Green facade for energy savings in buildings: The influence of leaf area index and facade orientation on the shadow effect

Highlights

• Leaf area index to measure the shadow potential of a green façade.

• Indirect method to measure LAI is suitable for green facades.

• GF provide comparable shadow factor for all orientations than artificial barriers.

• For a LAI of 3.5–4, 34% of energy savings was measured.

• Energy savings provided by GF are wall orientation dependent.

Abstract

To “green” building envelopes is currently one of the most promising ways to provide energy savings in buildings and to contribute to the urban heat island effect mitigation. The shadow effect supplied by plants is the most significant parameter for this purpose. One way to characterize the potential shadow effect of greenery is to calculate the facade foliar density by means of the leaf area index (LAI). As LAI is commonly used in horizontal crops, their use in vertical greenery systems (VGS) has generated dispersion and uncertainty in previous studies both in terms of methodologies and results obtained. In addition, a lack of data relating to the influence of the facade orientation in the final contribution of vertical greenery to the energy savings has been observed in previous studies.

This study aims at establishing a common and easy way to measure LAI and to lick it to the energy savings provided by VGS. Moreover, the energy savings achieved as well as the influence of facade orientation on the final thermal behaviour of two different VGS, a double-skin green facade and a green wall, was studied.

From the results, it can be stated that the most simple and quick procedure to measure LAI in order to characterize the foliar density of VGS is the indirect method based on the amount of light transmitted through the green screen. From the experimental tests interesting energy savings were obtained (up to 34% for Boston Ivy pant specie with a LAI of 3.5–4, during summer period under Mediterranean continental climate). Moreover, the dependence on facade orientation was confirmed with representative contribution over the whole energy savings from East and West orientation.

Introduction

Nowadays, buildings represent the largest energy-consuming sector in the economy, with over one-third of all energy and half of global electricity consumed there. As a result, they are also responsible for approximately one-third of global carbon emissions. With improvements in economic development and living standards expected to increase as the planet’s population grows by 2.5 billion by 2050, energy use in the building sector is also set to rise sharply by 50%, placing additional pressure on the energy system [1].

In most regions of the world, heating and cooling loads represent the largest building-sector energy end-use. The building envelope - the boundary between the conditioned interior of the building and the outdoors - can be significantly improved to reduce the energy needed to heat and cool buildings. Therefore, there is an urgent need to make building envelopes more energy-efficient, as 20–60% of all energy used in buildings is affected by the design and construction of the building envelope [1].

Among other innovative technologies to improve the thermal performance of building envelopes urban green infrastructure, that is green roofs and all vertical greenery systems (VGS), is standing out as one of the most promising [2]. These innovative and environmental friendly envelope systems not only contribute with thermal improvements to the building [3], but they provide also multiple ecosystem services at city scale, such as urban heat island mitigation [4], [5].

This research relates specifically to the thermal performance of vertical greenery systems in buildings. In this regard, it must be taken into account the several strategies to vertically “green” a building because clear differences have been previously described, not only related to the design but also to their thermal performance [6]. A first great differentiation takes place between green walls (living walls) and green facades, requiring the former higher levels of maintenance (intensive) than the second (extensive) [6]. Among green facades, in which climber species are mainly used, the so-called traditional green facades, when the building facade material is used by plants as support, can be distinguished from double-skin green facades, when a real double skin is created by means of lightweight support structures that allows the vertical development of a plant to happen at some distance from the building facade (Fig. 1). This contemporary adaptation of traditional green facades, based on easy designs, is very promising as far as it is basically extensive, and it implies low investments and interacts only superficially with architecture [7], [8]. In the present research, a double-skin green facade has been studied as passive tool for energy savings in buildings.

Referring to the contribution of these VGS to energy savings in buildings, this ecosystem service takes place essentially due to the shadow provided by the plants. Other effects that can contribute, although with minor magnitude, are cooling (evapotranspiration from plants and substrates), insulation (insulation capacity of the different construction system layers: plants, air, substrates, felts, panels, etc.), and the wind barrier effect (modification of the wind influence on the building surfaces due to the presence of plants and support structures) [3], [6].

From the previous research about the potential of double-skin facades as passive tool for energy savings in buildings it can be observed that the most interesting parameters to consider in their analysis are the period of study (cooling, heating or both), the species used, the facade orientation, the foliage thickness (or the coverage percentage), and the air gap thickness between the plant layer and the building facade wall. Referring to the contribution to energy savings, generally the reduction on the exterior surface temperature of the building facade wall ranged from 1 °C to 15.18 °C [3].

In particular, in Hoyano [9] the effectiveness of a vine sunscreen for sun shading was found, reaching reductions up to 60% on solar radiation and 1–3 °C air temperature reductions in the studied veranda. Stec [10] conducted a lab experiment in order to evaluate theoretically (simulation) the shading effect by an Ivy layer (Hereda helix) instead of the common blinds layer used in a double-skin green facade. The temperature of the cavity air behind the plants layer was significantly lower (20–35%) than behind the blinds layer. Wong et al. [11] concluded from a large experiment under tropical climate that the average wall surface temperature reduction under the double-skin green facade was 4.36 °C, finding maximum reductions during the afternoon. In Ip et al. [12] indoor air temperature reductions of 5.6 °C during the day and 3.5 °C during the Summer nights were obtained by comparing two identical rooms in an office building due to a sun screen placed in a window of an office building. Pérez et al. [13] measured reductions of 5.5 °C under a building wall shadowed areas of a double-skin facade in reference to sunny areas in August, reaching maximum values of 15.2 °C on the South-West facade in September under Mediterranean continental climate. In similar studies, Perini at al. [14] obtained average reductions of 2.7 °C, and Koyama et al. [15] reductions of 3.7–11.3 °C, with coverage between 15% and 54%. Recently Jim [16] obtained reductions of 5 °C on sunny days and 1–2 °C on cloudy days, standing out the importance of facade orientation on the thermal behaviour.

It can be observed that these previous studies highlight, on one hand, the big potential of these systems to intercept solar radiation and to reduce the building wall surface temperatures, and on the other hand, the relation between this shadow effect and the foliage thickness. However, available data are too sparse and no conclusion referring to the influence of foliage thickness on the thermal behaviour can be withdrawn from these studies. In addition, a lack of experimental data referring to total final energy savings provided by these systems can be noted [3].

A simple way to characterize the thermal benefit that a green facade provides at any time during its development can be to measure the relation between the leaf density of the green layer and the shadow effect and, consequently, the energy savings provided.

In this regard, the most used methodology to characterize the leaf mass of a plant or set of plants is the leaf area index (LAI). Traditionally, the concept of LAI has been used in agriculture and ecology to measure the development and yield of crops, to compare among them and to schedule irrigation and amendments during the crop development [17].

Although some previous authors have used the concept of LAI in order characterize the potential of green facades as a passive tool for energy savings, after a literature review, a lack of knowledge about LAI concept applied to this purpose has been found. Thus, key issues such as the way to measure LAI in VGS, the relation between LAI and the energy savings provided by the facade are not yet resolved, fact that justifies deeper research in this direction. The following paragraphs summarize the main findings from the scarce previous authors who have applied the concept of LAI to vertical greenery systems.

Wolter et al. (2009) designed an experimental double-skin green facade, made with a steel trellis support and Ivy plants (Hereda helix) in order to study the LAI in green facades. According to these authors, in the case of vertical greenery, LAI describes a relation between the leaf area and the square meters of facade instead of the relation between the leaf area and the square meters of floor as usual (e.g. for green roofs application). Moreover, it is necessary to take into account the fact that in a green facade the LAI value changes with the height. Although Wolter’s study do not consider the thermal benefits of green facades, as the LAI index has a direct influence on the foliage density, this value can be linked to the thermal behaviour of green systems. The LAI average measured at each exposition at the end of the testing period was between 7 (East) and 8.51 (South). These leaf area indexes lay in between or are even higher than those of conventional facade greenery with Hereda helix (2.6–7.7) [18].

Wong et al. (2009) conducted an interesting simulation about the effects of vertical greenery systems on the temperature and energy consumption of buildings. For this purpose, the authors tried to establish a correlation between LAI and the shading ratio (the ratio of the solar radiation beneath the plant and the bare wall) based on measurements carried out in an experimental set-up in which eight different VGS were compared. Although a correlation between these two parameters was found, it cannot be generalized neither considered concluding, because the measurements were few and were done in very different construction systems (some of them were green facades and the other ones were green walls). The general trend was the expected one, i.e. low solar radiation beneath the plant means that the plant shades the wall effectively. To conduct the simulations authors used specific data from plants, both at building level (Urechites lutea, Ophiopogon japonicas “Kyoto Dwarf” and Tradescantia spathacea “compacta”), with corresponding shading coefficients of 0.986 (high), 0.500 (medium) and 0.041 (low), and at city scale (Nephrolepis exaltata, Boston fern, with a LAI of 6.76, even though this plant is a fern, not a climbing plant). In this study, the equipment used to measure LAI and the shading coefficient (solar radiation) is described, but not the methodology neither the goodness of the supplied data [19].

In Ip et al. (2010), a coefficient which tries to characterize the shading performance of a climbing plant canopy over its annual growing and wilting cycle was proposed. The study was based on data from an experiment conducted during 2003 and 2004. In that experiment a double-skin green facade, made with modular trellis and Virginia creeper (Parthenocissus quinquefolia), was placed in a window of an office building located in Brighton (UK). A very interesting contribution of this study is the effort to characterize the shadow effect of double-skin green facades. With this aim, the leaf solar transmissivity evolution depending on the number of leaf layers was established from up to 2000 measurements under the green facade, in reference to the received radiation in front of the facade. In this experiment, measures for the solar radiation were carried out with the solarimeter in vertical position, fact which, according to the authors, implies the need to make corrections in the calculations in order to consider only the horizontal component of the radiation that is perpendicular to the facade [12].

Susurova et al. (2013) defined a mathematical model to characterize the thermal effects of plants on heat transfer through building facades. Leaf density, characterized by LAI, appears among the various parameters used for the simulation, being one of the most influential in reducing the building facade wall surface temperature. However, in this study LAI was estimated by measuring the area of a single typical leaf and counting the area of ivy in a picture of a traditional green facade under study [20].

Scarpa et al. (2014) proposed a mathematical model for the energy performance of living walls. Again, LAI was an important parameter to consider in the theoretical model. In this study the two values used for LAI were 3 for a living wall with a “vertical garden” made with different species of shrubs and 5 for a living wall that uses grass as vegetation, surprisingly higher than the first one. These values for LAI come from a previous study conducted by the authors, which were obtained by measuring LAI under the shrubs placed in horizontal position, in the nursery [21].

From this literature it can be concluded that LAI is a key parameter to characterize the foliar density and consequently the thermal behaviour of VGS and especially for green facades, due to the great influence on the shadow effect. Nevertheless, there is a lack not only of suitable data of this parameter but also related to the suitable methodology to measure LAI for these purposes. Thus, along these years of studies a common methodology to measure and use LAI for VGS has not been established. In addition, the LAI of the different species used for VGS, the influence of climate in the development of these species and consequently on LAI values, the variations of LAI according to the height, are questions still to be answered. Having real values of LAI for different plants in different climates, and to link these values to energy savings, can be suitable information to face the design necessities during the building project phase.

The present long-term research aims to study the potential of VGS as passive systems for energy savings in buildings, being one of the main focuses to measure the influence of leaf density, by means of the LAI value, on the thermal behaviour of the whole system. Since this leaf density is dependent on the typology of VGS, the type of plant species, as well as its stage of development and the climatic conditions, it is necessary to establish simple and generalizable working methodologies, in order to easily share and compare data from studies conducted around the world.

In a first phase of this research, an existing double-skin green facade located near to Lleida (Spain), under Mediterranean continental climate was yearlong monitored (Fig. 2). The facade consists in a steel modular trellis support and Glycine climber plants (Wisteria sinensis). Understanding the light transmission factor of the double green facade as the ratio between the intermediate space illuminance and the exterior illuminance, this value ranged between 0.04 in July to 0.37 in April, during the season with the foliage fully developed. The exterior building wall surface temperature behind a covered area was 5.5 °C lower than in an exposed area. This difference was higher in August and September, reaching maximum values of 15.2 °C on the South-West facade in September [6].

In addition, the transmission capacity of four different plant species well adapted to this climate was determined by means of a simple experimentation [13]. The species chosen were Ivy (Hereda helix) and Honeysuckle (Lonicera japonica), as perennial plants, and Virginia creeper (Parthenocissus quinquefolia) and Clematis (Clematis sp.), as deciduous plants. The results of this experiment showed light transmission factor values of 0.15 for Virginia creeper, 0.18 for Honeysuckle, 0.14 for Clematis and 0.20 for Ivy plants. These values are comparable to the best values of the shadow factor that can be obtained by using artificial barriers for the South orientation (Fig. 3).

In order to measure the energy savings associated with this shadow effect, the green facade was moved to an experimental cubicle in which the big capacity of the green facade to intercept solar radiation was confirmed with reductions in outside wall surface temperature up to 14 °C in July in Mediterranean climate [22]. Despite this achieved surface temperature reduction, for an indoor set-point of 24 °C, the obtained daily energy consumption reduction in the cubicle was only 1%. This was because at the time of this experimentation the green facade only covered 50% of the South orientation (Fig. 4).

In Pérez [7] the importance of using deciduous species in the regulation of solar gains along the different seasons of the year was considered. The conclusions highlighted the importance of knowing the biological cycle of different species under different climates, because this influences the moment when leaves fall (or grow) and therefore what amount of solar gains could be considered for the thermal balance of the building. This is particularly important in the transition seasons, that are Spring (when the leaves grow) and Autumn (when the leaves fall).

After these positive previous experiences and in view of the potential of the double-skin facade to provide shadow to the building, a new double-skin green facade covering the east, south and west orientations of the experimental cubicle, was built in 2012.

This paper summarizes the main results of the different experiments carried out in this new double-skin green facade addressed to measure the leaf area index (LAI) and to relate it to the shadow effect as well as to the energy savings provided. In addition, the paper attempts to establish a simple and generalizable working methodology for this purpose, so that it can be applied in further research and architectural projects. Finally, it also is an objective the study of how the green facade works depending on the orientation (East, South and West), with the aim to improve the design of these systems in the future.