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Vernacular Strategies
4.1 Vernacular Strategies
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Unlike the built environment today, Emirati vernacular architecture was developed to adapt to the harsh desert climate. Traditional liminal spaces or series of these spaces, such as the Sikkas (narrow streets), courtyards, and Liwan (loggia), were deployed to achieve a gradual change in indoor and outdoor temperatures while improving the comfort levels of the surrounding indoor areas. This chapter discusses the different levels of vernacular transitional spaces, some other passive cooling strategies, and examples of successful precedents in similar climates.
Compact Form
Research demonstrates that vernacular compact urban forms can improve thermal conditions in hot, dry regions like Sharjah and other parts of the UAE (Elkhazindar, Kharrufa, Sahar N, and Arar 2022). Traditionally, the houses in these areas were closely grouped, with some sharing as many as three walls, creating shade for both the built form and the nearby pedestrian walkways (AlBahar 1990) (Figure 4.1). With its narrow and winding streets (Sikkas), it can effectively reduce intense solar radiation and block dust storms from entering the city by allowing them to pass over.
A recent study by Elkhazindar, Kharrufa, Sahar N, and Arar (2022) evaluated and compared six different densities in traditional and modern urban forms in the UAE concerning temperature and thermal comfort. The results revealed that the low-density vernacular urban form exhibited the lowest air temperature and maximum pedestrian comfort in August (the hottest month) due to its high height-to-width ratio and low density. The highest ambient temperature was observed in the sites with medium density and the lowest height/width ratio. The results of this study are further discussed in Appendix B.
These old towns also included plenty of covered outdoor public spaces, known as sabat, which provided an exciting interplay of light and shade along with
Figure 4.1 The old town of Yazd with compact courtyard urban forms (Source: Foruzanmehr 2017)
Figure 4.2 Shaded alleys, light wells, sabat (Source: Weber and Yannas 2014)
Figure 4.3 Courtyard and Liwan(Source: Foruzanmehr 2017) cool breezes that flowed through the narrow alleyways (Figure 4.2). Below the sabat, these regions also featured the m’qad, benches with double-height plastered steps, allowing people to gather and rest in a cool, shaded place (Willi Weber and Simos Yannas 2014).
Courtyard and Liwan
Courtyards have been a prototypical transitional space in the Emirates community since its inception in the 1960s. Most of their vernacular architecture consisted of houses of mud or coral built around an open central courtyard (Figure 4.4). In addition, they adopted the layering of transitional spaces ranging from courtyards to shaded colonnades known as Liwan(Figure 4.3) to shield the impact of the outdoor climate on the indoors (Foruzanmehr 2017). They function as a microclimate regulator, enhancing the comfort levels of the surrounding spaces. Apart from the environmental aspect, the courtyard also contributed to the social factors crucial to the Arab society for various community activities along with privacy and solitude.
Yannas (2000) suggests that the thermal comfort in the courtyard results from surface temperatures, water bodies, vegetation, ambient air, and other microclimate modifiers. It largely depends on the aspect ratio, wind speeds, direction, and other factors.
Aspect Ratio
The courtyards aspect ratio is one of the most influential parameters for improving the microclimate of the surrounding indoor spaces. This section focuses on the geometrical part of the courtyard with the height-to-width ratio and consequent solar access on the thermal comfort of the courtyards. In
Sharjah’s hot desert climate (BWh), square courtyards are generally preferable to generate shadows on internal walls and avoid extreme heat stress along with wind-blown dust and sand. As evident in Figure 4.6, the courtyards with an aspect ratio (H/W) of 1 have better thermal performance than other narrow or wide courtyards. It allows for a circular whirlwind in the courtyard’s middle, creating a comfortable microclimate. These right proportions also help in the self-shading and shading of the adjoining facades, leading to a thermal lag. This potentially results in reduced heat gain and, thus, lower indoor temperatures (Bhargava 2018).
Figure 4.6 illustrates a comparison conducted by Yannas (2000) of the dimensionless temperature at three different vertical sections in a courtyard as a function of the H/W ratio. The curve shows that the minimum temperature occurs at the maximum intensity of the airflow vortex, which is found in a square-shaped courtyard with a H/W ratio of 1. This is because the air inside the courtyard is mixed well. In contrast, open courtyards with a H/W ratio of 0.1 have a lower dimensionless temperature, but it increases as the ratio increases to 0.3. In narrower courtyards with a H/W ratio greater than 1, the dimensionless temperature increases due to the decrease in intensity of the airflow vortex.
Vegetation
Vegetation holds a religious significance in UAE’s vernacular architecture apart from being a climate modifier. Every landscape form, ranging from grass to shrubs to trees, can mitigate heat stress and improve thermal comfort within the courtyard and the surrounding spaces in a specific capacity. They clean the air and lower the air temperature by providing shade, changing wind conditions, ventilating the buildings, and fending the courtyard against direct solar radiation. The watering of the plants and the transpiration also further assist in lowering the air temperature through evaporative cooling (Foruzanmehr 2017). For example, a study in a hot and arid zone, similar to Sharjah, comparing a paved, unshaded courtyard to a landscaped courtyard indicated the contribution of vegetation to improved thermal comfort (Figure 13). With shading by trees, the discomfort hours were reduced by over half, and when further combined with grass, both the shading mechanisms could yield comfortable conditions at all hours (Darvish, Eghbali, and Eghbali 2021).
In addition, the type, density, and geometry of vegetation are also crucial to determining the microclimate of the courtyard and, consequently, the indoor temperatures of the surrounding buildings. For instance, in the case of Sharjah, the local trees such as Ghaf (Prosopis cineraria), Samur (Acacia tortilis), and Garath provide more enhanced microclimate in the courtyard space and decreased annual cooling load when located near the internal thermal zones, lowering the temperature by almost 3oC (Figure 4.7) (Darvish, Eghbali, and Eghbali 2021). Therefore, vegetation and its optimal use in the courtyard can
Figure 4.4 Traditional courtyard housing in Sharjah and courtyard effect (Source: Khalifa and Ibrahim 2019)
Figure 4.5 ENVI output comparison of courtyard housing with and without trees (Source: Darvish et al. 2021)
Figure 4.6 Graphic of the non dimensional temperature and flow patterns related to the depth ratio of the courtyard (Source: Rojas et al. 2012) considerably affect the microclimate and the energy efficiency of courtyard housing.
Thermal Mass
Emirati Houses were usually constructed with thick walls, employing thermal mass principles to minimize heat gain and limit glare and hot winds. These walls were usually 50 to 80 cm thick and were built with local materials such as mud bricks and coral stones (Figure 4.7) (Foruzanmehr 2017). They act as insulation against external heat and have high heat-retaining capacities, providing stable interior conditions desirable in hot climates.
Due to their heat-retaining capacities, the thermal mass of the courtyard slows down the heat transfer process, creating a thermal lag. As a result, they absorb most of the heat received during the day slowly before passing it to the interiors during the night when the temperature drops, thus ensuring a thermally comfortable environment. In a recent study, vernacular courtyard housing in hot arid climates with high thermal mass demonstrated up to 27 percent building cooling benefits (St.Clair 2009).
However, while this can successfully mitigate daytime temperatures to an acceptable level, internal conditions at night could become uncomfortable due to the released heat during certain months. Fortunately, it could be resolved by combining these high thermal walls with high external or cavity insulation levels to insulate the interior further and even out the diurnal temperature range. They can also be cooled by natural and mechanical ventilation through wind towers (barjeel) in preparation for the next day (St.Clair 2009).
Openings and Fenestration
Traditional houses in the region were designed to be introverted, with most openings facing the central courtyard for daylight and ventilation (Dib 2013). Mashrabiyas and jaalis (wooden screens) were used as an additional layer to
Figure 4.6 Mud brick and coral stone walls in Old Sharjah Heritage Area (Source: Ibrahim 2017)
Figure 4.7 Mashrabiya and Olla (Source: Weber and Yannas 2014)
these modest-sized openings to provide shade and protection from the sun and also to allow breezes to flow into the building for cooling purposes. They had specific perforation sizes and patterns to accelerate the airflow, which was then directed through a clay jar or olla filled with water (Figure 4.8). The water transpired through the pores of the jar, cooling the air and improving indoor thermal comfort. This system is the oldest and simplest form of evaporative cooling, in which outdoor air is brought directly into the water, cooling the air by converting sensible heat to latent heat (Gourlis and Holzer 2022).
Migration
One of the most intriguing and common ways of coping with the harsh climate of Sharjah was migration, a behavioral adaptation of the inhabitants. Due to the wide range of microclimatic conditions and environmental qualities provided by the traditional Emirati courtyard house, the inhabitants migrated through the house during the year and did so throughout the day as well.
For instance, in summer, they slept on the roof and spent the daytime moving around from the courtyard to the summer quarters to the basement, depending on the outdoor temperature and requirements. This pattern of movement allowed for flexibility in the functions of the house and the “interchangeability” of domestic labels (Al-Bahar 1990). However, Knowles (2006) suggests that this diurnal migration, or “internal nomadism” (Foruzanmehr 2017), does not change the boundaries of the space; instead, it carries one through doorways and around objects, which changes the perception of a space.
