The urban heat island (UHI), which is the rise in temperature in urban environments, has increased in modern cities and communities. The demographic and activity makeup of metropolitan areas, in addition to the neighborhood layouts and designs of contemporary locations, all contribute to this effect (Detommaso et al., 2021). According to Salvati, Roura, and Cecere (2017) and Elmarakby et al. (2022), the UHI in modern districts increases building energy use and hastens climate change. Moreover, physical health, comfort, and development are facing difficulties as a result of the interaction between local UHI and global climate change. Because most people in the twenty-first century live in cities, preventing urban overheating is becoming more and more important (Santamouris et al., 2019). As Bueno et al. (2014) stated, UHI reduces heat evacuation through convection in metropolitan districts, particularly newer ones. In this situation, a variety of alternatives, including passive design solutions, such as increasing the shaded areas due to building height variation, can be incorporated into the planning of contemporary urban neighborhoods. UHI raises the temperature of urban areas, including urban centers, relative to rural and suburban areas, which raises the energy consumption of cities. Sangiorgio et al. (2020) mentioned that building height is the 8th of 11 factors that affect the absolute max UHI phenomenon.

Variations in building height can create a positive effect on the thermal performance of the area. Tall buildings could shade the streets and structures underneath, lowering the quantity of direct sunlight and, thus, the amount of solar radiation absorbed by these structures. It has been shown that spatial arrangement has a significant impact on air temperature. Moreover, the ventilation in a city can be impacted by tall structures. To improve ventilation and lessen the UHI effect, tall buildings can, for instance, produce a chimney effect where hot air is sucked up and away from the street level. Lan and Zhan (2017) stated that increasing the building height and density inevitably results in greater building shades. They added that building height is one of a group of parameters that together account for greater variance in outdoor air temperature. To reduce the UHI effect, they strongly advise that spatial organization be considered in future urban planning. Zheng et al. (2019) confirmed that vegetation and surfaces with tall structures of various heights are the two main types of land cover that have an impact on the thermal environment of the land surface and urban areas. They added that the land surface temperature fell from low-rise to high-rise residential buildings, and temperatures were lowest in high-rise residential areas. Shareef and Abu-Hijleh (2020) confirmed that compared to an urban layout with moderate variation in building heights, one with a large fluctuation in building heights led to a greater drop in outdoor air temperature.

Shishegar (2013) affirmed that in urban design, the variation in building height creates different height-to-width (H/W) ratios and urban layout and affects the outdoor air temperature and the wind speed. Moreover, she added that a street canyon’s H/W ratio has a major impact on how much solar energy is absorbed by buildings and the land between them. The amount of solar energy collected by the roadway surfaces rises as the H/W ratio is reduced. However, the varied surfaces of the urban roadway do not all receive this solar energy evenly. Basically, compared to vertical surfaces, the horizontal earth absorbs extra radiation from the Sun. Thus, the H/W ratio affects the earth’s surface temperature more than the facades and vertical surfaces. Placing a tall building next to a low-rise building causes the wind to increase by 90% and reduces the temperature by 1°C; thus, integrating a few high-rise buildings strategically will help increase the wind velocity inside the created canyon and thus decrease the air temperature (Priyadarsini and Wong, 2005). Heat is trapped in long, deep canyons with a high H/W ratio, and the air temperature rises. The airflow is effectively accessed by low H/W ratios of less than 0.3, and a significant amount of outdoor air flows through the canyon to lower the air temperature (Shareef and Abu-Hijleh, 2020).

Gamero-Salinas et al. (2021a) mentioned that considering the building form parameters (such as the height-to-depth ratio, open space ratio, and green plot ratio) that enhance air movement and lower the mean radiant temperature can provide thermally comfortable environments for typical social activities. The most thermally comfortable spaces for all activities during a normally warm hour are vertical and horizontal breezeways (Gamero-Salinas et al., 2021b). Dou (2014) stated that to enhance thermal comfort and lessen the consequences of climate change in urban settings, more compact urban forms with deeper urban canyons and green building coverings are recommended.

Li and Donn (2017) examined the relationships between urban heat islands, vertical temperature lapse rates, and the impact of tall structures on urban wind flows. They claimed that the outside temperature on the top floor of a 30-floor building would be 0.65° lower than on the bottom. They added that wind speeds also rise with height in the atmospheric boundary layer. In addition, tall buildings may experience an increase in heat loss from top stories as a result of simultaneous air pressure and velocity increases. Aflaki et al. (2014) mentioned that the air temperature inside the higher-floor unit is cooler than the lower-floor unit. The large difference in indoor air velocity between the lower and upper levels can explain this. According to Sangiorgia and Santamouris (2020), changes in building height have the most effect on whirlwind features because the height differences cause whirlwinds to grow proportionately, which in turn influences the air temperature and the weather. Salvati et al. (2017) stated that the average building height, which impacts structures’ solar gains, and maximum urban air temperatures, has emerged as the most crucial factor in urban morphology in regions with intense solar radiation and a narrow daily air temperature range. In the settings examined in such studies, a strong negative association (R2 = 0.7171) between the average building height and the cooling energy demand was discovered. Boudoukha and Zemmouri (2021) made an assessment based on measurements taken in situ throughout the summer and winter at various microclimate conditions. Their study compared two existing city apartment blocks and found that the taller one resulted in lower outside temperatures during the winter and summer. Also, the building’s passageway enhanced the accessibility and ventilation in a city block’s inside corner, but it may also increase wind speed, which could be uncomfortable for pedestrians. Zhang et al. (2020) stated that, in winter, a 10 m increase in street width resulted in a 0.17 °C increase in daytime temperature, whereas a 10 m increase in building height resulted in a 0.06 °C decrease in daytime temperature for the same street width. The number of floors in a building had a substantial impact on the daytime average temperature; however, when a building had more than 20 floors, the temperature did not change much with the height of the structure. Yang and Li (2015) mentioned that as building heights rise, the average urban albedo falls, and more solar radiation is generally absorbed as building height inequalities widen. Thus, a medium-density city with high-rise buildings and significant building height differences has the lowest average urban albedo.

The case study for this research is a plot area in the Sheikh Zayed Housing Programme (SZHP) (Figure 1) in the Al-Raqaib neighborhood in Ajman, United Arab Emirates, with a contemporary residential urban fabric. The 306 detached, two-story villas in this district are part of a modern neighborhood with modern architectural features, including setbacks and open areas. The layout of the neighborhood is made up of ordered and separated roadways and horizontal and vertical rows of residential buildings. Around 11 open boulevards and outdoor areas of various sizes and shapes are included in the fabric. The purpose of the boulevards is to promote neighborhood gatherings and safe social interaction away from the noise and danger of the primary automotive traffic routes, particularly for children and young adults. The individual units in this area are surrounded by 11 outdoor common spaces. Some of the outside open spaces are as wide as 15 m. The existing housing project’s broad roadways and low-height dwelling units reduced the quantity of shade, which increased the outdoor temperature, on one hand, and produced uncomfortable thermal conditions for the users on the other.

As the case study housing project for this research is located in the hot arid climate of the UAE, the outside temperature setpoint (OUT_SET) in the UAE differs according to the annum timing. OUT_SET refers to the desired temperature set by a building or heating or cooling system to maintain the indoor temperature, which is typically influenced by the outdoor temperature. In the UAE, during the hot summer months, the OUT_SET may be set lower to maintain a cooler indoor temperature, while in the cooler winter months, the OUT_SET may be set higher to maintain a warmer indoor temperature. Moreover, the value of the OUT_SET in the UAE depends on other factors, such as the type of building, the intended use of the space, and the prevailing weather conditions. Typically, in the UAE, the OUT_SET for commercial buildings such as offices, malls, and hotels is set to around 24°C–26°C, while for residential buildings, it may be set slightly higher at around 26°C–28°C. However, these values may vary depending on the specific requirements of the building and the preferences of the occupants. Additionally, during the summer months when outdoor temperatures can exceed 40°C, the OUT_SET may be set lower to maintain a cooler indoor temperature and ensure the comfort of the occupants. Thus, designing urban areas with passive strategies and decreasing the outdoor temperature can help improve the OUT_SET temperature. Moreover, Nie et al. (2022) mentioned that one of the key elements influencing the quality of life and livability of outdoor space is outdoor thermal comfort, which has received significant attention in urban planning and architecture. Hence, a variety of thermal indicators, such as OUT SET and the universal thermal climate index (UTCI), have been developed globally to measure it.

This research aimed to examine better thermal scenarios related to height variation in housing projects, which has rarely been discussed in previous research. Research on housing in hot arid climates such as the UAE was limited. This research is anticipated to assist in designing sustainable housing projects and fill the gap to improve the outdoor thermal conditions and thermal comfort for the users while also lessening the impact of the urban heat island.

The study was conducted on a housing project (Figure 3A) in Ajman Emirate, UAE (Figure 3B). Average temperatures in Ajman can soar as high as 41°C throughout the summer (Figure 3C) due to the region’s hot and dry climate. Due to its proximity to the seaside, it is often more temperate than other emirates in the UAE. The winters are pleasant, with average lows of 22°C. Ajman receives little rain on a yearly average, with the majority falling between December and March. The dominant wind in Ajman is the north-west wind (Figure 3D). Overall, Ajman has a desert climate distinguished by hot summers, moderate winters, and little precipitation.

A microclimate software program called ENVI-met was used to simulate thermal effects for the housing project case study. ENVI-met software is designed for use in many applications, including building design, urban planning, and climate change adaptation, for forecasting, analyzing, and visualizing the microclimate of cities, buildings, and green areas (ENVI-met 2023). With more than 3,000 independent studies, ENVI-met has been verified by a large number of researchers and practitioners, including in the urban climate and the urban microclimates (Salameh and Touqan, 2022; Salameh and Touqan, 2023; Wang et al., 2023; Alyakoob et al., 2023; Ghaffarianhoseini et al., 2015; Lee et al., 2016; Taleghani et al., 2014; Air Temperature, 2022; Forouzandeh, 2018).

The author conducted validation for the simulation using ENVI-met software on the same case study recorded in Salameh et al. (2022) as they used average measured air temperature data (Figure 4A) collected with an Extech meter (Figure 4B) from four points A, B, C, and D and compared those temperatures with their corresponding average air temperature data from the simulation using ENVI-met on 21 August 2020 (Figure 4C). The measured and simulated air temperature data were well correlated, as seen in Figure 4D. Despite some differences in the data sets, primarily due to the accuracy of the field measurement meter, the R ² score was 0.85 (Salameh et al., 2022). For more validation for the ENVI-met simulation results, the root mean square error (RMSE), a commonly used index for evaluating the accuracy of a model or software (Bande et al., 2019), was implemented in this study based on actual and predicted values gathered by the authors in Salameh et al. (2022). RMSE measures the difference between predicted values and actual values and is calculated by taking the square root of the average of the squared differences between predicted and actual values. A lower RMSE value indicates better agreement between predicted and actual values. For this study, the RMSE = 0.62 C° (see Figure 4E) and is considered valid, as higher RMSE values up to the levels of 4.83 obtained by Song and Park (2015) and 3.35 obtained by López-Cabeza et al. (2018)were deemed valid.

The research investigated the effect of the variation of the building heights in a housing project in the hot arid climate of the UAE on the outdoor thermal conditions in the related district. The investigation included two main parts: 1) The base case of the housing project plot (Table 1) and 2) the proposed scenarios for the height variations to be examined to identify the best configuration that can help in modifying the outdoor air (Table 2).

In the ENVI-met software application, the simulation plot area for the existing and proposed height configurations was in a working space with an air volume of 200 × 200 × 50 m³ and a height of 50 m for the air column based on a grid of 2 m in the x, y, and z directions (Table 3). These properties for the working space were considered fixed parameters for all the simulations of the different building height configurations for this research. The simulations were run for 36 h starting from 20 August at 6:00 p.m. until 21 August at 11:59 p.m. The extracted simulation data were for 21 August 2021. The selected site was simulated based on reality in terms of orientation, building materials (Table 3), built-up area, street geometries, street orientations, and 3D geometries. The date selected for the investigation was 21 August, the hottest month of the year in the UAE with an average high air temperature of around 41°C and an average low temperature of 34°C. These readings are above the thermal comfort defined by Khalfan and Sharples (2016), who claimed that the thermal comfort diagram is between 20°C and 27°C in accordance with Schnieders’ thermal comfort diagram.

When comparing and evaluating the thermal characteristics of the existing and proposed configurations for the housing project using ENVI-met, a variety of metrics and variables were used. These metrics included the following: 1-

The air temperature is a direct indicator of thermal performance in urban settings (Feroz, 2015).

The energy balance of user bodies within the fabricated microclimates is considered in PMV calculations. Using the ASHRAE scale, ENVI-met calculates PMV, with +3 meaning extremely hot weather and −3 denoting severely cold weather (Iowa Environment Mesonet, 2023). Additionally, a broader scale for open-air temperatures was included, ranging from +4 (extreme heat) to −4 (extreme cold) (PMV-ENVI-met, 2021). PMV is frequently linked to other aspects of the human body, such as the user’s age, metabolism, and gender, as well as external factors, including wind speed, humidity, air temperature, and radiant temperature (PMV-ENVI-met, 2021). Consequently, it has been established that a wide range of direct and indirect elements can affect thermal comfort; nonetheless, Lai et al. (2020) asserted that air temperature has a substantial impact on customers’ thermal comfort.

The base case and the two proposed configurations have building heights that vary from 8, 12, and 16, to 20 m. The first configuration group has unified building heights. In the first configuration, U1 has a unified height of 12 m, the U2 configuration is 16 m, and U3 is 20 m, as shown in Table 4.

The second group of configurations has diverse building heights. The D1 configuration has a gradual height decrease from the south direction starting from 20 m and reaching the lowest height of 8 m, with the decrease following this pattern: 20, 16, 12, 8, and 8 m. The D2 configuration has a gradual height decrease from the north-west direction, starting at 20 m and reaching the lowest point of 8 m, following the pattern of 20, 16, 12, and 8 m. D3 has a courtyard effect, with 20-m buildings on the site boundary and 8-m buildings on the inside. Finally, D4 has a semi-courtyard effect, with 20-m building blocks facing the north-west and south directions, while the height gradually decreases inward to 12 m and 8 m, as shown in Table 5.

The H/W ratio increases with the increase of the building heights as does the built-up area. The urban configurations were simulated with fixed factors such as the UAE location, climatic data (arid and hot climate), building materials (default concrete wall with moderate insulation for walls and roofs, asphalt road, red stones for pedestrian paths and sidewalks, and loamy soil for unpaved areas). The independent factors were building height, H/W ratio, SVF, and built-up area-to-plot area ratio. The simulation results, including outdoor potential air temperature, wind speed, relative humidity, mean radiant temperature, building average temperature, and PMV, are the dependent factors to identify the impact of each proposed configuration.