• July 12, 2018
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Abstract – Since more and more development of concrete buildings in the urban, the landed property has become scarce and significantly reducing the quantity of green spaces. To overcome this issue, vertical greeneries in building could be introduced. Although the space is limited, transitional spaces could be optimised. Transitional space is a space that is not directly occupied and located in between the interior and exterior environment. This place acts as both buffer spaces and physical links and consumes more energy to maintain the thermal condition to be acceptable for the occupants. Hence, this paper reviews the potential of vertical greeneries for improving the thermal condition in transitional spaces.

Keywords – Transitional space, Thermal comfort, Vertical Greeneries, Plant selection

 

I. INTRODUCTION

The rapid growth of the dwellers has resulted the vast development of concrete buildings in the urban. This phenomenon has been a critical issue since the past decade and resulting the landed property has become scarce. Open space for greeneries that are moist and permeable have been replaced into dry and impermeable surface. This effect commonly happened in urban areas where the lands have been replaced by buildings resulting higher temperature than nearby rural areas, or known as Urban Heat Island (UHI). Numbers of buildings are currently being recommended to be environmentally conscious, such as lowering energy consumption and greenhouse gas (GHG) emissions, reducing the effect of conduction heat gain and thermal radiation on buildings, improving air quality, as well as mitigating the UHI effect. Their main goals are to provide better liveable building and better sense of well being for the occupants.

The integration of greeneries in building has been introduced since the last ten years. There were numbers of newly developed low and high-rise building that starting to consider the importance of integrating greeneries in the building, be it is in the form of roof garden, vertical or horizontal greeneries. This trend is considered as the key element of urban transformation [1]. Coma et al. [2] also agreed that the presence of greeneries could be defined as a set of man-made elements that provides multiple ecosystem services at building and urban scales or known as the ‘Urban Green Infrastructure’. Moreover, certain amount of greeneries in building is required to fulfil the criteria of green building assessments [3]. This phenomenon has become one of the most innovative and rapidly developing features of city planning, architecture and ecological landscaping in the scope of built environment. Despite of the space limitation, many spaces in buildings could be maximised its function as the ‘green space’, such as in the transitional space. This space is located in-between outdoor and indoor environments acting as both buffer space and physical link such as the lobby, corridor, atrium, courtyard, and many others circulatory routes in the building [4], [5]. However, there was still lack of studies that discusses the performance of greeneries in these spaces, particularly in high-rise building. Therefore, this paper aims to study the potential of transitional spaces for improving thermal comfort through greeneries application.

As stated previously, due to the scarcity of urban land, greeneries now is commonly integrated with the building in the form of horizontal or vertical greeneries. The presence has initiate many scholars for investigating the benefits of having greeneries on the building especially in Singapore, Malaysia, and Thailand where the climate is hot and humid. For the last decade, many previous scholars have been conducting research regarding the benefits of greeneries in building. It is significantly proven that vertical greeneries could reduce the energy consumption on building and improve the thermal comfort [1]–[3], [5]–[12], reduce the carbon sequestration by growing edible plants [6]–[8], acts as a noise barrier [9], and alternatively as a food supplier [10], [11]. The importance of greeneries in building has influenced many researchers in many countries to develop and innovate on the method, technology, as well as optimizing the benefits of vertical greeneries in buildings.

 

II. TRANSITIONAL SPACE

As stated previously, green spaces are reducing fast because of the development of the properties and infrastructures. Due to that, applying vertical greeneries on building are one of the promising solutions to this issue, where people who live and work in building need the presence of greeneries as a restorative element. Although the space is scarce, usable spaces in building could be maximised with the best method to obtain a cost-effective and easy maintenance greenery systems. Transitional space could be an alternative location for applying both horizontal and vertical greenery systems. In general, transitional space or transient space could be defined as a buffer space, or spaces in between the interior and exterior environment as in Figure 1 (a-f). This area could be the entrance canopies, foyers, lift lobbies, corridors, and stairwells of a building, and many others.

Referring from Pitts and Jasmi [12], transitional space is not directly occupied by any major activities in the building however it has a large implication to occupants experience and building energy consumption. Generally, the space allocated for a transitional space in a building is up to 10% to 40% of total volume in different types of buildings and considered as one of the alternative means for optimising building performance. Although, it is not directly occupied, Hui and Jiang [13] agreed that transitional space consumes more energy compare to other occupied parts of building of similar size when being conditioned to achieve the desirable thermal comfort for the users.

Transitional spaces are often related with open area and it is easily influenced by the variable weather conditions as it is close to natural environment. This is different with the enclosed area where totally separated from the exterior environment and generally equipped with air conditioner [13]. By optimising the passive cooling strategy by integrating plants on the building, the space will be cooler, and at the same time, it will reduce the energy use for cooling the building. Therefore, the potential vertical greeneries in transitional space need to be further explored.

 

III. TYPE OF VERTICAL GREENERIES SYSTEM (VGS)

Both horizontal and vertical greeneries system could be applied in the transitional space depending on the space allocated. When the space is limited, the most common method is the vertical system. In general, VGS is classified based on their growing mechanisms, living wall and green façade [14]. Perini et al. [15] described on their research that living walls are commonly constructed from modular panel that contains natural or artificial growing medium (felt, foam, perlite, and mineral wool) on each of its panel and using the hydroponic system to provide the plants’ water and nutrition (refer to Figure 2). Due to the flexibility of the system in living walls, this system could accommodate single or multiple plant species, which are commonly low height perennials or herbaceous plants. Combining multiple plant species in one system could be more efficient in terms of the space used. In large scale, living wall is widely known as a microclimate modifier and it could be an artistic element due to the composition from the various plants use.

a. LWS based on planter boxes
b. LWS based on foam substrate
c. LWS based on felt layer

Another type of VGS is green façade. They also stated green façade is categorized into three major types: direct greening, indirect greening, and indirect greening combined with planter box. The first type is where plants are climbing directly on the building façade from the ground. The second type is using cables or meshes to support the plants for climbing, and the last is the combination of both support cables and planter box. Plants are grown on the planter box before climbing on the cables provided. Figure 3 shows the illustration of the direct, indirect, and indirect greening system with planter box.

a. Direct greening system
b. Indirect greening system
c. Direct greening system with planter box

Many people are glad to have greeneries in building to enjoy the contact with the nature that represented by plants. Plants could be one of the architectural aesthetic elements and the beautiful patterns from the variety of plants used has successfully creating a new perspective of sustainable art that could enhance the visual appeal for the public. Hence, the presence of greeneries is important although the space is scarce. This limitation creates a challenge to create a greenery system that is feasible to be applied in small spaces, including in the transitional space. To enhance the efficacy of the performance, Shahidan and Jones [16] suggest that multiple layer of plant canopy could be introduced since multiple canopy layers are known effective in modifying microclimate.

 

IV. PLANT MORPHOLOGY

In general, thermal comfort is strongly affects by the quality and the quantity of plants. There are four key microclimate elements can be manipulated through landscape design, those are radiation, humidity, wind and temperature. According to Shahidan and Jones [16], plants could be ideally used as a climate controller, such as windbreakers, shades, and also from the precipitation and evaporation activities. The morphology of plants determines the quality of the shade produced to filter the radiation.

A. Plant Species Selection

There are several important factors to be considered before selecting the appropriate plant species. Selecting the suitable plants for any greeneries application is a very crucial matter where the plant choice strongly affects the performance as well as the visual quality of it. Plant selection is an important aspect for the overall performance of living wall and other greeneries, as stated by Amir, Abdullah, and Malek [11]. Inappropriate plant selection can lead to various problems such as excessive growth, plant losses, pest or disease problems, high maintenance, poor public perception, and unattractive structures. These problems might fail to provide the benefits from the greeneries.

In tropical countries, plants must able to withstand the hot and humid conditions. Hopkins and Goodwin [17] emphasized on the importance for selecting suitable plants before application, particularly when applied in different weather conditions. The plants used should be available from the local market for the ease of maintenance and replacement [7]. For living wall, small herbaceous species, perennial flowers, low shrubs, ferns and grasses are suitable for living wall application. The leaf size also determines the efficacy of a living wall which is determined by the Leaf Area Index (LAI) value. LAI can be defined as the one-sided leaf surface area per ground surface area, which is important for determining the density of the foliage. The substrate also contributes on the growth of the plant, where 5-10 cm is suitable for living wall application. Table 1 describes the summary of the possible plants that could be used in vertical greening on the building along with the benefits.

 

TABLE I. SUMMARY OF PLANTS USED AND THE BENEFITS

Type of Plants Species	Benefits	Author, Location
Edible and medicinal plants – Blue trumpet vine (Thunbergia grandiflora) – Ivy gourd (Coccinia grandis) – Mexican creeper (Antigonon leptopus)	Air temperature reduced by maximum 4.71 C	Sunakorn and Yimprayoon [3] Location: Bangkok
Winged bean (Psophocarpus tetrogonobulus)	Average of carbon sequestration 2.35 µmole CO2 m-2  s-1  was converted to kg CO2 per year per m2, equal to 9357.83 kg CO2 year-1 hectar-1	Amir et al. [6] Location: Malaysia
Ornamental plants – False heather (Cuphera hyssopifola H.B.K.) – Princess flowers (Tibouchina urvilleana) – Chinese croton (Excoecaria cochinchinensis)	– C. hyssopifola H.B.K. has the best performance in temperature reduction – Large living wall has better thermal performance	Charoenkit & Yiemwattana [7] Location: Thailand
Edible and medicinal plants Winged bean (Psophocarpus tetrogonobulus)	Outdoor wall surface temperature reduced by maximum 11 oC	Rahman, Yeok, and Amir [8] Location: Malaysia
Edible plant Winged bean (P. tetrogonobulus)	Maximum surface temperature drop until 6.4oC	Basher et al. [10] Location: Malaysia
Ornamental plants – Dragon’s breath (Hemigraphis repanda) – Ceylon myrtle (Phyllanthus myrtifolius) – Oyster plant (Tradescantia spathacea ‘Compacta’ )	Wall temperature reduced by maximum 11.58 oC	Wong et al. [18] Location: Singapore
Ornamental plants – King’s mantle (T. electa) – Mondo grass (Ophipogan verigated)	Temperature change occurs consistently on average 1.5 oC at the high-rise building due to the usage of modular system in large quantities.	Jaafar et al. [19] Location: Malaysia
Edible and medicinal plants Blue trumpet vine (T. grandiflora)	– The living wall & green façade can reduce indoor temperature up to 3oC & 4oC respectively – The living wall and green façade reduce the wall cavity temperature by 8oC and 6.5oC respectively	Safikhani et al. [20] Location: Malaysia
Edible and medicinal plants – Red spinach (Iresine herbstii hook) – Cat’s whiskers (Orthosiphon aristatus)	Plants with red colored leaves are more effective in air temperature reduction.	Asikin, Nugroho, Hasyim [21] Location: Indonesia
Ornamental plants – Moth orchid (Phalaenopsis sp.) – Striped dracaena (Dracaena warneckii) – Local climber plants	Interior surface temperature on the façade can be reduced up to 2.1oC	Widiastuti, Prianto, and Budi [22] Location: Indonesia
– Winged bean (P. tetrogonobulus) – Kidney bean (Phaseolus vulgaris) – Long bean (Vigna unguiculata sesquipedalis) – Sweet pea (Pisum sativum)	– Four Legume plants are suitable for biofaçade application in hot and humid climate	Amir et al. [23] Location: Malaysia
Yellow trumpet vine (Anemopaegma chamberlaynii)	The shading coefficient is 0.21, A. chamberlaynii is suitable climbers for green façade for reduction of solar radiation transmission towards behind the wall	Sulaiman et al. [24] Location: Malaysia

 

From the Table 1, it could be described that there are many native plants that are widely available in tropical countries could be applied for VGS. Beside ornamental plants, edible and medicinal plants are also possible to be grown vertically along with its benefits in improving the thermal comfort. Some plants, such as Winged bean (Psophocarpus tetrogonobulus) are also effective in carbon sequestering.

B. Foliage Density

In order to obtain the optimum performance of the greeneries in building, foliage density also needs to be considered since it is significantly related with the shadow effect. The density of plants gives an impact to the occupants’ comfort as well as to the microclimatic condition. To create a good quality shadow, it needs a plant with dense foliage, multiple layers of plant canopy, or with dense canopy [16]. Hui and Zhao [25] are also agreed that effective greeneries depend on the density, thickness, and the type of plants selected.

In a single layer, leaf could approximately absorb 50% visible and infrared radiation, 30% reflected and only 20% transmitted [26]. From that, it could be concluded that more layers of leaves could be more effective at reducing solar radiation. The most common measurement for foliage density is Leaf Area Index (LAI). According to Perez, Coma, and Cabeza [27], LAI is a key parameter to characterize the foliar density as well as the thermal behavior for vertical greeneries due to the shadow effect created by the foliage. Charoenkit and Yiemwattana [7] defined LAI as the ratio of total-one sided leaf area to the ground surface area. In plants, the large LAI value effectively cools the building and lower the cooling load due to the evaporation effect. The larger the LAI value, the higher the effect of shading quality, magnitude of transpiration process and higher effect of radiation filtration.

Multiple layer canopy, or multi-layer canopy, consists of several layers of vegetation such as ground covers and dense plant canopy could be introduced to enhance the efficacy of greeneries in building. The basic concept of multiple layering plant canopies is a form of forest mimicry, which consists of three main layers in the forest: upper layer, middle layer, and bottom or field layer (refer to Figure 4).

The upper layer focuses on the radiation filtration and shade by providing loose density plants and broad/spreading form (LAI value <5), the middle layer plant requires dense tree canopy (LAI value >5), and the bottom layer should be grown with full shaded green plants such as thick low shrubs or turf. Previous research has been done regarding the potential of multi-layer plant canopy in Malaysia by Shahidan and Jones [16]. They agreed that three layers of plant canopy will create a high quality shaded area, promote a high humidity level, provide an optimum evaporative comfort cooling, as well as to reduce the overall surrounding air temperature in urban area. Nevertheless, there is still lack of studies regarding the application of VGS using multiple layer plant canopy concepts in low and high-rise building. This could be a promising method since the growing space is limited as in the transitional space.

C. Foliage Colour and Size

From the previous research, it has been known that plants with relatively dark coloured leaf potentially to absorb the solar heat increases significantly. Red coloured leafs are more effective in reducing air temperature [21]. Nevertheless, further studies on the leaf colour and thermal comfort must be considered in the future. Besides of its colour, leaf performance also depends on the amount of stoma particularly if applied in small spaces. It has been proven that small leaf plants like Lanceolate, Spatulae, and Linear is significantly lowering the air temperature and relative humidity, including absorbing many types of chemical pollutants. While the wide leaf plants like Cordate and Elliptical are potentially reducing only the air temperature. It is also proved that by combining the leaf sizes, it will contribute a better thermal comfort for the occupants [28]. Moreover, the colourful foliage and the size variety on VGS can be considered as a living artwork that could enhance the quality of life [29]. Figure 5 shows the implementation of various foliage colours and sizes as the façade cover by growing the plants vertically. The combination of plants on each layer is believed to be effective in improving the microclimate, however there is a need to study regarding the most suitable plant species to be applied on each layer in order to maximise the effect of the greeneries application.

V. VERTICAL GREENERIES SYSTEM IN TRANSITIONAL SPACES

The VGS applied on building is not a new issue for the last decade. In the ‘eco-conscious’ building, VGS and other landscape features are commonly applied. There are minimum requirement of the amount of greeneries applied on building since it is one of the assessment points for obtaining the green building rating mark. Beside as a climate modifier, VGS could be as a visual barrier, space divider, windbreaker, air filter, as well as a decorative element, as in Figure 6.

Although VGS could be applied in the transitional space, not all buildings able to provide ample spaces for the greeneries due to the economic reason. Regarding to this issue, the green space inside the building could be reduced, and in some buildings, there are no presence of greeneries. The people who work and live in the urban area, especially for high-rise building occupants, are lack of having their own greeneries. In spite of the limitation and privacy, balcony could be optimized for applying the VGS. However, there are many considerations for balcony planting. Since many people tend to utilise their balcony for domestic purposes, the efficacy of the VGS must be optimised. The view to the outdoor must be unobstructed since view is one of the selling points in property business. In this situation, the concept of multi-layering plant could be implemented. The multi-layer plants are purposely arranged based on the layer and the selection of plant species are based on the foliage density, which is determined by the LAI value.

To obtain the best performance from the VGS, the structure and maintenance of the VGS is another important issue. One of the prominent examples of VGS in the transitional space is in Entopia, Penang, as figured in Figure 7. The façade of the building are covered with felt pocket VGS. Different plant species are applied to create a beautiful sustainable work of art. The temperature surrounding the site is significantly cooler due to the moist felt and provides comfort for those who are nearby. Therefore it is proven that VGS could improve the thermal comfort.

Regular maintenance is very crucial since plants are living things. There are possibilities to overgrow or die. Regular watering is required, where excessive watering could harm the plants and triggers the unwanted species to grow (Figure 8a). The less exposure to the daylight also contributes a negative effect, where plants could not optimally grow and leaving empty patches as in Figure 8b. The study of plant characteristic is needed to select the appropriate species if the VGS is under exposed to daylight or applied indoor and fully rely on artificial lighting.

VI. CONCLUSION

Optimising the transitional space could be a promising solution for applying greeneries on building where the growing space is limited. By introducing the multi-layer canopy and selecting the suitable plant species, it is expected to improve the thermal comfort in transitional space. It has been proven that thermal performance is the most influenced by the presence of greeneries application in building. The variety of foliage density will modify the microclimate through the different layers as measured by the LAI value. Foliage colour and size also contribute in improving the thermal comfort as well as enhancing the visual aesthetic value. It is suggested that foliage size could be combined in order to obtain higher efficacy especially in limited space. Since it is not fully enclosed, planting in transitional space could offer more variety in terms of the plants species. However, maintenance issue needs to be taken into consideration to keep the plants grow healthily. Finally, applying VGS in transitional space is effective in modifying the thermal comfort and at the end it could helps to reduce the energy used for cooling as well as to achieve the desirable thermal condition for the occupants.

ACKNOWLEDGMENT

The authors would like to thank to Universiti Sains Malaysia as the founding body of this research under the grant number 1001/PPBGN/814286.

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