1 Introduction

Modernization and urbanization caused rapid growth rates of cities [1, 2]. Previous literature stated that as a result of the urbanization phenomenon and migration from rural areas to cities, structural density has increased [3, 4], leading to a decrease in the green areas in the urban environment and many negative environmental developments [5, 6]. And the consumption of non-renewable energy resources in cities has increased very rapidly [3, 7].

It was found internationally that the best way to increase energy efficiency in cities would be to preserve energy in buildings. For this reason, different and new ways have been used to protect and increase energy efficiency in buildings [7, 8], as the building sector is highly responsible for energy efficiency, global warming, environmental pollution, and many global problems that follow [9], developments were promoted such as ecological approaches [3, 10,11,12], building envelope retrofitting [13], renewable energy resources [14, 15], environmental design [16], green architecture [9], and smart buildings [17, 17,18,19].

The concepts of ecology and sustainability have recently been used to create strong reflections as an environmental approach to decrease energy efficiency [8, 9, 20, 21]. Accordingly, this study tackles the application of green roofs (GR) and green walls (GW) as an ecological approach in the built environment toward increasing energy efficiency. Recent studies presented an environmental assessment of each system proving its contribution to energy efficiency and thermal comfort enhancements in arid environments [12]. However, no comparisons were achieved. Accordingly, this study aims to investigate the most effective system as an environmental approach to enhance energy efficiency, relying on a comparative analysis of the environmental impact of green walls and green roofs in Cairo Egypt as a case study for hot arid climate, through an experimental research design through model simulation.

2 Environmental challenges in arid environments

Previous literature mentioned the significance of green areas as an ecological approach to positively impact cities toward a cooler environment [12, 22]. Therefore, the decrease in green areas results in negative impacts on cities’ environmental situations and affects climate change [8].

Adding to the multiple challenges resulted in decreasing green areas and therefore affecting energy efficiency in the built environment, which requires users to increase HVAC systems usage to achieve their thermal comfort [9, 16]. As was previously discussed the HVAC demand is increasing due to climate change causing great concerns [23], and consequently, increasing the economic burden on the user, the more optimized energy efficiency, the lower the cost of the economic life cycle of the building.

The building envelope is one of the main influencers in building energy consumption [24, 25]. Previous literature proposed the implementation of green roofs and green walls in the building envelope [11, 26,27,28] could contribute to increasing green areas in cities and enhancing energy efficiency and thermal comfort [29,30,31].

In addition to the Window-to-wall Ratio (WWR), it was proved through literature that when there is a positive correlation between the WWR and the annual cooling load, energy consumption, cost, and thermal comfort; As the translucent materials are the only area of the building that receives direct solar gain [32] Therefore, putting this component of the building exterior into consideration, especially in areas with intense solar radiation seem significant for better environmental enhancements. Ideal WWR value was suggested to be 23.5 and 25.9% [33, 34] in Mediterranean regions. Moreover, Recent literature tested the optimal use of WWR in arid environments using Design-builder simulation and it indicated that 20–30% WWR for rectangle and square office building result in energy conservation between 41 and 61% [35].

3 Green walls and green roofs as ecological approaches

Green walls are designed vegetation covering the façade of the building imitating natural cliffs in natural environments [36]. Green walls could be established by different systems, it could be 1-green façades as support systems relying on climber plants and the growing media is on the top or bottom of the support system, or 2-living walls, where the growing media is inserted on the façade of the building [37,38,39,40]. Green walls, having a high percentage of exposure area in the urban environment [12], yet acting as a shading device, it was mentioned through literature as an appropriate ecological strategy for hot and dry climates [12, 27, 30, 37, 41,42,43,44], and could also help reduce the urban heat island effect and the urban environment [10, 36, 45]. Moreover, Green roofs are planted roofs on top of buildings, it could be intensive, extensive, or semi-extensive [46, 47]. Previous studies also highlight the significance of green roofs in enhancing energy efficiency in arid climates [47,48,49].

Various concerns were documented concerning the technical understanding of system application, the economic costs, and maintenance of green walls application in Egypt [37], therefore, experts stated that trellis systems as green facades could be one of the most applicable systems to be appropriated in Cairo, Egypt for its low costs and practicality in construction [37, 50]. While green roofs could appear less complex in construction, in addition, Previous research stated that extensive green roofs could be more common for their low maintenance and low costs [46, 47].

Both green roofs and green walls were proved to be important thermal insulation layers for buildings and thus significantly reduce energy consumption [12, 30, 51,52,53], as well as green walls [11, 12, 54], and this can greatly lower building operation costs. It was previously discussed how green roofs could have a high impact on urban climate enhancement as roofs represent a high percentage of the area exposed to the climate in urban environments [12]. While green walls could have a higher percentage of exposed areas [55]. Previous literature presented a comparison of energy saving between green roofs and green walls in tropical climates [56, 57]. it indicated that annual energy savings were enhanced by green walls better than green roofs. However, there is a research gap showing a comparative analysis to assess the most efficient strategy in terms of application to maximize energy efficiency in arid climates, and research studies on other climates could not be generalized. Previous literature recommended a comparison of their contribution to energy efficiency in arid climates [12, 53]. Accordingly, the study method will aim to fill this gap and will use a comparative case study analysis as below.

4 Method

The main aim of this study is to investigate the most effective ecological approach applied to enhance thermal comfort and energy efficiency in arid environments. This study is focusing on the context of Cairo, Egypt as an arid environment with different environmental and economic challenges. Therefore, considering the study literature, this study will rely on an experimental research strategy using simulation models to measure quantitative and statistical analysis concerning thermal comfort and energy efficiency as environmental impacts affected by different optimizations in an office building. This study will rely on design-builder as an industry-standard Building Energy Simulation tool, as it gives users access to the most often needed simulation capabilities for building materials and thermal mass. Financial analysis, glazing, shading, renewable energy, and HVAC.

A benchmark model developed by the Department of Energy DOE and simulated using Design Builder software. The Doe medium office building prototype model is used in energy simulation. This prototype model has been used to investigate building performance and energy savings [58,59,60,61,62].

At the beginning of the experiment, environmental performance optimization of the wall-to-wall ratio will be applied to the conventional model, this is considering several research emphasizing the importance of wall-to-wall optimization in arid climates [63]. This also ensures the ecological systems’ efficiency and a non-biased comparison between the two ecological systems and the ultimate building optimization. The comparative analysis will be performed between the ultimate identified optimizations of the 3 models, the conventional building, the green wall model, and the green roof. Analysis and results will be presented in descriptive statistics as graphs and tables.

4.1 The case of Cairo, Egypt

Egypt’s population is in speedy growth since the twentieth century [64]. Population densities within Cairo are some of the highest in the world [65, 66] which is the 11th largest urban population on earth [67]. Leading to environmental challenges [12, 13]. Moreover, Cities in Egypt are experiencing a major reduction in the percentage of green areas [5]. The rapid urban growth in Cairo has resulted in an obvious increase in car use due to limited transportation services [4]according to the traffic index for the year 2023, Cairo is ranked as one of the highest countries with traffic with a traffic index of 244 and Co2 coefficient index 9364.95 [68].

The government relied on reducing the existing green areas towards the expansion of traffic roads. Figure 1 shows the reduction in green areas over the years in Cairo and even new satellite cities. This has happened without considering the environmental and social aspects of the design of public space and building property despite all previous literature [69]. Which resulted in huge environmental challenges and energy consumption in the urban environment.

Fig. 1
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Green area reduction in Egypt, the above photographs are from before 2015 and the below are from 2022. Photos from Google Earth Pro

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Moreover, Egypt is facing an economic crisis, [67] with the rapid decline in overall costs. According to statistical analysis in Table 1, Cairo’s cost of living index increased by 77.5% in five years (decreased from 27.35 to 21.2 and was ranked 547th out of 558 cities in the world). This unstable economic descent can affect residents’ quality of life as well as their thermal comfort in their buildings.

Table 1 Cost of living in Cairo
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Most middle-income countries as Egypt, are at an early stage towards achieving sustainable development goals (SDGs) [64]. Previous literature mentioned different proposed strategies by the government for better urban development in Cairo. Such as the project “Cairo 2050” which was introduced in 2008, aimed to increase greenery through the green belt project [6, 70]. However, it was limited due to the political, social, and economic crisis in 2011 [64, 71]. Later after the crisis, Egypt began the establishment of a sustainable development strategy: Egypt Vision 2030 to afford energy (SDG 7), improve urban sustainability (SDG 11), and take urgent actions to maintain climate change (SDG 13) [64]. Moreover, it aims to apply a green pyramid grating system [72]. In addition, Political efforts also established “The technical committee for adapting the Egyptian code indicator for the quality of life” to improve the quality of life in Egypt, including environmental improvement strategies such as proposing green roofs and green walls on buildings in cities [73].

In light of previous literature, the proposed systems need constant maintenance [37, 46, 47]., and there is no governmental system for providing economic and maintenance management for a sustainable application [65], Moreover, according to previous literature, office buildings could have an important role in enhancing energy efficiency in Egypt [74, 75] Therefore, the simulated model and optimization for the study will be for an office building, and as shown in Fig. 2, the simulation models will be used through different phases, the First phase where the model is set to fit the activity, lighting, construction, openings, WWR and HVAC settings that fit the medium of an office building in Egypt. The simulation settings are adjusted to have annual results. The output data focus was thermal comfort (discomfort hours), total site energy (Electricity), Heating, and cooling loads.

Fig. 2
figure 2

Medium Office Building Simulation Methodological chart

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In the second phase, the main building output data was used as the main objective for the optimization process, The presented model with the main building was optimized into three building envelopes according to thermal comfort, energy demand, heating, and cooling. The three envelopes will vary according to 3 main variables 1- the wall-to-wall Ratio (WWR), ranging from 20 to 80%, 2-the roof construction whether conventional or with the application of a green roof and 3- the wall construction whether conventional or with green walls applied.

4.2 Model setting and input data

The required simulations were performed using Design-builder software for a base case model for a medium-sized office. The medium office building geometry is extracted from the database of the US Department of Energy (DOE) building simulation models as shown in Table 2 and Fig. 3. The medium office first was set on the conventional building materials of construction in Egypt according to the energy construction code. The model consists of 3 floors; each floor consists of 5 zones.

Table 2 Setting the design-builder model and used construction materials
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Fig. 3
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Optimization analysis—Minimize Total site energy consumption on the horizontal axis, and discomfort of all clothing on the vertical axis—Pareto chart

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4.3 Model output data

As shown in Table 3, The conventional model was sensitive to the construction material and simulation settings (the total site energy 296,747.16 Kilo Watt Hour (kWh), the heating = 3878.34 kWh, the cooling = 205,686.83 kWh, and ASHRAE Discomfort hours = 1743.17 kWh). The model thermal comfort (ASHRAE Discomfort hours) showed an initial indication reflecting a high number of hours (= 1743), however, the model should be less than 300 discomfort hours.

Table 3 Model Simulation output data
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4.4 Thermal comfort optimization and design variables

The optimization model relied on a multi-objective optimization (MOO) algorithm. Optimization was carried out with separate groups of design variables as shown in Table 4 and conducted by using the MOO algorithm with the associated outcomes being analyzed.

  • The first model is considered the conventional model (with a conventional roof and conventional walls).

  • The second model used the application of green walls on all building façades.

  • The third model used the application of green roofs. The green roof construction is supported by Design Builder. Gaetano Sciuto provided a validated model of Design-builder simulations of green walls and green facades using an experiment. The outcomes were proved reliable, and the experimental results matched the simulated output. from Design Builder [76] Therefore this study relied on the optimization in the experiment; As a variety of characteristics for any vegetated layer, including the depth of the soil, the conductivity of the soil, the density and height of the plants, and the amount of embodied carbon could be determined [77, 78].

Table 4 Optimization Setting According to Conventional Model
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The optimization processes are based on building WWR for ultimate environmental performance, changing the WWR from 20 to 80% using 2 steps. At the end of the study, the optimum designs were analyzed to identify the difference in thermal comfort and energy consumption. The simulation model design is explained below.

5 Analysis and results

5.1 Findings of WWR optimization and comparisons of all cases

Analysis 1 is the main optimization solution objective that identifies the minimum discomfort hours and total site energy, with cooling and heating additional outputs. The optimum cases’ outputs could identify and illustrate the suitable application of green roofs and green walls with different WWR, less discomfort hours, and low energy consumption while monitoring the heating and cooling loads. Figure 3 shows the simulation for different cases optimized. After simulating the models, the models with the green roof application and the green wall, A total of 10 optimum solutions among 59 iterations between the conventional and GW and 16 optimum solutions among 59 iterations between conventional and GR were performed.

As The design-builder with higher WWR provides fewer comfort hours, Findings in the design-builder optimization, as shown in Table 5, show the minimum discomfort hours with GW construction are for Window-to-wall percentage range between 20 and 52%. While Green wall optimization shows the minimum discomfort hours with a Window-to-wall percentage range between 20 and 56%. It is also visualized through parallel coordinates in Fig. 4, as the optimum total site energy and unmet hours of heating and cooling loads are presented in red, which refers to green roof application.

Table 5 Green wall and green roof with different WWR optimum solutions
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Fig. 4
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Parallel coordinate plot analysis for the green walls and envelope showing all variables

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A comparison shows the optimum solutions among the 2-optimization processes. Two optimum cases from the simulation were selected as indicated in Table 5. The conventional case and optimum GW optimization showed an overlapped result with high energy consumption and discomfort hours, in comparison to GR optimum optimization as shown in Fig. 3, which showed better performance in energy consumption and discomfort hours. The optimum GW showed 879 discomfort hours and a total site energy of 70068 kW/h, which is lower than the optimum conventional. Compared to the optimum GR, there is a clear decrease in the discomfort hours 648 h and 58,310 kW/h, while the optimum conventional roof optimum discomfort hours 891 h and 72,594 kW/h).

The optimum green wall optimization showed 1.4% less discomfort hours and 3.4% less Total site energy than the optimum conventional wall construction. Moreover, the optimum green roof showed 27% less discomfort hours and 20% less in all site energy than the optimum roof wall construction.

Analysis 2, optimization objectives are compared to optimization parameters, and the difference is measured in percentage of decrease is presented above each column in Fig. 5. Two optimum designs from the GW and two others from the GR were chosen for comparison, they show no difference in discomfort hours (The GR opt 1 showed 648 discomfort hours, and the conventional case showed 829 h), which made GR Thermal comfort more efficient by 22% than the conventional case. The Total Site Energy for the GR is 58,310 kW/h, while the conventional case is 725947 kW/h, therefore the GR makes 20% better in energy consumption. Moreover, more analysis was developed to compare the results with the conventional case below.

Fig. 5
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Parallel coordinate plot analysis for the optimum WWR in the conventional case, 2 optimum green walls (GW) cases, and 2 optimum green roofs (GR) cases

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5.2 Findings of comparative analysis of GW, GW, and base case

According to the comparative analysis between the conventional case with the 4 optimum solutions of green walls and green roofs (GW1, GW2, GR1and GR2), as shown in Fig. 5, the parallel coordinate plot shows the impact of the optimum green roof and optimum green wall cases when applying different WWR on the building discomfort hours and electricity demand, monitoring the cooling and heating loads. All green roof lines shown in red are at the bottom level showing different WWRs.

The second analytical study is a comparison between optimization objectives and optimization parameters, then the difference is measured in the percentage of decrease presented above each column in Fig. 6. It was found that two optimum designs from the GW and two others from the GR were chosen for comparison and showed a significant difference in discomfort hours (The GR opt 1 showed 648 discomfort hours, and the conventional case showed 1743 h), which made GR Thermal comfort more efficient by 269% than the conventional case. The Total Site Energy for the green roof is 58,310 kw/h, while the conventional case is 296747Kw/h, therefore the GR makes 509% better in energy consumption.

Fig. 6
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Comparing the 4 optimization objectives for the conventional case versus 2 optimum green walls (GW) and 2 optimum green roofs (GR). Comparing the percentage of decrease of each parameter to the conventional case for each objective

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The percentage of decrease between the output of each parameter (Thermal Comfort, Total Site Energy, Heating, and cooling) on all objectives (WWR, GW opt 1, GW opt 2, GR opt 1, GR opt 2) compared to the conventional case as shown in Fig. 6. Then the performance increases in measured in percentage for each parameter.

GR1 and GR2 showed better results than both optimum GW1 and GW2 options and conventional models on all tested parameters. The percentage of increase in the performance is higher on all output parameters. However, the WWR is a parameter that changes in all cases, therefore the ranges of WWR from 20 to 56% could be optimum design selection and provide convenient indoor thermal comfort, according to the given cases different energy consumption loads.

5.3 Findings of economic study

According to the economic study, the current price of 1 kWh is 1.60 LE for more than 1000 kWh for commercial buildings, according to the Electricity Holding Company in Egypt in early 2023. Figure 7 presents the electricity consumption and the percentage of cost change effect concerning conventional. The green roof opt. 2 shows a 527% difference in the cost. This case consumes a total site energy of 56,271 Kw/h for 90,033 L.E annually, while the conventional consumes 296,747 for 474,795 L.E annually. The green walls showed 412.7 and 423.5% difference.

Fig. 7
figure 7

Building Electricity consumption cost for conventional case, 2 optimum Green wall (GW), 2 optimum Green roof (GR) cases versus cost percentage represented on the line for the 4 new cases

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6 Discussion and conclusion

Filling the gap in research concerning the application of green walls versus green roofs aiming to enhance energy efficacy and thermal comfort; This study aimed to investigate the most effective system as an environmental approach to enhance energy efficiency, relying on a comparative analysis between the environmental impact of green walls & green roofs in hot climates, relying on a benchmark commercial office building simulation in Cairo, Egypt as a case study. This experimental research relied on investigating the application of green walls in comparison with green roofs in addition to the WWR optimization and cost, to reach the maximum thermal comfort and optimum total site energy for commercial buildings.

Green walls could be as simple or as complex according to the applied system, however, green roofs are less complex in construction and require lower maintenance.

According to the literature, green walls have a higher percentage of exposure area, acting as a shading device on the building envelope. Moreover, previous research in different climates stated that green walls positively affect energy consumption compared to green roofs. However, applying this study in arid climates, the opposite results were shown stating that green roofs have more energy reduction than green walls with better energy savings, and thermal comfort; it demonstrated low consumption of energy with 508% enhancement compared to conventional cases and enhanced thermal comfort for occupants by 269%. Green roofs are proven to be a more viable approach in the case of arid environments for better building performance.