Assessing the thermal performance of living wall systems in wet and cold climates during the winter

Energy & Buildings 208 (2020) 109680

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Assessing the thermal performance of living wall systems in wet and cold climates during the winter Xinge Nan, Hai Yan, Renwu Wu, Yan Shi, Zhiyi Bao∗ School of Landscape Architecture, Zhejiang A & F University, Hangzhou 311300, China

a r t i c l e

i n f o

Article history: Received 25 June 2019 Revised 12 November 2019 Accepted 4 December 2019 Available online 5 December 2019 Keywords: Living wall system Indoor thermal environment Insulation effect Orientation Plant species

a b s t r a c t As eastern China rapidly urbanizes, living walls are spreading swiftly across cities and consequently resulting in its stronger influence on the urban thermal environment. The objective of this study is to assess the effect of external living wall systems (LWSs) on indoor thermal environments in winters with low temperatures and high humidity levels. Four containers with different outer facades were arranged in Hangzhou and a comparative analysis was conducted on changes in the internal and external thermal environments of these containers. Results reveal that both the soil-filled planter pots and plants positioned on the LWSs play a role in indoor insulation while pots have a more stable effect. Good thermal performance of living walls can improve the energy efficiency of buildings. When the north-facing walls of the containers were equipped with planter pots, a bamboo living wall or a living wall with other plants, the internal temperatures were 0.4 °C, 1.7 °C and 1 °C higher during the insulation period, respectively, relative to those of the reference container, and the all-day insulation period of the north-facing LWSs lasted more than 20 h a day. By contrast, thermal benefit of the north-and south-facing LWSs was weaker. Compared to more widely used plant species, dwarf bamboo increased insulation and wind resistance levels on the living walls, creating significantly higher temperatures on living wall surfaces and within containers in winter, indicating that plant features have a strong positive impact on the insulating features of living walls. This study not only makes up for the lack of research on the thermal performance of LWSs in the researched area during the winter, but also is of great significance to the development and application of living walls with better thermal benefits, such as orientation setting, plant selection, building energy saving, etc. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Humanity today is experiencing a dramatic shift toward urban living, especially in populous countries such as China and India [1]. While China’s urban development in recent years has improved standards of living, on the flip side of the coin rapid urbanization has also had numerous negative effects on the environment. Vast areas of natural vegetation have been transformed into concrete infrastructure, and regional climate shave changed accordingly. The aggravation of environmental concerns related to urban heat islands and high levels of building energy consumption has drawn people’s attention and has led scholars to conduct more research in this area. Numerous studies have reported that vegetation has a positive impact on urban ecosystems and on human health, and hence with an increasingly abnormal climate, the development of green infras∗

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

https://doi.org/10.1016/j.enbuild.2019.109680 0378-7788/© 2019 Elsevier B.V. All rights reserved.

tructure will play a momentous role in regulating urban climate [2–9]. In addition, many studies have shown that whether urban trees, green walls (GWs) or green roofs (GRs) all can improve the internal environment of buildings and conducive to energy saving [10]. However, due to the value of urban land and as not all areas can be greened, improvements made to harsh environments have been limited. Land shortage has prevented the expansion of urban trees. Based on the above background, vertical greening that using the building structural surfaces, are employed more and more frequently in the city. GRs and GWs are among the most important forms of such structures [11]. In general, a large portion of the roof area is often occupied by assorted types of building services and potential of GWs to improve urban microclimate and buildings’ ecological footprint is huge. Meanwhile, many people’s knowledge of GWs plants is limited to climber [11–15]. GWs are greening structures with vegetation covering a vertical surface. There are two main terms relating to GWs: green facades (GFs) and living wall systems (LWSs) [12,16]. Green facades

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as the establishment of climbing vegetation that is rooted in the ground or in planters and that is then trained to grow directly on wall surfaces or on an overlying wire or trellis framework [17,18]. LWSs have been likened more to vertical living roofs, covering a broader rooting area. Living walls differ from green facades in that they support vegetation that is either rooted on the walls or in a substrate attached with the wall itself [17,18]. Most living wall systems are modular and consist of an irrigation system and an encased growing medium [17]. Each planter pots contains its own soil or other growth media based on hydroponic cultures (soil, felt, perlite, etc.), while using balanced nutrient solutions to provide all or part of the plant’s nutrient and water needs [18]. Regardless of the type of GWs considered, all have great potential to offer thermal environmental benefits. Green facades have been used in buildings more frequently in the recent past and more research on the thermal behavior of GWs has focused on green facades than on LWSs [19]. It is noticeable, however, that living walls have many potential advantages, especially enrich the plant species of GWs or add greenery to high-rise buildings, and more LWSs are likely appear in urban vertical green spaces in the foreseeable future [20]. Therefore, research on the regulation of thermal environments by LWSs is of great value. 1.1. Thermal environmental effects of LWSs In some hot and humid countries and regions, research on the thermal benefits of LWSs have been booming in recent decades. In Singapore, where wall greening is highly developed, eight different vertical greenery systems (7 LWSs, 1 GF) were demonstrated that they all benefit to reduce the surface temperatures of buildings facades in a tropical climate summer [21]. They have different cooling effects but all will lead to a corresponding reduction in the cooling load and energy costs. A similar cooling and humidifying effect has been manifested in Wuhan, China [22]. Satisfactory results like these have been mentioned all the time [23] and considerable cooling potential has led to a greater focus on summer patterns, especially in subtropical [15,22,24,25], tropical [21,26–28], desert [29], oceanic [30,31], and Mediterranean climates [30,32– 36]. In contrast, only a few studies have been conducted in the winter or were only mentioned in studies of the year-round thermal performance of living wall. Under an oceanic climate, for instance, researchers found the energy balance of green wall facades to be 20% higher than that of a reference facade in the winter, demonstrating the insulating effect of green walls in such a climate [31]. This thermal benefit is related to the orientation of the LWS and has been studied in some regions under Mediterranean climate. Living walls are effective at lowering thermal losses when they are north-, east- or west-facing, thus we can improve the energy efficiency of buildings mainly by insulating these orientations [37]. And reductions of temperature on south-facing walls were found to be much more significant than those of east- and west-facing walls as a result of plants blocking solar energy [35]. Most of these studies focus on the western region of continental Europe while related thermal effects of LWS in the winter in China [11,15] have been explored less. Moreover, scholars have been more likely to regard LWSs as single facades as a major variable. Fewer comparative studies have been conducted with the number of facades used as a variable. 1.2. Environmental factors and self-factors affecting LWS thermal environment regulation In analyzing the thermal benefits of LWSs, including shade, cooling, insulation, and wind barrier effects, Pérez et al. [38] found that the main influencing factors include constructive systems, the

climate and plant species. These influencing factors are mainly classified into environmental factors and self-factors, and their effects on the thermal performance of LWSs are multifaceted. Environmental factors refer to solar radiation, wind, rainfall, etc. A study on the thermal performance of climber in summer showed that strong levels of irradiance drive high surface temperatures, but also support considerable vegetation surfaces and adjacent ambient air cooling by transpiration [14]. Effective green wall cooling must maintain solar intensity to a threshold of 300 W m−2 and summer weather conditions must be sunny or rainy with sunny episodes [14]. These conclusions about GFs are also valuable for LWSs. During cold winters, solar radiation is extremely important as it affects indoor and outdoor temperature and humidity levels and human thermal comfort. The thermal effect of an LWS is also affected by its unique features (e.g., modular systems, plants, air layers, etc.). Green modular systems on walls can reduce heat losses and form a warmer insulating air layer between the wall and prototype, thus reducing for both species the amount of energy crossing the wall by 37– 44% [36]. For an LWS, the impact of matrix-filled planter pots on wall insulation remains to be explored. Evergreen species will create an external layer of insulation and contribute to energy savings and the loss of heat in colder climates [39]. In Greece, researchers employed a thermal network model to simulate the capacity for green walls to improve thermal environments in the summer. As the percentage of plant foliage cover increases, the resulting positive effect is enhanced [40]. The air layer between LWS and wall is also an important factor and its smaller distance has better cooling effect during summer [22]. Through sorting out the relevant literatures on the thermal effect of green walls, it can be found that most of the researches were in the fields of architecture and engineering [41]. Consequently, an evaluation of the thermal performance and applicability of LWSs from perspectives of plant selection and allocation is necessary.

1.3. Study objectives As underlined by previous studies, the thermal performances of LWS are excellent and conducive to building energy saving. However, most of the current research on the thermal performance of living walls focused on addressing high temperatures in the summer while studies on winter conditions remain scarce. And they were more of a single orientation measurement comparison. Furthermore, for the role of modular system, substrate and vegetation in building insulation, a lot of research was based on calculation of heat flux. They need to be measured separately to demonstrate their thermal benefits. Moreover, in terms of experimental design, most studies tend to be a comparative study of with or without wall greening and lack of variation in plant species. Therefore, in this paper, we have made experimental measurements to address the deficiencies abovementioned. Based on the results, an LWS layout might have better thermal benefits in winter was proposed. China is currently experiencing rapid urbanization with highly concentrated populations and a rapid increase in building density and area. Under these conditions, the combined use of modular and plant bag LWSs will serve as very suitable means of urban vertical greening. Such features are safe and easy to maintain, plants can be easily replaced, and resulting potential ecological and thermal environment effects are considerable, especially for large populations and high levels of building density. Therefore, this study takes Hangzhou, a high-density city in China, as a case to investigate the specific adjustment capacities of external LWS for indoor thermal environment of containers under low temperature and humid winter conditions. The study examines the following:

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(1) Whether the main factors affecting indoor temperatures in the winter are the substrate or plant, or both and which is the dominant factor. (2) The influence of the quantity and orientation of LWSs (including modular systems and plants) on the capacity for theirs to regulate the thermal environment within a container. (3) Thermal performance of LWSs composed of different plants and the causes of the differences.

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Table 1 Container configurations used for each experiment. Experiment

Date

Experimental group settings

a

Jan 19– 23

b

Jan 24– 28

C1-a: C2-a: C3-a: C4-a: C1-b: C2-b: C3-b: C4-b:

2. Methodology

Bare wall 1040 soil-filled pots (N&S) 1040 pots of bamboo with 3 varieties (N&S) 1040 pots of plants with 10 varieties (N&S) Bare wall 520 soil-filled pots (N) 520 pots of bamboo with 3 varieties (N) 520 pots of plants with 10 varieties (N)

2.1. Study area Located in eastern China, Hangzhou is the capital of Zhejiang Province and a central city of the Yangtze River Delta. China’s building climate demarcation classifies Hangzhou as hot in the summer and cold in the winter. According to the Koppen-Geiger climate classification, the climate in Hangzhou is of the Cfa category, meaning that it occupies a warm temperate zone characterized by hot summers and year-round humidity [42]. In sum, the low temperatures and high humidity levels are typical of the winter season in Hangzhou. The experimental site is situated in Lin’an District, Hangzhou at 30°19 N latitude and 119°82 E longitude. The site is flat and surrounded with farmland, which is far positioned away from the city’s main roads. As there are no industrial facilities around the site, external disturbances are minor (Fig. 1). We obtained meteorological data for Lin’an for nearly 10 years from the China Meteorological Administration, which shows that 2016 was a typical year in terms of changes in air temperature and humidity. As is shown in Fig. 2, the winter in Lin’an spans from December to February when overall temperatures were at the lowest level of the year. The average daily outdoor air temperature was 7 °C and the mean relative humidity was roughly 80%. These conditions continued until the end of March. January was the coldest month of the winter, when the average daily temperature was only 3 °C and with the largest range of fluctuation (the maximum temperature was 19.6 °C and the minimum temperature was −14.2 °C). Under the influence of low temperature and rainfall levels, the average air relative humidity level reached as high as approximately

89%. Continuous low temperature weather occurred in late January, which proved to be the most valuable period for research on the insulating effects of LWSs. 2.2. Experimental design Four containers were set up and successively numbered for the experiment while controlling for multiple variates to explore the specific impact of LWSs on the internal temperature and humidity levels of each container (Table 1). The outer walls of Container No. 1 (C1) were bare, whereas the north- and south-facing walls of Container No. 2 (C2) were fully covered with suspended planter pots filled with soil matrix. Container No. 3 (C3) was equipped with the same planter pots as those used for C2 on its north-and south-facing walls with all pots planted with bamboo. C3 differed from Container No. 4 (C4) by the plant species grown in the pots. We transplanted 10 varieties of living wall vegetation commonly used in the experimental area to the C4 LWS surfaces. The containers were placed in areas of the experimental site where they would not affect sunlight or wind conditions, and equal amounts of water were then sprayed onto the north- and south-facing walls (Fig. 3). The doors and windows of the four containers were closed to monitor indoor thermal environments. Three days later on January 19, 2019, experimental data were continuously collected as part of Experiment a (north- and southfacing modular systems or LWSs). After data collection was completed on January 23, planter pots on the south-facing walls of C2,

Fig. 1. Location of the study area.

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Fig. 2. Daily variation of temperature and relative humidity in Lin’an in 2016.

Fig. 3. Layout of the experimental site.

C3, and C4 were removed. We then started Experiment b (northfacing modular systems or LWSs), which recorded the same data as those collected for Experiment a from January 24 to January 28. The capacities for the different living wall systems to regulate internal thermal environments was analyzed by comparing all monitoring data taken from the 4 containers and from outdoor areas throughout the 2 experiments. 2.3. Preparation of experimental materials The four containers were of the same size (3.2 m × 3.2 m × 7.2 m) and orientation. The wall structures were also identical, composed of a sandwich structure with 0.5 mm-thick steel on both sides and with nearly 50 mm-thick foam in the center. Ferrous metal frames were welded onto the north-and south-facing outer walls and each could hang 520 pots (40columns × 13rows). For the pots we used vertical greening universal soil matrix uniformly purchased on the market. The matrix included peat: coconut husks: yellow mud and volcanic stone with a ratio of 3:3:3:1 and with a total porosity level of more than 60%. Meanwhile, the matrix pH ranged from 5 to 8, conductivity levels ranged from 0.35 to 1.5 mS/cm, and relative water content levels were measured at less than 35%. Further information on the LWS and containers is given in Fig. 4.

Living wall plants with ideal landscapes for the winter season were planted onto C3 and C4 (Fig. 5). The LWS of the C3 walls included three types of bamboo suited for wall greening: Shibataea kumasasa, Indocalamus tessellatus and Sasa pygmaea. Species used the C4 LWS were determined based on the proportion of plant growth forms used for living walls in eastern China. As is shown in Fig. 5, we selected three woody plants (Lonicera nitida ‘Maigrun’, Nandina domestica ‘Firepower’ and Aucuba japonica var. variegata), four herbs (Acorus gramineus var. Pusillus, Euryops pectinatus, Heuchera micrantha ‘Red Sun’ and Sedum sarmentosum), two climbers (Hedera nepalensis var. Sinensis and Muehlenbeckia complexa) and one variety of bamboo (Shibataea kumasasa). 2.4. Microclimate parameter measurements Eight humidity temperature meters (TES1365) were placed at a height of 1.5 m in the center of the four containers to continually monitor internal thermal environment variations. After a calibration test running from January 15th to January 16th, 2019 was applied, the temperature error was measured at less than 0.1 °C and the humidity error was measured at less than 1%, showing improved experimental accuracy. Each room included 2 instruments allocated to avoid deviation and recording intervals were set to

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Fig. 4. Details of the living wall system and container (Notes: The actual metal frame is black while the drawing is marked red for easy identification.).

5 min. Three weather stations (Watchdog-2900ET) were installed at corresponding locations (Fig. 3) outside of the containers to continuously measure meteorological parameters. The recording interval was to 5 min and the results were averaged. During the experiments, the surfaces of the containers were photographed every 2 h with a thermal imaging camera (ST9450) and temperatures were recorded. The characteristics of abovementioned instruments are listed in Table 2.

3. Results and analysis 3.1. Air temperatures 3.1.1. Experiment a A continuous five-day internal and external thermal environment test was performed from January 19, 2019, to January 23, 2019. During this period, the last four days were sunny, and

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Fig. 5. Plants for experiment (a) Shibataea kumasasa, (b) Indocalamus tessellatus, (c) Sasa pygmaea, (d) Hedera nepalensis var. sinensis, (e) Muehlenbeckia complexa, (f) Acorus gramineus var. pusillus, (g) Lonicera nitida ’Maigrun’, (h) Nandina domestica ’Firepower’, (i) Aucuba japonica var. variegata, (j) Euryops pectinatus, (k) Heuchera micrantha ’Red Sun’, (l) Sedum sarmentosum, (m) Container No.3, (n) Container No.4.

diurnal variations of solar radiation intensity were obvious. We studied the period running from 6:00 on January 21 to 6:00 on January 22 as a typical day for analysis. Hourly environmental parameter changes presented in Fig. 6 show that air relative humidity levels ranged between 37% and 98.9%. The lowest temperature of −2.6 °C appeared at 6:00 on January 21, and the highest tem-

perature of 11 °C appeared at 16:00 on January 21. In addition, soil moisture levels were maintained at below 17.1% and the soil temperature was always higher than the air temperature. According to the illustration (Fig. 7), internal trend changes in temperature for the 4 containers were basically the same as the outdoor air temperatures throughout the day. From the morning

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Table 2 Characteristics of the instruments. Parameters

Instrument

Measuring range

Accuracy

Indoor air temperature ( °C) Indoor relative humidity (%) Outdoor air temperature ( °C) Outdoor relative humidity (%) Soil temperature ( °C) Soil moisture (%) Solar radiation intensity (W/m2 ) Wind speed (km/h) Wind direction (°) Surface temperature ( °C)

TES1365 TES1365 Watchdog-2900ET Watchdog-2900ET Watchdog-2900ET Watchdog-2900ET Watchdog-2900ET Watchdog-2900ET Watchdog-2900ET ST9450

−20 to 60 °C 10–95% −32 to 100 °C 10–100% −30 to 100 °C 0~saturation 0–1500 W/m2 0.3–241 km/h 0–360° −25 to 450 °C

±0.5 °C ±3–5% ±0.6 °C ±3% ±0.6 °C ±3% ±5% ±5% ±4° ±2 °C/±2%

Fig. 6. Environmental status of Experiment a.

Fig. 7. Change of temperature inside and outside containers from 6:00 on Jan 21 to 6:00 on Jan 22.

to midday, as air temperatures rose, relative relationship of internal temperatures were successively measured as C1>C2>C3=C4 where the former two changed more significantly. C1 internal temperature reached a maximum temperature of 15.4 °C at 13:25, roughly 1 h after the timing of peak solar radiation intensity at measured 12:30 (Fig. 6), and maximum internal temperatures for C2, C3, and C4 were postponed to 14:20, 16:10, and 16:15, respectively. The walls, metal frames and LWSs of the containers all will affect the internal temperatures rise, especially in the south orientation of containers. Such as thermal conductivity, shielding area and other differences are closely related to the rate of temperature rise. The internal temperature of C1 and the intensity of solar radiation show similar curves, especially from

13:20 to 16:20 in the afternoon, indicating that indoor temperature is mainly affected by the intensity of solar radiation without any shielding. Meanwhile, the thermal conduction between ferrous metal frames with extremely high heat absorption rate and container surfaces constituted important sources of internal heat, further accelerating heating for C1. The thermal conduction effects of C2, C3, and C4 were almost negligible, as the metal frames of these structures were blocked by pots and plants with smaller thermal conductivity. Despite there being no direct contact between the modular systems and wall surfaces, an air layer formed. The main heat sources for containers with the modular systems were identified as thermal convection between the air layers and thermal radiation from the plants, pots and ground to the walls, which greatly extended the length of time required for internal temperatures peak. At the same time, plants of the LWS and their shadows blocked heat absorption on the container walls during the day, resulting in slow heating. However, longwave radiation and heat retention contained heat within the containers as solar radiation declined. Thus, internal temperature changes observed within the 4 containers during the study period differed considerably as follows: The internal temperature of C1 decreased sharply while that of C2 presented as slower upward trend and a more limited range of fluctuations. The internal temperatures of C3 and C4 continued to rise while the highest point was significantly lagged. The outdoor area began to cool at nightfall, spurring drops in temperature within the containers. Scales of temperature variation were similar and the gradient was always maintained at C3>C4>C2>C1. We thus find that LWS plants and soil-filled pots under low temperature conditions preserve heat at night, as LWSs enhance the thermal resistance of container walls. Taken together, the insulation period for C2 lasted from 18:10 to 8:40, during

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Fig. 8. Change of relative humidity inside and outside containers from 6:00 on Jan 21 to 6:00 on Jan 22.

Fig. 9. Environmental status of Experiment b.

which the internal temperature was 0.4 °C higher than that of C1 on average. Insulation periods for C3 and C4 ran from 16:30 to roughly 8:40 (C4 ends at 8:25), and internal temperatures were respectively 1 °C and 0.6 °C higher than those of C1 on average. As shown by Fig. 7, temperature variations range for C2, C3 and C4 were smaller than C1. Relative to outdoor conditions, air relative humidity levels with the 4 containers were stable and relatively similar. Overall trends observed are the inverse of temperature curves (Fig. 8). Affected by temperatures, minimum relative humidity levels for C3 and C4 were also delayed. Similarly, relative humidity change ranges for C2, C3, and C4 grew less pronounced, and more stable conditions than those of the bare wall container were observed. We can thus conclude that an LWS can alleviate discomfort caused by wet and cold environmental conditions over a period of time. 3.1.2. Experiment b From January 24, 2019 to January 28, 2019, similar to Experiment a, Experiment b involved a container thermal environment test conducted on a clear day. We studied the period running from 6:00 on January 26 to 6:00 on January 27 as a typical day for analysis. Fig. 9 shows the hourly environmental changes of this typical day. Relative air humidity levels ranged from 39.8% to 96.2%. The minimum temperature of −5.3 °C appeared at 6:00 on January 27,

and the maximum temperature of 9.4 °C appeared at 15:00 on January 26. Soil moisture levels did not exceed 1.2% and soil temperatures were always higher than air temperatures. For Experiment b, the south-facing walls of the four containers were unobstructed, rendering the main heat source within the containers identical. Therefore, rates of temperature increase were more similar after dawn. Differences emerged from thermal radiation levels transferred from the ground and atmosphere into the northern walls of the containers. As a result, C1 and C2, which had fewer occlusion area during the warming phase, presented faster internal temperature upward trends while that of the C3 and C4 rose slightly slow (Fig. 10). From 8:25 to 10:55, the internal temperature for C1 was basically equal to that of C2 and higher than those of C3 and C4. Average temperature differences were respectively measured as 1.4 °C and 2 °C. Clouds blocked the sun at 10:55, and solar radiation levels as a result showed instantaneous troughs and peaks. Levels returned to equilibrium after 11:30. Due to heat transfer delays created by the walls, effects of solar radiation variations on internal temperatures of the containers were observed after half an hour. At 11:25, the internal temperatures of C1 and C2 started to fall but those of C2 declined slower due to the sheltering and thermal convection of north-facing wall pots. By 13:00, the internal temperatures of the two containers had risen again. Plants on north-facing walls of C3

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Fig. 10. Change of temperature inside and outside containers from 6:00 on Jan 26 to 6:00 on Jan 27.

Fig. 11. Change of relative humidity inside and outside containers from 6:00 on Jan 26 to 6:00 on Jan 27.

and C4 still transferred heat to their interior areas with higher levels of overall thermal resistance, causing internal temperatures to continue to rise. Solar radiation then continued to decline, causing the internal temperatures of the four containers to decline. However, C3>C4>C2>C1 conditions were maintained until sunrise on the following day. Eventually, the internal temperature curves of the four containers tended to be same. Over the course of the day, the period of C2 insulation ran from 10:15 to 9:10, excluding fluctuations lasting for 30 min after 15:00. The internal temperature of C2 was 0.4 °C higher than that of C1 on average during the insulation period. Insulation periods for C3 and C4 ran from 11:25 to roughly 8:25 (C4 ends at 8:10), and internal temperatures were 1.7 °C and 1 °C higher than those of C1, respectively. The daily internal temperature change curves for C2, C3 and C4 are shifted upwards relative to that of C1 as shown in Fig. 10. During the daytime, overall trends of internal relative humidity for the four containers are the opposite of their temperature changes (Fig. 11). As observed from Experiment a, due to the insulating effects of modular systems and plants, the timing of minimum humidity was delayed. In general, the relative humidity levels of C1 and C2 were similar while those of C3 and C4 were similar, and internal relative humidity levels of C3 and C4 were 4.4% higher than those of C1 and C2 on average. Daily variation curves for the internal relative humidity of containers in which the planter pots and plants were suspended do not

show a reduction in the fluctuation range or a downward shift in the curve. 3.2. Surface temperatures 3.2.1. Experiment a We can also illustrate the patterns of thermal performance in experiment a from a thermal distribution map of north- and southfacing walls of the 4 containers created using a thermal imaging camera (Fig. 12, Fig. 13). Wall temperatures of the four containers followed the same trends as internal temperatures (Fig. 7). In the solar radiation enhancement phase (Fig. 6), C1 wall temperatures were always higher than those of C2, C3 and C4. As solar radiation levels decreased, C1 south-facing wall temperatures dropped by more than twice the levels observed for the other three southfacing walls from 15:00 to 17:00. Since the north-facing walls were not exposed to direct sunlight, their surface temperature changes were minor and similar. Differences were observed from 13:00 to 15:00. As temperatures of the north-facing walls of C1 and C2 began to decrease, temperatures of the north-facing walls of C3 and C4 continued to rise. This occurred due to the enhanced thermal resistance of LWSs, which effectively limited heat losses from air layers. From the images, high temperature areas of C1 north- and south-facing walls can be identified as areas of the metal frame while C2 stored more heat in soil of the modular system. Pots on the south-facing walls of C3 and C4 show higher temperatures

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Fig. 12. Thermal imaging of north- and south-facing walls of C1 and C2 in Experiment a.

while north-facing walls show different patterns. During the day, high temperatures on the north-facing wall of C3 were distributed across plant surfaces and were then transferred to the modular system at night. The north-facing wall of C4 did not present clearly distinguishable high and low temperature areas during the day, and at night the same characteristics as those of C3 were observed. We speculate that higher levels of illumination would encourage plant transpiration on a south-facing wall, resulting in faster rates of surface temperature decline while cooling effects of

north-facing wall plant transpiration should be weaker than corresponding heat preservation effects. Hence, C3 north- and southfacing walls presented different high-temperature zones in the day and night. Walls temperature change trend for C4 was observed to be similar to that of C3. However, the taller and denser bamboo plants installed on C3 generated higher levels of thermal insulation than those observed for C4. Hence, the north-facing wall of C4 showed a more uniform distribution within its high temperature region.

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Fig. 13. Thermal imaging of north- and south-facing walls of C3 and C4 in Experiment a.

3.2.2. Experiment b From 11:00 to 13:00, surface temperature declines of C3 and C4 north-facing walls exceeded those of C1 and C2 by more than 1 °C also can explain the temperature change inside the 4 containers after 11:25 (Figs. 14 and 15). According to thermal distribution maps of the north- and south-facing walls of the 4 containers, high-temperature areas of south-facing were all concentrated at metal frames while those of north-facing walls showed signifi-

cantly different patterns. For north-facing walls, high-temperature areas of C1 were concentrated to the metal frame and those of C2 were concentrated to pot soil. Those of C3 and C4 were the same as those observed in Experiment a at noon while at other times they were located within the pots. Thus, heat absorbed by the plants was maintained within the plants and air layers and was then transferred to internal areas of the containers. A decline in solar radiation intensity for C3 and C4

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Fig. 14. Thermal imaging of north- and south-facing walls of C1 and C2 in Experiment b.

only limited growth in their internal temperatures. The superior thermal insulation properties of bamboo relative to those of C4 LWS plants caused C3 internal temperatures to increase faster. At 13:00, as solar radiation levels began to fall, the internal temperatures of the four containers continued to rise until 15:15 due to the delay of thermal conduction (Fig. 10). In this period of

time, when the temperatures of north-facing walls of C3 and C4 increased again, the temperatures of the other six walls decreased. This occurred because C3 and C4 had higher internal temperatures, and heat storage in the north-facing LWS and air layers will also reduce the temperature drop. Afterwards the surface temperatures of the north and south walls of the 4 containers basically showed downward trends, but the north walls of C2, C3 and C4 decreased

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Fig. 15. Thermal imaging of north- and south-facing walls of C3 and C4 in Experiment b.

less and had better thermal behavior at night than the other five walls. 3.3. Computation and comparison of accumulated temperature Accumulated temperature differences observed between the interior and outdoor of the four containers in Experiments a and b were compared to explore the influence of LWS quantity and

orientation on container thermal benefit in the winter (Fig. 16). Areas enclosed by temperature difference curves during the day, night and all day are calculated separately, that is, the accumulated temperature of each time period, then compared and analyzed. Whether there is solar radiation as a standard for distinguishing between day and night. Table 3 shows that the thermal benefit of the north-facing LWS is superior to that of the north- and southfacing LWS in the winter, especially during the day. This is mainly

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Fig. 16. The charts of accumulated temperature difference between inside and outside the containers. Table 3 Accumulated temperature and difference of containers. Experiment a

Experiment b

Difference value (b-a)

Number

Day

Night

All day

Number

Day

Night

All day

Day

Night

All day

C1-a C2-a C3-a C4-a

449.7 254.2 222.7 195.8

316.5 376.9 476.7 412.1

766.2 631.1 699.4 607.9

C1-b C2-b C3-b C4-b

474.7 496.2 704.9 581.2

299.1 344.5 437.7 372.8

773.8 840.7 1142.6 954

25 242 482.2 385.4

−17.4 −32.4 −39 −39.3

7.6 209.6 443.2 346.1

due to the difference in heat inflow from the south walls. Although thermal insulation levels observed in Experiment b at night are not as strong as those observed from Experiment a, the accumulated temperatures do not differ considerably. The same approach to accumulated temperature area calculation was used to quantify and further illustrate the thermal benefit of LWSs in the two experiments. Accumulated temperature values of the other three containers were divided by that of C1 to demonstrate their thermal benefit differences intuitive by percentage. For Experiment a, when an LWS was installed on the exterior container wall, regardless of whether plants were applied or not, the thermal benefit was not as strong as that of the bare wall container during the day and differences in values nearly doubled. On the contrary, thermal insulation effect of LWSs can be more clearly reflected at night with low temperature and no sunlight. The heat inside C1 was continuously escaping outward and its accumulated temperature decreases compared with the daytime, while the other three containers showed upward trends. During the whole typical day in Experiment a, accumulated temperature values for C2, C3, and C4 were measured as 82%, 91%, and 79% of C1, respectively. In Experiment b, C2, C3, and C4 had better thermal benefits than the bare wall container both during the day and night, reaching levels 9%, 48%, and 23% higher than those of C1 throughout the day, respectively.

We also compare accumulated temperatures for C2, C3 and C4 measured from Experiments a and b. Periods in which solar radiation intensity levels exceeded a value of zero are analyzed first. When south-facing walls were obscured, internal accumulated temperature of the soil-filled modular system container was slightly higher than those of the LWS containers, as C2 included less blocked area. In the meantime, C3 and C4 LWS south-facing plants showed enhanced levels of transpiration under strong light conditions that led to corresponding cooling and humidification effects. And when south-facing walls were unobstructed, C3 and C4 preserved more heat than C2 during the daytime. Once night fell, thermal benefits of C3 and C4 were superior than C2 in both experiments. 4. Discussion 4.1. Differences in thermal performance of different LWS components in the winter From the above experimental results, we confirm that soil-filled pots and plants of the LWS both played an important role in indoor insulation during the winter. It also indirectly proves that they act as passive tools for energy savings in the winter. The LWSs not only enhanced wall insulation but also extended the thermal delay

X. Nan, H. Yan and R. Wu et al. / Energy & Buildings 208 (2020) 109680

between exterior and interior areas. As a result, the time at which internal temperature reached extreme values was delayed consistent with results generated for the Geogreen living wall system [34]. There are many influencing factors for the thermal performance of LWSs including thermal resistance, shadow, wind barrier, evaporation and transpiration [35], which have different effects on the modular system and plants. The container with a wall covered with soil-filled pots maintained the most stable internal temperature curve throughout the day in the two experiments, and internal temperatures increased by 0.4 °C during the insulation period relative to the bare wall container. The shape, thermal resistance, thermal absorption and other relatively constant characteristics of modular system determine the stability of its thermal insulation capacity. Although its blocking of solar radiation will reduce heat entering the container and delay the time when the internal temperature reaches the peak, it also blocks the leakage of internal heat and acts as a wind barrier, and soil releases heat into walls when external temperatures decrease. Thermal benefits of modular system could be further improved by a higher integration within the building envelope [43], for the frames and walls that still have room to narrow. As was observed from the modular system, plants offer insulation value in cold winter conditions. Compared to those of the container with the soil-filled modular system, the average internal temperatures of LWS containers were 0.2--1.3 °C higher under the same conditions in the 2 experiments. However, plant species exhibit varying levels of insulation capacity [36] through different abilities in thermal resistance, shadow and wind barrier in contrast to soil-filled modular system. The cooling effect of transpiration will reduce its thermal benefits to some degree, but overall thermal performance of vegetation is still excellent. Due to the high air humidity level in the study area, whether it is substrate, pots or plants surface, the cooling effect generated by evaporation on LWS may be unavoidable, especially in the south walls that experience strong solar radiation during the day. The moisture in the substrate depend on the intensity of irrigation, and an increase in moisture will also enhance soil thermal conductivity [44], thereby increasing heat transfer to the container. In this study, irrigation was conducted only three days before the beginning of Experiment a, so the thermal benefit of Experiment b may be slightly lower than the actual value though this error can be ignored due to continuous sunny days. Meanwhile, LWSs reduce relative humidity levels by increasing indoor temperatures, providing heat for the human body. Consequently, in Hangzhou, an area characterized by high humidity levels and low temperatures in the winter, LWS layouts will affect the internal thermal environments of containers. For the same confined space, an LWS can preserve heat and reduce humidity levels for much of the year and can enhance indoor thermal comfort for human populations in the winter. 4.2. Effects of the orientation and quantity of walls on LWS thermal performance in the winter Although living walls improve interior thermal performance in the winter, this is influenced by the orientation and quantity of LWSs present, which are crucial factors that must be considered when developing urban greening applications. In Experiment a, plants and pots installed on north- and south-facing walls reduced internal temperature ranges within the containers and rendered them more stable, i.e., fluctuations declined, verifying previous research [21,45]. However, indoor temperatures of the LWS containers were higher than those of the bare wall container only 15 h per day on average. The average daily accumulated temperatures of the three containers with modular systems and plants on walls were only 84% of those of the bare wall containers, and average

15

insulation values were 1 °C lower than those of the bare wall container. LWS reductions of internal relative humidity were also limited to the nighttime due to increased heat levels. Therefore, from benefits observed all day when modular systems and plants are installed on both north- and south-facing walls, improvements to indoor thermal environments were not considerable. When modular systems and plants were installed on a northfacing wall alone, internal container thermal environments were better regulated, and more heat was preserved. In Experiment b, relative to the bare wall container, overall curves of internal temperature changes for containers of the modular systems and plants on north-facing walls increased toward the Y-axis, and corresponding temperatures were slightly lower than those of the bare wall container only 2–3 h each day. This is an ideal situation. In comparing the results of Experiment a and b by accumulated temperature, we can find that the north- and south-facing LWS can store more heat than north-facing LWS, and warm up containers interior quite effectively at night when the air temperature drops. However, in terms of all-day thermal benefits, the north-facing LWS is better. According to the results of two experiments, southfacing LWS will isolate external heat radiation and limit heat flux [37,43,46], while intense solar radiation also increases evapotranspiration of plants and soil and lowers ambient temperatures. In addition, LWS requires regular irrigation in practical applications, which has further cooling effect though the soil thermal conductivity increased. By contrast, north-facing LWS is not only weak in heat gain and plant activity due to weak solar radiation, but it is also extremely effective against wind. Based on above comparative analysis, we believe it is necessary for an LWS to adjust the thermal environment better without shading south-facing walls. Furthermore, we envisage a new north- and south-facing LWS layout that might have better thermal benefits. North orientation is still an LWS. Some specific perennial plants (the aboveground parts died in the winter such as some chrysanthemum) are planted on the south-facing pots and can be regarded as a modular system. It has higher thermal absorption and less shadow area on the south wall than regular LWS, and its life activities are weaker. These perennial plants may have corresponding cooling effect when they grow in the warm season. It also reduces the cost of plant replacement. Such a layout may lower in insulation time or value compared to north-facing LWS, but its overall thermal benefits may increase, which is worth exploring later. 4.3. Effects of different plants on the thermal performance of LWSs in the winter In our 2 experiments, temperature and humidity levels measured within the containers of different plant walls were very similar. They facilitated heat preservation and dehumidification at the same time and applied a barrier effect. However, they conferred different insulation and humidity reduction effects. According to the internal thermal performance results of C3 and C4, heat preservation and dehumidification effects of the bamboo LWS always exceeded those of the LWS with other plant species. The principle of heat preservation of living walls is covering plant foliage onto a wall to reduce the surface in direct contact with outside air, enhancing the heat dissipation resistance of the container. We here analyze differences between the 2 studied plant LWSs to explore factors that affect their improvement of indoor thermal environments. The systems included the same number of plants exhibiting strong growth and covering 100% of the walls. In addition, species planted included both large and small leaf area indexes, limiting overall differences. C3 plants were yellow and green in color with moderate levels of sun absorption while C4 plants exhibited both high and low absorption rates. For example, Hedera nepalensis var. sinensis, Nandina domestica ’Firepower’ and

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Heuchera micrantha ’Red Sun’ are red, and absorb less heat from sunlight. Muehlenbeckia complexa, Lonicera nitida ’Maigrun’, Euryops pectinatus and Sedum sarmentosum in contrast are dark green in color and absorb more heat from the sun. Corresponding rates of sunlight heat absorption are the same overall. The outdoor area was low in temperature during the experiment and plant transpiration levels were also low. Hence, differences of transpiration in varieties had limited effects on the overall internal thermal environment. An analysis of plant morphologies shows that plants of the two LWSs mainly differed in terms of height and leaf coverage. Living wall systems and green facades have different characteristics that can influence their benefits (e.g., their cooling and insulating properties). The thickness of leaves constitutes an important factor by creating a stagnant air layer and by shading the facade [43]. C3 included 3 varieties of bamboo with an average height of 60 cm. C4 plant species included taller plants such as Shibataea kumasasa and shorter plant such as Heuchera micrantha ’Red Sun’, with an average height and thickness half that of C3 at roughly 25–30 cm. When the heights of plants are doubled, the overlapping area is expanded, and the covered area and horizontal surface is also expanded. This will increase the thermal resistance of containers in dissipating heat, and heat radiation released into the container will also increase. Certainly, taller vegetation will also need to grow more leaves on their stems, i.e., higher leaf coverage. Dense foliage will reduce wind flow around a facade and thus also help building cooling [43,46]. C3 leaf coverage was significantly greater than that of C4, blocking more wind. Living wall systems with planter pots serve as the most effective wind barriers, as the resulting reduction of wind velocity affects the thermal resistance of a building envelope and thus its efficiency [39]. Furthermore, from our thermal distribution maps we find that an increase in plant height and leaf coverage will improve the capacity for air layers to retain heat. According to the current application of LWS in the Yangtze river delta, people prefer some exotic plants to the local abundant bamboo resources. As more building surfaces may began to green in the future, bamboo is actually a very good choice for LWS in the winter to reduce costs and energy saving. In addition to the surrounding area of the study site, the conclusion is also of reference value for northern China, where annual heating energy consumption is huge. These areas have low temperatures throughout the year, and most of research now focused on the summer months may be less hospitable to them. In the meantime, the relative lack of plant resources also limits their vegetation selection on LWS. Although bamboo may not be suitable for growing in these areas, the results may help them select native plants with similar characteristics on LWS. 4.4. Limitations and potentials of the study Due to the extremely hot summer in and around this region, people are more interested in studying the cooling effect of LWS in summer. However, the winter here is also very cold, and the occasional La Nina can even exacerbate the cold. In the meantime, there have been calls for South China to be able to provide heating in winter, which may become a reality in the future and the huge energy consumption problem that brings along with it needs to be considered. This study is very necessary for the region and is conducive to the application of LWS in winter. People sometimes like to install the building with LWS in all orientation for better cooling effect in summer, but this is not necessarily in winter. Our results bear this out. However, studies of GRs show that it can offer building thermal protection but cannot replace the insulation layer [47]. In a sense, the insulation capacities of living walls are similar to those of green roofs and cannot serve as a substitute for wall insulation

despite their capacities for thermal protection. Living walls are aesthetically pleasing and act mainly as a landscape [16]. In addition, for containers with simple structures and weak thermal insulation properties, heat sources mainly relate to solar radiation, exacerbating heat loss. For modern buildings, strong insulation and internal heating capacities can improve the thermal performance of LWSs, thus greatly reducing building energy consumption brought about by winter heating [11,31,37,39]. Our results show that living walls including different plants can improve indoor thermal environments in the winter whereas the thermal performance of specific species was not yet determined. Different physiological and morphological indexes of plants affect the internal thermal performance of LWSs, and this can be explored further, as most living wall studies conducted in the fields of landscape and ecology have focused on summer conditions. We plan to compare different single variety installed on the outer walls of containers to complete our experiments of heat preservation effects of different plants used on living walls in the winter. LWSs can serve as an unconventional means of insulating walls. However, optimizing living wall performance for winter alone seems to be inappropriate, as in the summer Hangzhou can be very hot and unpleasant. The studied LWSs differ from some LWSs closely integrated with architectural forms, for which an air layer exists between walls and planter pots, leading to more severe heat loss. Therefore, such structures may be more suitable for cooling and humidifying in the summer. Further field experiments conducted in other seasons are likewise necessary to obtain data to offer a comprehensive assessment of the thermal effects of this LWS throughout the year.

5. Conclusions The thermal performance of LWSs in wet and cold climates during the winter was studied with a focus on the insulation capacities of LWS and corresponding influencing factors. Our main findings are as follows. (1) Soil-filled LWS planter pots and plants both played an important role in container heat preservation. When modular systems were installed on north- and south-facing walls or on north-facing walls of containers alone, indoors mean temperatures were both 0.4 °C higher than those of the bare-walled container and indoor temperatures of the container with the bamboo living wall averaged at 1 °C and 1.7 °C higher, respectively, while those of another botanical wall averaged at 0.6 °C and 1 °C higher, respectively. (2) The soil-filled modular system increased the thermal resistance of wall surfaces, and the thermal environment within the container covered with planter pots proved more stable. As plants, in addition to increasing thermal resistance, can reduce air temperatures through shading and transpiration. Thermal regulation capacities are greatly affected by the given period and insulation values are unstable. (3) For buildings without internal heating in the winter, northand south-facing LWSs only have an insulating effect from the evening to the early morning, which can limit fluctuations in overall temperature and humidity. North-facing LWSs have significant insulating effect almost all day long with overall temperature and humidity ranges better catering to human needs. (4) Plants have a certain impact on the thermal performance of LWSs. The higher the plant heights and leaf coverage of green walls are, the better insulating and moisture reduction effects on indoor buildings become mainly by increasing wall thermal resistance, raising the temperature within the air layer and reducing the influence of wind.

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In conclusion, we can confirm that LWSs installed in Hangzhou, which is characterized by low temperature and high humidity winter conditions, can improve indoor thermal environments, thus exhibiting strong potential applications to high-density cities of eastern China. Reasonable orientation selection of LWSs in winter can be made in practice according to the results of the above two experiments. When there is no internal heating, buildings occupied throughout the day are more suitable for north-facing LWSs, especially in hospitals, stations, convenience stores and other buildings that are manned 24 h a day, or areas such as schools, office buildings with large crowds during the day. For buildings that are empty in the daytime and occupied by night such as some dormitory, residential buildings, nightclubs, etc., either north-facing or north- and south-facing LWSs can be applied. Moreover, at present, plants used for LWS in eastern China have a high degree of similarity. This experiment proves that some dwarf bamboos have good application prospects in the winter. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (51508515) and Natural Science Foundation of Zhejiang Province (LY19C160 0 07). We gratefully acknowledge the support of Jingbang Chen, Chunbing Song, Gaomin Chen, Wenchao Chen, and Wenhong Ge for their invaluable assistance in the field experiments. Authors also respectfully thank Xiaohui Guo, Chenwei Zhang, Shimin Yang for their help in the research. References [1] N.B. Grimm, S.H. Faeth, N.E. Golubiewski, C.L. Redman, J. Wu, X. Bai, J.M. Briggs, Global change and the ecology of cities, Science 319 (2008) 756–760, doi:10. 1126/science.1150195. ´ [2] K. Tzoulas, K. Korpela, S. Venn, V. Yli-Pelkonen, A. Kazmierczak, J. Niemela, P. James, Promoting ecosystem and human health in urban areas using Green Infrastructure: a literature review, Landsc. Urban Plan. 81 (2007) 167–178, doi:10.1016/j.landurbplan.20 07.02.0 01. [3] C. Yu, W.N. Hien, Thermal benefits of city parks, Energy Build. 38 (2006) 105– 120, doi:10.1016/j.enbuild.20 05.04.0 03. [4] C.Y. Sung, Mitigating surface urban heat island by a tree protection policy: a case study of The Woodland, Texas, USA, Urban For. Urban Green. 12 (2013) 474–480, doi:10.1016/j.ufug.2013.05.009. [5] G. Papangelis, M. Tombrou, A. Dandou, T. Kontos, An urban “green planning” approach utilizing the Weather Research and Forecasting (WRF) modeling system. a case study of Athens, Greece, Landsc. Urban Plan. 105 (2012) 174–183, doi:10.1016/j.landurbplan.2011.12.014. [6] D.E. Bowler, L. Buyung-Ali, T.M. Knight, A.S. Pullin, Urban greening to cool towns and cities: a systematic review of the empirical evidence, Landsc. Urban Plan. 97 (2010) 147–155, doi:10.1016/j.landurbplan.2010.05.006. [7] D. Armson, P. Stringer, A.R. Ennos, The effect of tree shade and grass on surface and globe temperatures in an urban area, Urban For. Urban Green. 11 (2012) 245–255, doi:10.1016/j.ufug.2012.05.002. [8] G.Y. Qiu, Z. Zou, X. Li, H. Li, Q. Guo, C. Yan, S. Tan, Experimental studies on the effects of green space and evapotranspiration on urban heat island in a subtropical megacity in China, Habitat Int. 68 (2017) 30–42, doi:10.1016/j. habitatint.2017.07.009. [9] H. Yan, X. Wang, P. Hao, L. Dong, Study on the microclimatic characteristics and human comfort of park plant communities in summer, Procedia Environ. Sci. 13 (2012) 755–765, doi:10.1016/j.proenv.2012.01.069. [10] Y. Wang, F. Bakker, R. de Groot, H. Wörtche, Effect of ecosystem services provided by urban green infrastructure on indoor environment: a literature review, Build. Environ. 77 (2014) 88–100, doi:10.1016/j.buildenv.2014.03.021. [11] Q. Xing, X. Hao, Y. Lin, H. Tan, K. Yang, Experimental investigation on the thermal performance of a vertical greening system with green roof in wet and cold climates during winter, Energy Build. 183 (2019) 105–117, doi:10.1016/j. enbuild.2018.10.038. [12] M. Köhler, Green facades – a view back and some visions, Urban Ecosyst. 11 (2008) 423, doi:10.1007/s11252- 008- 0063- x. [13] M. Manso, J. Castro-Gomes, Green wall systems: a review of their characteristics, Renew. Sust. Energy Rev. 41 (2015) 863–871, doi:10.1016/j.rser.2014.07.203. [14] C.Y. Jim, Thermal performance of climber greenwalls: effects of solar irradiance and orientation, Appl. Energy 154 (2015) 631–643, doi:10.1016/j.apenergy.2015. 05.077.

17

[15] Y. He, H. Yu, A. Ozaki, N. Dong, S. Zheng, An investigation on the thermal and energy performance of living wall system in Shanghai area, Energy Build. 140 (2017) 324–335, doi:10.1016/j.enbuild.2016.12.083. [16] E.V. White, B. Gatersleben, Greenery on residential buildings: does it affect preferences and perceptions of beauty? J. Environ. Psychol. 31 (2011) 89–98, doi:10.1016/j.jenvp.2010.11.002. [17] R.A. Francis, J. Lorimer, Urban reconciliation ecology: the potential of living roofs and walls, J. Environ. Manag. 92 (2011) 1429–1437, doi:10.1016/j.jenvman. 2011.01.012. [18] N. Dunnett, N. Kingsbury, Planting Green Roofs and Living Walls, second ed., Timber, Portland, 2008. [19] H.F. Di, D.N. Wang, Cooling effect of ivy on a wall, Exp. Heat Transf. 12 (1999) 235–245, doi:10.1080/089161599269708. [20] B. Riley, F. de Larrard, V. Malécot, I. Dubois-Brugger, H. Lequay, G. Lecomte, Living concrete: democratizing living walls, Sci. Total Environ. 673 (2019) 281– 295, doi:10.1016/j.scitotenv.2019.04.065. [21] N.H. Wong, A.Y.K. Tan, Y. Chen, K. Sekar, P.Y. Tan, D. Chan, K. Chiang, N.C. Wong, Thermal evaluation of vertical greenery systems for building walls, Build. Environ. 45 (2010) 663–672 https://doi.org/10.1016/j.buildenv.20 09.08.0 05. [22] Q. Chen, B. Li, X. Liu, An experimental evaluation of the living wall system in hot and humid climate, Energy Build. 61 (2013) 298–307, doi:10.1016/j.enbuild. 2013.02.030. [23] S. Charoenkit, S. Yiemwattana, Living walls and their contribution to improved thermal comfort and carbon emission reduction: a review, Build. Environ. 105 (2016) 82–94, doi:10.1016/j.buildenv.2016.05.031. [24] C.Y. Cheng, Ken K.S. Cheung, L.M. Chu, Thermal performance of a vegetated cladding system on facade walls, Build. Environ. 45 (2010) 1779–1787, doi:10. 1016/j.buildenv.2010.02.005. [25] C.Y. Jim, H. He, Estimating heat flux transmission of vertical greenery ecosystem, Ecol. Eng. 37 (2011) 1112–1122, doi:10.1016/j.ecoleng.2011.02.005. [26] N.H. Wong, A.Y.K. Tan, P.Y. Tan, N.C. Wong, Energy simulation of vertical greenery systems, Energy Build. 41 (2009) 1401–1408, doi:10.1016/j.enbuild.2009.08. 010. [27] T. Safikhani, A.M. Abdullah, D.R. Ossen, M. Baharvand, Thermal impacts of vertical greenery systems, Environ. Clim. Technol. 14 (2014) 5–11, doi:10.1515/ rtuect- 2014- 0 0 07. [28] C.L. Tan, N.H. Wong, S.K. Jusuf, Effects of vertical greenery on mean radiant temperature in the tropical urban environment, Landsc. Urban Plan. 127 (2014) 52–64, doi:10.1016/j.landurbplan.2014.04.005. [29] M. Haggag, A. Hassan, S. Elmasry, Experimental study on reduced heat gain through green façades in a high heat load climate, Energy Build. 82 (2014) 668–674, doi:10.1016/j.enbuild.2014.07.087. [30] R. Djedjig, E. Bozonnet, R. Belarbi, Analysis of thermal effects of vegetated envelopes: integration of a validated model in a building energy simulation program, Energy Build. 86 (2015) 93–103, doi:10.1016/j.enbuild.2014.09.057. [31] R. Djedjig, R. Belarbi, E. Bozonnet, Experimental study of green walls impacts on buildings in summer and winter under an oceanic climate, Energy Build. 150 (2017) 403–411, doi:10.1016/j.enbuild.2017.06.032. [32] U. Mazzali, F. Peron, P. Romagnoni, R.M. Pulselli, S. Bastianoni, Experimental investigation on the energy performance of Living Walls In a temperate climate, Build. Environ. 64 (2013) 57–66, doi:10.1016/j.buildenv.2013.03.005. [33] F. Olivieri, L. Olivieri, J. Neila, Experimental study of the thermal-energy performance of an insulated vegetal façade under summer conditions in a continental mediterranean climate, Build. Environ. 77 (2014) 61–76, doi:10.1016/j. buildenv.2014.03.019. [34] M. Manso, J.P. Castro-Gomes, Thermal analysis of a new modular system for green walls, J. Build. Eng. 7 (2016) 53–62, doi:10.1016/j.jobe.2016.03.006. [35] J. Coma, G. Pérez, A. de Gracia, S. Burés, M. Urrestarazu, L.F. Cabeza, Vertical greenery systems for energy savings in buildings: a comparative study between green walls and green facades, Build. Environ. 111 (2017) 228–237, doi:10.1016/j.buildenv.2016.11.014. [36] L. Bianco, V. Serra, F. Larcher, M. Perino, Thermal behaviour assessment of a novel vertical greenery module system: first results of a long-term monitoring campaign in an outdoor test cell, Energy Effic. 10 (2017) 625–638, doi:10.1016/ j.ufug.2012.05.002. [37] J.S. Carlos, Simulation assessment of living wall thermal performance in winter in the climate of Portugal, in: Build. Simul, Tsinghua University Press, Beijing, 2015, pp. 3–11, doi:10.1007/s12273- 014- 0187- 2. [38] G. Pérez, J. Coma, L.F. Cabeza, Vertical greening systems to enhance the thermal performance of buildings and outdoor comfort, in: Nature Based Strategies for Urban and Building Sustainability, Butterworth-Heinemann, Oxford, 2018, pp. 99–108, doi:10.1016/B978- 0- 12- 812150- 4.0 0 0 09-4. [39] K. Perini, M. Ottelé, A.L.A. Fraaij, E.M. Haas, R. Raiteri, Vertical greening systems and the effect on air flow and temperature on the building envelope, Build. Environ. 46 (2011) 2287–2294, doi:10.1016/j.buildenv.2011.05.009. [40] K.J. Kontoleon, E.A. Eumorfopoulou, The effect of the orientation and proportion of a plant-covered wall layer on the thermal performance of a building zone, Build. Environ. 45 (2010) 1287–1303, doi:10.1016/j.buildenv.2009.11.013. [41] A.M. Hunter, N.S. Williams, J.P. Rayner, L. Aye, D. Hes, S.J. Livesley, Quantifying the thermal performance of green façades: a critical review, Ecol. Eng. 63 (2014) 102–113, doi:10.1016/j.ecoleng.2013.12.021. [42] M.C. Peel, B.L. Finlayson, T.A. McMahon, Updated world map of the Köppen– Geiger climate classification, Hydrol. Earth Syst. Sci. 11 (2007) 1633–1644, doi:10.5194/hess- 11- 1633- 2007. [43] M. Ottelé, K. Perini, A.L.A. Fraaij, E.M. Haas, R. Raiteri, Comparative life cycle analysis for green façades and living wall systems, Energy Build. 43 (2011) 3419–3429, doi:10.1016/j.enbuild.2011.09.010.

18

X. Nan, H. Yan and R. Wu et al. / Energy & Buildings 208 (2020) 109680

[44] N.H. Abu-Hamdeh, R.C. Reeder, Soil thermal conductivity effects of density, moisture, salt concentration, and organic matter, Soil Sci. Soc. Am. J. 64 (20 0 0) 1285–1290, doi:10.2136/sssaj20 0 0.6441285x. [45] F. Olivieri, F.J. Neila, C. Bedoya, Energy saving and environmental benefits of metal box vegetal facades, WIT Trans. Ecol. Environ. 127 (2009) 325–335, doi:10.2495/RAV090291.

[46] S.W. Peck, C. Callaghan, M.E. Kuhn, B. Bass, Greenbacks from Green Roofs: Forging a New Industry in Canada, Canada Mortgage & Housing Corporation (CMHC), Ottawa, 1999. [47] E. Eumorfopoulou, D. Aravantinos, The contribution of a planted roof to the thermal protection of buildings in Greece, Energy Build. 27 (1998) 29–36, doi:10.1016/S0378-7788(97)0 0 023-6.