Nature-Based Solutions: Green and Smart Façade with an Innovative Cultivation System for Sustainable Buildings and More Climate-Resilient Cities

Abstract

To address the challenges linked to climate change, rapidly increasing urbanization, and food security necessity, this study explores the potential of smart, low-cost innovative cultivation systems for modules on facades as nature-based solutions (NBSs) to improve building energy efficiency, urban food production, and sustainability. Innovative cultivation systems were studied and implemented in the horizontal experimental setup, with a focus on sub-irrigation techniques with terracotta pots, ozonated water, and IoT use. The best eco-smart irrigation system was selected considering both plant growth and the water savings obtained (up to 57.14%) in comparison to the traditional method. With the implementation of this system, a vertical green module (VGM) was designed, allowing for efficient distribution and water savings. The positive effects in terms of temperature reduction and energy behavior were validated by comparing two office rooms: one without VGM and the other with VGM in a Mediterranean city. The drop in internal temperatures achieved was up to 3–4 °C during the hot days of the experimental campaign. The uptake of this low-cost and smart prototype can be useful to support the enhancement of energy-efficient, eco-sustainable, and self-sufficient buildings and urban spaces, contributing to creating more climate-resilient cities and promoting sustainable urban agriculture.

1. Introduction

In recent decades, cities have expanded rapidly, with major effects on climate stability, the availability of natural resources, and food security. Extreme weather events due to climate change are now constant, and the further increase in global warming brings serious risks, from large to small scale [1,2]. Additionally, the increasing global urban population, as expected by the United Nations to reach 10.2 billion people by 2050, with over 55% residing in urban areas, has contributed to accelerated land use for cities even to the detriment of agricultural and forest land [3]. The urbanization process has removed most people from food production and made them dependent on food imported from increasingly far territories [4]. Uncontrolled urbanization and climate change together pose a major global challenge, contributing to the loss of cultivated land due to population growth, the conversion of farmland to urban use, and climate impacts like desertification, altered rainfall, soil degradation, shifting growing seasons, and more extreme weather events [5].

In this scenario, nature-based solutions (NBSs) represent innovative strategies capable of responding to these challenges in an integrated manner, promoting interventions that restore or mimic natural processes to generate environmental, economic, and social benefits [6,7,8]. Some of their most promising applications include the integration of NBSs into buildings through green facades and vertical farm systems, which can improve air quality, regulate microclimate, support local food production, and reduce building energy consumption [9,10,11]. The vertical green systems represent sustainable solutions to restore the environmental quality of urban areas by re-introducing vegetation, aiming at enhancing the esthetic value of buildings and leading to several benefits in terms of reduction in the environmental impacts caused by urbanization and climate change [12,13,14,15].

In parallel, the City Region Food System (CRFS) paradigm promotes a systems approach to urban and peri-urban food planning, linking production, distribution, consumption, and resource management in a circular and sustainable manner [16]. Integrating food production into built spaces through the application of vertical green modules on façades [17], contributes to enhancing urban food security, reducing transportation-related emissions, and improving the connection between citizens and nature, thereby strengthening the socio-ecological resilience of cities [18,19].

With this perspective, it becomes crucial to adopt an integrated view of the water–energy–food nexus (WEF), a concept increasingly central to debates on sustainable urban development, considering the issue of water scarcity. This problem is very serious, especially in the Mediterranean region, where it is exacerbated by the impacts of climate change (e.g., rising temperatures, changing precipitation patterns, and increasing frequency of extreme events, etc.) and requires the research of a solution from the point of view of water economy [20,21]. The WEF nexus emphasizes the interdependence of the water–energy–food nexus and the need to design their use synergistically, especially in high-density urban environments. Green façades with smart growing systems are a concrete solution to promote such integration by optimizing water use through advanced irrigation techniques, such as sub-irrigation systems that are used in agriculture [22,23]. A number of scientific evidence [24,25] points to a lesser-known system, which refers to another sub-irrigation technique: the olla method. It is an ancient irrigation technique practiced since Roman times and is still used today in the warm countries of the Mediterranean basin [26]. It is an effective method of underground micro-irrigation that employs simple baked clay ampoules, crudely named indeed ollae. These capacious pots were buried almost completely in the ground and were filled with water, turning into large water reserves for plants (Figure 1).

Specifically, this method takes advantage of the terracotta’s transpiration capacity, providing controlled irrigation through capillary flow to the surrounding root system [27]. This phenomenon occurs due to the porous nature of terracotta, which allows water to seep through gradually, creating a moisture reserve in the soil around the olla.

In this way, it is possible to reduce water loss by increasing the water directly used by the root system, which is usually estimated only from 10% to 30% [28]. Moreover, Agriculture is currently facing a new technological revolution, defined as “Agriculture 4.0”, as highlighted by Rose et al. [29]. This transition is the result of the integration of the Internet of Things (IoT) [30,31] and digital technologies in field and urban agriculture. The IoT facilitates automated monitoring and management of environmental and crop parameters through a network of distributed sensors that collect real-time data [32] on critical variables, such as moisture, temperature, and nutrient levels, both environmental and soil. These sensors can be strategically placed on different structures, such as balconies, terraces, and building facades, thus improving the efficiency and breadth of data collection. This monitoring and automation architecture, combined with intelligent irrigation systems [33], enables highly efficient cultivation, minimizing the consumption of resources such as water and energy.

In order to ensure the health and well-being of plants, it is crucial to prevent the proliferation of any diseases and pathogens that may affect their growth. One potential solution is the integration of ozone in the water used for irrigation; in fact, ozone (O₃) is a triatomic oxygen molecule with powerful oxidizing and disinfecting properties. In particular, in agriculture, the use of ozone dissolved in water offers several benefits for crop growth, including the disinfection of water, control of plant diseases, post-harvest preservation of fruits and vegetables, and improvement of soil quality [34]. According to several studies [35,36,37], ozone can significantly reduce the microbial load in irrigation water [38,39], thereby improving overall plant health and reducing the incidence of disease.

In order for the use of ozone in water treatment systems to provide the desired effectiveness, the operating times and temperatures should be taken into consideration. Indeed, studies have shown that the amount of ozone dissolved in water remains in an inversely proportional relationship to temperature, e.g., ranging from about 32 min at 40 °C to 10 h at 20 °C [40].

However, despite the increasing attention to NBS and the adoption of advanced technological strategies for greening buildings, a methodological gap persists in the testing and integrated evaluation of the performance of such solutions. In particular, the literature shows a lack of studies that systemically combine agronomic, water, and energy parameters in relation to the effectiveness of green façades on an urban scale. Furthermore, there is a significant discrepancy between the theoretical and actual performance of many green solutions applied to buildings. For example, experimental analyses conducted on low-impact buildings in Italy show a deviation of up to 43% between the thermal transmittance values measured in situ and those predicted in the design phase [41], underlining the need for more robust and multidimensional experimental approaches.

Moreover, recent studies on Urban Heat Island (UHI) show how, in cities such as Rome, temperature differences between urban and rural areas can exceed 3 °C, substantially influencing the energy demand for summer cooling of buildings [42].

In this context, green façades not only mitigate the effects of UHI but also act as multifunctional devices capable of reducing energy demand, supporting climate adaptation, and enhancing the residual spaces of vertical architecture.

The present study fits into this conceptual framework. Firstly, it proposes the development and experimentation of innovative cultivation systems for green facades, which integrate three main technologies: the use of sub-irrigation, the use of ozonized water for its benefits in preventing the proliferation of any diseases and pathogens, and facilitating crop growth to be used in both horizontal and vertical patterns. This system was identified through a preliminary experimental campaign, which integrated all of these innovative technologies, with the aim of optimizing water efficiency and improving plant growth conditions.

2. Materials and Methods

Based on the three thematic pillars described in the introduction (sub-irrigation, ozonized water, and the use of IoT), a method for defining innovative nature-based solutions for facades is developed. The methodological framework can be structured into four phases. The first phase consists of the development of an innovative cultivation system across an experimental campaign conducted on a balcony (horizontal cultivation). The second phase involves the development of a green and smart façade module. Then, combining the results of phase 1, which identifies the best eco-smart irrigation system, and the design of the façade module (FM) (phase 2), a prototype is implemented (phase 3) to conduct real-scale experimentation (vertical cultivation). This campaign is aimed at validating the prototype’s effectiveness in reducing indoor temperatures (phase 4).

The transition from horizontal to vertical cultivation makes it possible to achieve intermediate results useful to implement sustainable cultivation in horizontal partners on balconies, and then to assess the transferability of the selected system’s performance under more complex operating conditions. At the same time, it aims to verify the applicability of the system in vertical development to optimize the use of space and encourage greater efficiency in the use of available surfaces.

The diagram below summarizes the methodological framework of the study and how it integrates with the overall research process (Figure 2).

The first phase of the work includes several key steps: in particular, it consists of a detailed analysis of sub-irrigation systems available, aiming to identify the most suitable system to apply on building façades and aiming to optimize the use of water resources. At the same time, a study of ozone production and distribution has to be conducted, evaluating its potential applications in different crops. Then, the IoT sensors and actuators required to configure a smart system must be identified in order to ensure optimal plant growth. The next steps focus on defining the experimental setups (horizontal cultivation), considering different irrigation combinations, and analyzing experimental results. The decision to start with horizontal experimentation is motivated by its greater simplicity of implementation and management in a controlled environment. At the same time, this makes it possible to evaluate the effectiveness of different combinations of irrigation systems (standard sub-irrigation vs. sub-irrigation optimized with IoT and/or ozonized water) and to identify the best eco-smart irrigation system, through an evaluation of the data collected during the experimental campaign concerning plant growth and water consumption. The evaluation criteria used in this phase include standardized plant growth indicators (e.g., height, biomass quality, and quantity) and precise measurement of water consumption by sensors integrated into the IoT irrigation system. The analysis is based on collecting empirical data during the whole experimental campaign, comparing the performance of different irrigation sets.

The second phase of the work can be divided into two key steps. The first one involves prototype design, considering the green wall and vertical farm principles as a model and, at the same time, the use of low-cost and lightweight materials and systems to maximize the replicability and scalability of the solution. The design includes construction details, façade attachment systems, and the integration of the selected irrigation system. The second phase focused mainly on the study of the plant species to be included in the module, considering the site’s environmental criteria (e.g., Mediterranean climate, sun exposure, drought resistance) and plant characteristics (e.g., growth rate, water requirements, thermal insulation capacity). The selection of plant species will be based on a review of scientific literature and practical considerations regarding maintenance and esthetic appearance [43].

Phase 3 involves the implementation of an innovative prototype with the selected eco-smart irrigation system according to the results of the previous phases. This vertical prototype is applied in a case study in a Mediterranean city (vertical cultivation).

In phase 4, the validation of the prototype’s effectiveness in lowering indoor temperatures is carried out through the application of a kit of sensors to measure the indoor and outdoor environmental parameters. To ensure data reliability, a set of three temperature sensors is considered for each survey point. The results will be reported in the text by means of tables and figures illustrating temperature variations over time and between different conditions (with and without prototype).

3. Method Application and Results

3.1. Phase 1: Development of an Innovative Cultivation’ System

Firstly, the sub-irrigation technique, the system for producing ozonated water, and IoT use for cultivation were analyzed for their application. Among the different sub-irrigation techniques, the system based on the use of terracotta pots (the ollae technique) was chosen for the following reasons: this technology is low-cost and particularly suitable not only for vegetable gardens and fruit orchards but also for the irrigation of pot plants on terraces and balconies. Due to the porous conformation of the terracotta, the olla gradually releases water by dripping, allowing the roots to directly absorb the necessary moisture.

Therefore, for the experimental campaign, low-cost ollae were made using commercial terracotta pots and plant saucers.

Concerning the system for the ozonization of water, several methods of ozone generation were examined. The corona-effect ozone production was found to be the most cost-effective of the techniques analyzed [44]; the main benefit comes from the inherent ability of ozonizers to be designed with high performance and compact size. The ability to generate ozone with simultaneous minimization of the production of additional irritant gases is highly advantageous. This approach improves the efficiency of the ozonation process but also helps to extend the operational life of the cells used. The corona-effect ozonizers are devices that operate among an electrical discharge in gas to produce ozone (O₃) from oxygen (O₂) in the air. For these reasons, the corona-effect ozone production was used to ozonize water. In detail, the water ozonation process for the experimental campaign was carried out through a purpose-built system consisting of an insulated, heatproof box. Inside this box, there is a small control unit installed on the lid, which operates an ozone producer with a capacity of 0.5 g/h and two small pneumatic diaphragm pumps. The use of two pumps has been designed to manage two distinct hydraulic circuits, each with a specific function. The first circuit is dedicated to the circulation of ozonized water inside underground terracotta pots, ensuring a controlled release to the plant roots. On the other hand, the second circuit is for surface irrigation and uses the ozonized water pushed at a specific pressure toward a system of atomizing nozzles, commonly known as sprayers. Both circuits can be activated independently or combined, depending on agronomic needs and operating conditions, offering a high degree of flexibility in optimizing irrigation strategies.

During this phase, the choice of site for the preliminary experimentation was oriented toward the Metropolitan area of Bari (Puglia Region, in the south of Italy) because of its distinctive climatic characteristics, particularly critical during the summer. This region, which has a Mediterranean climate, is characterized by hot and dry summers, with temperatures often above 30 °C and extremely low rainfall. The experimental period was from 25 June to 27 August 2022, during which the average maximum temperature was 36.3 °C in the middle hours (between 12 p.m. and 4 p.m.), highlighting an exceptional heat wave for the period. In this critical climatic condition, the experimental campaign also aimed at comparing traditional and innovative irrigation techniques: thus, seven installations were realized with different equipment systems. For each experimental setup, three plants were used to observe the replicability of the treatment and irrigation type effects on the plants. The common element in all installations was the use of PVC pots measuring 100 × 40 cm, placed in series (Figure 3) and used as experimental plant containers.

Therefore, the seven installations (Figure 4) are the following: without ollae, Traditional Watering (TRW): the PVC was filled with soil, and the traditional irrigation system was applied. This installation worked as the reference case for comparison with all others; Nebulized Ozonated Water (NEBOW): the PVC pot, filled with soil, was equipped with three atomizers mounted along the edge, installed for programmed delivery of ozonated water (0.5 g/h for 15 min in 5 L of water); Ollae with Ozonated Water + Nebulized Ozonated Water (OLOW-NEBOW): the PVC pot, filled with soil, was equipped with three buried terracotta pots (ollae) that were connected in series by rubber hoses, through which ozonated water circulated, while three atomizers, programmed for controlled delivery of ozonated water (0.5 g/h for 15 min in 5 L of water), were installed along the edge; Ollae with Ozonated Water (OLOW): the PVC pot, filled with soil, was equipped with three buried terracotta pots (ollae) that were connected in series by rubber hoses, through which ozonated water circulated (0.5 g/h for 15 min in 5 L of water); Ollae with Water Normally Filled (ONW): the PVC pot, filled with soil, was equipped with buried terracotta pots (ollae) were filled manually with normal water; Pots Immersed in Ozonated Water (PIOW): the PVC pot were filled with ozonated water (0.5 g/h for 15 min in 5 L of water), inside which three large terracotta pots were completely submerged, filled with soil and painted at the top. These pots were firmly anchored to the bottom by a metal frame, ensuring an airtight seal. In order to prevent water evaporation and contamination by insects or other external agents, the entire installation was covered with a sheet of white polycardboard with three 10 cm-diameter holes, corresponding to the openings of the pots. The pot immersed in normal water (PIW) has the same configuration as PIOW, with the only difference being the use of normal water.

To take full advantage of the potential of the Internet of Things (IoT) in agriculture, even on a small scale, such as a balcony, a low-cost system has been developed. The key element of this system is an AZ-MEGA2560 Arduino board, equipped with Wi-Fi connectivity, which manages a set of sensors and actuators for the production of ozonized water. With this type of board, it was possible to implement a monitoring system, which was able to measure air temperature, air relative humidity, soil moisture sensor, and Ph. These data, acquired every 30 min, provided a detailed picture of the environmental conditions in which the plants developed. In order to optimize water use, irrigation interventions have been scheduled mainly in the evening and night hours, when temperatures are lower and evaporation is minimal. Only in the event of a significant drop in soil moisture during the day was an additional afternoon intervention planned. This strategy, in addition to reducing water wastage, helps to create an ideal microclimate for plant growth.

Lastly, for each installation, the cultivation of Lactuca sativa (a Roman variety) was chosen. This crop type is generally considered a hardier variety than others because it can tolerate a wider range of climatic and soil conditions.

Results of Phase 1

Concerning the first phase, below are the results as graphs of the data analysis, collected over an observation period from 9 July to 27 August, which includes these variables: plant height (max.), leaf width, and length (max.). The objective, therefore, is to identify the most efficient and sustainable technique based on the parameters evaluated (Figure 5).

Data analysis revealed significant differences in vegetative growth parameters due to the irrigation methods and treatments used for each experimental set. The systems with ozonated water, both nebulized and submerged, positively influenced plant growth. In particular, the OLOW-NEBOW system showed the greatest height and width of the leaf area. This observation could indicate that the presence of ozone in the water provides an antibacterial action or stimulates physiological processes in the plants, promoting more vigorous growth. The OLOW-NEBOW system suggests that controlled irrigation by means of ollae, in combination with ozonated and atomized water, is particularly efficient in the support of optimal vegetative growth. Immediately after OLOW-NEBOW in the ranking for greater height and size of leaves, there is the observation of OLOW with automatic sub-irrigation with ozonized water, where the presence of the ozone in the water provides an antibacterial action or stimulates physiological processes in the plants, promoting more vigorous growth. In contrast, irrigated sets without ozone, such as TRW and PIW, show lower values in leaf height, width, and length than ozone water treatments. This finding indicates that normally filled water without ozone treatment does not provide comparable impulses for fast and vigorous growth, highlighting the importance of ozone as a potential nutrient supply for plant development. The moderate growth observed in the ONW (pots with normal water) and PIW systems may also suggest that the pots provide slower benefits in the absence of ozone, although they may help stabilize water availability. However, the overall effect of ozone seems to contribute more to the increase in leaf area, which is crucial in vertical growing systems, where increased light exposure and photosynthetic efficiency are crucial.

Moreover, an in-depth analysis of the following features was conducted about the installation process, such as the cost of installation and difficulty of realization (

Table 1. Cost and complexity of installation for the different experimentation setups.

Installation	TRW	NEBOW	OLOW-NEBOW	OLOW	ONW	PIOW	PIW
Cost installation (€)	30 €	60 €	105 €	80 €	50 €	125 €	85 €
Realization difficulty	2/10	6/10	6/10	5/10	5/10	9/10	8/10

); parameters for maintenance, such as water consumption, energy consumption, and human time for green maintenance (

Table 2. Parameters for green maintenance per week in the observation period.

Parameters for Maintenance	TRW	NEBOW	OLOW-NEBOW	OLOW	ONW	PIOW	PIW
Water consumption (L)	3.5	4	5.5	2.5	2	1.5	1.5
Energy consumption (W)	0	9	18	15	0	5	0
Human time for green maintenance (minutes)	5 min	3 min	3 min	0 min	7 min	1 min	1 min

). The objective is to identify the best eco-smart solution between OLOW and OLLOW-NEBOW, considering that both have reached the greatest heights and leaf dimensions, which are prioritized both in terms of increased productive cultivation and mitigation effects. At the same time, Table 1 and Table 2 show a comparison of the different characteristics of all installations for their possible use in horizontal cultivation on balconies.

In particular, the installation cost includes materials, equipment, and devices. The difficulty of realization is obtained by normalizing the installation’s realization time on a base of ten, which considers a minimum of 15 min to a maximum of 180 min. Water consumption was measured by considering the level in the irrigation tanks. Energy consumption was measured by means of a calculation that considers the operating time of the pump for water circulation and human time for green maintenance by means of observations during the experiment.

As can be seen from the summary table, the techniques show significant variations in resource consumption. The ONW system is characterized by a particularly low water consumption of 2 L, while the OLOW and NEBOW systems require 2.5 and 4 L, respectively. In terms of energy consumption, ONW and TRW systems are undoubtedly the most sustainable, due to zero energy consumption, as there is no ozone production process and the water supply is contributed manually. In contrast, the OLOW-NEBOW system shows a consumption of 18 watts, indicating higher energy consumption than the other techniques, since it is designed as a stand-alone system. In short, compared to water consumption in TRW, there was a reduction of 28.57% for OLOW, 42.86% for ONW, and 57.14% for PIOW and PIW; on the contrary, an increase in water consumption occurred in NEBOW (14.29%) and OLOW-NEBOW (57.14%).

The human time required also varies between techniques: those requiring the most effort are ONW, at 7 min, and TRW, at 5 min. In comparison, the other techniques considered have virtually no maintenance time. In terms of visual quality, evaluated in relation to both overall esthetics and plant growth, OLOW-NEBOW offers the most comprehensive and satisfactory results, followed by NEBOW and OLOW techniques. In contrast, the ONW and TRW techniques present less appealing visual quality and growth. Regarding the quantity of production, it is observed that the techniques employing ozonated water, which are NEBOW, OLOW, and OLOW-NEBOW, resulted in significantly higher development as early as the first half of the experimental month, compared to the techniques not employing ozone dilution in water. Moreover, installation costs and difficulty of implementation are two quite complex issues. TRW turns out to be the cheapest solution, with a cost of €30, and the easiest to implement. In contrast, OLOW-NEBOW has the highest installation cost (€105) and the highest difficulty of implementation, while NEBOW and OLOW show intermediate cost and difficulty, respectively.

Concerning the parameters measured by the PIOW and PIW installations, which are characterized, respectively, by the immersion of the vessels in ozonated and non-ozonated water, the comparative analysis highlights some relevant differences. Both parameters present similar values in terms of water consumption and runtime. However, significant differences are observed in other parameters. PIOW has a higher energy consumption than PIW, which is not very energy-consuming. In terms of installation costs and difficulty of implementation, PIOW is slightly higher than PIW. Despite these differences, both offer comparable results in terms of visual quality and production quantity. Therefore, the optimal choice between PIOW and PIW depends on the specific application requirements. If the primary objective is to minimize costs and system complexity, the PIW is the most suitable solution. Conversely, if a slightly higher level of performance is required against a higher initial investment and implementation complexity, the PIOW may be preferable.

In conclusion, comparing the results in Table 1 and Table 2, in relation to the two most effective configurations (OLOW and OLOW-NEBOW) for lettuce growth and plant well-being, OLOW shows the best performance in terms of cost-effectiveness (80€ vs. 105€) and less difficulty in implementation (5/10 vs. 6/10) (Table 1), as well as water consumption (2.5 L vs. 5.5 L), energy consumption (15 vs. 18 W), and human time for green maintenance (0 min vs. 3 min) (Table 2).

From the comparison of these results, in relation to different configurations and scenarios for lettuce, OLOW is shown to be the most effective solution for plant well-being. This configuration offers dual action: ozone is delivered both through the nebulized ozonated water to the leaves and by the osmotic effect from the ollae to the soil. This approach provides a significant improvement in plant growth conditions and accelerates development time.

3.2. Phase 2: Development of a Façade Module Design

Based on the vertical farm [45] and green façade concept [11,46], which are characterized by low systemic technology, and few constituent elements [47], a lightweight, sustainable, and low-cost vertical module was produced. This vertical module (Figure 6a) was defined to be self-build and host a little vertical farm for application on balconies or building façade. In particular, the structure consists of five tubular sub-modules, which have a length of 160 cm, a height of 40 cm, and a depth of 10 cm. These sub-modules are designed to be assembled in a self-supporting system, reaching a total height of about 235 cm. This modular arrangement offers significant flexibility in system configuration, allowing users to adapt it to their specific needs. The use of standardized sub-modules allows for easy assembly and disassembly, making the system versatile and adaptable to various contexts, both internal and external (Figure 6b).

The tubular structure in chromed iron provides strength and stability to the disposition of rectangular vases in plastic (38 × 8.5 × 9 cm). In these planters, a simple soil stratigraphy was made: a layer of expanded clay on the bottom and then a layer of potting soil. Due to its porosity, expanded clay improves soil drainage and prevents water stagnation, reducing the risk of root rot. It also creates a good environment for the aeration of the roots, promoting their healthy growth. The top-soil layer provides the nutrient-rich substrate necessary for proper plant development.

With reference to the decision and choice of plants, an analysis was conducted of the species considered most suitable for vertical experimentation in relation to the long-lasting shading effect. In particular, a thorough analysis of the physiological characteristics of vegetation and its environmental requirements was carried out, with a specific focus on aromatic plants. Based on these studies, a number of typical species of the Mediterranean area were selected, characterized by high resistance to arid climatic conditions and low water requirements, which are perfectly compatible with the developed vertical cultivation system. The selected species, with their respective water and exposure requirements, are summarized in the table below (

Table 3. Analysis of aromatic plants.

Scientific Name	Plant Characteristics	Environmental Requirements	Other
Menta piperita officinalis	Perennial herbaceous Bushy plant Height max: 70 cm Leaves: lanceolate, purplish green Flower: VII–IX, pink	Water: regularly (moderate) Light: full sun exposure	Aromatic plant Grows in moist and well-drained soils Soil pH: 7
Ocimum basilicum	Semi-resistant annual plant Upright plant Height max: 60 cm Leaves: lanceolate, intense green Flower: VI–IX, white	Water: regularly (moderate) Light: full sun exposure	Aromatic plant Grows in moist and well-drained soils Soil pH: between 6.0 and 7.5
Origanum vulgare	Perennial herbaceous Bushy and upright plant Height max: 80 cm Leaves: lanceolate and opposite, intense green Flower: V–IX, white/lilac	Water: regularly (moderate) Light: full sun exposure	Aromatic plant Grows in moist and well-drained soils Soil pH: between 5.0 and 7.5
Origanum majorana	Perennial herbaceous Bushy plant Height max: 70 cm Leaves: lanceolate and opposite, green Flower: VII–IX, withe/pink	Water: regularly (moderate) Light: full sun exposure	Aromatic plant Grows in moist and well-drained soils Soil pH: between 6.0 and 7.5
Rosmarinus erectus officinalis	Evergreen shrub Upright plant Height max: 120 cm Leaves: linear-lanceolate, dark green Flower: III–IX, light blue	Water: regularly (moderate) Light: full sun exposure	Aromatic plant Grows in moist and well-drained soils Soil pH: between 5.5 and 8.5
Salvia officinalis latifolia	Evergreen undergrowth Bushy plant Height max: 50 cm Leaves: oblong, silver gray-green Flower: V–IX, violet	Water: regularly (moderate) Light: full sun exposure	Aromatic plant Grows in moist and well-drained soils Soil pH: between 6.5 and 8.5

).

3.3. Phase 3: Implementation of the Green and Smart Façade Module

From the Phase 1 results of experimentation on horizontal cultivation, the OLOW system—such as the sub-irrigation system with ollae and water ozonated—was chosen to be applied in the vertical module. For this system, terracotta pots with the following dimensions were used: 6 cm in height and 7 cm in diameter. Each small pot was sealed at the top with its corresponding plant pots (diameter: 7 cm, Figure 7a) to create a small water container. Additionally, these pots were placed inside the planters, with one positioned centrally and the other two laterally (Figure 7b).

All terracotta pots were connected in series through the use of a rubber hose and tee fittings. This allows water to be distributed to each mini olla via a single fitting, which has a dedicated outlet and inlet for water flow. Specifically, the fountain system, which was designed to fill large, hermetically sealed containers, using two separate circuits for supply and return, was studied and subsequently adapted. In this case, to ensure uniform circulation and total filling of each container, a small rubber tube of the length and diameter required to fill the small container was introduced into the fitting. This small tube was passed through the threaded short side of the fitting, following an elbow path (Figure 8a) to avoid bottlenecks. In this way, the fountain system would work as follows: the fluid enters at the bottom of the container, allowing for a gradual inflow. During filling, the air initially present inside the terracotta pot is evacuated through a vent circuit, the purpose of which is to prevent the formation of air bubbles that could compromise the hydraulic efficiency of the system. This vent circuit consists of an extension that extends to the inner top of the vessel, allowing the air to escape in a controlled manner and guaranteeing the continuity of the flow. In this way, the liquid can continue uninterrupted to the next terracotta pot, ensuring stable and efficient operation of the entire system (Figure 8b).

Following the design and implementation of the sub-irrigation system and water circulation circuit, the plants were placed in the spaces between the buried terracotta pots. This configuration was adopted to maximize water absorption by the roots, exploiting the natural porosity of the terracotta, which allows a gradual and controlled release of water into the substrate. A total of twenty planters were installed in the module, distributed in a number of four on each level. Inside these, forty-two plants belonging to the previously described aromatic species were placed (Figure 9).

To ensure adequate hydration of the crop substrate, the irrigation system was designed to operate through programmed cycles of water circulation and ozonation. The management of these processes was entrusted to an electronic microcontroller belonging to the Arduino family, such as the Arduino Mega 2560 + ESP8266, which performed a dual function. On the one hand, it performed timed programming of the system’s activity, adjusting the frequency and duration of irrigations; on the other, it integrated an environmental monitoring system based on low-cost sensors capable of detecting critical parameters for the optimization of operating conditions. To improve the sustainability of the system and ensure its energy self-sufficiency, a photovoltaic panel was integrated into the vertical green module to power the electronic components and reduce dependence on external power supplies. The total cost incurred for the development of the vertical green module was approximately EUR 780. This cost was broken down into three main components: the gardening section, which included plant materials, substrates, and irrigation elements, amounted to EUR 230; the structural section, which included the modular support frame and fastening systems, amounted to EUR 160; the electronic section—dedicated to environmental monitoring through sensors, data acquisition systems and the use of a photovoltaic panel—represented the most significant investment, with an approximate cost of EUR 390.

During this phase, the following environmental parameters were monitored: external relative humidity and temperature; soil humidity, and temperature. The acquisition of these data was transmitted in real-time through a Wi-Fi connection to a specially developed webpage for the visualization and monitoring of the information, allowing continuous control of the system and enabling timely intervention in the event of anomalies or malfunctions.

3.4. Phase 4: Validation of Prototype’s Effectiveness in Reducing Indoor Temperatures

At the end of the implementation phase of the structure, which was also conceived as a transportable module, it was installed in the selected experimental site, located in the metropolitan area of Bari (in the same city as the experimentation of phase 1—Climate Zone C with 1185 degree days). Unlike phase 1, the study focused on two office rooms on the second floor (Room A and Room B), each with a surface area of 17 m² and equipped with a French door in aluminum without a thermal break measuring 1.45 × 2.50 m, and with a balcony connecting the two rooms (Figure 10).

The choice of these rooms was influenced by their lack of air-conditioning systems and their solar exposure (south–east), which allows for the effectiveness of the proposed system to be analyzed when comparing indoor temperatures between Room A and Room B under the same usage conditions. This configuration makes it possible to assess the system’s impact on thermal performance and indoor comfort.

Both rooms are located on the same intermediate floor (on the second floor) of a building with mixed residential and office use and are characterized by hollow block masonry perimeter walls with cavity (U-value = 1.42 W/m²K) and thickness of 0.30 m, internally plastered with cement mortar and without thermal insulation. The windows are not fitted with internal or external sun-shading during the monitoring period, and the rooms have no mechanical ventilation or artificial air-conditioning systems.

In this phase of the experiment, the structure was positioned in front of one of the two French windows in order to maximize the interaction between the green module and the interior environment (Figure 11). This configuration allows for the influence of the system on microclimatic conditions to be analyzed. In order to more accurately quantify the impact of the system, a comparative approach was adopted between the environment equipped with the green module and the one without it. To achieve this, a temperature sensor was also installed in the indoor environment to monitor thermal variations and determine the effectiveness of the system in influencing the indoor microclimate.

The observation period was conducted in the month of June 2024, characterized by an intensification of extreme thermal conditions, with a high point recorded in the last week. In the period between 23 and 30 June, the average maximum temperature reached 37 °C, and the average maximum relative humidity was 81%, highlighting a particularly warm climatic context, relevant to the analysis of the impact of the green module on microclimatic conditions. During this period, data were collected on outdoor temperatures (T. outdoor), indoor temperatures (T. indoor), and indoor temperatures with the application of a vertical green module (T. indoor with vertical green module). The objective of the analysis is to evaluate the thermal variations between these three conditions, with a focus on the effectiveness of the vertical green module in mitigating indoor temperatures.

Results of Phase 4

As previously mentioned, an intensification of temperature extremes was recorded during the observation period, with a particularly high value concentrated in the last week of June. Therefore, it was deemed appropriate to focus the analysis on the data collected during this period (from 23 to 30 June 2024); the results are presented in the following graphs (Figure 12). In particular, the data reported refer to the average air temperature values measured (T. indoor and T. outdoor) by the three sensors for each measurement point, and the standard deviation.

During the observation period, the outdoor temperature showed a wide range of variability, with values between 19 °C and 39 °C. In particular, the maximum temperature peak was recorded on 30 June, a day when the temperature reached 39 °C at 4 p.m., highlighting a climatic context characterized by strong solar radiation and heat waves. The indoor temperature of the room without mitigation systems (Room A) followed a similar trend, ranging between a minimum value of 20 °C and a maximum of 44 °C, with particularly high peaks in the central hours of the day.

In comparison, the indoor temperature of Room B, where the vertical green module (VGM) was installed, showed a systematic reduction compared to the other room, with values between 20 °C and 41 °C. This difference suggests that the presence of the green module helps to moderate the heat build-up within the room, mitigating more intense thermal oscillations and ensuring a relatively more stable indoor climate. The analysis of the data showed that the adoption of the VGM resulted in a constant lowering of the indoor temperature, with a maximum difference of 3–4 °C during the daytime hours and 0.5–1 °C at night.

The mitigating effect is particularly pronounced in the middle hours of the day, i.e., when solar radiation is at its highest and building surfaces tend to reach high temperatures. For example, on 23 June at 2 p.m., the indoor temperature in the room without VGM mitigation reached 33 °C, while in the room with the vertical green module, the temperature recorded was 30 °C, showing a reduction of 3 °C. A similar behavior was observed on 30 June, during one of the hottest days of the observation period: with an outdoor temperature of 39 °C, the indoor temperature in Room A reached a maximum value of 44 °C, while in Room B with VGM reached a value of 41 °C.

Considering that VGM allowed a reduction in internal temperature up to 3–4 °C, it can be helpful to reduce air-conditioning use and so it can support the improvement of energy-efficiency of buildings and urban space.

The results obtained in this study are in line with those documented in the literature in similar climatic contexts, confirming the effectiveness of vertical green systems in reducing the internal temperature of building environments. Although it is a localized intervention and not applied on extended surfaces, a significant thermal reduction was nevertheless observed, with values comparable to those found in other larger-scale case studies. For example, the study conducted by de Jesus et al. [48] evaluated the effectiveness of green facades in improving the thermal behavior of buildings, through on-site measurements at the Caixa Forum Museum in Madrid, Spain. The results showed a reduction in air temperature between 2.5 °C and 2.9 °C during the summer season, and up to 1.5 °C in the autumn season. These reductions are mainly attributed to the shading provided by the vegetation and the evapotranspiration, which contribute to a cooler microclimate around the building.

4. Discussion and Conclusions

This research has facilitated the collection of data on horizontal cultivation techniques, establishing a critical knowledge base for assessing the performance of different irrigation systems and their impact on crop growth. All the sub-irrigation systems allowed for a crucial water saving (from 28.57 to 57.14%) compared to Traditional Watering systems (TRWs). Considering both the optimization of water use and the enhancement of plant growth and health, the experimental analysis identified the sub-irrigation system with ozonized water (Ollae with ozonized water—OLOW equipped by IoT sensors and actuators controlled by Arduino Mega) as the best eco-smart solution. A further replication of these innovative treatments (with ozone) and sub-irrigation systems could be necessary in future research in order to test them under similar conditions.

Anyway, these results constituted a fundamental step in the design and implementation of a low-cost vertical green module (VGM) integrating the OLOW system. The VGM was developed as a nature-based solution, integrating advanced technologies, with an intelligent system based on sensors and actuators for monitoring and control of growth parameters, as well as a sub-irrigation system with ozonated water. This solution with a sub-irrigation system has demonstrated the potential to significantly reduce water waste and improve crop quality, limiting the incidence of plant diseases and reducing the need for chemical treatments. Furthermore, the module was designed to be completely energy self-sufficient, reducing dependence on the electricity grid (thanks to photovoltaic panels) and improving the overall sustainability of the system. VGM with its shading effects proved to be able to mitigate indoor temperatures and so reduce air-conditioning use, enhancing the energy-efficient behavior of buildings and urban space. Despite the fact that the VGM has been shown to contribute to a reduction in indoor temperature of up to 3 °C, the analysis of the data suggests that this effect, although significant, could be insufficient to ensure optimal thermal comfort conditions in the presence of extreme heat waves. In particular, on days with high outdoor temperatures, the indoor temperature still remained above 40 °C. This suggests that the cooling effect of the VGM, although advantageous, needs additional passive cooling strategies, such as natural ventilation, in order to improve indoor comfort.

Both horizontal and vertical cultivation solutions show how the redesign of an ancient technique—sub-irrigation systems (ollae)—with technological innovation and ozonated water, supported by an experimental campaign, can significantly improve the efficiency of urban cultivation, minimizing human intervention and promoting the transformation of urban spaces into vertical production systems. Moreover, the proposed solutions, integrated directly into the façades and balconies of buildings, offer an innovative and cost-effective model to enhance the energy behavior of buildings, reducing the use of air conditioning and thus air pollution. Indeed, this approach provides multiple benefits, including reducing CO₂ emissions, increasing oxygen production, and promoting “0 m” horticulture. In addition to the ecological and productive benefits, the introduction of these systems can generate psychological and social advantages, encouraging a connection with nature in highly urbanized contexts and stimulating greater awareness of the importance of local food production. On a larger scale, the adoption of VGM could contribute not only to improving food security but also to mitigating the negative effects of urbanization on natural resources.

Moreover, the results obtained during the second phase of the experimentation, conducted under real operating conditions, suggest that VGMs can serve as an effective strategy for improving indoor thermal comfort in urban settings, while also contributing to the mitigation of the urban heat island effect and enhancing overall environmental quality. This type of intervention is particularly strategic in densely built environments, where available ground space for horizontal greenery is extremely limited.

From this perspective, the application of VGMs may positively influence urban policies and design practices, encouraging the adoption of regulations that require or incentivize a minimum amount of green surface per building. Such requirements could be met through technical solutions such as green roofs, green facades, or modular systems like the one tested in this study. The introduction of incentive mechanisms—such as increased allowable building volume or tax benefits—could act as a significant driver for the widespread integration of green infrastructure in the built environment.

In addition, the proposed VGM combines agronomic parameters (use of aromatic plants for productive purposes), water (rational management of irrigation through sub-irrigation technique), and energy (monitoring of indoor and outdoor temperatures), presenting itself as a system replicable on an urban scale but also adaptable to small-scale residential contexts. In this way, the module demonstrates the potential of green facades not only for thermal mitigation but also for domestic production and sustainable resource management.

In terms of transferability, the modular nature of the tested structure allows it to be adapted to different building typologies and climatic contexts. Its lightweight, ease of installation, and potential for disassembly and reuse make it a scalable solution suitable for both new construction and retrofitting of existing buildings. The integration of these systems into urban regeneration strategies and climate resilience plans, therefore, represents a concrete and desirable direction for future regulatory and design developments.

Further studies will be necessary to explore additional parameters, including effectiveness in sound insulation, CO₂ absorption potential, and impact on indoor environmental quality. Moreover, the reliability and precision of data collection could be significantly improved by employing higher-quality instrumentation. The low-cost sensors used in this study, although functional for preliminary assessments, exhibit limited accuracy and durability over time, which may affect the robustness of the recorded data. Therefore, future research should consider the use of professional-grade monitoring equipment, despite the higher costs, to enhance the validity of the results. Additionally, extending the duration of the experimental campaign beyond the summer season is essential to evaluate the system’s performance under a broader range of climatic conditions. Monitoring over multiple seasons would allow for a more comprehensive assessment of the system’s long-term effectiveness, adaptability, and contribution to user comfort across different environmental contexts.