thus reducing or delaying the demand for indoor heating. The heating effect behind green façades during the evening/night period of warm days requires further attention in relation to internal comfort at night. A solar radiation threshold value of 200 Wm−2 emerged as necessary to incite notable cooling functions on the walls behind the green layers during daytime, with a direct dependence of the surface temperature on the level of solar radiation. This result is in general agreement with the findings of another experimental study, which assessed a solar radiation minimum value of 300 Wm−2 for mobilizing effective transpiration cooling for indirect green façades forming a 10 cm air gap between wall and support mesh [23]. Data in Table 7 show that differences between species in lowering the temperature of the external plaster did not statistically differ. The statistical analysis allowed also to evaluate the significance of the influence of the wind speed and air relative humidity of the experimental site on the reduction of the external surface temperature of the two green façades. An indirect dependence of the surface temperature on the increase of external air relative humidity was shown; this trend can be attributed to the plants evapotranspiration rate. Maximum effects of the wind speed were shown in the range 3–4 ms−1. 5. Conclusion Façades covered with vegetation can be used in passive design of buildings where the plants increase energy efficiency by functioning as a natural shading and thermal screen. A thorough knowledge of the thermal behaviour of green façades in different climate scenarios is important to provide effective design tools for building engineers and architects. Energy balance simulation models of buildings can be improved with tools describing the thermal behaviour of the green façades. In this study, a long-term monitoring activity was carried out on two green façades and on a control wall in Mediterranean climatic conditions. The thermal behaviour of two different climbing plants,

Acknowledgements The contribution to programming and conducting this research must be equally shared between the Authors. The present work has been carried out under the “Piano triennale della Ricerca 2015–2017 nell'ambito del Sistema Elettrico Nazionale, Progetto D.1 ‘Tecnologie per costruire gli edifici del futuro’, Obiettivo: 12

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Tecnologie “green” per gli edifici, Piano Annuale di Realizzazione (PAR) 2015″, Accordo di Programma Ministero dello Sviluppo Economico – ENEA, funded by the Italian Ministry of Economic Development, Decreto Ministeriale of 21/4/2016.

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