A Literature Review on Facade Greening: How Research Findings May Be Used to Promote Sustainability and Climate Literacy in School

Abstract

The promotion of Climate Literacy is a central concern of our time. To achieve this ability, one can draw on different content areas. One possible area is Nature-Based Solutions (NBS), such as Vertical Greening Systems (VGS), and their effectiveness in mitigating climate change. However, VGS is not yet an established topic in environmental education, even if the pro-environmental effectiveness of VGS has been proven from a scientific point of view and this topic is close to everyday life. To facilitate the transfer of knowledge from research to school, this paper presents an example of a possible procedure. This procedure starts with a narrative review of the scientific literature on VGS. Then, the main results of this review are related to the Sustainable Development Goals, Climate Literacy, and general educational goals to capture its educational relevance. Finally, a flow chart for a teaching sequence is developed, with the phase sequence derived from the performed narrative review. Thus, a parallelism between the structure of a scientific review and an action-oriented environmental education becomes visible. To what extent this parallelization may be generalized, and whether teaching based on it is effective, will have to be tested.

1. Introduction

Today’s young generation increasingly claims the right to choose their future and expresses concerns about environmental deterioration. Among the environmental problems that will have to be addressed, one of the most urgent is climate change. Large cities have a substantial ecological footprint for CO₂ emissions and energy use, and rapid increase in urbanization and energy consumption has profound environmental consequences [1]. Science teachers should accompany young people on this difficult path to a more environmentally conscious and sustainable life, which requires transferring scientific knowledge from research to school. Promoting Science Literacy and Climate Literacy is, in fact, crucial to this extent, but this is not enough. Practical experience of nature is also necessary to develop pro-environment attitudes and behaviors [2]. Facade greening is a topic that allows students to experience nature and, at the same time, learn about climate change and basic climate protection measures. However, facade greening is not yet considered a topic in school curricula. To make this topic better known and show the breadth of possible focal points, we prepared a narrative review of the findings on facade greening.

The goal of this narrative review is partially different from the goals of conventional reviews. On the one hand, the purpose here (as in conventional reviews) is to summarize the findings and emphasize focal points of research. On the other hand, the focus of this review is less on identifying research gaps and more on inferring the educational relevance of the scientific findings. Therefore, at the end of each content section of the review, there is a small paragraph which discusses implications for teaching. In a next step, the main research findings on effects of VGS (based on the review) are mapped against significant educational objectives, namely the UN Sustainable Development Goals (SDG) and the principles of Climate Literacy. In addition, the extent to which the topic of VGS meets general pedagogical requirements is assessed. Finally, a proposal for an action-oriented teaching unit is developed, inspired by the review in terms of content and structure. Thus, we provide an example of a scientific review to become a template for the instructional design of the corresponding topic in school. An instructional flow chart is then developed (Section 3.3), which in its first two phases reflects those perspectives by which the review was structured: a design, social, and natural scientific perspective. Due to the general structure of the flow chart, it can be used for other science topics to be taught in classroom, especially those that are application-based and socially controversial. In summary, this article provides an example of a strategy for transferring knowledge from research to school.

As a lead-in to the narrative review, we will start with some definitions of central terms, which will guide us to the main topic, the greening of facades.

A possible solution to climate change and other environmental problems could be provided by nature itself. The International Union for Conservation of Nature (IUCN) defines Nature-Based Solutions (NBS) as: “… actions to protect, sustainably manage and restore natural or modified ecosystems, which address societal challenges (e.g., climate change, food and water security or natural disasters) effectively and adaptively, while simultaneously providing human well-being and biodiversity benefits” [3] (p. XII). The European Commission defines NBS in somewhat more detail still as “solutions that are inspired and supported by nature, which are cost-effective, simultaneously provide environmental, social and economic benefits and help build resilience; such solutions bring more, and more diverse, nature and natural features and processes into cities, landscapes and seascapes, through locally adapted, resource-efficient and systemic interventions.” It is emphasized that “NBS must benefit biodiversity and support the delivery of a range of ecosystem services.” [4] (p. 5). NBS may be a tool to achieve climate change mitigation and adaptation and sustainable management of urban areas, where more than half of the global population lives. [5].

Urban Green Infrastructures (UGI) can be defined as the interlinking of two elements: green spaces (and their associated ecosystem services) and urban planning [5]. Green spaces such as urban parks, green roofs, green facades, and green walls are all examples of UGI in the general framework of the NBS. Their manifold benefits consist of the mitigation of and adaptation to climate change effects, as well as the improvement of the physical and psychological well-being of the entire community [6]. These green spaces also constitute oases for other living beings that are advantageous in preserving biodiversity [7]. This literature review will focus on a particular type of NBS: Vertical Greenery Systems (VGS), such as green facades and living walls.

2. Findings of Scientific Research on Façade Greening

Most of the studies on VGS have been conducted by engineers and architects. They are primarily focused on the engineering aspects of construction techniques, energy saving, cooling of building interiors, etc. However, while addressing their interests in their own technical fields, the researchers have also defined general concepts and models that allow a more comprehensive understanding of the VGS topic. In particular, findings of general interest are related to the classification/characteristics of the various types of VGS, and the plant species most suitable for use in VGS. Other relevant findings are on the public acceptance of VGS and the effects of VGS on buildings, climate and environment, and health and well-being.

2.1. Different Types of Vertical Greenery Systems (VGS)

According to Wong et al. [8], VGS is any way of setting plants on the vertical facade of a building. A clear, easy-to-follow VGS classification was proposed by Pérez, Coma, Martorell, Cabeza [9]. This approach distinguishes two main categories of VGS (

Table 1. Classification of VGS.

VGS Type	Subtype
Green facades	In traditional green facades, climbing plants grow directly on the wall/facade of buildings. In general, these plants are planted directly in the ground around the building. In double skin green facades, there is a separation between the wall and the foliage cover. The plants can grow, for example, with the support of meshes, cables, wires, etc. In perimeter flowerpots, hanging shrubs are planted in a structure on top of the building and constitute a green curtain around the building perimeter.
Living walls	In living walls, plants are grown in panels or geotextile felts, sometimes pre-cultivated, which are fixed to a vertical support or on the wall structure.

): green facades (GF) and living walls (LW).

A similar and more recent classification is based on the functional elements of VGS, consisting of: (i) supporting elements; (ii) growing media; (iii) vegetation; (iv) drainage; and (v) irrigation [10]. In this classification, green facades are characterized by a low systemic technology. Instead, living walls require more materials and more technologically complex structures and may be continuous or modular. Green facades can be further divided into two subgroups: direct green facades and indirect green facades. In the first case, the plants grow directly on the wall, whereas in the second case, there is a structural support for plant growth [10].

Implications for teaching. Two main VGS classification systems have been identified. Due to its lower complexity, we consider the first classification system more appropriate for teaching purposes, because it can be easily used to classify VGS in the immediate vicinity of the students, and possibly design or set up a VGS in their school.

2.2. Suitable Plant Species

The choice of plants to be used in a VGS depends firstly on the VGS type.

Traditional green facades require climbing plants, which may be deciduous or evergreen species. Two species are mainly used: Hedera helix (an indeciduous species), and Parthenocissus tricuspidata (a deciduous species) [9].

Green facades with supporting structures mounted on the wall allow a greater variety of plant species in the VGS. This is because the climbing species that use filiform tendrils, spines, and other strategies for vertical development can take advantage of the support structures to grow.

An even greater variety of species may be used with living walls, also in combination. Shrubs, grasses, perennials, and herbaceous plants (well adapted to local conditions) may be used, but shrubs and herbaceous plants, which are evergreen, are most commonly found [9].

Since the plant species chosen should be suitable to the environment of the geographical area in which they grow [11], the choice is also conditioned by the climate of the place where the green facade is set up. The climate determines plant species’ survivability and their growth. To compare and standardize the scientific literature findings, we follow the Köppen-Geiger classification (Figure 1) [9,12].

This classification divides the various climates into five main climate groups, with each group being categorized on the basis of seasonal precipitation and temperature patterns. Examples of a selection of plant species based on the VGS type and climate are reported in

Table 2. Examples of plant selection depending on VGS type and climate according to the terminology of Köppen-Geiger.

Type of VGS	Climate (Köppen Geiger Classification)		Plant Species
Traditional green facade	Cfa	Warm temperate; fully humid; hot summer	Parthenocissus tricuspidata (Boston Ivy) [13]
	Cfb	Warm temperate; fully humid; warm summer	Hedera helix(Ivy) [14] Parthenocissus tricuspidata (Boston Ivy) [15] Stachys byzantine (Lambs’ Ears) [16]
	Dfa	Snow, fully humid, hot summer	Parthenocissus tricuspidata (Boston Ivy) [13]
Double skin green facade	Cfa	Warm temperate; fully humid; hotsummer	Phaseolus vulgaris (Common Bean) [13]
	Cfb	Warm temperate; fully humid; warm summer	Hedera helix (Ivy) [13] Parthenocissus quinquefolia (Virginia Creeper) [13] Phaseolus vulgaris (Common Bean) [13]
	Csa	Warm temperate, summer dry, hot summer	Clematis sp. [13] Hedera helix (Ivy) [13] Lonicera japonica (Japanese Honeysuckle) [13] Parthenocissus tricuspidata (Boston Ivy) [17] Pandorea jasminoides (Bower of Beauty) [18] Rhyncospermum jasminoides (Star Jasmine) [18] Wisteria sinensis (Chinese Wisteria) [13]
	Cwb	Warm temperate, desert, warm summer	Clitoria ternatea (Asian Pigeonwings) [19] Pentalinun luteum (Hammock Viper’s-Tail) [19] Sedum sp. [19]
	Dfc	Snow, fully humid, cold summer	Parthenocissus inserta (Thicket Creeper) [20]
Living wall	Cfa	Warm temperate; fully humid; hot summer	Carex brunnea (Greater Brown Sedge) [21] Fatsia japonica (Japanese Aralia) [21] Geranium sanguineum (Bloody Crane’s-Bill) [21] Rosmarinus officinalis (Rosemary) [21] Sedum spurium (Caucasian Stonecrop) [21] Salvia nemorosa (Woodland Sage) [21] Zoysia matrella (Manila Grass) [21]
	Cfa/ Cfb	Warm temperate; fully humid; hot/warm summer	Several shrubs, herbaceous, climber species. [13]
	Csb	Warm temperate, summer dry, warm summer	Several shrubs, herbaceous, climber species [13]
	Cwa	Warm temperate, winter dry, hot summer	Euphorbia sp. [9] Zoysia japonica (Korean Lawn Grass) [9]
	Csa	Warm temperate, summer dry, hot summer	Sedum sp. [13]

.

In addition to the above general criteria of VGS type and climate, other criteria for the selection of plants are related to energy efficiency, particularly their performance in terms of cooling and energy saving in buildings [11]. The energy efficiency depends on four characteristics of the plant: structural parameters of the plants such as height and Leaf Area Index (LAI), radiative properties (albedo and emissivity), plant traits (e.g., leaf hairiness, color, thickness), and biological processes (stomatal conductance and resistance, leaf water loss) [22]. Some of these technical terms are explained in

Table 3. Definition of selected plant parameters affecting the energy efficiency of VGS.

Plant Parameter	Definition
Leaf Area Index (LAI)	The one-sided green leaf area per unit ground surface area (LAI = leaf area/ground area, m2/m2) in broadleaf canopies [17].
Albedo	The reflectivity of the leaves: i.e., the fraction of incident solar radiation reflected by the leaves (while leaf emissivity is the thermal radiation emitted from leaf surface, via outgoing long-wave radiation) [22].
Stomatal conductance	The rate of water vaporization on leaf surface, i.e., water vapor exiting through the stomata of a leaf and changing with environmental conditions to conserve moisture in plants (while stomatal resistance is the opposite) [22].

.

Mosses are also used sometimes in green walls and may be a low-cost and low-maintenance option because of their low requirements of growing substrates, water, and nutrients, and due to their high desiccation tolerance [23].

Implications for teaching. A large variety of plant species may be used in VGS in relation to the VGS type, climate and plant specific traits. Depending on where students live and the climatic conditions in that area, the VGS plants to be studied in class are selected. These plants should be part of the local environment and easy to find in the proximity of schools. Thus, more relevance to students’ life can be ensured.

2.3. Public Acceptance of VGS

A key point in designing and setting green facades in an urban environment is how citizens perceive them.

A study conducted in 2020 [24] investigated the pleasantness of spaces with or without green facades, through an online survey aimed at two target groups (ordinary citizens and architecture and urban planning specialists) in two European countries (Slovenia and the Netherlands). The study supports the initial hypothesis: people in general perceive walls covered with greenery as more pleasant than bare walls, although with differences between the citizens of the two countries interviewed and also between the two target groups (general public and professional community). In addition, the reasons for appreciation are not always the same between different countries: the authors mention as an example that, while in Greece an opinion poll showed citizens particularly attentive to the aesthetic aspect, a Malaysian study revealed that citizens see green facades as a form of “street art”, while in hot and humid cities such as Singapore, vertical green is perceived as a type of urban greening.

In another study conducted in Genoa (Italy) in 2015 [25], a survey was carried out to understand the perception of citizens on the setting of green facades on the walls of buildings before their installation. By doing the survey first, the socio-cultural environment of the area can be considered in a better way. The study showed that the citizens of the area were more favorable than the employees working in the building on which the green facade would be set, the latter being worried about the possibility of maintenance work or renovation of the wall due to damage to the facade by the climbing plants. However, all the respondents considered the contribution to biodiversity and air quality a particularly valuable effect. Previous studies had highlighted ambivalent feelings: if in Sydney (Australia) green envelopes are now known and accepted, other studies made across Europe showed wariness of possible negative effects, such as damage to the facade, need for maintenance, or presence of insects. The improvement in air quality and the possibility of a habitat for birds was perceived as a positive effect.

In another study conducted in Lisbon in 2018 [26], a survey tool was created to estimate the socio-economic feasibility of setting up green walls in public buildings, analyzing the economic costs and benefits (cost/benefit analysis). The building in question was a primary school, so the attention was focused on the users of the building: children in the development phase for whom the negative and positive effects are crucial. As a major benefit, it was considered that the application of VGS could play a significant positive role in the physical, psychological, and social well-being of children, in their cognitive development, and consequently in their academic performance. Since it may be assumed that green facades positively affect children’s well-being and improve student learning, schools could be the ideal location to set VGS.

Implications for teaching. The presence of VGS is generally accepted by the public, however it may also raise concerns and controversies. A survey might be prepared in class and then carried out by students to understand the public perception of VGS pros and cons. The survey results may be analyzed in the class with regard to possible barriers to the installation of VGS, whether these barriers are valid and how one could try to reduce these barriers for the benefit of climate protection.

2.4. Effects of VGS

2.4.1. Effects on Buildings (Urban Ecology)

Research in the last decade has focused on VGS as a good solution for saving energy inside buildings, in the cooling as well as in the heating period, obtaining positive evidence. Energy efficiency is linked to environmental sustainability as demonstrated by econometric models [27].

A comprehensive literature review was carried out in 2014 [9] in which the authors organized the literature findings on the effectiveness of VGS for energy saving in buildings. The different operating mechanisms of VGS to save energy are:

• Shade effect, i.e., the solar radiation interception provided by plants.
• Cooling effect, due to evapotranspiration from the plants and substrates.
• Insulation effect, due to the insulation capacity of the different layers (the vegetation layer, the substrate layer, the air between the various layers, etc.).
• Wind barrier effect, i.e., the capacity to modify the direct wind effect over the building facade.

Quantitative findings on energy saving attained by the different VGS types are as follows [9,28]. For traditional green facades, the reduction on the exterior surface temperature of the building facade wall ranges from 1.2–1.7 °C to 13 °C in warm temperate climate (C), and between 7.9 °C and 16 °C in snow climate (D) during the summer period. The exterior surface wall temperature reduction for double skin green facades covers a marginally broader range than for traditional green facades: 1 °C to 15–18 °C in a warm temperate climate (C). The most effective, however, seem to be living walls, that reduce the exterior surface temperature of the building wall from 12 °C to 20.8 °C in the summer period of the warm temperate climate (C). In all cases, the effect depends on the species used, the facade orientation, the foliage thickness, the substrate typology and thickness, and the air gap thickness between the plant layer and the building facade wall. Simulation studies on VGS showed an even higher efficiency: reductions in energy consumption between 5% and 50% were observed, the most frequent being between 20% and 30% in warm temperate (C) and arid (B) climates.

These findings led the authors of the review [9] to conclude that the external wall surface temperature reduction of a building is the most relevant parameter when determining the effectiveness of different VGS types, because it is the most direct effect of the presence of a sunscreen. In fact, “neither the heat fluxes through the wall nor the interior surface wall temperature are comparable due to the differences between constructive systems of the facade building wall” [28] (p. 102). Furthermore, the climate also influences the energy saving performance of the whole system (building and VGS). The climate has an effect not only on the thermal performance of the building but also on specific aspects of the plants used in the VGS, such as their growth (foliage density, plant height) and their physiological responses (transpiration, position of leaves) [28]. The authors also mentioned the main gaps in VGS research at the time of their study: although the potential of VGS to reduce energy consumption in buildings in the cooling period was demonstrated, a lack of data was highlighted for the heating period. Moreover, other aspects could be studied in more depth, such as which species are the most suitable for different climate types, the influence of facade orientation, foliage thickness, presence of air layers, and finally, substrate layer composition and thickness, in the case of green walls [9].

Other studies have tried to fill the above-mentioned gaps. Research conducted during the heating period has highlighted that:

• A green coating of Hedera helix covering the north wall of a building in Manchester (UK) increased the mean external wall temperature in winter by 0.5 °C, with energy losses reduced by around 8% [14]. In another experiment covering brick cuboids with Hedera helix in Reading (UK) for two winters, green facades were able to enhance wall surface temperatures up to 3 °C and increase energy efficiency by 40–50% [29]. It was also found that the colder the temperature the more effective the insulation [14], and that the largest energy savings were associated with more extreme weather conditions, such as cold temperatures, strong wind or rain [29].
• “As Sternberg et al. pointed out, since the insulation provided by climbers can reduce the temperature fluctuations of walls, they can reduce the chances of outer walls freezing in winter, preventing frost deterioration of the building materials” [14] (p. 32).
• In an experiment carried out in Puigverd de Lleida (Spain), under Mediterranean continental climatic conditions (Csa), living walls made with evergreen species demonstrated higher heating and cooling performance than green facades covered with deciduous vegetation (Parthenocissus tricuspidata). In the heating period, the energy saving was approximately 4.2%, due to the thermal stability supplied by the polyethylene modules [13].

Finally, several studies have focused on the beneficial effect of VGS concerning evapotranspiration in the cooling period and concerning the moisture balance. Through evaporation, incoming energy converts water into water vapor, rather than increasing the sensible heat. If the water is within the plant and the plant releases water vapor, then the term is modified to “evapotranspiration” [30]. The key findings are the following:

• The cooling potential of different plant species used in VGS was investigated, showing that certain species are more effective in cooling through evapotranspiration than through the shade effect. Evapotranspiration cooling is due to the exiting of water vapor from the stomata, i.e., to stomatal conductance, whereas the shade effect is due to leaf size, leaf morphology, and the leaves’ arrangement along the stem. For example, Fuchsia is more effective in evapotranspiration cooling, while Jasminum and Lonicera provide a greater shade effect [16].
• A lack of moisture balance may be a risk for buildings in the weather conditions of the northern hemisphere. One of the traditional and frequent ornamental species used in Scandinavia, Parthenocissus inserta, was selected to investigate the impact of the climbers on the moisture performance of facades, using a double skin green facade. VGS provides effective protection from wind-driven rain and improves moisture safety [20].

Implications for teaching. VGS has been demonstrated to have a significant effect on energy saving in buildings through several operating mechanisms. This part of the literature review is important for two reasons: the first reason is that research on VGS started here, intending to demonstrate the energy saving potential of VGS in buildings and other beneficial effects. Thus, this is basic knowledge to build on for teaching/learning about sustainable development in cities. The second reason is that this kind of knowledge may serve as a basis for developing practical activities for science lessons (for example, the measurement of relevant parameters such as temperature and humidity). This can only occur by knowing which parameters to measure and what to compare.

2.4.2. Effects on Climate and Environment

Climate change poses one of the greatest threats to society. In terms of heat waves, droughts and flooding, their effects on people and the environment are particularly evident in cities where about half of the earth’s population lives [31]. Urban temperature and local warming depend on large-scale climatic changes and localized phenomena such as the Urban Heat Island (UHI) effect, as a function of urban building density without vegetation area. This effect is caused by three main factors: “(1) increasing amount of dark surfaces such as asphalt and roofing material with low albedo and high admittance, (2) decreasing vegetation surfaces and open permeable surfaces such as gravel or soil that contribute to shading and evapotranspiration and (3) release of heat generated through human activity (such as cars, air conditioning, etc.)” [5] (p. 17).

Potential side effects should also be highlighted. Phenomena such as global warming and UHI increase energy consumption in more extreme weather events, i.e., more heating is needed for extremely cold temperatures and more air conditioning/cooling for extremely hot temperatures, thus increasing air pollution and greenhouse gas emissions [32,33].

The environment in cities is also threatened by air pollution. The main air pollutants are particulate matter (PM) and nitrogen oxides (NO_x, NO₂, and NO). A secondary pollutant, resulting from chemical reactions of NO_x with volatile organic chemicals (i.e., VOCs), is ozone (O₃) at a low level in the atmosphere. PM with a diameter of 10 μm and below poses a great danger to human health if inhaled. In fact, these very small particles (PM₁₀, PM_2.5, PM_0.1) may penetrate the lungs, then enter the circulatory system, and reach different organs of the human body. These ultra-fine particles may also enter via the nose the olfactory nerves, and thus be transported directly to the brain. NO_x, and in particular NO₂, can have serious effects on health. They may increase mortality risk and asthma, decrease lung function in children and adults, determine low birth weight and affect cognitive development [32]. Recent research indicates that fine dust also gets into the fetus of pregnant women.

In this part of the review, the focus will be on the effects of NBS (greenness, parks, trees, green facades, and green roofs) and, in particular, the effects of VGS on the city’s climate and environment. We will try to extend the evidence about the beneficial effects of NBS to VGS, because the amount of greenery in an area is increased in both cases. Therefore, similar outcomes can be expected.

Vegetated areas are generally cooler than sealed surfaces and, therefore, they are useful in an urban context to cope with the UHI effect and climate warming. The most important mechanisms through which vegetation contributes to heat reduction are the higher albedo (i.e., lower absorption of radiant heat from the sun during the day and lower re-irradiation at night) and the cooling effect due to evapotranspiration [28]. Furthermore, trees and VGS reduce heat by providing direct sun shading [34].

Among the various possibilities of using NBS in cities, VGS is particularly promising, especially where there is not a lot of space available for a park, an urban forest or a green area. The use of VGS may allow the formation of “green street canyons”, by covering with plants the vertical surface of buildings, which is much larger and available than the horizontal [28,35].

Since VGS contributes to temperature regulation and energy savings in buildings, it potentially contributes to mitigating the Urban Heat Island (UHI) effect [16,28] and also global warming [33]. As an indirect effect, these positive outcomes lead to lower greenhouse gas emissions [32,35].

Concerning the experimental evidence of the effectiveness of VGS on the urban microenvironment, findings in the scientific literature are not always concordant. On the one hand, many authors indicate an important potential of lowering urban temperatures when the building envelope is covered with vegetation [36], depending on the VGS features [8]. On the other hand, a very significant temperature reduction due to VGS influence on the surrounding built environment is not always demonstrated, pointing out the necessity of more studies [28]. Anyway, some specific research findings on the effects of NBS and VGS are presented below.

• Sequestration of CO₂ from the atmosphere
  Photosynthesis in all types of plants leads to an increase of O₂ level and a decrease of CO₂ level, improving air quality. In a wider view, green infrastructures such as NBS may be used to adapt cities to climate change, since urban vegetation removes CO₂ from the atmosphere and stores it in its structure and in the soil, so that its release is delayed [32]. For instance, a recent study conducted in Rome highlighted that CO₂ sequestration by the vegetation developing in parks of historical residences was equivalent to 3.6% of the total CO₂ emissions for the city in 2010 [37]. Several studies indicated that CO₂ sequestration is also possible with VGS [35,38]. More specifically, in a study conducted in 2014 in the Mediterranean climate, a dynamic model was developed to simulate the total sequestration of CO₂ by a VGS, from its setup until its composting in the soil (the accumulated carbon being finally stored in the soil). It was estimated that an area of 98 m² of VGS captures an average flux of CO₂ between 13.41 and 97.03 kg CO₂eq per year. More specifically, an area of 98 m² of living wall constituted by Sedum spurium, Salvia nemorosa, Rosmarinus officinalis, Geranium sanguineum, Carex brunnea, and Fatsia japonica could capture an average CO₂ flux of 60.87 kg CO₂eq per year. The results were validated by comparing the data with the literature [21]. In another study [19] conducted in 2017, a double skin facade was considered to evaluate how much carbon (C) is captured per m² of this VGS (set in Acapulco, Mexico) with two typical plants of this region (Clitoria ternatea and Pentalinon luteo). Both species covered an area of 1 m² per individual plant. The carbon content in the plant biomass was evaluated by triturating a plant and analyzing the powders with an organic elemental analyzer. Then it was estimated that for every gram of assimilated carbon, 3.66 g of CO₂ were removed from the atmosphere [19]. The results are in line with the study of 2014 mentioned above.

2.

Improvement of air quality

Greenness in a city introduces extra surfaces and allows larger deposition velocities per unit area (compared with buildings or sidewalks or parking lots) for airborne particles, pollutants, and dusts. However, the presence of plants and trees may also constitute a barrier for airborne particles’ dispersion. The barrier effect influences the air flows, so an assessment of the position of this barrier should be made before setting plants. Whereas leaf surfaces collect particulate matter, some volatile chemical compounds (i.e., VOCs) can be directly absorbed through the cuticle, and some gases such as NO₂ can be captured via the stomata. This all improves air quality [32,39,40,41]. On the contrary, air pollution can negatively affect the growth of plants and the species of plants that can grow in a given area [35,41]. Studies have confirmed that VGS has the above-described effects on airborne particles uptake (dusts and airborne pollutants such as NO_x and O₃). Influencing parameters for pollutants’ reduction are: leaf morphology, the leaf area index (LAI), foliage density and shape, plant’s health, VGS structure, VGS type, plant species, site, building canyon’s structure, season, and climate [33,35,42].

3.

Effects of evapotranspiration

The evapotranspiration phenomenon in plants increases latent heat (energy transferred in a process without changing the body’s temperature) rather than sensible heat (energy transferred in a process with a change of the body’s temperature). While the measured temperature indicates sensible heat, latent heat cannot be perceived as warmth, because it transforms liquid water into water vapor. The air humidity is thus a measure for the latent heat energy. Thus, an increased air humidity (for instance, energy consumption for evaporation) supports the downregulation of air temperature [30] and counteracts the dry air effect of the UHI. Several studies confirmed this effect for VGS [33,39,43].

4.

Regulation of temperature

In addition to the evapotranspiration effect, plants contribute to temperature control through the absorption of radiant heat and the shading effect. In the specific case of VGS, the shading effect works together with the wind barrier effect and the insulation effect [9], reducing the temperature inside/nearby the building (as described in 2.4.1). If a green facade has been set on a yard’s fence, the temperature regulation would also be outside.

5.

Regulation of water flow

In general, trees and greenness provide better water management by absorbing water through the roots. The presence of permeable surfaces and rainfall interception by plants reduces water runoff and allows better water flow regulation. In addition, VGS can positively contribute to water management at the building level and urban scale. This effect has been indicated by several studies that, in addition to the mentioned rainwater runoff control mechanism, also highlighted a positive contribution to water management of the evapotranspiration effect [30,38,39,44].

6.

Generation of urban breeze

Parks in a city may generate urban breezes, due to cooler air, that goes from green to built spaces, counteracting the UHI effect. Furthermore, at the urban boundary layer (the layer of the atmosphere immediately above the ground), a cooling effect is also caused by greenness increasing surface roughness and improving convection efficiency [39]. “In vegetated areas, the coupling between the land and the atmosphere becomes more efficient because an increase in surface roughness lowers aerodynamic resistance, generates more turbulence and higher sensible and latent heat fluxes, and leads to a wetter, cooler atmospheric boundary layer” [41] (p. 4). However, this effect has not yet been observed for VGS.

7.

Muffling noise

Vegetation and green areas in cities muffle noise, especially for sites adjacent to roads. Studies on this aspect for VGS are limited. Still, some initial findings indicate that VGS may constitute an effective passive acoustic insulation system, showing similar or better acoustic absorption characteristics than common building materials [40,45].

8.

Safeguarding biodiversity

Biodiversity gives value to an ecosystem, both in the city and beyond. This concept should be taken into account in the re-greening of a city. A study carried out in 2018 analyzed the opportunities to protect and increase biodiversity in a city. For example, green roofs, green walls, and facades may support animal life in the city [7].

A doctoral thesis [46] focused entirely on the contribution to biodiversity by green facades and living walls, analyzing which species benefited from this kind of green infrastructure. Birds, snails, spiders, insects, in general, find in these urban infrastructures oases or corridors to move from one green area to another, within the city and outside. Species’ movement and the number of species also depend on the particular characteristics of the VGS considered: area of the vegetated vertical surface, plant richness and density, type of foliage (evergreen vs. deciduous), the surroundings (adjacent land, vehicle, and pedestrian traffic), and the seasonality [46].

Other studies have shown that among arthropods, good dispersers such as winged insects and species carried by the wind, such as spiders, are over-represented, depending on the resource provided (nesting habitat, food, and protecting areas) [47,48]. Living walls show the highest occurrence of beetles and spiders. At the same time, green facades (both traditional and double skin) are ideal habitats for generalist and common invertebrates. They are also highly visited by specialist flower visitors (honeybees, bumblebees, bristly flies, and butterflies) for nectar and pollen [48,49]. At last, the abundance of detritivores, herbivores, and predators depends on the climbing species and the wall age [46,48].

One research study estimated the public’s perceived value of green walls for urban biodiversity, in the form of their willingness to pay for it. Results indicated a willingness to pay [50]. Although with little money, willingness to pay demonstrates the feelings of citizens towards biodiversity, which VGS may improve.

Implications for teaching. VGS has been demonstrated to have a positive effect on the environment of urban areas by contributing to the improvement of the air quality, regulation of temperature and humidity, and safeguarding biodiversity. This part of the literature review can be closely linked to the concept of teaching/learning based on the possibility of making some of the described effects tangible through student experiments in school. This allows direct experiences with nature and provides insights into environment protection measures.

2.4.3. Effects on Physical Health and Mental Health

The biophilia hypothesis suggests that humanity has an innate tendency to seek connections with nature, trees, forests, plants, flowers, and animals. Edward O. Wilson defines biophilia as “the urge to affiliate with other forms of life” [51].

With cities becoming increasingly crowded over time, humanity suffers from a lack of nature, threatening mental and physical health. In this regard, several effects may be observed, such as depression and anxiety, linked to heart diseases, high blood pressure, stroke [52]. Air pollution is linked to respiratory diseases and lung cancer, as well as those already mentioned in Section 2.4.2, with regard to the various types of air pollutants.

According to the World Health Organization (WHO), anxiety and depression may also lead to harmful behaviors such as smoking or drinking alcohol. Enhancing greenness in cities may reduce those effects and improve citizens’ health. Activities with increasing levels of engagement with nature are: watching nature, walking, running, biking in a park, caring for gardens or vegetable gardens [52]. Even the simple view of a green landscape may improve psychological and physiological well-being [53,54]. Greenness has direct and indirect beneficial effects [55,56,57,58,59].

Direct effects include psychological relaxation, stress reduction and recovery, more opportunities for physical activity, reduced exposure to noise (which is a stress factor), reduced exposure to air pollution, reduced exposure to heat, and boosting the immune system by the chemical and biological agents of plants.

Indirect effects include reduced blood pressure, reduced mortality rates from circulatory diseases, reduced headaches, positive effect on cognitive ability or functions, positive effect on mental processes [57], positive effect on attention restoration, faster healing, and better social ties [60]. Furthermore, the improvement of sleep (a side effect of psychological relaxation and stress reduction) has an important effect on well-being. The underlying mechanisms of the above effects are [60]:

• The view of greenness (even the simple experience of silent contemplation of nature) is linked to a better physiological and psychological state since images of nature reduce sympathetic nerve activity and increase parasympathetic activity, causing psychological relaxation.
• In turn, psychological relaxation lowers inflammatory cytokines, which are risk factors for diabetes, cardio-vascular diseases, and depression. It also counteracts the negative effects of stress on energy metabolism, insulin secretion, and inflammatory pathways.
• Many plants release phytoncides, antimicrobial volatile organic compounds, boosting immune functioning and reducing blood pressure.

A study examines the associations between the duration, frequency, and intensity of exposure to nature and health in an urban population. It concludes that there is a minimum dose of 30 min where some effect of nature on health might be observed [61].

The possibility to access nature is also important because a significant increase in stress was observed for people living more than 500 m away from green areas compared to those living in proximity of green areas [62].

Once again, the evidence of the positive health effects of NBS may be extended to VGS. In a recent study on VGS, the beneficial effect of viewing a green facade landscape instead of a bare building wall was demonstrated in enhancing physiological and psychological relaxation [63]. But why do people feel more comfortable with plants? It was found that around 2.000 shades of green color can be distinguished by the human eye, while 100 shades of the color red can be distinguished [52]. A possible explanation could be that recognizing a plant’s shade of green was important to know whether to eat it or use it for shelter or medicine throughout human evolution.

Implications for teaching. The literature review reveals that nature and various forms of greening directly affect the health and well-being of people and, therefore, of students. This aspect is important to know, but difficult to be made directly tangible for students. Thus, this kind of information is more suitable to be acquired in knowledge-based, theoretically oriented lessons.

2.4.4. Effects on Teaching/Learning

Increasing evidence shows the close link between nature, learning, and personal development (having a view on a natural landscape, studying in a park or a garden, doing outdoor recreation, gardening, moving around in a natural environment) [64]. Several studies aim to show how green areas provide a “remedial” effect against stress to improve young people’s cognitive performance [65,66,67]. Even just the classroom view of green landscapes produces better results on tests of attention, due to students’ recovery from stressful experiences [68,69], therefore improving learning, behavior, and academic performance [68,69,70,71].

A study conducted in 2005 in Spain, in primary schools, found an improvement in cognitive development associated with surrounding greenness, particularly with greenness at schools, also mediated by air pollution reductions. Contact with nature plays a crucial and irreplaceable role in brain development. It positively influences cognitive development inciting engagement, risk taking, discovery, creativity, mastery and control, strengthening the sense of self, inspiring basic emotional states including the sense of wonder, and enhancing psychological restoration [72].

A recent study aims to prove that nature and the direct experience with nature improve learning. Nature increases attention, improves concentration, increases intrinsic motivation and self-discipline, relieves stress, and provides a better context for learning [2]. Furthermore, nature gives children more freedom to engage with one another. This leads to improving students’ cooperative relationships with peers and adults, and, in turn, improves their behavior and their development as a person. The authors also highlighted how experience with nature improves students’ behavior towards nature, also promoting students’ development as an environmental steward [2]. A positive childhood nature experience appears to play a key role in fostering pro-environmental behavior in adulthood [73].

Several other studies highlight the social utility of learning in a natural context [74], in preschool (concerning the development of cognitive abilities, social skills, and motor development) [75,76], for young people from low-income neighborhoods [70,77,78], and for students with special needs or disabilities, where a decrease of hyperactivity and inattention was observed when conducting activities in various types of green settings (“green outdoor”, “built outdoor” and “indoor” settings) [79,80].

However, most cities do not necessarily allow for the implementation of strategies for education based on nature. A study [81] uses the term “nature-deficit disorder” to describe the loss of nature in the experiences of children who live in cities without parks or wild edges. Over the years many studies have pointed out the necessity of planning cities with greenery integrated at multiple levels (from landscaping around homes, schools, and childcare centers, to linked systems of urban trails, greenways, parks, and ‘‘rough ground’’ for children’s creative play). Unfortunately, this is difficult to realize in increasingly crowded cities, especially in the poorest suburbs or economically deprived areas. The use of VGS and green roofs could at least partially compensate for this lack.

The term “Nature Based Learning” (NBL) has been introduced in recent years. NBL occurs through exposure to nature or by implementing nature based activities, both in natural contexts (excursions to green spaces) and in built environments, such as a school in which elements of nature have been introduced (plants, animals, fungi, algae, water containing microorganisms) [82].

The definition of NBL fits VGS in its introduction of natural elements at school. It might be argued that the beneficial effect of nature may be in part extended to the case of VGS, set up on the school’s wall, in the school garden, in the surrounding neighborhood, or in a botanical garden outside the school. Still, there are a few studies on this topic. Some examples are the following.

Based on the idea of the environment as a teacher, one author assumes that a school garden (including green facades) could improve students’ ecological awareness. The author proposes then to include greenery elements in the design of school buildings [83].

Furthermore, Montessori’s idea of the school environment as a “third teacher” may be extended to identify sustainable schools as a vehicle for interest and awareness of environmental issues and sustainability [84]. VGS may have a manifold effect in a sustainable school: raising awareness about the importance of ecology, acting as 3D teaching textbooks on sustainability, being a perfect tool to teach about the environment, being used by students in biology or art classes, inspiring real world thinking [59].

A recent study finally highlights that implementing green walls inside schools provides environmental health benefits and enables hands-on investigations with students in the classroom, making lessons more interesting and inspiring critical thinking. A connection can be established between the indoor environment (where students are) and the outdoor environment (where nature is). Green walls can inspire critical thinking through a combination of project-based learning strategies and environmental education [85].

Implication for teaching. There is a close link between nature and learning/personal development. This link has been translated into the concept of NBL, which occurs through exposure to nature or by implementing nature-based activities. Practical activities with VGS may be proposed in school to extend the general concept of NBL to VGS. This is particularly beneficial in those contexts where a lack of nature could be partly compensated by using VGS as a way to introduce nature in schools.

3. VGS Research Contents Suitable for Science Teaching

As highlighted in the introduction, this literature review aims to use a nature-based solution such as a VGS (a green facade or a living wall) as an example of knowledge transfer from research to school, selecting research contents with a real life relevance for the students. This section will show how the selected research contents may be used to introduce crucial issues such as sustainability and climate change.

3.1. Support for Sustainable Development Goals and Climate Literacy

UNESCO coordinates and promotes Education for Sustainable Development (ESD) in all countries of the world, to give young people the opportunity to make informed decisions and act for environmental integrity, economic sustainability, and an equal society for present and future generations [86,87].

During the United Nations Conference on Sustainable Development (Rio+20) held in Rio de Janeiro in June 2012, the member states of the United Nations agreed on a three-year path finally reaching the definition of 17 Sustainable Development Goals (SDG), whose purpose is to ensure a sustainable, peaceful, prosperous and equitable life on Earth for all, now and in the future.

Education for Sustainable Development (ESD) follows the 2030 Agenda for Sustainable Development (UN, 2015). This means that “ESD aims at developing competencies that empower individuals to reflect on their own actions, taking into account their current and future social, cultural, economic and environmental impacts, from a local and a global perspective. Individuals should also be empowered to act in complex situations in a sustainable manner.” [86] (p. 7).

VGS can be a topic of ESD. In fact, among the 17 goals, VGS suits the following three:

• Goal no. 11: Sustainable city and communities: Make cities and human settlements inclusive, safe, resilient and sustainable.
  With this objective, students can learn the basic principles of sustainable city planning and sustainable buildings.

• Goal no. 13: Climate action: Take urgent action to combat climate change and its impacts.
  With this objective, students can learn about adaptation and mitigation strategies for climate change.

• Goal no. 15: Life on land: Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss.
  With this objective, students will understand the importance of biodiversity and ecosystem services provided to humanity.

Among the various goals, no. 13 (climate action) is of particular interest due to the urgency of the problem and the emotions and reactions that this topic inspires among adolescents.

In 2009, the U.S. Global Change Research Program had proposed Climate Literacy as a guide for individuals and communities, giving the following definition: Climate Literacy is an understanding of your influence on climate and climate’s influence on you and society [88]. Climate Literacy is founded on a guiding principle, “Humans can take actions to reduce climate change and its impacts”, in addition to seven essential principles of Climate Science. These principles may work as discussion starting points for scientific inquiry and promote Climate Science Literacy.

Some of the principles of Climate Literacy may be linked to the effect of VGS on climate. For instance, Principle no. 2 states: “Climate is regulated by complex interactions among components of the Earth system.” The key point of this principle is that the abundance of carbon dioxide in the atmosphere is reduced by accumulating plant biomass, which acts as a counterweight to burning fossil fuels. Setting up a traditional or double skin green facade may act as an example to show how the plants of a VGS affect the (micro-) climate. This example is also relevant to Principle no. 3: “Life on Earth depends on, is shaped by, and affects climate.”. The interdependence between climate and plant species used for a VGS, and other species living in a VGS, can also be shown. Finally, Principle no. 6 states: “Human activities are impacting the climate system.” The key point of this principle is that, as indicated with large consensus in the scientific literature, the rise in global average temperatures since the latter part of the 20th century is due to human activities. Deforestation also played an important role in this process by removing CO₂ sinks. However, VGS may act as partly compensatory CO₂ sinks.

Several effects of VGS (Section 2.4) may be related to the Sustainable Development Goals of the UN, as well as to several principles of Climate Literacy.

Table 4. How VGS fits the Sustainable Development Goals no. 11, 13, 15 and some of the principles of Climate Literacy (guiding principle, principles no. 2, 3, 6).

	Sustainable Development Goal [86]			Climate Literacy Principle [88]
	Goal no. 11: Sustainable city and communities: Make cities and human settlements inclusive, safe, resilient and sustainable. Goal no. 13: Climate action: Take urgent action to combat climate change and its impacts. Goal no. 15: Life on land: Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss.			Guiding principle: “Humans can take actions to reduce climate change and its impacts.” Principle no. 2: “Climate is regulated by complex interactions among components of the Earth system.” Principle no. 3: “Life on Earth depends on, is shaped by, and affects climate.” Principle no.6: “Human activities are impacting the climate system.”
VGS key points	Goal no. 11.	Goal no. 13.	Goal no. 15.	Guiding principle	Principle no. 2	Principle no. 3	Principle no.6
VGS regulates the temperature inside a building (VGS is a tool to adapt to climate change), see Section 2.4.1.	X	X		X			X
VGS allows energy saving, less energy consumption, and less greenhouse gases emitted into the atmosphere (VGS is a tool to mitigate climate change), see Section 2.4.1.	X	X		X	X	X	X
VGS removes CO2 from the atmosphere (VGS is a tool to mitigate climate change), see Section 2.4.2.	X	X		X	X	X	X
VGS improves the air quality (VGS reduces dusts and gas pollutants), see Section 2.4.2.	X		X
VGS counteracts the dry effect of UHI and climate warming with evapotranspiration, see Section 2.4.2.	X	X	X	X		X	X
VGS safeguards biodiversity in the city, see Section 2.4.2.	X		X
VGS muffles noise, see Section 2.4.2 and Section 2.4.3	X
VGS regulates the water flow, see Section 2.4.2.	X
VGS safeguards human well-being in cities, see Section 2.4.2 and Section 2.4.3	X

summarizes these relations between VGS, the Sustainable Development Goals of UN and the principles of Climate Literacy.

Table 4 also shows how VGS key points may be linked to environmental sustainability and the ecological footprint, an indicator to estimate the supply and demand of nature, how much nature we consume and how much nature we have. Currently, the increasing human exploitation of natural resources significantly impacts the environment and the earth’s capacity to renew these resources [89].

In Section 2.4.4, the importance of VGS for teaching/learning was assumed, extending the beneficial effects of greenery in school to VGS, based on the definition of Nature-Based Learning (NBL). According to the studies reported in Section 2.4.4, experiencing nature plays a key role in promoting pro-environment attitudes and behaviors. This suggests that theoretical knowledge of sustainability or climate change issues is insufficient to change students’ behavior. Instead, practical experience with nature is linked to becoming aware of environmental issues and acting as an environmental steward [2]. Additionally, evidence from the literature on how to introduce the topic of climate change to young people suggests that educational practices that involve actions and feelings, besides knowledge, are particularly effective for becoming aware of the problem and coping with it [90].

As an example, if students are asked to memorize the definition of “sustainability”, stating that “Humanity has the ability to make development sustainable to ensure that it meets the needs of the present without compromising the ability of future generations to meet their own needs” [91] (p. 16), they probably will. Yet, they will not be able to internalize and implement this concept.

Similarly, for the ESD, the teaching of goal no. 13 (climate action: take urgent action to combat climate change and its impacts) will not be effective by solely relying on the guide for Climate Literacy [88], but will also need practical experiences to be better understood and learned. Another study [92] showed that educational interventions which use an interdisciplinary approach, based on direct experience and promoting discussions, are very effective among adolescents by reducing the psychological distance to climate change. Furthermore, it was demonstrated [93] that the combination of theoretical and practical activities in a primary school within a project where scientists shared research results on the effects of climate change (in this case on sea life) produced a clear gain in knowledge, teamwork skills, and communication. Positive educational outcomes also resulted from participation of schools in research activities [94] on climate change and from cooperation between research and education [95]. These findings seem to validate the approach chosen in this paper to transfer knowledge on VGS from research to school, based on practical experience and interaction with others, as a possible strategy for teaching climate literacy.

3.2. Criteria to Select Research Contents Suitable for School

Before the practical activities, it is necessary to establish the criteria that make the topic “VGS” valid in the curriculum of a class. To this purpose, the following criteria, with the first three referring to Wolfgang Klafki [96,97], a very well known German pedagogue, will be applied.

1

Exemplarity [96,97]:

• VGS, such as a green facade or a living wall, is an example of NBS to make cities more sustainable. Therefore, VGS may be used in teaching/learning in the general framework of the UN goals for sustainable development (goals no.11, 13, 15).
• The literature provides evidence regarding the importance of experiencing nature for young people. Such experiences may be conducted with VGS.
• Young people need to have a problem solving-oriented approach to questions and issues. VGS can be considered as one possible response to address the climate change problem, and is therefore well suited for inquiry-oriented lessons and problem-solving approaches.

2

Students’ current life meaning [96,97]:

• Students are confronted with information about air pollution and air quality in everyday life. Similarly, students often hear about the necessity to sustainably manage a city and to take mitigating/adapting actions against climate change, and about the behaviors citizens need to have and the tools to use for such purposes. VGS is a valid tool to improve air quality and to make a city more sustainable. VGS is also eye-catching because it changes the appearance of houses, gives them a special look, and thus is interesting for students.
• Students need to connect themselves with actual life around them, comparing ideas and sharing experiences and behaviors with peers, family members, and people in the neighborhood or city. VGS may stimulate this interaction, trigger discussions, and put into practice sustainable behavior and pro-climate actions.

3

Students’ future life meaning [96,97]:

To become responsible citizens and adults who can take a position on socially important issues, such as climate change and environmental protection, students should know the consequences of climate change, pollution and other threats to the environment, and then develop critical thinking to assess strategies to solve these problems. To come up with a solution to a problem, responsible citizens should consider both the benefit to the community (in this case, limiting further global warming) and the needs and feasibility for the individual (additional costs). The aim is to find a compromise that is acceptable to all parties. VGS may be a tool to foster these important objectives for their future (change of perspectives, weighing of interests, finding compromises).

4

Inclusivity:

As highlighted in Section 2.4.4, the literature suggests that nature-based lessons are the ideal context to create an inclusive learning environment offering more cooperative relations [2] and different learning approaches according to students’ abilities, with the learning approaches ranging from descriptive, observational, and measurement activities, to uncovering and interpreting theory–practice connections. VGS may be a good solution to set up a green space in a school or a nearby location when natural greenery is not available, thus creating a suitable context for Nature Based Learning and the inclusion of all students.

5

Link to school curricula:

There are several possible links between VGS and the topics of school curricula in the natural sciences. Examples include photosynthesis and CO₂ sequestration, knowledge of plant species and plant parts, the atmosphere and greenhouse effect, energy and the environment, evapotranspiration, the water cycle, the solar irradiation and how this energy is transformed within a natural system, sensible heat and latent heat, the temperature and the thermometer, measurements and charts, the human body and health (nervous system, stress recovery in the presence of greenness, respiration, homeothermy connected with UHI, the immune system), ecology and interactions between living beings and the environment, and the exchange of matter and energy between plants and environment.

6

Feasibility [98]:

Lessons/activities on VGS have to be possible with affordable instruments in nearby or easy to reach locations and in a reasonable time.

3.3. Possible Practical Activities with VGS

In the following pages, we will provide examples of actions and practices using VGS as a tool to act, articulated in various phases: a descriptive, investigative, and, finally, communicative phase (as shown in Figure 2).This sequence of phases corresponds to the structuring of subtopics in the narrative review (when referring to phases 1 and 2).Thus, the structure of a scientific review can be used as a guide for structuring activity-based lessons, in this case, on the topic of VGS.

The descriptive phase includes creating an inventory of already existing examples on the topic of interest (VGS) and structuring them according to established criteria. The investigative (inquiry-based) phase may be split into two sets of activities, one with a social-scientific focus and one with a natural-scientific focus, with the aim to look for inter-relations between the respective results of each one. The communicative phase aims instead to public outreach, to share what has been learned and raise awareness in the public of important issues, such as climate change, and the effectiveness of certain strategies (such as VGS) to counteract them.

3.3.1. Design

Initially, students could collect photos of existing VGS in their city (photo safari) to compare them with reference pictures of VGS types and plants to recognize what they have found (Figure 3).

Shooting photos around the city is a way to observe and “take home” pieces of the city and analyze them. Before studying green facades, it makes sense for students to observe what is already around them, with the eye of the scientist, observing in a targeted way. Green facades do not exist only in books, but also on the walls of their cities.

Students should identify different VGS types, how they differ, and compare them with theoretical knowledge on VGS. For instance, the teacher may bring various photos of VGS to class, including close-up/detailed photos. In the photos, the structure of the VGS and the plant species used should be clearly visible. With the help of additional material (for instance, a simplified identification key), the students should be able to determine the type of facade, the plant species, and the climate requirements of these plants.

In doing so, students will get an idea of the most common green facade types in their area and the plant species in use. These activities should also enable them to estimate which kind of facade could be suitable for their own school (in order to design a VGS and eventually set it up).

3.3.2. Social Aspects

Students should get an impression of what people think about VGS and whether they are willing to support the installation of a VGS. To do so, students may conduct interviews to collect people’s beliefs and acceptance of VGS (Section 2.3). Such a survey should present the advantages and problems of VGS, to show a controversy for which a solution must be found.

Students can then extract ideas and arguments from these interviews regarding the climate and environmental relevance of VGS. These interviews can thus serve as a basis for deriving hypotheses that will be taken up in the scientific orientation of the lesson. An example of a possible structure and content of a survey, inspired by the literature [25] and adapted to the situation of a student who interacts with family members or friends, is shown in Figure 4.

3.3.3. Scientific Aspects

To introduce the topic “VGS” at the level of the teaching staff in school, a model could be used as a starting point, such as the one in Figure 5, which summarizes all the important effects a VGS may have, in particular the exchanges of energy and matter between a VGS and the outside environment, using oriented arrows. This model serves as an orientation for the teacher on which aspects can be addressed in class and investigated experimentally.

The graph should initially contain only the VGS. The students can subsequently add the various environmental factors that are changed by the influence of the VGS as possible assumptions/hypotheses. Some of the assumptions may be derived from the results of the survey carried out when studying the social aspects. These assumptions can then be investigated experimentally by the students. In this way, students can test their hypotheses, which allows inquiry-based learning.

One way to investigate the scientific aspects is to measure relevant parameters depicted in the model. By doing so, students try to quantify and get insights into the natural processes and phenomena happening in a VGS (see Section 2.4.1 and Section 2.4.2). For example, regarding the temperature, the building external wall surface temperature reduction is the most obvious parameter because the VGS acts like a sunscreen (as mentioned in Section 2.4.1).

The measurement of relevant parameters should be made with affordable and easy-to-use instruments, suitable for schools.

Table 5. Examples of relevant parameters and possible measuring instruments.

Parameters	Measuring Instruments
Surface temperature of green walls compared with bare walls	Temperature logging i-buttons Thermohygrometer with sensor
Surface temperature of walls “indoor” compared with bare walls	Temperature logging i-buttons Thermohygrometer with sensor
Moisture on green walls compared with bare walls	Humidity logging i-buttons Thermohygrometer with sensor
CO2 sequestered	CO2 sensor
Dusts, airborne particles captured	Laser egg
NO2 uptaken	NO2 sensor
Biodiversity	Direct observation

gives an overview of some relevant parameters and their possible measuring instruments.

In addition, students should learn how to interpret the collected data, for instance, by looking at comparative measurements on greened and bare facades and by comparing their data with published ones (examples in Figure 6 for the temperature and Figure 7 for the humidity).

After understanding the significance of the collected data, the teacher should guide students to put this new knowledge into the more general framework of environmental issues, such as climate change, and draw meaningful conclusions that will be communicated in the next phase.

3.3.4. Knowledge Transfer to the Public

Communicating and discussing what has been learned and experienced about climate change is important for students, who thus become “agents of change” for spreading awareness and knowledge of environmental issues in society, for influencing public opinion, for promoting climate-friendly policies, and for motivating people to adopt more sustainable behaviors [100,101]. Researchers have highlighted the multiplier effect of education, which means that entire families and communities have a benefit when individuals share what they have learned. This is particularly true in the case of climate change education: what has been learned about climate change mitigation and adaptation can be passed on to other citizens, so that they are more able to participate in civil society and influence decision making [102].

Therefore, the discussion with peers and adults will be an important phase to multiply and reverberate knowledge, beliefs, and actions into society. Since in this review the focus has been on using VGS as an example of a tool to learn about and act against climate change, the knowledge transfer should be focused on VGS as a starting point for a broader discussion on climate change.

Examples of public outreach may be: discussions among peers (in class, with other classes of the same school, or outside with friends), discussion with adults (family, neighbors, etc.), open days at schools, participation in public events.

Another way to communicate what has been learned externally could be the setting up of a VGS as a living example of a Nature Based Solution (adapted to the climate and environment where students live) to counteract climate change.

4. Conclusions

This literature review analyzed the status of scientific knowledge on VGS and suggested a way of implementation in schools as an example of knowledge transfer from research to school to raise awareness of environmental problems (particularly sustainability and climate change) in young people, as well as for the general purpose of teaching/learning natural sciences. The structure of this narrative review is based on the different content areas of publications, which range from design principles of VGS to research on social acceptability, to research on the effectiveness of VGS. The chosen structure for the review proves also to be an appropriate structure for the design of a teaching sequence on VGS, that can also be applied to other NBS such as green roofs, or in general to green areas in a city.

Moreover, it widens the perspective from the specific object of interest (VGS) to more general issues of social relevance such as environmental protection and climate change. In this regard, the principles of Climate Literacy constitute a guide for educators and teachers to help students in bridging the gap between the specific VGS contents they have learned and more general scientific issues. At the same time, practical experience should facilitate more effective learning and long-lasting learning outcomes.

The design of a prototypical teaching unit (derived from the structure of the review) includes three different phases: a descriptive, an investigative, and a communicative phase. The first phase is an inventory of the object of investigation (VGS) in one’s own living environment and a classification of the different forms found. The second phase refers to an inquiry-based approach. Thereby, a distinction is made between two sub-phases: a social-scientific and a natural-scientific research approach. The order in which these two sub phases is performed can vary. In our view, it makes sense to first ask about public opinion and, based on this, then to conduct natural-scientific investigations to determine the extent to which the expressed assumptions are appropriate. Alternatively, however, the natural-scientific investigations could be carried out first, and then the extent to which these findings are known to the public could be surveyed. In this latter case, the attitude/belief questionnaire (Figure 4) could even be designed by the students themselves. In the third phase, which is the communicative one, the results of the second, investigative phase, are processed so that they can be presented to a larger audience (e.g., at an open day in the school). In this way, students are doing public outreach and see themselves as ambassadors for climate literacy and sustainable development. All proposed instructional activities are consistent with action-oriented, student-centered learning and promote different areas of competence, such as subject knowledge acquisition, inquiry competence, and communication competence. However, the proposed classroom activities will need to be further developed and subsequently tested to analyze whether or not students’ scientific knowledge has increased, whether their awareness and behaviors towards sustainability and climate change have improved, and any effect on inclusion observed. These will be future steps to take in this research project.