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Experimental investigation on the energy performance of Living Walls in a temperate climate
Building and Environment 64 (2013) 57e66
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Experimental investigation on the energy performance of Living Walls in a temperate climate Ugo Mazzali a, *, Fabio Peron a, Piercarlo Romagnoni a, Riccardo M. Pulselli b, Simone Bastianoni b a b
Department of Design and Planning in Complex Environments, IUAV University of Venice, Dorsoduro 2206, 30123 Venice, Italy Department of Earth, Environmental and Physical Sciences, University of Siena, Siena, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2013 Received in revised form 1 March 2013 Accepted 5 March 2013
Living Walls, a type of vertical greenery system, are relatively light structures for architectural green cladding. They embed a thick curtain of plants nurtured by an automated watering system. Three Living Wall field tests are presented for investigating potential effects of the energy behavior on building envelopes. In particular, Living Walls were monitored in a Mediterranean temperate climate context at the latitudes of Northern and Central Italy. As a result, the dependence on the solar radiation forcing came out clearly. During sunny days, difference in temperature (monitored on the external surface) between the bare wall and the covered wall ranges from a minimum of 12 C (case C) to a maximum of 20 C (case A). The analysis was extended also to heat flux. The incoming (positive) heat flux through the bare wall was found to be higher compared to the Living Wall. Considering an overall thermal balance during the monitoring period, the outgoing heat flux through the Living Wall was higher. These results indicate that the use of green architectural cladding can significantly contribute to cooling energy reduction and offer a valuable solution for retrofitting existing buildings. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Living Walls Field measurements Green cladding Sustainability
1. Introduction In recent years building green claddings have spread widely as architectural and constructive elements to design façades and roofs. Vertical green claddings are increasingly present in the international architectural panorama, namely Green Walls. Green Walls can be classified into two categories: typical Green Façades, made up of natural creepers which creeps over the building envelope or a support grid structure; Living Walls, made up of plants or grass embedded in a specific cladding structure anchored on the building façade. Differences in Green Walls may concern construction systems, type of plants, assembling and maintenance, and can affect their behavior as passive energy saving systems [1]. Many kinds of Living Walls are available on the market, and it is possible to distinguish them according to their constructive technology - such as, for example, made of aluminum frame and Pvc panel with geotextile felt or of recycled polypropylene boxes and to the type of green cover which can be grass or plants in dependence of climate and cladding orientation. A clear distinction between Green Façades and Green Walls is proposed by [2] * Corresponding author. Tel.: þ39 0412571302. E-mail address: [email protected] (U. Mazzali). 0360-1323/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.buildenv.2013.03.005
together with a microclimate analysis of the intermediate space of a double skin green façade found with lower temperatures and higher humidity compared to external ambient air. Another classification is proposed by [3] together with an historical overview and a pro et contra analysis of facade greenery. Green Façades field measurements were performed by [1] in dry Mediterranean Continental climate focusing on shading properties of four climbing species and resulting in a light transmission factor comparable to that of artificial barriers and estimated in a range from 0.15 to 0.41. Living Walls were found to have healthy effects on the human environment as well as environmental, economic, and social benefits [4] such as, for example, reduction of air temperature, improvement of the air quality by filtering airborne particles, water runoff reduction and consequent decreasing of drainage infrastructure costs, decreasing of the climatic stress on the envelope with the reduction of maintenance costs, increase of plants and green areas leading to an increase of livable spaces to play, relax and improve social relationships [5]. In this context, Green Walls are conceived as valuable measures for retrofitting buildings as well as urban public spaces. Specifically on microclimate, Green Walls may contribute in a considerable way to the mitigation of the so called Urban Heat
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Island (UHI) effect which leads to an increase of air temperature in urban areas up to 2 Ce5 C compared to peripheral and less populated areas. For example the green covering of parking urban area was estimated to mitigate the UHI effect and lead to a surface temperature reduction up to 6 Ce9 C [6]. Shading trees [7] have a potential cooling effect on the surrounding air up to 1.5 C during hottest hours of the day for a mediterranean sub-tropical climate of the Los Angeles bay (Köppen classification: Csb). A number of thermal analysis of green claddings was published. Screening from direct solar radiation by Green Walls is among the most important factors affecting the thermal performance of building envelopes. Green shading leads to an external surface temperature reduction and to a consequent reduction of the heat flux entering through the wall and the summer cooling energy consumption. A further distinguishing element from other shading systems is the evapotranspiration of plants, that is supposed to increase heat losses. The Leaf Area Index, i.e. the ratio of leaves surface to ground surface [8], the solar absorption coefficient and the leaves emissivity [9] also affect thermal performance of Green Walls. Based on these factors, a simulation model developed in Greece [10] showed a surface temperature reduction of 1 Ce2 C on the North and 16 Ce17 C on the West oriented facade with a consequent reduction of summer cooling energy consumption (in a reference room) from 4.65% (North) to a 20.08% (West). Microclimatic monitoring campaigns in Greece [11] show surface temperature reduction, compared to a bare wall, varying from 1.9 C to 8.3 C. Also in equatorial climate [12], eight different green cladding field measurements highlighted reductions in surface temperature up to 11.6 C and in air temperature up to 3.3 C at 15 cm from the external layer, depending on assembling technology, substrate, wall insulation, humidity content and Leaf Area Index. The present paper aims to provide a further description from a thermophysical point of view of Living Walls (LWs). Among different typologies we have considered the following: a) Living Wall A: a threefold felt layer fixed on PVC sheets supported by an aluminum frame. Evergreen or seasonal plants are embedded in pockets on the external felt layer. This system is anchored on the building façade leaving an open air cavity behind the cladding. b) Living Wall B: same structure of LWA with grass directly sown on an external felt layer. This has a closed air cavity behind the cladding. c) Living Wall C: turf grass sown inside little bowls filled with soil. The recycled polypropylene panel with little bowls is
supported by an aluminum structure. This system has an open air cavity behind the cladding. The three kinds of Living Wall are visible in Fig. 1. Details of the vegetation anchor system on the cladding structure - felt pockets, felt layer, and recycled polypropylene bowls - are shown. This analysis was performed by means of a microclimatic field measurement of the LWs above installed in Northern and Central Italy. Controlled microclimatic rooms were considered behind the LWs. Two of three field measurements (LWA and LWB) were performed in this condition which gave the possibility to have a more detailed analysis of the monitored variables. With respect to LWA and LWB, rooms’ microclimate behind LWC was not controlled and the indoor air temperature was subject to fluctuation depending on the external ambient air. 2. Materials and methods In this section the three LWs field tests will be described in detail, including location, probe positioning and monitoring equipment. In the present work the field measurement of the case study A (LWA) was performed in Lonigo, the case study B (LWB) in Venice and case study C (LWC) in Pisa (Fig. 2). The list of probes used in the three monitoring campaigns and the corresponding accuracy is reported below, in Table 1. The B and LW subscripts mean that measurements refer to a Bare Wall and a Living Wall, respectively. 2.1. Living Wall case study A Measurements were performed on a South-West oriented 3 m 3 m prototype installed in Lonigo (45 230 N). Fig. 3 shows the described cladding. This kind of Living Wall is made up of an aluminum structure, a PVC panel installed on it, and three felt layers with different functions. The first layer let the water flow between the Pvc panel and the felt itself, the second layer allows the roots of the plants to propagate and the third felt has a mechanical function, as a support during plant rooting, becoming a unique compact structure with the roots. The aluminum frame is fixed on a further aluminum structure directly installed on the building wall by means of plugs. At the top of the LWA a flexible pipe for the irrigation is present. This is able to provide through each nozzle 2.1 l/h of water. Water flow is in dependence of many variables such as, for example, orientation and kind of plants. The distance between each nozzle is 15 cm and the surplus of water drained off by the plants is collected
Fig. 1. Three kinds of Living Walls.
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Fig. 3. Living Wall A e the monitored wall.
The monitoring campaign was performed between June and September 2011. Probes were positioned in correspondence of the bare wall and the LWA along four monitoring sections: two sections at 0.75 m and two sections at 2.25 m height. For the two 2.25 m height monitoring sections a heat fluxmeter was added as shown in Fig. 4. Fig. 2. Location of the three monitored sites.
on a little basin at the bottom of the structure. The percentage of water not collected by felts or not used by the plants for evapotranspiration and growth is less than 5%e10%. The LWA is installed on an external not insulated wall in concrete blocks of 40 cm thickness included internal and external plaster. The most important plant species embedded in the LWA are here reported in latin names: Juniperus communis Sedum spurium, Geranium sanguineum, Geranium Johnson’s blue, Anemone sp., Viva minor, Parthenocissus tricuspidata, Heuchera micrantha Palace Purple, Salvia nemorosa, Lonicera pileata, Pittosporum tobira, Rosmarinus officinalis, Alchemilla mollis, Bergenia cordifolia, Oenothera missouriensis, Plumbago capensis.
Table 1 Probes and accuracy summary. Monitored microclimatic variable
Probe accuracy
Surface temperature External air temperature and relative humidity
0.2 C 0.18 C accuracy at 25 C for the temperature and 2.5% from 10% to 90% for the relative humidity 0.05 m/s accuracy 0.4 Ce0.45 C accuracy at 20 C for the temperature and 3% accuracy at 25 C for the relative humidity accuracy 5% accuracy 20 W/m2 directional error
Air velocity Internal air temperature and relative humidity Heat flux Solar radiation Variables are coded as follows: - Surface temperature: STB, STLW - Air temperature: AT - Air relative humidity: ARH - Air velocity: AV - Heat flux: HFB, HFLW - Solar radiation: SR.
2.2. Living Wall case study B Measurements were performed on a South-West oriented 3 m 3 m prototype installed in Venice, at the headquarter of the IUAV University of Venice (45 260 N). This cladding is made up of an aluminum structure, a PVC panel installed on it, and a vertical turf grass which has been specially made to grow on the felt support. The prototype of the described Living Wall is visible in Fig. 5. The kind of sown grass belongs to Zoysia species and is uniformly seeded on the felt layer. The monitoring campaign was performed between June and September 2012 in the Applied Physics Laboratory of University of Venice. The external not insulated wall over which the LWB is installed is made up of hollow brick with a total thickness of 40 cm including internal and external plaster. Probes for the monitoring of surface temperatures and heat fluxes and other microclimatic variables were positioned on the field in the proximity of the vertical green cladding as visible in Fig. 6. Internal and external surface temperatures and heat fluxes were recorded for both the bare wall and the LWB of the building wall. A little weather station provides values of the microclimatic variables in the proximity of the field measurement. 2.3. Living Wall case study C The monitored wall, as visible in Fig. 7, is 10.80 m long and 2.80 height and is made up of 126 recycled polypropylene of 60 cm long 40 cm height installed in 7 lines of 18 panels. The wall over which the LWC is installed is made up of not insulated concrete blocks with a total thickness of 30 cm. All the polypropylene panels are provided with many kinds of grass species among which: Zoysia matrella ‘Zeon’, Zoysia tenuifolia, Zoysia japonica ‘El Toro’, Cynodon dactylon X Cynodon trasvalensis ‘Patriot’ Stenotaphrum secondatum, Dicondra, Paspalum vaginatum, Cynodon transvalensis. About 15% of the panels was expressly without vegetation with the aim to reproduce a bare wall without cladding.
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Fig. 4. Living Wall A - bare wall and Living Wall section with probes.
The monitoring campaign started September 5th 2009 in Pisa (43 430 N). Nine surface temperature probes were positioned in correspondence to the Living Wall naturalized with Dicondra. This particular specie of grass is a herbaceous suitable to all climatic zones of Italian latitudes and although it prefers hot climates it is requested for low maintenance turf grass design thanks to its particular features such as low water and nutrition demand and the reduced growth height, not over 4 cme5 cm. Probes were positioned on the inside surface, on the outside surface, in the air cavity, and adjacent to the soil under the turf grass. Other 4 probes were positioned in correspondence of the bare wall. The whole probe positioning is visible in Fig. 8. A pyranometer was also installed in the field with the aim to monitor the solar radiation behavior on the wall. 2.4. Overview of the monitored Living Walls The most important features of the three monitored LWs are reported in Table 2. 3. Results and discussion For each LW, particular climatic analysis periods were identified with the aim to better understand the thermophysical behavior of
Fig. 5. Living Wall B - the monitored wall.
the cladding. Days with highest and lowest solar radiation impinging on the walls were considered because of the clear importance of the solar radiation as climate forcing factor which influences the green cladding behavior. In the next sections the behavior of the most important climatic variables will be analyzed during the reference days.
3.1. Living Wall case study A The monitoring campaign started on July 6th 2011 and was concluded at the end of the summer period on September 21st 2011. During this period, days with significant values of the climatic forcing variables were selected. The following days were selected: - Day with strongest solar radiation: September 20 2011 e 4338 Wh/day (total impinging radiation) - Day with lowest solar radiation: September 18 2011 e 732 Wh/ day (total impinging radiation) Being almost contiguous, these days will allow for studying the continuity of temperature trends. The radiation during representative days reported above is the total amount of global solar radiation incident on the vertical plane, throughout the day, as measured by the pyranometer positioned on the wall. As emerged from the meteorological data, solar radiation is minimal on September 18 at the beginning of a rainy day. The next day the rainfall increases significantly, however, solar radiation is still higher than the previous day. On September 20, finally the storm ends and the radiation turns to a maximum. An initial analysis shows that the difference in surface temperature at different heights is not significant and on sunny days does not exceed 1.5 C. Further analysis will be focused on the difference in surface temperatures between the covered and the bare wall. Fig. 9 shows the hourly temperature difference trend between STB,2 (external surface temperature of the bare wall) and STLW,1 (external surface temperature of the Living Wall). A positive DeltaT value indicates that the external surface temperature of the bare wall is higher than the Living Wall. The dependence on the solar radiation, reported on the right axis of the figure, is clear and in particular the temperature difference reaches values from 12 C to 20 C during sunny days and much lower values around 5 C, during cloudy days. Moreover during night-time the bare wall will tend to cool itself more than the covered wall up to around 2 Ce3 C. As shown in Fig. 10, in correspondence of the Living Wall heat flux on the external surface measured by the heat flux meter HF_LW oscillates between values of 18 W/m2 and 30 W/m2 while
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Fig. 6. Living Wall B e wall section with probe positioning.
for the bare wall heat flux oscillates between values from 90 W/ m2 and þ100 W/m2. This clear difference in terms of incoming (positive) or outgoing (negative) energy from the wall is mainly due to the large shielding effect provided by the green cladding which reduces drastically the amount of energy coming from the sun, but is also due to other factors, typical of a vegetation coating, such as the latent heat of evapotranspiration, and the absorption coefficient. The water latent heat of evaporation is around 2450 kJ/kg. This means, in the case of LWA, about 100 W/m2 of heat removed from the wall surface thanks to the evaporation of water [13]. The visible absorption coefficient affects the absorption of solar radiation in the range between 380 nm and 780 nm and coefficient for the plants can be considered near to 0.4 according to [14]. The difference between the heat fluxes outgoing from the wall behind a Living Wall compared to those outgoing from a bare wall is in the order of 70%e80%. 3.2. Living Wall case study B The monitoring period for LWB starts from July 5th 2012 and it concludes in the second half of September. The days identified for the study of the thermal behavior are as follows: - Day with strongest solar radiation: September 9 2012 e 4341 Wh/day (total impinging radiation)
- Day with lowest solar radiation: September 3 2012 e 560 Wh/ day (total impinging radiation) The external surface temperature difference between the wall portion not covered with vegetation and the one behind the plant coating reaches 16 C, as shown in Fig. 11. During the night, however, the bare wall tends to cool its external surface temperature much more than the covered one, also thanks to the emission of heat by radiation towards the sky. This temperature is 6 C lower than the corresponding temperature of the covered wall. This cooling problem of the Living Wall is due to the presence of the architectural cladding itself which, as a matter of facts, prevents the wall surface cooling during the night. This particular behavior was already observed in the first field measurement. The heat flux analysis, as visible in Fig. 12, shows that during days of high values of solar radiation, the incoming heat flux (positive) on the bare wall, is always greater than the corresponding on the green wall. In an overall thermal balance, discussed in detail in the following sections, this situation will result in an energy benefit for the Living Wall which, as a matter of fact, reduces the incoming energy. During the sunny days heat fluxes ranging from a maximum of 6 W/m2 and 6.2 W/m2 and a minimum of 9.7 W/m2 and 11.2 W/m2 respectively for the green wall and the bare wall. 3.3. Living Wall case study C The monitoring period of the case study C was performed between September 10 2009 and October 01 2009. Within this period, two representative days with opposite climatic conditions (maximum and minimum solar radiation) were identified as follows: - Day with strongest solar radiation: September 12 2009 e 4404 Wh/day (total impinging radiation) - Day with lowest solar radiation: September 16 2009 - 552 Wh/ day (total impinging radiation)
Fig. 7. Living Wall C e the monitored wall.
The behavior of the internal and external surface temperatures is very clear, especially if related to the solar radiation trend. During warmer days, as shown in Fig. 13, it is clear that during the daytime the bare wall see its surface temperatures rise higher compared to those of the green wall; the temperature difference reaches values up to 12 C. An opposite behavior is visible during night time for previously already highlighted reasons, where the temperature difference reaches values up to 3 C indicating, even in this case, that the bare wall tends to cool itself more than the covered wall
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Fig. 8. Living Wall C e wall section with probe positioning.
Table 2 Living Walls features summary.
Living Wall A Living Wall B Living Wall C
Substrate type
Substrate thickness [cm]
Air cavity type and thickness [cm]
Back wall type and thickness [cm]
Latitude
Orientation
Felt Felt Soil
1 cm 1 cm 5 cm
Open e 5 cm Closed e 3 cm Open e 5 cm
Concrete blocks - 40 cm Brick wall - 40 cm Concrete wall - 30 cm
45 230 N 45 260 N 43 430 N
South-west South-west East
during night. Equally evident is the trend of surface temperatures during cloudy days in which the surface temperature of the bare wall is higher than the same temperature on the green wall with differences up to 2 C. In this case a cloudy day with values of solar radiation close to other case studies was chosen, but the weather data analysis reveals that September 14 2009, the day with less solar radiation (equal to less than 300 W/m2), the surface temperature of the bare wall is lower than the corresponding on the covered wall. This situation is different from previous cases in which, even in cloudy days, the bare wall kept surface temperature values higher than the covered one. This is partly due to the lower amount of solar radiation impinging on the wall during cloudy days
in this location. During night time the bare wall cool itself of around 2 Ce3 C compared to the covered wall regardless the conditions of the previous days. No measurements of heat flux were taken for this case study, but in the following sections a value will be estimated in order to make a comparison with the other two types of Living Wall. 3.4. Comparison between the Living Walls In general, as visible in Table 3, surface temperature difference between covered and bare walls is significant and in some cases it can reach values up to 20 C. This result is very important if related
Fig. 9. Living Wall A - temperature difference between not covered (ST_B_2) and covered (ST_LW_1) wall in dependence of solar radiation.
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Fig. 10. Living Wall A e heat flux behavior.
to thermal stress of building materials. A reduction in temperature fluctuation, thanks to shading devices such as a Green Wall, leads to a lengthening of the material life. From the analysis of the three Living Walls it is possible to see that during sunny days the temperature difference between the external surfaces ranges from a minimum of 12 C for the LWC to a maximum of 20 C for the LWA.
During cloudy days, with lower values of solar radiation, temperature difference reduce to values of 1 Ce2 C. Through a heat flux analysis calculated on the inside face of the monitored walls, in correspondence of both the Living Wall and the bare wall, it was possible to perform a first energy comparison of the three monitored systems.
Fig. 11. Living Wall B - temperature difference between not covered (ST_B_2) and covered (ST_LW_3) wall in dependence of solar radiation.
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Fig. 12. Living Wall B e heat flux behavior.
Preliminary analysis were performed for case studies A and B in order to compare monitored heat flux to calculated heat flux. In case study A, solar radiation impinging on the external surface was also taken into consideration. The aim of this initial test is to validate the method to calculate the heat flux on the internal surface starting from internal/external temperature difference data. The general equation was:
q ¼ hDT
(1)
where h is, according to the case, the internal or external surface heat transfer coefficient [W/m2 K]and DT [ C] is the temperature difference between the internal or external surface and the surrounding ambient air. The surface heat transfer coefficient h is the sum of hc and hr, the radiant and convective components respectively,
Fig. 13. Living Wall C - temperature difference between not covered (ST_B_1) and covered (ST_LW_4) wall in dependence of solar radiation.
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Table 3 Living Walls comparison- temperatures.
Case study A Case study B Case study C
Total radiation on sunny day [Wh/day]
Total radiation on cloudy day [Wh/day]
Surface Dt on sunny day [ C]
Surface Dt on cloudy day [ C]
4338 4341 4404
732 560 552
20 16 12
5 1 2
Fig. 14. Comparison between monitored and calculated heat fluxes for Living Wall A and B.
calculated according to EN ISO 6946:2008 [15] as in the following relations:
For the case study A the equation (1) was modified introducing the contribution of solar radiation as follows:
hc; est ¼ 4 þ 4v
(2)
q ¼ hDT þ av SR
3 hr ¼ ε4sTm
(3)
where v is the wind velocity [m/s], ε is the surface emissivity, conventionally 0.9, s Stefan Boltzmann constant and Tm [ C] mean thermodynamic temperature of the surrounding surfaces.
(4)
where av is the solar absorption coefficient and SR [W/m2] is the solar radiation impinging on the external surface of the wall. The results of this preliminary comparison test are visible in Fig. 14 that shows the correspondence between measured and calculated heat flux for the external and internal surface of the LWA
Fig. 15. Comparison between heat fluxes of the 3 monitored Living Walls during a sunny day.
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Table 4 Living Walls comparisone heat fluxes.
Case A Case B a
Incoming heat fluxes [Wh/m2] LW
Outgoing heat fluxes [Wh/m2] LW
Incoming heat fluxes [Wh/m2] BW
Outgoing heat fluxes [Wh/m2] BW
Total balance [W/m2] LW
Total balance [W/m2] BW
e 31
87a 68
46a 54
16a 78
87 37
30 23
Hypothetical values.
and LWB respectively. The correspondence looks good especially in the case study A. The preliminary comparison described above was necessary to estimate with better accuracy the inside heat flux for LWA and LWC because of the lack of values directly collected from the monitoring campaign. Final comparison of hypothetical inside heat fluxes through the investigated walls is visible in Fig. 15 and in Table 4. As can be seen from the graph there is a typical thermal behavior, common to all three cases, in which is identifiable an outgoing (negative) heat flux from the wall during middle hours of the day during which solar radiation reaches high values. The three case studies have massive walls and this feature can explain the time lag with respect to solar radiation forcing. The incoming heat flux peak, in fact, occurs during night hours and the solar radiation is at its maximum during middle hours of the day. During the night the heat flux is positive except for case A. If a daily thermal balance is performed, as in Table 4, it is possible to note that for the case study A and B, the outgoing heat fluxes from the Living Wall are predominant compared to those from the bare wall. For the case study C, the total balance is 0, but it could be explained considering the ambient behind the wall. This room, as outlined in the introduction, was not controlled by a conditioning system as for the other two case studies, hence, heat balance results of the case study C were not reported in the summary on Table 4 because of its lack of significant information. However, as mentioned before, the thermal behavior of LWC preserves the typical heat flux trend and is consistent with LWA and LWB. The prevalence of outgoing heat fluxes is a significant advantage during the summer season because it represents a clear reduction of the cooling load supplied by the Hvac system with a direct reduction in the primary cooling energy consumptions. 4. Conclusions In this work three field microclimatic studies were performed with the aim of understanding the thermophysical behavior of a particular architectural wall cladding called Living Wall in different climatic contexts and with the aim to extend the results of previous work. In particular the Living Walls were monitored in typical Mediterranean Temperate climate context at latitudes corresponding to those of Northern Italy. Moreover the test facilities were very similar to real installations on building facades thus, in two of the three cases, with a Hvac conditioned rooms behind the monitored walls. The dependence on the solar radiation forcing came out clearly and the three vertical gardens showed a similar behavior in similar climatic conditions. During sunny days external surface temperature differences between the bare wall and the covered wall from a minimum of 12 C (case C) to a maximum of 20 C (case a) were recorded. During cloudy days the temperature differences reduce their values to 1 Ce2 C. The analysis was then extended also to heat fluxes through the monitored wall comparing heat fluxes through bare wall and heat
fluxes through covered wall. Even in this case it was possible to point out how the incoming (positive) heat fluxes are greater in correspondence to the bare wall if compared to those through the Living Wall. Moreover considering an overall thermal balance of the inside wall heat fluxes, in the two cases with the conditioned room behind the wall, the outgoing heat fluxes were higher in correspondence of the Living Wall. The LWA shows an overall outgoing heat flux of 87 W/m2 against the incoming heat flux of 30 W/m2 of the corresponding bare wall. The LWB shows an overall outgoing heat flux of 37 W/m2 against the outgoing heat flux of 23 W/m2 of the corresponding bare wall. These results lead to direct considerations on summer cooling energy consumptions indicating that the use of green architectural cladding can significantly contribute to cooling energy reduction by offering a valid alternative of retrofit also in existing buildings. Acknowledgments This paper presents results from a research project, namely GREENED, funded by the Tuscan Region (Italy), within the program PAR FAS Regione Toscana 2007e2013 - line 1.1.a.3. We gratefully thank the Sundar, Verde Profilo and Tecology companies for making available the vertical greening setup for our experimental measurements and for the collaboration and constant availability. References [1] Pérez G, Rincón L, Vila A, González JM, Cabeza LF. Behaviour of green facades in Mediterranean Continental climate. Energy Convers Manag 2011;52:1861e7. [2] Pérez G, Rincón L, Vila A, González JM, Cabeza LF. Green vertical systems for buildings as passive systems for energy savings. Appl Energy 2011;88:4854e9. [3] Köhler M. Green facadesda view back and some visions. Urban Ecosystem 2008;11:423e36. [4] Sheweka S, Magdy AN. The living walls as an approach for a healthy urban environment. Energy Proced 2011;6:592e9. [5] Givoni B. Impact of planted areas on urban environmental quality: a review. Atmos Environ Part B Urban Atmos 1991;25:289e99. [6] Onishi A, Cao X, Ito T, Shi F, Imura H. Evaluating the potential for urban heatisland mitigation by greening parking lots. Urban For Urban Green 2010;9: 323e32. [7] Rosenfeld AH, Akbari H, Romm JJ, Pomerantz M. Cool communities: strategies for heat island mitigation and smog reduction. Energy Build 1998;28:51e62. [8] Barrio EPD. Analysis of the green roofs cooling potential in buildings. Energy Build 1998;27:179e93. [9] Rubio E, Caselles V, Badenas C. Emissivity measurements of several soils and vegetation types in the 8e14 mm wave band: analysis of two field methods. Remote Sens Environ 1997;59:490e521. [10] Kontoleon KJ, Eumorfopoulou EA. The effect of the orientation and proportion of a plant-covered wall layer on the thermal performance of a building zone. Build Environ 2010;45:1287e303. [11] Eumorfopoulou EA, Kontoleon KJ. Experimental approach to the contribution of plant-covered walls to the thermal behaviour of building envelopes. Build Environ 2009;44:1024e38. [12] Wong NH, Kwang Tan AY, Chen Y, Sekar K, Tan PY, Chan D, et al. Thermal evaluation of vertical greenery systems for building walls. Build.Environ 2010;45:663e72. [13] Allen R, Pereira L, Raes D, Smith M. Crop evapotranspiration - guidelines for computing crop water requirements. FAO Irrigation and drainage paper 56. Rome: FAO - Food and Agriculture Organization of the United Nations; 1998. [14] Stec WJ, van Paassen AHC, Maziarz A. Modelling the double skin façade with plants. Energy Build 2005;37:419e27. [15] EN ISO 6946. Thermal resistance and thermal transmittance. 2008.
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Experimental investigation on the energy performance of Living Walls in a temperate climate Ugo Mazzali a, *, Fabio Peron a, Piercarlo Romagnoni a, Riccardo M. Pulselli b, Simone Bastianoni b a b
Department of Design and Planning in Complex Environments, IUAV University of Venice, Dorsoduro 2206, 30123 Venice, Italy Department of Earth, Environmental and Physical Sciences, University of Siena, Siena, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2013 Received in revised form 1 March 2013 Accepted 5 March 2013
Living Walls, a type of vertical greenery system, are relatively light structures for architectural green cladding. They embed a thick curtain of plants nurtured by an automated watering system. Three Living Wall field tests are presented for investigating potential effects of the energy behavior on building envelopes. In particular, Living Walls were monitored in a Mediterranean temperate climate context at the latitudes of Northern and Central Italy. As a result, the dependence on the solar radiation forcing came out clearly. During sunny days, difference in temperature (monitored on the external surface) between the bare wall and the covered wall ranges from a minimum of 12 C (case C) to a maximum of 20 C (case A). The analysis was extended also to heat flux. The incoming (positive) heat flux through the bare wall was found to be higher compared to the Living Wall. Considering an overall thermal balance during the monitoring period, the outgoing heat flux through the Living Wall was higher. These results indicate that the use of green architectural cladding can significantly contribute to cooling energy reduction and offer a valuable solution for retrofitting existing buildings. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Living Walls Field measurements Green cladding Sustainability
1. Introduction In recent years building green claddings have spread widely as architectural and constructive elements to design façades and roofs. Vertical green claddings are increasingly present in the international architectural panorama, namely Green Walls. Green Walls can be classified into two categories: typical Green Façades, made up of natural creepers which creeps over the building envelope or a support grid structure; Living Walls, made up of plants or grass embedded in a specific cladding structure anchored on the building façade. Differences in Green Walls may concern construction systems, type of plants, assembling and maintenance, and can affect their behavior as passive energy saving systems [1]. Many kinds of Living Walls are available on the market, and it is possible to distinguish them according to their constructive technology - such as, for example, made of aluminum frame and Pvc panel with geotextile felt or of recycled polypropylene boxes and to the type of green cover which can be grass or plants in dependence of climate and cladding orientation. A clear distinction between Green Façades and Green Walls is proposed by [2] * Corresponding author. Tel.: þ39 0412571302. E-mail address: [email protected] (U. Mazzali). 0360-1323/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.buildenv.2013.03.005
together with a microclimate analysis of the intermediate space of a double skin green façade found with lower temperatures and higher humidity compared to external ambient air. Another classification is proposed by [3] together with an historical overview and a pro et contra analysis of facade greenery. Green Façades field measurements were performed by [1] in dry Mediterranean Continental climate focusing on shading properties of four climbing species and resulting in a light transmission factor comparable to that of artificial barriers and estimated in a range from 0.15 to 0.41. Living Walls were found to have healthy effects on the human environment as well as environmental, economic, and social benefits [4] such as, for example, reduction of air temperature, improvement of the air quality by filtering airborne particles, water runoff reduction and consequent decreasing of drainage infrastructure costs, decreasing of the climatic stress on the envelope with the reduction of maintenance costs, increase of plants and green areas leading to an increase of livable spaces to play, relax and improve social relationships [5]. In this context, Green Walls are conceived as valuable measures for retrofitting buildings as well as urban public spaces. Specifically on microclimate, Green Walls may contribute in a considerable way to the mitigation of the so called Urban Heat
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Island (UHI) effect which leads to an increase of air temperature in urban areas up to 2 Ce5 C compared to peripheral and less populated areas. For example the green covering of parking urban area was estimated to mitigate the UHI effect and lead to a surface temperature reduction up to 6 Ce9 C [6]. Shading trees [7] have a potential cooling effect on the surrounding air up to 1.5 C during hottest hours of the day for a mediterranean sub-tropical climate of the Los Angeles bay (Köppen classification: Csb). A number of thermal analysis of green claddings was published. Screening from direct solar radiation by Green Walls is among the most important factors affecting the thermal performance of building envelopes. Green shading leads to an external surface temperature reduction and to a consequent reduction of the heat flux entering through the wall and the summer cooling energy consumption. A further distinguishing element from other shading systems is the evapotranspiration of plants, that is supposed to increase heat losses. The Leaf Area Index, i.e. the ratio of leaves surface to ground surface [8], the solar absorption coefficient and the leaves emissivity [9] also affect thermal performance of Green Walls. Based on these factors, a simulation model developed in Greece [10] showed a surface temperature reduction of 1 Ce2 C on the North and 16 Ce17 C on the West oriented facade with a consequent reduction of summer cooling energy consumption (in a reference room) from 4.65% (North) to a 20.08% (West). Microclimatic monitoring campaigns in Greece [11] show surface temperature reduction, compared to a bare wall, varying from 1.9 C to 8.3 C. Also in equatorial climate [12], eight different green cladding field measurements highlighted reductions in surface temperature up to 11.6 C and in air temperature up to 3.3 C at 15 cm from the external layer, depending on assembling technology, substrate, wall insulation, humidity content and Leaf Area Index. The present paper aims to provide a further description from a thermophysical point of view of Living Walls (LWs). Among different typologies we have considered the following: a) Living Wall A: a threefold felt layer fixed on PVC sheets supported by an aluminum frame. Evergreen or seasonal plants are embedded in pockets on the external felt layer. This system is anchored on the building façade leaving an open air cavity behind the cladding. b) Living Wall B: same structure of LWA with grass directly sown on an external felt layer. This has a closed air cavity behind the cladding. c) Living Wall C: turf grass sown inside little bowls filled with soil. The recycled polypropylene panel with little bowls is
supported by an aluminum structure. This system has an open air cavity behind the cladding. The three kinds of Living Wall are visible in Fig. 1. Details of the vegetation anchor system on the cladding structure - felt pockets, felt layer, and recycled polypropylene bowls - are shown. This analysis was performed by means of a microclimatic field measurement of the LWs above installed in Northern and Central Italy. Controlled microclimatic rooms were considered behind the LWs. Two of three field measurements (LWA and LWB) were performed in this condition which gave the possibility to have a more detailed analysis of the monitored variables. With respect to LWA and LWB, rooms’ microclimate behind LWC was not controlled and the indoor air temperature was subject to fluctuation depending on the external ambient air. 2. Materials and methods In this section the three LWs field tests will be described in detail, including location, probe positioning and monitoring equipment. In the present work the field measurement of the case study A (LWA) was performed in Lonigo, the case study B (LWB) in Venice and case study C (LWC) in Pisa (Fig. 2). The list of probes used in the three monitoring campaigns and the corresponding accuracy is reported below, in Table 1. The B and LW subscripts mean that measurements refer to a Bare Wall and a Living Wall, respectively. 2.1. Living Wall case study A Measurements were performed on a South-West oriented 3 m 3 m prototype installed in Lonigo (45 230 N). Fig. 3 shows the described cladding. This kind of Living Wall is made up of an aluminum structure, a PVC panel installed on it, and three felt layers with different functions. The first layer let the water flow between the Pvc panel and the felt itself, the second layer allows the roots of the plants to propagate and the third felt has a mechanical function, as a support during plant rooting, becoming a unique compact structure with the roots. The aluminum frame is fixed on a further aluminum structure directly installed on the building wall by means of plugs. At the top of the LWA a flexible pipe for the irrigation is present. This is able to provide through each nozzle 2.1 l/h of water. Water flow is in dependence of many variables such as, for example, orientation and kind of plants. The distance between each nozzle is 15 cm and the surplus of water drained off by the plants is collected
Fig. 1. Three kinds of Living Walls.
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Fig. 3. Living Wall A e the monitored wall.
The monitoring campaign was performed between June and September 2011. Probes were positioned in correspondence of the bare wall and the LWA along four monitoring sections: two sections at 0.75 m and two sections at 2.25 m height. For the two 2.25 m height monitoring sections a heat fluxmeter was added as shown in Fig. 4. Fig. 2. Location of the three monitored sites.
on a little basin at the bottom of the structure. The percentage of water not collected by felts or not used by the plants for evapotranspiration and growth is less than 5%e10%. The LWA is installed on an external not insulated wall in concrete blocks of 40 cm thickness included internal and external plaster. The most important plant species embedded in the LWA are here reported in latin names: Juniperus communis Sedum spurium, Geranium sanguineum, Geranium Johnson’s blue, Anemone sp., Viva minor, Parthenocissus tricuspidata, Heuchera micrantha Palace Purple, Salvia nemorosa, Lonicera pileata, Pittosporum tobira, Rosmarinus officinalis, Alchemilla mollis, Bergenia cordifolia, Oenothera missouriensis, Plumbago capensis.
Table 1 Probes and accuracy summary. Monitored microclimatic variable
Probe accuracy
Surface temperature External air temperature and relative humidity
0.2 C 0.18 C accuracy at 25 C for the temperature and 2.5% from 10% to 90% for the relative humidity 0.05 m/s accuracy 0.4 Ce0.45 C accuracy at 20 C for the temperature and 3% accuracy at 25 C for the relative humidity accuracy 5% accuracy 20 W/m2 directional error
Air velocity Internal air temperature and relative humidity Heat flux Solar radiation Variables are coded as follows: - Surface temperature: STB, STLW - Air temperature: AT - Air relative humidity: ARH - Air velocity: AV - Heat flux: HFB, HFLW - Solar radiation: SR.
2.2. Living Wall case study B Measurements were performed on a South-West oriented 3 m 3 m prototype installed in Venice, at the headquarter of the IUAV University of Venice (45 260 N). This cladding is made up of an aluminum structure, a PVC panel installed on it, and a vertical turf grass which has been specially made to grow on the felt support. The prototype of the described Living Wall is visible in Fig. 5. The kind of sown grass belongs to Zoysia species and is uniformly seeded on the felt layer. The monitoring campaign was performed between June and September 2012 in the Applied Physics Laboratory of University of Venice. The external not insulated wall over which the LWB is installed is made up of hollow brick with a total thickness of 40 cm including internal and external plaster. Probes for the monitoring of surface temperatures and heat fluxes and other microclimatic variables were positioned on the field in the proximity of the vertical green cladding as visible in Fig. 6. Internal and external surface temperatures and heat fluxes were recorded for both the bare wall and the LWB of the building wall. A little weather station provides values of the microclimatic variables in the proximity of the field measurement. 2.3. Living Wall case study C The monitored wall, as visible in Fig. 7, is 10.80 m long and 2.80 height and is made up of 126 recycled polypropylene of 60 cm long 40 cm height installed in 7 lines of 18 panels. The wall over which the LWC is installed is made up of not insulated concrete blocks with a total thickness of 30 cm. All the polypropylene panels are provided with many kinds of grass species among which: Zoysia matrella ‘Zeon’, Zoysia tenuifolia, Zoysia japonica ‘El Toro’, Cynodon dactylon X Cynodon trasvalensis ‘Patriot’ Stenotaphrum secondatum, Dicondra, Paspalum vaginatum, Cynodon transvalensis. About 15% of the panels was expressly without vegetation with the aim to reproduce a bare wall without cladding.
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Fig. 4. Living Wall A - bare wall and Living Wall section with probes.
The monitoring campaign started September 5th 2009 in Pisa (43 430 N). Nine surface temperature probes were positioned in correspondence to the Living Wall naturalized with Dicondra. This particular specie of grass is a herbaceous suitable to all climatic zones of Italian latitudes and although it prefers hot climates it is requested for low maintenance turf grass design thanks to its particular features such as low water and nutrition demand and the reduced growth height, not over 4 cme5 cm. Probes were positioned on the inside surface, on the outside surface, in the air cavity, and adjacent to the soil under the turf grass. Other 4 probes were positioned in correspondence of the bare wall. The whole probe positioning is visible in Fig. 8. A pyranometer was also installed in the field with the aim to monitor the solar radiation behavior on the wall. 2.4. Overview of the monitored Living Walls The most important features of the three monitored LWs are reported in Table 2. 3. Results and discussion For each LW, particular climatic analysis periods were identified with the aim to better understand the thermophysical behavior of
Fig. 5. Living Wall B - the monitored wall.
the cladding. Days with highest and lowest solar radiation impinging on the walls were considered because of the clear importance of the solar radiation as climate forcing factor which influences the green cladding behavior. In the next sections the behavior of the most important climatic variables will be analyzed during the reference days.
3.1. Living Wall case study A The monitoring campaign started on July 6th 2011 and was concluded at the end of the summer period on September 21st 2011. During this period, days with significant values of the climatic forcing variables were selected. The following days were selected: - Day with strongest solar radiation: September 20 2011 e 4338 Wh/day (total impinging radiation) - Day with lowest solar radiation: September 18 2011 e 732 Wh/ day (total impinging radiation) Being almost contiguous, these days will allow for studying the continuity of temperature trends. The radiation during representative days reported above is the total amount of global solar radiation incident on the vertical plane, throughout the day, as measured by the pyranometer positioned on the wall. As emerged from the meteorological data, solar radiation is minimal on September 18 at the beginning of a rainy day. The next day the rainfall increases significantly, however, solar radiation is still higher than the previous day. On September 20, finally the storm ends and the radiation turns to a maximum. An initial analysis shows that the difference in surface temperature at different heights is not significant and on sunny days does not exceed 1.5 C. Further analysis will be focused on the difference in surface temperatures between the covered and the bare wall. Fig. 9 shows the hourly temperature difference trend between STB,2 (external surface temperature of the bare wall) and STLW,1 (external surface temperature of the Living Wall). A positive DeltaT value indicates that the external surface temperature of the bare wall is higher than the Living Wall. The dependence on the solar radiation, reported on the right axis of the figure, is clear and in particular the temperature difference reaches values from 12 C to 20 C during sunny days and much lower values around 5 C, during cloudy days. Moreover during night-time the bare wall will tend to cool itself more than the covered wall up to around 2 Ce3 C. As shown in Fig. 10, in correspondence of the Living Wall heat flux on the external surface measured by the heat flux meter HF_LW oscillates between values of 18 W/m2 and 30 W/m2 while
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Fig. 6. Living Wall B e wall section with probe positioning.
for the bare wall heat flux oscillates between values from 90 W/ m2 and þ100 W/m2. This clear difference in terms of incoming (positive) or outgoing (negative) energy from the wall is mainly due to the large shielding effect provided by the green cladding which reduces drastically the amount of energy coming from the sun, but is also due to other factors, typical of a vegetation coating, such as the latent heat of evapotranspiration, and the absorption coefficient. The water latent heat of evaporation is around 2450 kJ/kg. This means, in the case of LWA, about 100 W/m2 of heat removed from the wall surface thanks to the evaporation of water [13]. The visible absorption coefficient affects the absorption of solar radiation in the range between 380 nm and 780 nm and coefficient for the plants can be considered near to 0.4 according to [14]. The difference between the heat fluxes outgoing from the wall behind a Living Wall compared to those outgoing from a bare wall is in the order of 70%e80%. 3.2. Living Wall case study B The monitoring period for LWB starts from July 5th 2012 and it concludes in the second half of September. The days identified for the study of the thermal behavior are as follows: - Day with strongest solar radiation: September 9 2012 e 4341 Wh/day (total impinging radiation)
- Day with lowest solar radiation: September 3 2012 e 560 Wh/ day (total impinging radiation) The external surface temperature difference between the wall portion not covered with vegetation and the one behind the plant coating reaches 16 C, as shown in Fig. 11. During the night, however, the bare wall tends to cool its external surface temperature much more than the covered one, also thanks to the emission of heat by radiation towards the sky. This temperature is 6 C lower than the corresponding temperature of the covered wall. This cooling problem of the Living Wall is due to the presence of the architectural cladding itself which, as a matter of facts, prevents the wall surface cooling during the night. This particular behavior was already observed in the first field measurement. The heat flux analysis, as visible in Fig. 12, shows that during days of high values of solar radiation, the incoming heat flux (positive) on the bare wall, is always greater than the corresponding on the green wall. In an overall thermal balance, discussed in detail in the following sections, this situation will result in an energy benefit for the Living Wall which, as a matter of fact, reduces the incoming energy. During the sunny days heat fluxes ranging from a maximum of 6 W/m2 and 6.2 W/m2 and a minimum of 9.7 W/m2 and 11.2 W/m2 respectively for the green wall and the bare wall. 3.3. Living Wall case study C The monitoring period of the case study C was performed between September 10 2009 and October 01 2009. Within this period, two representative days with opposite climatic conditions (maximum and minimum solar radiation) were identified as follows: - Day with strongest solar radiation: September 12 2009 e 4404 Wh/day (total impinging radiation) - Day with lowest solar radiation: September 16 2009 - 552 Wh/ day (total impinging radiation)
Fig. 7. Living Wall C e the monitored wall.
The behavior of the internal and external surface temperatures is very clear, especially if related to the solar radiation trend. During warmer days, as shown in Fig. 13, it is clear that during the daytime the bare wall see its surface temperatures rise higher compared to those of the green wall; the temperature difference reaches values up to 12 C. An opposite behavior is visible during night time for previously already highlighted reasons, where the temperature difference reaches values up to 3 C indicating, even in this case, that the bare wall tends to cool itself more than the covered wall
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Fig. 8. Living Wall C e wall section with probe positioning.
Table 2 Living Walls features summary.
Living Wall A Living Wall B Living Wall C
Substrate type
Substrate thickness [cm]
Air cavity type and thickness [cm]
Back wall type and thickness [cm]
Latitude
Orientation
Felt Felt Soil
1 cm 1 cm 5 cm
Open e 5 cm Closed e 3 cm Open e 5 cm
Concrete blocks - 40 cm Brick wall - 40 cm Concrete wall - 30 cm
45 230 N 45 260 N 43 430 N
South-west South-west East
during night. Equally evident is the trend of surface temperatures during cloudy days in which the surface temperature of the bare wall is higher than the same temperature on the green wall with differences up to 2 C. In this case a cloudy day with values of solar radiation close to other case studies was chosen, but the weather data analysis reveals that September 14 2009, the day with less solar radiation (equal to less than 300 W/m2), the surface temperature of the bare wall is lower than the corresponding on the covered wall. This situation is different from previous cases in which, even in cloudy days, the bare wall kept surface temperature values higher than the covered one. This is partly due to the lower amount of solar radiation impinging on the wall during cloudy days
in this location. During night time the bare wall cool itself of around 2 Ce3 C compared to the covered wall regardless the conditions of the previous days. No measurements of heat flux were taken for this case study, but in the following sections a value will be estimated in order to make a comparison with the other two types of Living Wall. 3.4. Comparison between the Living Walls In general, as visible in Table 3, surface temperature difference between covered and bare walls is significant and in some cases it can reach values up to 20 C. This result is very important if related
Fig. 9. Living Wall A - temperature difference between not covered (ST_B_2) and covered (ST_LW_1) wall in dependence of solar radiation.
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Fig. 10. Living Wall A e heat flux behavior.
to thermal stress of building materials. A reduction in temperature fluctuation, thanks to shading devices such as a Green Wall, leads to a lengthening of the material life. From the analysis of the three Living Walls it is possible to see that during sunny days the temperature difference between the external surfaces ranges from a minimum of 12 C for the LWC to a maximum of 20 C for the LWA.
During cloudy days, with lower values of solar radiation, temperature difference reduce to values of 1 Ce2 C. Through a heat flux analysis calculated on the inside face of the monitored walls, in correspondence of both the Living Wall and the bare wall, it was possible to perform a first energy comparison of the three monitored systems.
Fig. 11. Living Wall B - temperature difference between not covered (ST_B_2) and covered (ST_LW_3) wall in dependence of solar radiation.
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Fig. 12. Living Wall B e heat flux behavior.
Preliminary analysis were performed for case studies A and B in order to compare monitored heat flux to calculated heat flux. In case study A, solar radiation impinging on the external surface was also taken into consideration. The aim of this initial test is to validate the method to calculate the heat flux on the internal surface starting from internal/external temperature difference data. The general equation was:
q ¼ hDT
(1)
where h is, according to the case, the internal or external surface heat transfer coefficient [W/m2 K]and DT [ C] is the temperature difference between the internal or external surface and the surrounding ambient air. The surface heat transfer coefficient h is the sum of hc and hr, the radiant and convective components respectively,
Fig. 13. Living Wall C - temperature difference between not covered (ST_B_1) and covered (ST_LW_4) wall in dependence of solar radiation.
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Table 3 Living Walls comparison- temperatures.
Case study A Case study B Case study C
Total radiation on sunny day [Wh/day]
Total radiation on cloudy day [Wh/day]
Surface Dt on sunny day [ C]
Surface Dt on cloudy day [ C]
4338 4341 4404
732 560 552
20 16 12
5 1 2
Fig. 14. Comparison between monitored and calculated heat fluxes for Living Wall A and B.
calculated according to EN ISO 6946:2008 [15] as in the following relations:
For the case study A the equation (1) was modified introducing the contribution of solar radiation as follows:
hc; est ¼ 4 þ 4v
(2)
q ¼ hDT þ av SR
3 hr ¼ ε4sTm
(3)
where v is the wind velocity [m/s], ε is the surface emissivity, conventionally 0.9, s Stefan Boltzmann constant and Tm [ C] mean thermodynamic temperature of the surrounding surfaces.
(4)
where av is the solar absorption coefficient and SR [W/m2] is the solar radiation impinging on the external surface of the wall. The results of this preliminary comparison test are visible in Fig. 14 that shows the correspondence between measured and calculated heat flux for the external and internal surface of the LWA
Fig. 15. Comparison between heat fluxes of the 3 monitored Living Walls during a sunny day.
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Table 4 Living Walls comparisone heat fluxes.
Case A Case B a
Incoming heat fluxes [Wh/m2] LW
Outgoing heat fluxes [Wh/m2] LW
Incoming heat fluxes [Wh/m2] BW
Outgoing heat fluxes [Wh/m2] BW
Total balance [W/m2] LW
Total balance [W/m2] BW
e 31
87a 68
46a 54
16a 78
87 37
30 23
Hypothetical values.
and LWB respectively. The correspondence looks good especially in the case study A. The preliminary comparison described above was necessary to estimate with better accuracy the inside heat flux for LWA and LWC because of the lack of values directly collected from the monitoring campaign. Final comparison of hypothetical inside heat fluxes through the investigated walls is visible in Fig. 15 and in Table 4. As can be seen from the graph there is a typical thermal behavior, common to all three cases, in which is identifiable an outgoing (negative) heat flux from the wall during middle hours of the day during which solar radiation reaches high values. The three case studies have massive walls and this feature can explain the time lag with respect to solar radiation forcing. The incoming heat flux peak, in fact, occurs during night hours and the solar radiation is at its maximum during middle hours of the day. During the night the heat flux is positive except for case A. If a daily thermal balance is performed, as in Table 4, it is possible to note that for the case study A and B, the outgoing heat fluxes from the Living Wall are predominant compared to those from the bare wall. For the case study C, the total balance is 0, but it could be explained considering the ambient behind the wall. This room, as outlined in the introduction, was not controlled by a conditioning system as for the other two case studies, hence, heat balance results of the case study C were not reported in the summary on Table 4 because of its lack of significant information. However, as mentioned before, the thermal behavior of LWC preserves the typical heat flux trend and is consistent with LWA and LWB. The prevalence of outgoing heat fluxes is a significant advantage during the summer season because it represents a clear reduction of the cooling load supplied by the Hvac system with a direct reduction in the primary cooling energy consumptions. 4. Conclusions In this work three field microclimatic studies were performed with the aim of understanding the thermophysical behavior of a particular architectural wall cladding called Living Wall in different climatic contexts and with the aim to extend the results of previous work. In particular the Living Walls were monitored in typical Mediterranean Temperate climate context at latitudes corresponding to those of Northern Italy. Moreover the test facilities were very similar to real installations on building facades thus, in two of the three cases, with a Hvac conditioned rooms behind the monitored walls. The dependence on the solar radiation forcing came out clearly and the three vertical gardens showed a similar behavior in similar climatic conditions. During sunny days external surface temperature differences between the bare wall and the covered wall from a minimum of 12 C (case C) to a maximum of 20 C (case a) were recorded. During cloudy days the temperature differences reduce their values to 1 Ce2 C. The analysis was then extended also to heat fluxes through the monitored wall comparing heat fluxes through bare wall and heat
fluxes through covered wall. Even in this case it was possible to point out how the incoming (positive) heat fluxes are greater in correspondence to the bare wall if compared to those through the Living Wall. Moreover considering an overall thermal balance of the inside wall heat fluxes, in the two cases with the conditioned room behind the wall, the outgoing heat fluxes were higher in correspondence of the Living Wall. The LWA shows an overall outgoing heat flux of 87 W/m2 against the incoming heat flux of 30 W/m2 of the corresponding bare wall. The LWB shows an overall outgoing heat flux of 37 W/m2 against the outgoing heat flux of 23 W/m2 of the corresponding bare wall. These results lead to direct considerations on summer cooling energy consumptions indicating that the use of green architectural cladding can significantly contribute to cooling energy reduction by offering a valid alternative of retrofit also in existing buildings. Acknowledgments This paper presents results from a research project, namely GREENED, funded by the Tuscan Region (Italy), within the program PAR FAS Regione Toscana 2007e2013 - line 1.1.a.3. We gratefully thank the Sundar, Verde Profilo and Tecology companies for making available the vertical greening setup for our experimental measurements and for the collaboration and constant availability. References [1] Pérez G, Rincón L, Vila A, González JM, Cabeza LF. Behaviour of green facades in Mediterranean Continental climate. Energy Convers Manag 2011;52:1861e7. [2] Pérez G, Rincón L, Vila A, González JM, Cabeza LF. Green vertical systems for buildings as passive systems for energy savings. Appl Energy 2011;88:4854e9. [3] Köhler M. Green facadesda view back and some visions. Urban Ecosystem 2008;11:423e36. [4] Sheweka S, Magdy AN. The living walls as an approach for a healthy urban environment. Energy Proced 2011;6:592e9. [5] Givoni B. Impact of planted areas on urban environmental quality: a review. Atmos Environ Part B Urban Atmos 1991;25:289e99. [6] Onishi A, Cao X, Ito T, Shi F, Imura H. Evaluating the potential for urban heatisland mitigation by greening parking lots. Urban For Urban Green 2010;9: 323e32. [7] Rosenfeld AH, Akbari H, Romm JJ, Pomerantz M. Cool communities: strategies for heat island mitigation and smog reduction. Energy Build 1998;28:51e62. [8] Barrio EPD. Analysis of the green roofs cooling potential in buildings. Energy Build 1998;27:179e93. [9] Rubio E, Caselles V, Badenas C. Emissivity measurements of several soils and vegetation types in the 8e14 mm wave band: analysis of two field methods. Remote Sens Environ 1997;59:490e521. [10] Kontoleon KJ, Eumorfopoulou EA. The effect of the orientation and proportion of a plant-covered wall layer on the thermal performance of a building zone. Build Environ 2010;45:1287e303. [11] Eumorfopoulou EA, Kontoleon KJ. Experimental approach to the contribution of plant-covered walls to the thermal behaviour of building envelopes. Build Environ 2009;44:1024e38. [12] Wong NH, Kwang Tan AY, Chen Y, Sekar K, Tan PY, Chan D, et al. Thermal evaluation of vertical greenery systems for building walls. Build.Environ 2010;45:663e72. [13] Allen R, Pereira L, Raes D, Smith M. Crop evapotranspiration - guidelines for computing crop water requirements. FAO Irrigation and drainage paper 56. Rome: FAO - Food and Agriculture Organization of the United Nations; 1998. [14] Stec WJ, van Paassen AHC, Maziarz A. Modelling the double skin façade with plants. Energy Build 2005;37:419e27. [15] EN ISO 6946. Thermal resistance and thermal transmittance. 2008.