Measurement of Innovative Green Façades in the Central European Climate

Abstract

Green structures, such as green roofs or green façades, are great examples of climate change mitigation. Their impact is mainly focused on roofs in the area of overheating reduction. In this paper, initial measurement results of a green façade experimental test setup are provided. The green façade uses an innovative board from recycled materials with vegetation rooted directly on the board. The tested green façade is divided into three segments. These segments differ from each other in their watering regimes, which are crucial for cooling effectiveness. Watering operates with the assistance of gravity; water flows from the top gutter through the boards. In this paper, these three segments are compared to each other with respect to temperatures on the surface of a regular external thermal insulation composite system façade (ETICS) during two summer days. The green façade showed an impact on the temperature in the ventilated air gap, where the temperature is almost the same as the outdoor air temperature in the morning with direct solar radiation on the façade and lower than the outdoor air temperature in the afternoon. At the peaks, the surface temperatures within the air cavity surface are up to 8 °C lower than those on a new white ETICS coating. This demonstrates a cooling potential, although the surface temperatures are always higher than the outdoor air temperatures during daylight hours.

1. Introduction

The urban heat island (UHI) effect is a negative consequence of rapid urbanization, and it has been long investigated since its first mention [1,2]. To reduce the rising temperatures in urban areas, green infrastructure is incorporated. Green infrastructure, or greening structures, mainly includes green roofs and green façades. Green roofs are considered one of the best ways to mitigate climate change, reduce summer overheating, accumulate water, pollutants, etc. The number of studies is increasing [3,4,5], and the outcomes differ from each other within the outdoor climate. Vertical greenery systems (VGSs), green walls, and green façades are passive strategies for reducing the UHI and other negative effects. All of the mentioned systems consist of a load-carrying structure and a layer of vegetation. The impact of green walls can be assessed from the building [6] up to the urban scale [7]. Benefits and cost savings can include energy saving [8,9,10], greywater treatment, pollutant removal, urban noise mitigation [11], urban heat islands mitigation, urban ecosystem conservation or improvement, property value enhancement, beautification, and well-being enhancement. More than 600 case studies were analyzed by T. Susca et al. in [9] to determine whether the green façade is a universal solution for reducing building energy use and urban temperatures. The answer is that the results depend significantly on the climate zone [3,12,13]. Meeting the individual environmental policies of different municipalities, the green infrastructure is described in [4].

The comprehensive state-of-the-art summary of greening structures can be found in these publications [5,9,14]. All the benefits of the roof can be achieved with the use of a green façade. Moreover, with the use of full-scale façades [8], the covered area can be larger than that of the roof. Another benefit is that the façade directly helps to avoid overheating in the street canyons [7,15,16]. There are several classifications of green façades [5,9,14,17,18], initially distinguishing between direct and indirect face greenery (use of ventilated air gap or a double-skin façade). The second classification is as follows:

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green façade, where plants are rooted on the ground in soil, climb up the façade (supporting structure with a fence), and form a living wall;

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living wall, where some type of planter boxes [17], pre-vegetated sheets, or more recent cork-based boxes [19,20] that are attached to a structural wall or frame are used.

Green walls offer similar benefits to green roofs, along with some additional ones. The thermal benefits of green walls, such as the reduction of the outdoor ambient air temperature, were quantified by several authors, ranging from 1 to 3 K [10,21,22,23] and up to 15 K [23]. Based on different case studies, the surface temperature reduction varies from 6.1 °C on sunny days to 4 °C on cloudy days [23]. A greater reduction (up to 13 °C) was measured during a one-week-long measurement, depending on the leaf area [22,23]. The green façade reduced the building wall temperature by up to 9 °C during the summer and reduced the thermal amplitude in the building wall by 50% [24]. In a study evaluating the thermal effects of nine types of green wall systems, Wong et al. [10] measured a reduction of 3.3 °C in the ambient air temperature, which corresponded to a 1.1–11.6 °C decrease in the façade surface temperature immediately behind the vegetation, depending on the vegetation type. The transformation of the thermal energy to achieve building energy efficiency is described in [25], where the author achieved an external wall surface temperature cooler than ambient air by up to 6.6 °C. Most of the articles, such as [25,26], focus on hot and humid climates and do not address the Central European climate. The impact of short-wave radiation and other energy benefits are discussed in [26].

According to [27], the indoor operative temperature can be decreased by up to 3.6 °C. Among other things, the building’s acoustic insulation and urban noise reduction should be considered. Typically, vegetation has been used for acoustic isolation of urban areas, especially from traffic noise. The impact of a green façade is evaluated in [11].

The use of recycled materials in the circular economy within civil engineering is analyzed in the in situ measurement of a modular living wall [28]. An analysis of a green building and the mitigation probability of using the microclimate-neutral building is presented in [29].

Unlike green roofs, a green façade is highly dependent on watering or irrigation, which has to be artificial. Even in locations with higher wind-driven rain loads, only some windward-oriented façades could obtain enough rainwater. The irrigation system usually consists of a pipe and hose system (such as drip irrigation) horizontally dividing the system of pots or boxes [5,12]. With artificial irrigation, the potential for evapotranspiration—incorporating evaporation from the wet surface and transpiration from plants—is higher than on roofs, where irrigation is not as common. Lately, more articles have analyzed different irrigation scenarios [8]. Also, evapotranspiration simulations for urban green areas at the district scale have been conducted [30]. Nowadays, research is focused on the quantification of evapotranspiration, incorporating various types of greenery in different climates [31,32]. Evapotranspiration is more essential in green-roof-surface cooling than meteorological factors and substrate moisture [32]. Overall, evapotranspiration and different watering regimes have not been sufficiently investigated yet.

To the best of the author’s knowledge, there is no system using the vegetation board (VB), which consists of a board made from recycled materials, with succulents rooted into the board itself. Regarding the board [33] and its production [34], the board is comprised of a mixture of basic components with admixtures, including recycled material from textile fibers acquired through a defibering process from hard-to-defiber multi-composite textiles. This can be supplemented with recycled materials from textile pulp acquired through tire recycling processes. The basic components advantageously include plaster or polyurethane. During manufacturing of this building segment, textile waste is defibered into textile fibers, which are processed to the required length and are homogenized into an even layer, and, then, it is added to the mixture of basic components containing other elements. The main improvement compared to the other commonly used systems is that the vegetation is rooted directly into the VB structure. The plants, or the greenery on the board, are planted horizontally in a garden for about a year. The final customer receives the VB ready for installation, with rooted plants. The exact number of succulent species is not known, but it is considered a regular one, consisting of four to eight species.

In this study, the possibility of using a vegetation board as the outer skin for a double-skinned green façade is assessed. This board can also be used in green roofs, providing an easy and simple solution; notably, its benefits have been summarized in previous research [35,36].

This study follows outcomes based on measurements obtained using a climate chamber having similar conditions [6] to those of the real climate experienced by the façade of a building.

There are two basic reasons why we created a full-scale façade sample for experimental measurements: The first was to investigate the impact of the wall regime through direct comparison with other wall compositions (including coating and aerated cladding). As the green façade is of the double-skin type, its composition slightly differs, for example, from that of the coated wall. The second aim was to quantify its impact on the outdoor microclimate, which is not analyzed in this paper.

This study presents the initial measurement results based on a two-month period (including sunny and cloudy weather), as well as the impacts of different irrigation setups. Temperature courses are analyzed, and possible outcomes for the future development of the considered façade are assessed.

2. Test Site, Materials, and Methods

The experimental green façade segment was built on the eastward-oriented façade of one of the University campus buildings. This building mostly contains laboratories, and, therefore, the wooden experimental wall was placed directly above the green façade. The original façade of the building is masonry, and, prior to the construction of the façade, it was insulated with an ETICS including mineral wool of thickness 160 mm. The same thickness was used for the green façade segments. Unlike the ETICS, the insulation is covered not with a coating but, instead, with a waterproof diffuse membrane. The orientation of the façade is almost eastward (12° to the south).

The green façade is based on the double-skin aerated façade design, where the outer skin is made of the vegetation board (VB) with an open (ventilated) air gap behind it. Considering the irrigation scheme—where the water flows through the boards—it is almost impossible to make the façade a contact façade. The abovementioned membrane is located behind the outer skin and air gap.

The green façade is divided into three vertical segments (Figure 1): two segments have an air gap with the same thickness (50 mm), while the third one has a wider air gap (100 mm). Usually, the minimum thickness for an air gap to ensure air flow is 40 mm. In this case, the load-carrying structure and the backside of the VB (which is not smooth) can lead to an air flow disturbance, and, therefore, a 50 mm air gap was used. Furthermore, doubling the gap in the third segment should increase the effectiveness of the ventilation. The width of every segment is 1.2 m, based on the regular dimensions of the VB (1.2 × 0.6 m). The height of the segment is 2.7 m. The VBs were mounted onto a load-carrying substructure, located in front of the regular wall.

The watering system consists of a top gutter with drainage holes. The water flows through the holes, then through the VBs, and later onto the surfaces due to the force of gravity. The bottom gutter collects the water into a tank, from which it is pumped using a small DC pump back to the top gutter. The watering regime was experimentally set up based on the previous climate chamber measurement [6]. During this initial measurement, the second and third segment pumps are powered with photovoltaic (PV) panels (Figure 2), such that the pump works when there is sufficient direct solar radiation. The water tanks were filled regularly. In this initial measurement, no evaporation rate was measured. For the first and second segments, with same air gap thickness, different irrigation schedules were used. The first segment was watered once a week with a hose for a few minutes in order to prevent the plants drying out (less than 1 L/m²). This led to a much dryer surface and plants, as can be seen from Figure 1 and Figure 2 (left).

The experimental green façade was equipped with sensors, as illustrated in Figure 3. During reconstruction of the building façade, thermocouples were incorporated between the original hollow-brick masonry wall and the new thermal insulation.

Another set of thermocouples (TC), the measurements obtained with which are mostly analyzed in this study, were placed on the outer surface of the insulation. Later, TCs were also added in the joints between the individual VBs and under the vegetation (named VB 1–3, used in watering impact section). Ahlborn T190 NiCr-Ni type K thermocouples with a temperature range from −25 to 400 °C were used [37] while, under the vegetation, sheathed thermocouples were used, which were also type K from Ahlborn [38]. All TCs were connected to a Fluke Hydra datalogger with a recording interval of one minute. Combined temperature/relative humidity sensors were also placed within the air gap, but their results are not presented in this study because of a malfunction: due to the almost constant irrigation, most of the day the relative humidity in the air gap is close to 100%. The outdoor climate data were obtained from a nearby weather station [39]. There was also a façade meteorological station [40], as shown in Figure 1, which was mostly used for radiation measurement (data not used in this study). Above the façade sample, there was an eastern-oriented lightweight wooden wall with different-colored coatings, as well as wooden ventilated cladding [39,41] (comparison not detailed here). The scheme of the façade sample, including sensor positions, is shown in Figure 3. The abbreviations refer to the number of segments: S stands for the surface of the membrane behind the air gap at three different heights (bottom, middle, and high), IS stands for the position under the insulation of the masonry wall, and VB stands for the surface position on the vegetation board.

The façade construction process requires drier VBs, considering their ease of manipulation and lower weight (Figure 1b). After a few sunny days and watering, the vegetation became more greenish and looked more natural (Figure 1a and Figure 2a).

3. Outdoor Boundary—Outdoor Climate

To investigate the impact of the green wall on the surface temperature, several time periods were considered. Temperature measurements were carried out in August of 2021. Courses for the air temperature, second segment, and the white ETICS coating temperatures are in Figure 4. From these whole-month observations, two periods were chosen: sunny days (represented by 7–16 August) and cloudy or overcast days (26–31 August).

The average air temperature during this month was 17.1 °C, with a minimum of 6.2 °C and maximum of 31.2 °C. A closer statistical analysis of these two different periods is detailed in the results section of this paper.

The boundary conditions for three consecutive days (starting with 6 August 2021) chosen for analysis of the temperature courses are shown in Figure 5. During these days, the minimum air temperature was 12.7 °C, the maximum was 26.4 °C, and the average was 18.9 °C. The barometric pressure was increasing (Figure 5). The first day was cloudy, while the two other days were sunny. During the second night, there was a period with increased air temperature caused by passing clouds reducing the night cooling, as can be seen according to the temperature and pressure fluctuations; overall, this was a warm night.

To analyze the impact of direct watering, three further consecutive days were considered, starting with 11 September 2021 (Figure 6). These days were also sunny, with a short cloudy period on the second day. The minimum air temperature was 6.8 °C, the maximum was 26.1 °C, and the average was 16.2 °C. The barometric pressure was steady.

4. Results and Discussion

The results for this initial measurement period are divided into subsections, involving comparisons of the temperatures of the three green façade segments and the white wall surface coating based on the proposed irrigation setup and direct irrigation of the segment during the day.

4.1. Temperature Measurement in Sunny and Cloudy Periods

The results from the one month of green wall measurement (August 2021; as represented in Figure 4) are shown in

Table 1. The calculated minimum, maximum, and average of measured temperatures for August 2021 and duration of sunshine influence on the green wall.

	Whole Month (0:00–23:59)			Whole Month (6:00 to 16:00)
	Min	Max	Average	Min	Max	Average
air temperature	6.2	31.2	17.1	7.9	31.2	19.2
coating-white	5.1	41.8	18.6	6.0	41.8	22.6
IS 1	18.8	22.8	21.1	19.2	22.7	21.1
IS 2	19.6	23.1	21.7	19.7	23.1	21.7
IS 3	19.0	22.7	21.0	19.0	22.7	20.9
S 1.L	7.4	33.9	18.0	7.8	33.9	19.5
S 2.L	8.1	29.5	17.0	8.1	29.5	18.1
S 3.L	8.0	28.9	16.6	8.1	28.9	17.4
S 1.M	7.9	32.0	18.0	8.0	32.0	19.2
S 2.M	8.5	29.3	17.4	8.7	29.3	18.4
S 3.M	8.1	28.8	17.0	8.1	28.8	17.9
S 1.H	8.0	34.6	18.8	8.3	34.6	20.4
S 2.H	8.7	29.6	17.6	9.1	29.6	18.8
S 3.H	8.3	29.8	17.6	8.4	29.8	18.8
VB 1	7.0	37.2	17.9	7.0	37.2	19.7
VB 2	7.9	30.6	17.4	7.9	30.6	18.7

. To observe the influence of the green wall, two time periods in the analyzed month were chosen:

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During sunny/hot weather (Figure 5,

Table 2. The calculated minimum, maximum, and average temperatures, as well as the difference from the ambient air temperature for 7–16 August (whole days) and the duration of sunshine influence on the green wall (6:00–16:00).

	Warm				Warm 6:00 to 16:00
	Min	Max	Average	Delta	Min	Max	Average	ΔΘ
air temperature	11.30	31.20	20.80	0.00	12.80	31.20	23.46	0.00
coating-white	10.39	41.76	23.11	−10.56	10.77	41.76	28.93	−10.56
IS 1	20.14	22.84	21.74	8.36	20.14	22.70	21.67	8.50
IS 2	21.08	23.14	22.19	8.06	21.08	23.11	22.15	8.09
IS 3	20.24	22.71	21.53	8.49	20.24	22.71	21.53	8.49
S 1.L	12.57	33.94	21.77	−2.74	12.64	33.94	23.98	−2.74
S 2.L	12.23	29.51	19.93	1.69	12.23	29.51	21.62	1.69
S 3.L	12.77	28.91	19.52	2.29	12.83	28.91	20.71	2.29
S 1.M	13.05	32.03	21.66	−0.83	13.09	32.03	23.55	−0.83
S 2.M	12.86	29.33	20.39	1.87	12.86	29.33	21.87	1.87
S 3.M	12.77	28.77	19.87	2.43	12.77	28.77	21.19	2.43
S 1.H	13.45	34.57	22.82	−3.37	13.45	34.57	25.14	−3.37
S 2.H	12.89	29.58	20.83	1.62	12.89	29.58	22.46	1.62
S 3.H	12.86	29.78	20.82	1.42	12.87	29.78	22.62	1.42
VB 1	12.37	37.19	21.79	−5.99	12.37	37.19	24.62	−5.99
VB 2	12.48	30.64	20.43	0.56	12.48	30.64	22.58	0.56

);

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During cloudy or partly cloudy weather (

Table 3. The calculated minimum, maximum, and average temperatures, as well as the difference from the ambient air temperature for 26–31 August (whole days) and the duration of sunshine influence on the green wall (6:00–16:00).

	Cloudy				Cloudy 6:00 to 16:00
	Min	Max	Average	Delta	Min	Max	Average	ΔΘ
air temperature	6.20	18.30	12.42	0.00	7.90	18.30	13.90	0.00
coating-white	5.10	31.99	13.22	−13.69	6.22	31.99	15.35	−13.69
IS 1	19.36	20.36	19.82	−2.06	19.45	20.09	19.80	−1.79
IS 2	19.95	21.20	20.47	−2.90	19.97	20.88	20.44	−2.58
IS 3	19.25	20.26	19.70	−1.96	19.32	19.98	19.68	−1.68
S 1.L	7.36	21.81	13.32	−3.51	7.75	21.81	14.07	−3.51
S 2.L	8.08	19.60	12.96	−1.30	8.08	19.60	13.41	−1.30
S 3.L	7.99	17.40	12.63	0.90	8.14	17.40	12.88	0.90
S 1.M	7.88	20.37	13.45	−2.07	8.03	20.37	13.98	−2.07
S 2.M	8.46	19.69	13.47	−1.39	8.73	19.69	13.83	−1.39
S 3.M	8.10	18.46	13.00	−0.16	8.13	18.46	13.39	−0.16
S 1.H	8.04	21.55	13.83	−3.25	8.28	21.55	14.55	−3.25
S 2.H	8.70	19.68	13.55	−1.38	9.07	19.68	13.99	−1.38
S 3.H	8.31	19.39	13.40	−1.09	8.36	19.39	13.89	−1.09
VB 1	7.00	21.06	13.22	−2.76	7.01	21.06	13.89	−2.76
VB 2	7.87	20.41	13.29	−2.11	7.89	20.41	13.69	−2.11

).

The first part is represented by the period from 7 to 16 August (Table 2), while the second part is represented by that from 26 to 31 August (Table 3). To determine the impact of the green wall more comprehensively, minimum, maximum, and average temperatures were calculated, as provided in the tables, as well as the duration of daylight (or the duration of direct sunshine on the wall), which ranged from 6:00 to 16:00 (10 h). In all cases, the temperature difference (ΔΘ) was calculated via subtraction from the ambient air temperature, for which a positive value indicates that the temperature at the analyzed position is cooler than the outdoor air temperature, while a negative value means that the position is warmer.

Graphic representations of Table 2 and Table 3, in terms of the average temperatures during the warm and cloudy weather periods, are shown in Figure 7. Furthermore, differences from the outdoor air temperature are shown in Figure 8, where the best results are positive (i.e., temperature at the position is lower than that of the outdoor air).

In Figure 9, daily temperature averages based on the duration of sunlight (6:00–16:00) are described; therefore, the averages are higher as the night-time data are excluded. Lower temperatures than the outdoor air during the sunny days can be observed, demonstrating the effect of the green wall cooling, whereas the averages for the white coating are much higher. During the cloudy weather, with low levels of direct solar radiation on the wall, the differences are minimized and the positive outcome is not as clearly visible as for sunny days.

Taking the white coating of the ETICS wall as the basis, the green wall lowered the surface temperature (i.e., at the surface of the membrane under the green façade), depending on the segments, with maximum values ranging from 9.8 to 15.5 °C. The reduction in the average surface temperature on the selected days varied from 0.6 to 1.6 °C. When considering only the crucial time of the day (i.e., with incident solar radiation), the temperature difference increased from 3.4 to 5.2 °C. Note that these values are the averages for the whole analyzed (one-month) period.

The results for sunny and cloudy weather periods are summarized in

Table 4. Results for individual days from the measured values for August. Calculated values are from averages (6:00–16:00). Analyzed position is on the membrane in the middle of the vertical dimension of the segment.

Date	Subtraction from Coating			Subtraction from Air Temperature
	S 1.M	S 2.M	S 3.M	S 1.M	S 2.M	S 3.M
7.8	6.28	7.45	8.86	0.26	1.43	2.84
8.8	2.72	4.12	5.11	−0.94	0.46	1.46
9.8	4.02	6.00	6.44	−1.13	0.86	1.30
10.8	7.31	8.40	9.42	0.67	1.76	2.79
11.8	4.23	5.43	6.49	−0.23	0.96	2.03
12.8	5.84	9.10	8.18	−0.88	2.39	1.46
13.8	4.90	6.78	7.75	0.35	2.23	3.20
14.8	5.38	7.30	8.24	0.39	2.31	3.25
15.8	5.85	7.76	8.40	0.18	2.08	2.72
16.8	7.29	8.30	8.49	0.37	1.39	1.57
26.8	0.05	0.08	0.32	0.40	0.43	0.67
27.8	0.78	0.99	1.22	−0.44	−0.24	−0.01
28.8	2.85	3.18	3.67	0.25	0.59	1.07
29.8	3.15	3.38	4.28	−0.58	−0.34	0.55
30.8	0.01	−0.02	0.32	−0.06	−0.09	0.25
31.8	0.40	0.58	0.84	0.36	0.54	0.80

, which confirm that the green wall obtained a benefit ranging from 5.4 °C (green wall S1) up to 7.7 °C (green wall segments S3) during the sunny weather. During the cloudy or semi-cloudy weather, the reduction was almost negligible, taking into account the accuracy of the used sensors. Furthermore, the difference from the air temperature was similar to the previous results. The outdoor air temperature was similar to the VB surface temperature during cloudy days and lower during the sunny days. Of the VBs, the worst results were obtained with the green wall segment S1, which was watered less. When considering the difference between the ventilated air gaps, segment S3 presented the best reduction in temperature.

4.2. Three-Day Temperature Measurements

More detailed temperature measurement courses are shown in Figure 10 and Figure 11. Figure 10 shows courses for three sections at the position of the upper part of the air gap, while Figure 11 shows temperature courses for the middle sections at three positions on the surface of the gap: bottom, middle, and top. Minimum, maximum, and average temperatures for the individual sensors and the outdoor air temperature are presented in

Table 5. The measured temperatures over the analyzed period at the selected positions.

6–8 August 2021	Min.	Max.	Average	ΔΘ (°C)
Air temperature	12.7	26.4	17.97	13.7
VB1	13.2	27.3	17.7	14.1
IS1	20.1	21.0	20.6	0.9
S1 L	13.6	27.1	18.2	13.6
S1 M	13.8	25.9	18.0	12.1
S1 H	14.2	28.3	19.0	14.1
VB2	13.4	27.2	17.3	13.8
IS2	21.1	21.5	21.3	0.5
S2 L	13.4	24.5	17.0	11.1
S2 M	13.9	24.5	17.4	10.6
S2 H	13.8	24.6	17.7	10.8
IS3	20.2	21.0	20.6	0.7
S3 L	13.6	23.1	16.5	9.5
S3 M	13.7	22.8	16.9	9.2
S3 H	13.7	24.9	17.7	11.1

.

Comparison of the results shown in Figure 10 indicates that the green façade lowered the temperatures on the membrane, where the used TCs were positioned. For the watered sections (S2 and S3), the courses in the morning were similar. Later, when the direct solar radiation was reduced with the movement of the sun, the air temperature still increased to the daily peak. At this time, the surface temperatures of the green façades were lower than that of the outdoor air. The only exception was the first segment (which was not watered), for which the air gap surface achieved a higher temperature than the outdoor air. The difference between the watered and non-watered structures was about 3 °C.

The courses in Figure 11 represent the basic courses at the locations influenced by the chimney effect in the ventilated air gap. The air temperature in the inlet had the lowest temperature, which increased vertically. The cooling effect of the watered façade was also observed during the night, which was visible mainly during the short warmer period during the second night (the temperature difference between the top and the bottom was up to 2.5 °C). The inlet and outlet temperatures had higher fluctuations due to the presence of openings. The middle position temperature course presented a smoother course, situated between those of the bottom/top positions. The use of watering and the cooling effect of the wet VB kept the temperature close to that of the outdoor air, with a shorter period of higher surface temperatures. The temperature of the outer surface of the VB was higher than that of the outdoor air, with a maximum of 2.2 °C.

4.3. Watering Impact Measurement

During this initial measurement, the same watering regime was used for the second and third segments. The first segment was watered once a week by spraying it with water (Figure 2c). The three-day temperature course for this segment, in accordance with the outdoor climate data shown in Figure 6 (11–13 September), is shown in Figure 12. During the time marked on the second day of this period, the VBs of the first segment were irrigated. A closer look at the courses on this day is shown in Figure 13. After irrigation, the temperature of the outer surface of the VB dropped by 8 °C. Later during the same day, its course almost matched that of VB 2.

If the irrigation occurred on the first day, the difference would be even higher, as the first day was characterized by higher radiation in the morning and the dryer surface was warmer than the white coating. This is normal, considering the different short-wave radiation absorptivity of different colors. The differences in temperature courses between the VBs and the white coating were very similar, due to their different absorptivities, and those between the VBs were due to their different water contents.

The temperatures (minimum, maximum, average) at selected positions for 12 September are summarized in

Table 6. Temperatures measured for analyzed periods (12 September) at selected positions.

12 September 2021	Min.	Max.	Average	ΔΘ (°C)
Air temperature	8.7	26.1	16.8	17.4
VB1	9.3	36.9	19.8	27.6
IS1	20.2	20.7	20.5	0.5
S1 M	10.5	24.8	17.5	14.3
VB2	9.0	24.5	17.3	15.5
IS2	20.5	20.9	20.7	0.3
S2 M	10.3	21.5	16.4	11.2
S3 M	10.1	21.5	16.3	11.4

.

The VB sensors (measuring the vegetation board surface temperatures) were added later and, therefore, are not included in the results presented in Figure 10 and Figure 11. The watering led to a sharp decrease in temperature. Although a relatively small amount of water was used (approximatively 0.6 L/m² once), compared to the other segments (with constant watering during the sunshine hours), this watering caused the temperature course to be almost identical to those for S2 and S3. On the first day, the temperature of the VB 1 surface was almost 20 °C higher than the surface of white ETICS coating, due to its darker surface and higher short-wave absorptivity. The decrease in VB temperature was also slower than that for the ETICS coating. Meanwhile, the watered segment surface (VB 2) was cooler than the white ETICS. The difference between the membrane surface temperatures was 4.8 °C (middle positions S 1.M and S2.M). On the third day, after watering, the temperature difference was 1.8 °C. This is a sharp decrease, although the cooling effect was reduced due to non-circulating water within the vegetation board.

The temperature reduction of the VB lasted only two days. Thus, if it provides sufficient moisture to prevent the plants from drying up, the boards can be irrigated every other day with a small amount of water (less than 1 L/m²).

A graphical representation of the temperature difference, considered as the crucial outcome, is shown in Figure 14. In particular, the temperature difference between the outdoor air temperature and the surface temperatures (at the middle positions of three segments with the coated surface as a reference) indicated the benefits of the second skin. During the peak sunlight on the eastward oriented wall, the difference between the white coating was up to 20 °C, while the green wall’s insulation temperatures remained lower than 6 °C (dry VB) or lower than 3 °C (irrigated VB). A small difference between the second and third segments was also observed, according to their air gap thicknesses, which indicated that the wider gap was more efficient. This requires further evaluation, as this result may have also been caused by the difference in water content. With this test setup, real outdoor measurements, and irrigation system, it is not possible to have the same water content in the morning. Therefore, the efficiency should be investigated over a longer time period.

Based on previous measurement of the façade with an outside ventilated cladding having the same orientation (Figure 1a), differences between the coating surface and surface under the cladding up to 10.1 °C were measured [41]. In this case, the difference between the temperature under the coating and that under the VBs was 15 °C for the S1. M (dry VB) and more than 16.5 °C for S2. M (wet VB). This comprises a difference of more than one-third but in various circumstances (e.g., different times, different boundary conditions). Furthermore, the cladding and coating used in the previous study [41] had different short-wave absorptivities due to weathering. However, at this time, the effectiveness of the green façade in reducing the membrane temperature, when compared to the previous cladding, is proven to be higher than 50%.

Considering the impact of irrigation on the surface of the insulation (S1, S2), the difference between the temperatures was almost doubled (Figure 14).

Comparing the results from the climate chamber measurement [6], which showed a reduction from the indoor surface by 2.8 °C, the maximum surface temperature of the dry VB was 66 °C and that of the bare, white-colored insulated metal panel was 55 °C. The decrease in the VB’s temperature after one-time watering, similar to that observed in the present outdoor in situ test (Figure 13), was 28 °C.

The maximum surface temperature achieved in the chamber was higher than that obtained in this study (Figure 8), by 14 °C. This is strictly dependent on the water content of the VB, as a higher water content leads to a lower temperature, given that it is harder for the sun to warm the surface.

The indoor surface temperature was not compared as, in this case, there can be hardly any differences in the disturbance of the heat flow. Therefore, the comparable reference layer was chosen at a position under the exterior insulation. The results presented in this case (Table 5 and Table 6) showed minimal differences within the segments. Unfortunately, the sensor within the ETICS coating was no longer operational. Due to the non-steady heat-air moisture transport, this temperature cannot be simply calculated; however, it will be simulated in further research in order to clarify the impact of the green wall.

The difference between the dry (or relatively dry) and wet VBs was 20 °C. This difference influenced the temperature around the façade, as well as within the air cavity. Due to the working principle of the climate chamber, it was not possible to measure the correct temperature within the cavity (as the chamber has to compensate the heat gains from the solar simulation system, for which it uses cooled air which flowed through the cavity). Therefore, the air temperature within the cavity was lower than the air in the chamber. In the presented outdoor measurements, the temperatures within the cavity surfaces were very close to the outdoor air temperature during the time of direct solar radiation on the wall and, later, were lower than the air by up to 4.5 °C (if the VB was wet).

The highest in situ measured temperature was 53 °C during the selected time period. Due to the eastward orientation, it is normal that the course does not present a perfect bell-shaped curve but, instead, has a steep initial phase with a short peak. The results revealed that the simulated temperatures in the chamber were close to the possible reality with a southward orientation and dry VBs. In this particular case, the one-time irrigation of the dry sample does not influence the VB temperature as much as the chamber measurement [6]; however, again, this strongly depends on the initial temperature of the VB and the temperature of the used water. In this case, it can be confirmed that the results are similar not in terms of the exact differences—which would be unrealistic—but, instead, in relation to the observed trends and shapes.

4.4. Limitations of This Study

The presented study has several limitations, providing directions for future studies.

The presented results showed the potential for reduction of the surface temperatures behind the air gap (membrane on the thermal insulation); however, the temperature course within the wall could not be analyzed due to a lack of temperature sensors on the indoor surfaces. The used masonry wall has a higher phase shift due to its thermal capacity and bulk density. As such, possible reductions in the indoor surface temperature based on that of the outside surface will be analyzed through HAM simulation in the future.

The influence of the indoor surface temperature should be higher with the use of low thermal capacity façades, such as the insulated metal panels investigated in the climate chamber test [6].

While the south-eastward orientation does not allow for the highest load of solar radiation, direct comparison with other measured segments is available. As the façade was located at the ground floor, there was some shade on the façade from other substructures and trees early in the morning, which could have had an influence on the results for the individual segments.

Watering should not only reduce the temperature but also increase the relative humidity of the air in the ventilated cavity and the amount of water in a liquid state within the air gap. This could create problems related to leaks into the thermal insulation, for example, if the waterproofing membrane does not have watertight joints. There is also the possibility of mold growing within the air gap, due to the wet and warm environment. The high relative humidity in the gap also reduced the ability to observe the relative humidity courses in the gaps, due to malfunctioning of the used sensors.

4.5. Future Work

Based on the one-time irrigation results, further investigation of the optimal volume of water needed to obtain the best results with the lowest irrigation will be conducted. With the PV panels powering the pump segments, the water tank lasted for approximately three days, which indicated a very high evaporation rate. Part of the water also ran off and was not collected at the bottom.

There were some flaws which showed up during this measurement, such as measurement of the microclimate near the façade or measurement of the temperature or relative humidity within the ventilated air gap not being possible, which was caused by the lack of sensors. Furthermore, data from the sheathed thermocouples for measuring the VB surfaces were not available from the beginning of this study. Implementation of the irrigation system with the gutter led to excessive water loss, as it was not collected back at the bottom. The last limitation is that the results obtained in the middle of summer (which would be usually during July in this region) were not usable due to a lack of sensors, missing PV panels, and other factors. Therefore, the presented results were selected from August and September.

This initial measurement led to several upgrades, such as adding the relative humidity sensors to measure the climate around the individual segments and changing the watering regime between the two PV-powered pumps. Higher measured temperatures and possible bigger differences during July (summer) are expected, which we intend to analyze next year. Both these results and the measurements obtained in winter will be published in the future.

The measured data within the gap will be also used for non-steady HAM simulation. In this way, different wall compositions can be simulated and the impacts of the green façades can be quantified. It is expected that these results will be easily transferable to other case studies.

The obtained results for the membrane temperature will be also used for whole-building energy simulation (e.g., in EnergyPlus or WUFI Plus), in order to transfer the results to buildings of different scales.

5. Conclusions

In this study, a test setup for a green façade based on a two-month study period was introduced. The obtained results are summarized as follows:

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The outdoor surface temperature in (behind) the aerated air gap (cavity) was lower than the temperature of the white ETICS wall;

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While the temperatures of the green façade segments presented similar courses, those of the irrigated segments were closer to that of the outdoor air;

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One-time watering of the vegetation board led to a temperature reduction that lasted roughly two days in warm weather—the outcome was a reduction in the VB temperature, but the reduction of the membrane surface temperatures was about half, when compared to the other (irrigated) segments.

The measured temperatures on the membrane, which were lower than those on the white ETICS and were close to the outdoor air temperature, were very positive, representing better results than those published in [24]. Furthermore, the thermal capacity of the VB in this type of green façade is more important than the actual leaf area. Therefore, the results are not as dependent on the leaf area as in the case of some direct façades [25,42].

The dependence between the white coating and the green wall, based on the analyzed results, depends on the following:

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The weather: on sunny days, the average difference ranged up to 13 °C, while, during cloudy weather, it was around 0–1 °C;

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The water content: the less-irrigated segment had the lowest impact but still presented lower temperatures than the coating;

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The air gap: the best results were achieved with the third segment, which had a thicker air gap than the other segments.

The findings of a previous climate chamber measurement study [6] were confirmed through the real-world experiment described in this study. The results for the considered eastward-oriented wall do not reflect the worst-case scenario, which has the advantage of validating that the green façade concept is useful for east-oriented façades but the disadvantage that the evapotranspiration effect may be reduced in other scenarios. Although construction of the façade was limited by the usable area on the building, the results can serve as a basis for further experimental measurements.