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Article

Shading Performance of Public Open Spaces: A Multi-Criteria Evaluation Framework for Housing Projects

by
Omar S. Asfour
1,2,*,
Osama Mohsen
2,3 and
Jamal Al-Qawasmi
1,2
1
Architecture and City Design Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
2
Interdisciplinary Research Center for Construction and Building Materials, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
3
Department of Architectural Engineering and Construction Management, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
*
Author to whom correspondence should be addressed.
Buildings 2023, 13(12), 3099; https://doi.org/10.3390/buildings13123099
Submission received: 30 September 2023 / Revised: 11 December 2023 / Accepted: 12 December 2023 / Published: 13 December 2023
(This article belongs to the Section Architectural Design, Urban Science, and Real Estate)
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Abstract

:
This study investigates the impact of building grouping patterns on enhancing shading in public open spaces, considering different solar orientations, housing densities, and the hot climatic conditions of Saudi Arabia. The study considered a set of environmental performance indicators, including the percentage of open space shaded areas, heat gains through the building envelope, open space surface temperature, and the natural ventilation potential of each grouping pattern of the buildings. The results showed that at a building height of five floors, the percentage of open space shaded areas ranged from 14% to 53%. However, using the suggested multi-criteria evaluation framework to consider the above-mentioned performance indicators revealed more in-depth observations. Configurations that included parallel rows of buildings staggered and centralized in the middle of the site offered the best performance in this regard. This shows that compact grouping patterns in which open spaces are decentralized offer more urban shading and protection against undesirable heat gains through the building envelope during summer. The results of this study will enable a wider exploration of different building grouping patterns, which are needed in the harsh hot climate of Saudi Arabia to respond to current rapid housing development plans.

1. Introduction

The air temperature in many cities is continuously increasing due to several factors, such as rapid global population increase, urbanization, and building and urban designs, that are irresponsive to climatic conditions. This has increased global concern about global warming and climate change and their potential impacts on cities, including flooding, heat waves, and sea-level rise [1]. In 2023, the hottest summer on record was recorded [2]. Rising temperatures make cities thermally stressful compared to the surrounding rural and underdeveloped areas. The higher temperatures in cities compared to the surrounding areas are explained by the urban heat island (UHI) effect, which affects the quality of our urban environments, including the usability of outdoor open places due to the lack of thermal comfort and increasing thermal stress. UHI also increases the energy demand of buildings, mostly for air conditioning, which in turn leads to an increase in UHI [3]. Several studies have highlighted that, in general, the UHI effect and thermal comfort in urban areas are significantly related to urban geometry [4,5,6], where public open spaces can play a major role.
Public open spaces can be effectively utilized to mitigate the UHI effect, especially when they are well-designed and integrated into residential neighborhoods. These spaces can be implemented considering a wide range of criteria such as adequate area, proximity, accessibility, space proportion, hardscape elements such as water features and furniture, safety, and attractiveness [7,8]. In hot regions, shading is a key design criterion for public open spaces as it has a major impact on mitigating high temperatures, thus enhancing the thermal quality of the place and its suitability for outdoor activities. Urban shading is a major passive cooling strategy that aligns closely with Sustainable Development Goal 11, which aims to encourage policies and practices related to the design and creation of sustainable cities and communities [9]. The shading of public open spaces can be enhanced through the geometry of surrounding buildings, orientation, and proportions of open spaces, and other factors, such as the use of vegetation [10,11]. Shading is essential in harsh hot climates, such as Saudi Arabia, where these parameters should be examined carefully to ensure that public open spaces receive less solar radiation during summer [12].
Therefore, the present study aimed to investigate how to enhance shading in public open spaces located within housing clusters, considering a variety of building grouping patterns, different levels of housing densities, and different housing cluster orientations. The study also used a multi-criteria evaluation framework to consider the impact of additional environmental performance indicators (heat gains through the building envelope, surface temperature of the examined open spaces, and natural ventilation potential of each grouping pattern) and proposed a more holistic approach for designing housing clusters. This investigation enabled a wider exploration of different grouping patterns of buildings that were too complex to be investigated in the field. Such studies are needed in the harsh hot climate context of Saudi Arabia, where major development programs and initiatives are underway to double the production of housing units and enhance the quality of life in cities [13].

2. Literature Review

2.1. Importance of Public Open Spaces

Public open spaces refer to freely accessible urban spaces, such as parks, green areas, and sports and leisure plots that offer opportunities for people to congregate, relax, participate, and engage in a wide range of daily activities [14]. Public open spaces should be available to all people, regardless of their age, sex, ethnicity, culture, or socioeconomic status. The literature has demonstrated a wide range of benefits that good public open spaces may offer for the city and its residents, including leisure, recreational, social, health, physical, psychological, environmental, and economic opportunities [15,16,17,18,19,20]. The attributes of good public open spaces include the promotion of social and cultural activities, comfort and responsiveness, high physical and aesthetic qualities, a sense of attachment and belonging, freedom of access, and a convenient lifestyle. The need for well-designed public open spaces that achieve these characteristics and qualities has increased because of the increased complexity and diversity of contemporary cities and the unprecedented extent of urbanization.
Public open spaces integrated into residential areas have a wide range of positive psychological, social, and cultural effects on residents and the quality and livability of residential neighborhoods [21]. As part of the Saudi Vision 2030, many projects and initiatives are underway to improve the country’s quality of life. To improve the livability and quality of life in cities, the National Transformation Program in Saudi Arabia has highlighted the importance of open spaces and the need for more efficient design and management of public open spaces and to raise the amount of public space from 3.4 m2 to 3.9 m2 per person by 2030 [13]. Several studies have been conducted to identify the characteristics and qualities of public open spaces that fit the needs and particularities of Saudi Arabia [18,22]. However, relatively more studies have focused on the environmental qualities of urban spaces and thermal comfort in these spaces [23,24,25,26]. This is expected because high temperatures are a major concern in outdoor urban spaces in regions with severe desert climates, such as Saudi Arabia [24].

2.2. Shading Performance of Public Open Spaces

To design high-quality public open spaces, it is necessary to develop and adopt a citywide open space strategy to plan and maintain a sustainable network of open spaces [27]. Although previous research has highlighted the importance of the physical qualities of public open spaces and their compatibility with users’ social and cultural needs, there is evidence that the nature of the activities and functions and their frequency are correlated with environmental factors, such as thermal comfort, microclimate conditions, and neighborhood contexts [28]. In severely hot climates, high outdoor temperatures pose a major threat to the core purpose and function of open public spaces. High temperatures result in thermally uncomfortable and underused outdoor spaces, diminishing their potential to fulfill their intended functions. This is affected by several factors such as the geometry of the surroundings, urban density, and the amount of greenery in the space [10,29,30]. In general, vegetation, green space, and water features can effectively improve the quality of open outdoor spaces [31]. Urban geometry influences daytime temperatures more than greenery at the microclimate level [11,32]. However, urban greenery requires frequent maintenance, which increases the associated costs, particularly in hot regions that suffer from limited rainfall. This can explain the tendency to use compact design and self-shading in severely hot climate regions, such as Saudi Arabia, which can create cool local islands and enhance the livability of public open spaces [32,33].
Several indicators can be used to evaluate the shading performance in open public spaces. This includes the Sky View Factor (SVF), which is commonly used to quantify urban shade. The SVF refers to the ratio of the sky hemisphere visible to the ground. Thus, a 100% SVF implies that no obstructions are preventing solar radiation from reaching the ground, such as buildings and trees [34]. Placing obstructions in a given open urban space reduces the SVF value. This value could be further reduced by increasing the density of these obstructions, such as by increasing building heights. This has a direct impact on urban shading, which directly affects outdoor thermal comfort [35].
The sunlit fraction (i.e., the percentage of open space shaded area) was used to evaluate the shading performance of public open spaces. The sunlit fraction can be estimated as the daily average value, considering the daily solar datasets for a specific location [36]. This depends on the aspect ratio of the open space, which is the ratio of the height of the adjacent building H to the open space width W. Public open spaces with a large solar obstruction angle (i.e., a high H/W ratio) are usually characterized by a higher percentage of shaded area at the ground level of the open space and surrounding building walls [10,37]. The orientation of the open space is another significant factor affecting the sunlit fraction of the space. For instance, open spaces oriented north–south (NS) are usually cooler than those oriented east–west (EW) because they have less sun exposure throughout the day [31,38]. In harsh hot climates such as Saudi Arabia, these two parameters are usually examined carefully during the design of public open spaces to ensure that the open space receives less solar radiation during summer [12].

2.3. Previous Studies

The impact of urban morphology and density on urban shading and outdoor thermal comfort has recently received considerable attention in the literature. For example, Liu et al. [39] investigated the impact of building shading on heating and cooling energy demands, considering different urban forms and climatic zones in China. Using a parametric simulation, they developed a numerical model that can be used to calculate the shading effect on the building energy demand. Wai et al. [40] assessed the relationship between urban form and outdoor thermal comfort using simulation models validated by field measurements. The geometrical variables included the building height, setback, and void use. However, the study was limited to parallel, continuous, and separate blocks. Bahgat et al. [41] reviewed the optimal features of open spaces in urban residential complexes in terms of energy efficiency and outdoor thermal comfort. This was accomplished by considering five urban patterns. Many urban features are considered in this review, including density, height-to-width ratio, building spacing, building orientation, and open space shape. This study considered two climate types: hot arid and hot humid. The findings of this comparative study are presented in tables to provide general recommendations for architects and urban designers in the early design stage. However, these findings are generic and sufficient only at the conceptual design stage. Soflaei et al. [42] studied the courtyard shading performance considering different solar orientations and building dimensions. This study analyzed ten traditional courtyard houses using DesignBuilder, considering the hot-arid climate of Iran. However, their analysis was limited to single houses and did not consider the urban context.
Some studies have considered the climatic and urban conditions in Saudi Arabia. Alznafer [26] examined the impact of different urban geometries in Riyadh on outdoor thermal comfort in these spaces. This study considered three variables: building height-to-street-width ratios, SVF, and urban canyon orientation. This study used surveys to investigate users’ thermal perceptions, field measurements at these urban locations, and thermal modeling of two hypothetical urban geometries to assess the microclimate effect. These included both isolated and parallel blocks. The results of the study provide a set of recommendations to enhance urban shading in relation to the investigated urban geometries and minimize the energy consumption of buildings. Masoud et al. [25] used numerical modeling to assess solar radiation in two urban layouts that represent the old and modern layouts of Jeddah City, Saudi Arabia. This study used the SVF to investigate the positive impact of city compactness on solar exposure and urban shading enhancement.
However, there is a need to expand these studies by considering commonly used urban patterns in housing design, where parametric investigation can be effectively used to examine the impact of urban forms on open space shading. Additionally, few studies have considered a larger set of variables using multi-criteria evaluations. Thus, this study aims to bridge this gap in the literature by offering a parametric investigation of the impact of residential building grouping patterns on enhancing shading in public open spaces located within housing projects, considering different solar orientations and housing densities. The study also considered the impact of heat gains through the building envelope, open space surface temperature, and natural ventilation potential of buildings to provide a more comprehensive framework for the multicriteria evaluation of the different building grouping patterns examined. The proposed methodology for achieving this goal is detailed in the following section.

3. Materials and Methods

Following the previous studies introduced in Section 2.3 above, this study uses a hypothetical case study approach to investigate the impact of housing density, building orientation, and grouping patterns on the shading performance in outdoor open spaces located within residential buildings. This has been conducted using parametric numerical modeling, which was experimentally validated in a previous study by Asfour [36]. This study used DesignBuider 5.5, which uses the EnergyPlus tool for energy calculations. Shading was estimated using the sunlit fraction of the open space ground surface, which indicated the percentage of shaded area. It is important to note that DesignBuilder calculates the daily average of the sunlit fraction considering all daily datasets, including night hours. Therefore, it was essential to use sunlit fraction hourly data during the daytime only to calculate the daily average sunlit fraction and percentage of open space shaded area. Hourly shading data were calculated during the daytime for the first and middle days of June, July, and August to represent summer and for the first and middle days of December, January, and February to represent winter.
To make the comparison between the different examined configurations more comprehensive, this study also considered the following additional three dependent variables.
  • Heat gains through building envelope: These were calculated using DesignBuilder considering heat gains through flat roofs, external walls, and windows. The U-values of these elements were assumed as follows [43]: 0.26 W/m2.K, 0.35 W/m2.K, and 1.98 W/m2.K, respectively. Building shadows mainly affect heat gains through external walls and windows. External walls are assumed to have two layers of 10 cm concrete blocks with an 8 cm extruded polystyrene layer in the middle, external rendering, and internal gypsum plastering. The concrete block has a specific heat of 1000 J/kg·K and a density of 1850 kg/m3, while the extruded polystyrene has a specific heat of 1400 J/kg·K and a density of 35 kg/m3. The solar absorptance of the external rendering layer is assumed to be 0.7. Windows are composed of two layers of 3 mm clear glazing with a 13 mm air layer in between. The Solar Heat Gain Coefficient (SHGC) of the window is 0.7. All the thermal properties of building materials were assumed fixed in the examined cases. The following settings were also considered in DesignBuilder: building type is residential, occupancy is 20 persons per floor (5 persons per housing unit), cooling setpoint temperature is 25 °C, heating setpoint temperature is 21 °C, model infiltration is 0.7 ach/h, and the window-to-wall ratio is 20%.
  • Public open space surface temperature: Open space here refers to the entire site area that is not covered by buildings. This is assumed as a tiled surface with a solar absorptance of 0.6. Open space surface temperature refers to the average temperature calculated over the top face of the outdoor thermal zone that represents the open space. This was calculated in DesignBuilder using the “Building Surface and Opening Outputs” function.
  • Natural ventilation potential of the examined grouping patterns using the total wind pressure difference across the building facades: Secondary data were obtained from Asfour [44], who examined the housing grouping patterns considered in this study.
The parametric numerical modeling was conducted with reference to the hot climatic conditions of Saudi Arabia. Saudi Arabia has five inhabited climatic zones including the hot-dry maritime zone. Weather data of Dharan city were used in this regard to represent this climatic zone [45]. Dhahran City (26.24° N) is located in the eastern region of Saudi Arabia, where the prevailing climate is hot and dry. Simulation weather data in DesignBuilder show an average annual direct solar radiation of 130 kWh/m2 and an average annual wind speed of 4.4 m/s. The average annual dry-bulb temperature is 27.4 °C. The average dry-bulb temperature in summer and winter is about 36 °C and 18 °C, respectively.
The relationship between the abovementioned four dependent variables, that is, shading, heat gain, surface temperature, and natural ventilation performance of buildings, was analyzed using a multicriteria evaluation framework. Multicriteria evaluation, or multicriteria decision analysis, is a research area in management science that involves analyzing various available options to achieve multiple objectives [46]. It is a generic term used to describe a group of systematic approaches that support the evaluation of alternatives based on multiple and often conflicting criteria [47,48].
Three independent variables were considered in this analysis. The first was the housing grouping pattern. Asfour [44] introduced five parametric urban configurations to examine the wind environment for different grouping patterns of housing blocks. This study adopted these common configurations by considering different levels of housing density. As shown in Figure 1, each configuration was composed of four buildings grouped using a variety of methods. The resulting open spaces for each configuration are as follows:
  • Configuration 1: Central linear open space enclosed by two parallel rows of buildings.
  • Configuration 2: Four corner square open spaces.
  • Configuration 3: T-shaped open space with an open side.
  • Configuration 4: Central open space with four staggered wings (Al-Mafrouka in Islamic art).
  • Configuration 5: Square central open space.
The second independent variable was housing density. This study assumed a square site that measures 70 × 70 m (4900 m2). To maintain the parametric nature of the investigated configurations, the site was divided into 196 cells, each measuring 5 × 5 m. As previously mentioned, each configuration included four typical housing blocks. Each measured 30 × 15 m (450 m2), that is 6 × 3 cells. The area of the open space enclosed by the four housing blocks was 3100 m2, which resulted in a fixed coverage ratio of approximately 37%. The four housing blocks were manipulated to introduce the different examined configurations, considering a fixed setback of 10 m. Three building heights were considered, namely one, three, and five floors, as listed in Table 1. These represent villas and apartments, which are the most common housing typologies in Saudi Arabia [49]. High-rise residential buildings are uncommon in the study area [50]. These three building heights resulted in housing densities of 32, 96, and 160 dwell units per ha, respectively. The minimum open space shared for each housing unit is 38.75 m2 or 6.6 m2 per capita, assuming a family size of 5.9 [51]. This is acceptable considering the World Health Organization (WHO) minimum recommendation of 9 m2 per capita, compared to the 2020 target of 3.9 m2 per capita in Saudi Arabia [52].
The third independent variable was site orientation. Four orientations are usually considered in parametric simulations to represent the four cardinal directions in addition to the four ordinal directions. The configurations examined were symmetrical about both axes in the Cartesian plane. This means that the three rotation angles of the site with respect to the north are sufficient for the simulation. The rotation angles were 0°, 45°, and 90° (Figure 1). However, as configuration 3 is symmetrical about one axis, 135° and 180° were used as the rotation angles in this configuration.

4. Results and Discussion

Parametric numerical modeling was implemented considering 45 cases (five configurations in three orientations and three densities). The bulk of the data was obtained from the simulation process. Data were processed to determine the impact of housing grouping patterns, density, and orientation on public open spaces’ shaded area, public open spaces surface temperature, and heat gain through the building envelope. The results are discussed below.

4.1. Open Space Shading Analysis

Figure 2 shows the daily average percentage of open space-shaded areas obtained by considering the summer climatic conditions of Saudi Arabia. In general, the percentage of shaded areas was proportional to the housing density. At the lowest density (i.e., building height of one floor), there was no significant amount of shading due to building massing in public open spaces. Thus, this low density can be considered a reference for this study. This difference became more significant as the building height increased to five floors. Additionally, the observed percentage of shaded open space areas in summer was generally limited, ranging from 14% to 53%. This is because of the relatively high solar altitude in Saudi Arabia, which reduces the amount of urban shade in the examined cases. This indicates that there is a need to use other shading elements in addition to building masses, such as vegetation. In contrast, the examined configurations offered a higher percentage of open space shaded areas in winter than in summer (Table 2) because of the lower angle of the sun. This percentage ranged from 14% to 67%. However, considering the prevailing hot climatic conditions in Saudi Arabia, summer conditions are given more focus in the following analysis, with the highest housing density as the best scenario.
In general, the examined building configurations offered different amounts of shade in the public open spaces. Configuration 1 included two parallel rows of residential blocks enclosing a linear open space along the site. This is a common urban form, in which parallel building masses are planned to create urban canyons and local microclimates. The results showed that this configuration performed better in the NS orientation when the site rotation angle with respect to the north was 0°. In this case, the low morning and evening suns hit the long eastern and western building façades, which increased the amount of shading in the examined open space. This resulted in an average shading percentage of 44% in summer, considering a building height of five floors (Table 2). However, the EW orientation, when the site rotation angle with respect to the north was 90°, offered a significantly lower percentage of shading at 14%, which was the lowest observed in this analysis. This is because the summer high sun in this case hits the southern long façades of buildings, which have limited shading potential (Figure 3).
Configuration 2 included two parallel rows of buildings, similar to Configuration 1. However, these two rows were staggered and centralized in the middle of the site, resulting in four open corner spaces. This building arrangement increased the exposure of long façades to the sun and, therefore, the amount of shading in the examined open spaces. When the site was rotated at 45°, all long façades were exposed to low sunlight in the morning or evening, depending on the direction of rotation (Figure 3). This resulted in a shading percentage of 53% in the open space, considering a building height of five floors (Table 2). This percentage was the highest observed in this analysis, making this configuration the most effective. However, the high solar exposure observed in this configuration also increased the heat gain through the building envelope, which is a concern under the hot climatic conditions considered in this study. This is discussed further in Section 4.3.
Configuration 3 included four housing blocks in a central open space. This open space was a semi-closed courtyard that had one open side directed to the north or south. However, both options offered similar behavior in terms of open space shading because the long eastern and western façades in these two options had similar solar exposure. In general, this configuration could offer open space shaded areas of 18%, 44%, and 39% for the three examined rotation angles of 0°, 45°, and 90°, respectively, given that the courtyard is open towards the north. When the courtyard is open towards the south, this configuration offered an open space shaded area of 19%, 41%, and 44% for the three rotation angles.
Configurations 4 and 5 included a central open space related to the buildings in a symmetrical pattern. Thus, the NS and EW orientations exhibited the same shading performance for each configuration. The percentages of open space shaded areas in both orientations were 30% and 33% for configurations 4 and 5, respectively. Similar values were obtained at a site rotation angle of 45°, as can be observed in Table 2.

4.2. Open Space Surface Temperature Analysis

Figure 4 shows the hourly average external air temperature during summer daytime in Dhahran City. It shows that the peak temperature approaches 44 °C, while the average temperature is about 38 °C. Higher temperature values were observed in the afternoon hours compared to the morning hours. Figure 5 shows the daily average open space external surface temperatures for the different cases examined, which ranged between 46.4 °C and 50.48 °C. To find out the impact of shading on open space surface temperature, hourly temperature data were calculated during the daytime and averaged for the first and middle days of June, July, and August to represent summer. The general trend of the graph shows a strong negative correlation with the open space shading measurements, as presented in Figure 2. The correlation factors between open space surface temperature and shaded area were −0.8, −0.9, and −0.9 at site rotation angles of 0°, 45°, and 90°, respectively. This demonstrates the direct impact of urban shading on reducing the open space surface temperature. In addition, the surface temperature decreased as building density increased in all cases. However, the highest reduction was observed in Configuration 2 at the 45° orientation of the site, where a temperature reduction of 3.4 °C was observed when building height was increased from one to five floors. This case also offered the highest shading percentage, as described in Section 4.1. In some other cases, the observed variation in the open space surface temperature was limited. This is because of the relatively high solar altitude in Saudi Arabia, which reduces the amount of urban shade in the examined cases. This limits the shading impact on the examined variables such as open space surface temperature.

4.3. Heat Gains through Building Envelope

It might be argued that buildings that provide more shadows in open spaces will also be exposed to more solar radiation and thus experience greater heat gains. To investigate this, the resulting heat gains through the building envelope in the examined configurations were examined. This included the heat gain through external walls, glazing, and roofs, considering a building height of five floors. The heat gain data were calculated and averaged for the first and middle days of June, July, and August to represent the summer season. Table 3 presents the results obtained in this regard, along with the corresponding percentages of open space shaded areas. The results showed that the observed variation in heat gains in some cases seems to be limited. However, it is important to note that the values presented here are average daily values that were calculated to represent summer. The following observations were also made:
  • When the site rotation angle was changed from 0° to 45°, the average daily heat gain through the building envelope during summer increased significantly because this oblique orientation increased the exposure of the long façades to the sun.
  • When the site rotation angle was changed from 0° to 90°, the average daily heat gains through the building envelope during summer remained the same in the configurations that included long façades in both the NS and EW orientations, that is, Configurations 3, 4, and 5. However, it increases in other configurations that include long façades in one orientation, that is, Configurations 1 and 2. When the site rotation angle was changed from 0° to 90°, the average daily heat gains in Configurations 1 and 2 increased from 724 to 760 kW and from 726 to 731 kW, respectively. This increase was more significant at a rotation angle of 45°, where the average daily heat gain increased from 724 to 774 kW and 726 to 757 kW in Configurations 1 and 2, respectively.
The results showed two conflicting design criteria: maximizing the percentage of open space shaded area in summer, which ranged between 14% and 53%, and minimizing the daily average heat gains through the building envelope in summer, which ranged between 724 and 777 kW. The comparison becomes more complicated when we consider the resulting open space surface temperature (discussed in Section 4.2) and natural ventilation potential of the examined cases, which were investigated in a previous study by Asfour [44]. The natural ventilation potential of the entire site was quantified using the gross wind pressure difference across the building façades (Table 3). This shows that there is a need to conduct a multi-criteria evaluation to figure out this comparison, which is presented in the following section.

4.4. Multi-Criteria Evaluation

As argued above, comparing different housing grouping patterns requires a multidimensional analysis that considers different related environmental factors. Four variables were considered in this study, as listed in Table 3. These variables were used to propose a multi-criteria evaluation framework based on the following objectives (listed in descending order from most to least important):
  • Reducing summer heat gain through the building envelope: This was assumed to be the most important objective given that air conditioning accounts for more than 50% of the total annual electricity consumption of buildings in Saudi Arabia [53].
  • Increase in shaded areas in the examined public open spaces: This was the second most important because people spend most of their time indoors [54]. Therefore, achieving thermal comfort inside buildings is more important than achieving outdoor thermal comfort.
  • Reduction in surface temperature of examined public open spaces This was assigned the same importance weight as the second objective because of the strong correlation between these two objectives, as discussed in Section 4.2.
  • Improving natural ventilation performance of residential buildings: This was assumed to be the least important objective as natural ventilation in buildings can be utilized over a limited period during the year, considering the hot climatic conditions examined in Saudi Arabia.
The study suggests two approaches in this regard as explained below:

4.4.1. Shortlisting Process of Design Alternatives Followed by Multi-Criteria Evaluation

This study aims to exclude the least promising alternatives based on the two most important design objectives among the four listed above, considering the hot climatic conditions of Saudi Arabia. These two objectives were to reduce the heat gain through the building envelope and increase the percentage of shaded areas in public open spaces. The following design thresholds are assumed to be acceptable compromises.
  • The daily average heat gain through the building envelope should not exceed the lowest observed value of 5%. This value, as shown in Table 3, was 724 kW, which was observed in Configuration 1 considering a site rotation angle of 0°.
  • The percentage of open space shaded areas should not be less than 30%.
Based on these two conditions, the following configurations could be shortlisted:
  • Configuration 1 at a site rotation angle of 0°.
  • Configuration 2 at a site rotation angle of 45°.
  • Configuration 3 at a site rotation angle of 90°.
  • Configuration 4 at a site rotation angle of 0° or 90°.
  • Configuration 5 at a site rotation angle of 0° or 90°.
A weighted analysis using a multi-objective evaluation matrix was developed to compare the shortlisted design alternatives, as shown in Figure 6. In comparing the two criteria, the preference for one over the other was scored as follows: 1 = slight preference, 2 = average preference, 3 = above-average preference, and 4 = major preference. In Figure 6, the four criteria are listed as A–D. For example, in the triangular portion of the matrix, the heat gains through the building envelope (A) were compared to the percentage of the open space shaded area (B). Criterion A had a “slight preference” over criterion B, A-1 was placed in a diamond-shaped cell linking the two criteria. Similarly, criteria C and D were compared and because they were equally preferred (no preference), C/D was recorded. In the lower part of the matrix, raw scores for the criteria were calculated based on the number of times the criteria appeared in the upper part of the matrix. For example, A was given a raw score of 5, whereas D received a score of 1. These raw scores were then converted into weights by normalization using Equation (1):
w e i g h t   o f   i m p o r t a n c e = 10 × x − x m i n x m a x − x m i n
where x is the raw score of the criterion, xmin is the minimum possible score of 0, and xmax is the maximum raw score (i.e., 5 in this case). Five shortlisted design alternatives were compared based on the total scores. Each criterion (A to D) was assigned a score between 1 (poor) and 5 (excellent) based on its values for each configuration, as shown in Table 3. For example, for the heat gain criterion, the scores were 724, 757, 751, 752, and 747 kW for Configuration 1 at 0°, 2 at 45°, 3 at 90°, 4 at 0°, and 5 at 0°, respectively. Because we sought lower heat gains through the building envelope, Configuration 1 scored 4 for this criterion, followed by Configuration 5, scoring 3; Configurations 3 and 4, scoring 2; and Configuration 2, scoring 1. The score of Configuration 2 was the lowest because it had the highest value for the heat gains through the building envelope, which was the least desirable value for this criterion. These scores were then multiplied by the weight of importance and summed to obtain the total score for each alternative. As shown in Figure 6, based on this weighted analysis, Alternative 1 (Configuration 1 at a rotation angle of 0°) was selected because it scored the highest total value of 72 points.

4.4.2. Merging Process of Design Alternatives Followed by Multi-Criteria Evaluation

This approach is based on the common practice of urban design of housing projects, which involves the use of typical housing clusters in different orientations within the same site. This is usually useful for reducing costs, utilizing the site area, and enhancing the visual variety of the project. Thus, from a practical perspective, this study suggests using the average value of the three orientations (i.e., 0°, 45°, and 90°) for each of the four variables considered in the multi-criteria evaluation process. The orientation-independent average values are listed in Table 4. Configuration 2 offered the highest shading potential while maintaining the lowest heat gains through the building envelope compared to the other configurations. This configuration included two parallel but staggered rows of buildings placed in the middle of the site, resulting in four open corner spaces. This shows that compact grouping patterns in which building rows are staggered and open spaces are decentralized offer more urban shading and protection against unwanted heat gains through the building envelope during summer. This is demonstrated in Figure 7, which shows a multiobjective evaluation matrix of different configurations based on the average values for each design criterion. It can be observed that Configuration 2 should be selected, as opposed to Alternative 1 in the previous evaluation approach discussed in Section 4.4.1.
It should be noted that the results had some limitations. Numerous parameters affect the shading performance of open public spaces. However, the proposed parametric analysis coupled with multi-criteria evaluation can form the basis of a wider environmental performance evaluation framework that considers additional parameters:
  • Climatic parameters: The results were limited to the hot climatic conditions of Saudi Arabia, including solar geometry and solar radiation.
  • Urban parameters: The results were limited to the five grouping patterns of housing blocks examined, considering three rotation angles of the site (0°, 45°, and 90°).
  • Building design parameters: The results were limited to the examined rectangular building shapes and building heights of one to five floors.
  • Multi-objective evaluation assumptions: The results were limited to the importance level assigned to each environmental performance evaluation criterion. Considering the hot climatic conditions in Saudi Arabia, priority was given to reducing the solar exposure of buildings. This may vary if the proposed evaluation framework is implemented at other locations.

5. Conclusions

Public open spaces are essential components of traditional and modern cities and a wide range of criteria, including shading, should be considered during their design. This is essential in hot climates, where urban shading is considered a key passive design strategy that can effectively improve outdoor thermal comfort and enhance the quality of a place. Thus, this study investigates the impact of different building grouping patterns on enhancing shading in open outdoor public spaces, considering different solar orientations and housing densities. This was implemented using numerical simulations, considering the hot climatic conditions of Saudi Arabia, and a multi-criteria evaluation framework to consider a variety of environmental performance indicators in addition to shading, including heat gains through the building envelope, open space surface temperature, and the natural ventilation potential of buildings. Shading analysis showed that the examined building configurations offered different shading potentials. However, it was observed that building configurations that provide more shadows in open spaces also experience more sun exposure, which increases heat gains through the building envelope. This shows that there are two conflicting design criteria in this regard: maximizing the percentage of open space shaded areas in summer, which ranged between 14% and 53%, and minimizing the daily average heat gains through the building envelope during summer, which ranged between 724 and 777 kW in the examined cases. Thus, this study conducted a multi-criteria evaluation of the investigated housing configurations, considering the following four design objectives (listed in descending order according to their importance in the investigated hot climate): reducing heat gains through the building envelope, increasing the shaded area in the open space, reducing the surface temperature of the open space, and improving the natural ventilation potential of the buildings.
The multi-criteria evaluation showed that Configuration 1, which included two parallel rows of residential blocks that enclosed a central linear open space, offered the best performance. Building masses in this configuration provided a relatively high open space shaded area in summer (44%) while maintaining the lowest daily average heat gains through the building envelope in summer (724 kW). This is true in the NS orientation when the site rotation angle with respect to the north is 0°. However, this was not the case when the configuration was rotated by 90°, which reduced the open space shaded area in summer to 14% and increased the daily average heat gain through the building envelope in summer to 760 kW. However, we cannot build our cities using only a single building orientation. Residential neighborhoods usually include typical housing clusters rotated at different angles in response to site requirements. Thus, this study conducted a multi-criteria evaluation of open space shading, building heat gains through the building envelope, open space surface temperature, and natural ventilation performance of buildings in the different examined building configurations, considering the average value of the three orientations (0°, 45°, and 90°). This shows that Configuration 2 is the best design alternative in this regard. This configuration is similar to Configuration 1, as it includes two parallel rows of buildings; however, these rows are staggered and centralized in the middle of the site. This creates four corner open spaces, instead of one centralized open space. This building arrangement offered a relatively high open space shaded area in summer (34%) while maintaining a relatively low daily average heat gain through the building envelope in summer (738 kW). It is important to mention that the observed percentage of shaded open space areas in most cases was generally limited due to the high solar altitude in Saudi Arabia. This requires further investigation into the use of other shading elements in addition to building masses, such as vegetation. Mitigation of the UHI effect and heat gains through building envelope through the use of building materials with low thermal capacity is also recommended for further investigation.
Using the key results of this study, it is possible to establish several design guidelines that can help improve the shading performance of public open spaces. These include the following:
  • Site orientations that provide more shadows in open spaces also experience more sun exposure, which increases heat gains through the building envelope. Thus, it is important to specify design priorities based on the climatic conditions.
  • Housing projects are typically designed using clusters planned in different orientations. Thus, from a practical perspective, this study suggests the use of an overall average value of open space shading that considers different orientations in each building grouping pattern as an orientation-independent indicator of shading performance.
  • Using the abovementioned overall average value of open space shading, the study found that fragmented, decentralized open spaces are preferred over large, centralized ones. In this case, configurations with staggered rows of buildings placed in the middle of the site offered a relatively higher amount of shading while maintaining the lowest heat gains through the building envelope compared to the rest of the examined configurations.
The results and guidelines derived from this study will help policymakers improve the quality of public open spaces located within housing environments and enable wider exploration of different housing grouping patterns that are too complex to be investigated in the field. The parameters that were restricted in this study are recommended for further research, including additional building grouping patterns, shading performance of high-rise buildings, impact of surface temperature on airflow temperature, and impact of other shading elements such as trees.

Author Contributions

Conceptualization, O.S.A.; Methodology, O.S.A. and O.M.; Software, O.S.A.; Formal Analysis, O.S.A. and O.M.; Investigation, O.S.A., O.M. and J.A.-Q.; Writing—Original Draft Preparation, O.S.A., O.M. and J.A.-Q.; Writing—Review and Editing, O.S.A., O.M. and J.A.-Q.; Visualization, O.S.A. All authors have read and agreed to the published version of the manuscript.

Funding

The Article Processing Charges (APCs) were funded by King Fahd University of Petroleum and Minerals (KFUPM).

Data Availability Statement

The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to privacy.

Acknowledgments

The authors acknowledge King Fahd University of Petroleum and Minerals (KFUPM) for financial support.

Conflicts of Interest

The authors declare no conflict of interest.

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Buildings 13 03099 g001
Figure 1. Different examined building configurations.
Figure 1. Different examined building configurations.
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Figure 2. Daily average percentage of open space shaded area in summer in the different examined configurations considering three rotation angles of the site.
Figure 2. Daily average percentage of open space shaded area in summer in the different examined configurations considering three rotation angles of the site.
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Figure 3. Shading patterns at different times during a typical summer day in Configuration 1 (with a rotation angle of 90°) and Configuration 2 (with a rotation angle of 45°), which offered the lowest and highest summer shading amount in the examined public open spaces, respectively.
Figure 3. Shading patterns at different times during a typical summer day in Configuration 1 (with a rotation angle of 90°) and Configuration 2 (with a rotation angle of 45°), which offered the lowest and highest summer shading amount in the examined public open spaces, respectively.
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Figure 4. The hourly average external air temperature during summer daytime.
Figure 4. The hourly average external air temperature during summer daytime.
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Figure 5. Daily average external surface temperature of the open space in the different examined configurations during summer considering three rotation angles of the site.
Figure 5. Daily average external surface temperature of the open space in the different examined configurations during summer considering three rotation angles of the site.
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Figure 6. Multi-criteria evaluation matrix to compare the five short-listed building configurations.
Figure 6. Multi-criteria evaluation matrix to compare the five short-listed building configurations.
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Figure 7. Multi-criteria evaluation matrix of all building configurations considering an average value of the three rotation angles in each evaluation criterion.
Figure 7. Multi-criteria evaluation matrix of all building configurations considering an average value of the three rotation angles in each evaluation criterion.
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Table 1. Different examined housing density levels.
Table 1. Different examined housing density levels.
No. of FloorsNo. of Housing UnitsDensity (Unit/ha)Open Space Area (m2)Housing Unit Share of Open Space (m2/unit)
116323100194
34896310065
580160310039
Table 2. Daily average percentage of open space shaded area in both summer and winter in the different examined cases considering a building height of five floors.
Table 2. Daily average percentage of open space shaded area in both summer and winter in the different examined cases considering a building height of five floors.
% of Shaded Area in Summer% of Shaded Area in Winter
0°45°90°0°45°90°
Configuration 1443414554345
Configuration 2265324321432
Configuration 3184439674952
Configuration 4303830524252
Configuration 5332833614661
Table 3. Examined variables in the examined configurations considering the daily average value during summer.
Table 3. Examined variables in the examined configurations considering the daily average value during summer.
Heat Gains through Building Envelope [kW]% of Open Space Shaded Area Open Space External Surface Temp. [°C]Natural Vent. Performance (Pressure Diff.) [Pa]
Orientation
Configuration		04590045900459004590
Config 172477476044341447.548.149.6182425
Config 272675773126532446.746.449.6261528
Config 375277575118443949.847.947.8332222
Config 475277775230383048.948.348.9272327
Config 574774574733283348.048.448.0281828
Table 4. Four examined variables presented as average values of all rotation angles in the different examined configurations.
Table 4. Four examined variables presented as average values of all rotation angles in the different examined configurations.
ConfigurationHeat Gains through Building Envelope [kW]% of Open Space Shaded Area Open Space External Surface Temp. [°C]Natural Vent. Performance (Pressure Diff.) [Pa]
Config 17533148.422
Config 27383447.623
Config 37593448.526
Config 47603348.726
Config 57463148.125
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Asfour, O.S.; Mohsen, O.; Al-Qawasmi, J. Shading Performance of Public Open Spaces: A Multi-Criteria Evaluation Framework for Housing Projects. Buildings 2023, 13, 3099. https://doi.org/10.3390/buildings13123099

AMA Style

Asfour OS, Mohsen O, Al-Qawasmi J. Shading Performance of Public Open Spaces: A Multi-Criteria Evaluation Framework for Housing Projects. Buildings. 2023; 13(12):3099. https://doi.org/10.3390/buildings13123099

Chicago/Turabian Style

Asfour, Omar S., Osama Mohsen, and Jamal Al-Qawasmi. 2023. "Shading Performance of Public Open Spaces: A Multi-Criteria Evaluation Framework for Housing Projects" Buildings 13, no. 12: 3099. https://doi.org/10.3390/buildings13123099

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