Passive Design for Residential Buildings in Arid Desert Climates: Insights from the Solar Decathlon Middle East

Abstract

This study investigates the effectiveness of passive design in low-rise residential buildings located in arid desert climates, using the Dubai Solar Decathlon Middle East (SDME) competition as a case study. This full-scale experiment offers a unique opportunity to evaluate design solutions under controlled, realistic conditions; prescriptive, modeled performance; and monitored performance assessments. The prescriptive assessment reviews geometry, orientation, envelope thermal properties, and shading. Most houses adopt compact forms, with envelope-to-volume and envelope-to-floor area ratios averaging 1 and 3.7, respectively, and window-to-wall ratios of approximately 17%, favoring north-facing openings to optimize daylight while reducing heat gain. Shading is strategically applied, horizontal on south façades and vertical on east and west. The thermal properties significantly exceed the local code requirements, with wall performance up to 80% better than that mandated. The modeled assessment uses Building Energy Models (BEMs) to simulate the impact of prescriptive measures on energy performance. Three variations are applied: assigning minimum local code requirements to all the houses to isolate the geometry (baseline); removing shading; and applying actual envelope properties. Geometry alone accounts for up to 60% of the variation in cooling intensity; shading reduces loads by 6.5%, and enhanced envelopes lower demand by 14%. The monitored assessment uses contest-period data. Indoor temperatures remain stable (22–25 °C) despite outdoor fluctuations. Energy use confirms that houses with good designs and airtightness have lower cooling loads. Airtightness varies widely (avg. 14.5 m³/h/m²), with some well-designed houses underperforming due to construction flaws. These findings highlight the critical role of passive design as the first layer for improving the energy performance of the built environment and advancing toward net-zero targets, specifically in arid desert climates.

1. Introduction

Passive design plays a key role in maintaining comfortable indoor environments while lessening the reliance on Heating, Ventilation, and Air Conditioning (HVAC) systems, referred to as active design. By lowering the demand for active systems, passive design strategies not only reduce energy use but also require minimal maintenance. Their beneficial impacts are particularly significant in extreme climates, such as the arid desert in Dubai, United Arab Emirates (UAE), where maintaining indoor thermal comfort comes with high energy costs.

In support of global climate goals, the UAE Net Zero strategic initiative aims to achieve net-zero emissions by 2050 [1]. In alignment with this national ambition, Dubai’s revised Clean Energy Strategy aims for 25% of electricity to come from clean energy by 2030 and 100% by 2050 [2]. These goals are particularly important given that the energy demand is expected to grow by 3% by 2030 [3,4] as a result of the increase in the urbanization rate. The building sector remains one of the largest consumers of energy worldwide, accounting for a significant portion of global resource demand and carbon emissions. This trend has led to extensive research, policy, and regulation efforts aimed at improving its energy performance, including the introduction of zero energy buildings (ZEB) as an aspirational target for high-performing buildings [5,6].

The climate has a direct influence on the energy demand of buildings. Ideally, an optimal, climate-responsive building design follows a hierarchical strategy that begins with passive design measures to minimize energy demand, such as optimizing orientation, form, and envelope performance. This should be followed by the integration of efficient active systems to reduce the energy demand for lighting, HVAC, and appliances [7]. Ultimately, it integrates renewable energy sources to offset consumption and achieve low or net-zero energy demand.

However, in their systematic review, Hu et al. identified a lack of reviewed publications on passive strategies for cooling buildings [8]. In many regional studies, the impact of passive design on energy performance remains underexplored, with a predominant focus on active systems and renewable energy sources. This limits the potential of those technologies, as passive strategies form the foundation of the energy efficiency hierarchy. Optimizing the passive design not only reduces the overall demand but also improves the effectiveness of active and renewable solutions. Additionally, passive measures are generally more cost-effective and require less maintenance, making them a critical first step in delivering high-performance, climate-responsive buildings.

This work reviews passive design strategies for cooling and quantifies their individual impacts on the energy demand of buildings in Dubai, a region with an arid desert climate. The analysis was conducted using houses from the Solar Decathlon Middle East (SDME) competition. This competition constrains all participants equally, providing a valuable opportunity to study real building energy performance in a controlled environment [9]. This study aims to provide a comprehensive overview of strategies and measures crucial for high-performance buildings in Dubai and similar arid regions.

Section 2 presents a literature review, starting with an overview of passive design strategies and measures. This is followed by an overview of the specific Dubai context, its climate, vernacular architecture, and current building regulations. Next, the SDME competition rules and the participating houses are summarized. Section 3 outlines the materials and methods used for this work, including prescriptive-based, modeled performance-based, and monitored performance-based assessments. Section 4 presents and discusses the results, followed by conclusions in Section 5, and limitations and future research work in Section 6.

2. Literature Review

2.1. Passive Design

Passive design, also known as passive solar design, aims to maintain comfortable indoor conditions while minimizing energy consumption. In moderate climates, it effectively maximizes winter solar gain for heating while minimizing it in summer to reduce cooling needs [10]. In hot climates, passive design, which is often referred to as passive cooling design, maintains indoor thermal comfort conditions by avoiding or dissipating heat gains generated from external heat sources, such as direct solar radiation and outdoor air temperature, as well as from internal heat sources, such as people, cooking, lighting, and appliances [11]. This is achieved by applying targeted passive design strategies and measures.

2.1.1. Passive Design Strategies

There are three strategic steps to apply passive design: protection, modulation, rejection, or dissipation [11,12]:

• Heat protection prevents external heat gains from entering the building’s inner space by using surrounding elements as heat barriers, such as water surfaces, shading vegetation, and neighboring buildings, as well as building components, such as orientation, shape, material properties, airtightness, and shading elements [11].
• Heat modulation uses the building’s thermal mass to absorb and store heat during the day and release it at night, smoothing temperature fluctuations. Heat storage is achieved by taking advantage of the thermal capacity of solid or liquid materials, either through sensible heat or latent heat. Materials with sensible heat storage change temperature as they store heat, whereas Phase Change Materials (PCMs) store latent heat during phase changes without a temperature change, offering higher storage capacity [11,13,14,15,16].
• Heat dissipation involves removing heat gains from both external and internal sources to an external body that acts as a cooling sink [11,15]. This requires a significant temperature difference and efficient thermal coupling between the building and the heat sink [12]. Methods include geothermal cooling (using the ground’s temperature) [12,14], evaporative cooling (using water to absorb heat from air) [11,15], convective cooling (utilizing natural ventilation) [11,15], and radiative cooling (emitting heat to the sky) [15,17].

The optimal implementation of these strategies minimizes the need for active cooling, ensuring low energy consumption while maintaining thermal comfort.

2.1.2. Passive Design Measures (Design Elements)

Passive design strategies maintain indoor comfort and reduce HVAC reliance by controlling heat exchange through the building envelope [7]. The adoption of suitable passive design measures requires an understanding of various strategies and their applicability. Below, potential passive measures are summarized along with findings from studies on low-rise residential buildings in the Middle East, particularly in the Gulf Cooperation Council (GCC) region:

• Compact building shapes minimize the heat exchange surface between the conditioned volume of the building and the outdoor environment. Two parameters describe building shape per the European Committee for Standardization: compactness (the ratio of the thermal envelope area to the building volume) and the shape factor (the ratio of the thermal envelope area to the conditioned floor area) [7,18]. A study in Baghdad’s hot desert climate reported that a square-shaped low-rise building saved 30% more energy than an L-shaped building did and 33% more energy than an H-shaped building did [19].
• The window-to-wall ratio (WWR) is the ratio of transparent to opaque vertical envelope areas, whereas the skylight-to-roof ratio (SR) applies to horizontal surfaces. Large transparent areas often lead to increased thermal transmittance, thermal bridges, and increased solar gains. In Muscat, a 10% increase in WWR led to a 4% increase in energy consumption [20]. In Al-Ain, reducing the WWR from 12% to 6% decreased cooling by 3.7% and electricity by 3.2%, whereas increasing the WWR to 20% increased cooling by 8.5% and electricity by 7.2% [21].
• Orientation affects exposure to solar radiation and wind [7]. In the hot climates of the Northern Hemisphere, maximizing the openings in northern façades and minimizing them in the western and eastern facades [22], placing service spaces in the west as thermal buffers, and positioning the main entrance to the north with a foyer to reduce heat gains are recommended [7].
• Shading elements can be fixed or movable and vertical or horizontal, such as overhangs, louvers, light shelves, and blinds [11,22]. In Dubai, low solar angles in the east and west make shading challenging, whereas southern orientations with higher angles are easier to shade. Shading generally saves more energy in commercial buildings than in residential buildings, as commercial buildings have larger glazing areas [22].
• Thermal bridges are created when the building envelope is interrupted, causing a change in thermal resistance [23]. They form where materials with different insulation properties meet or where the geometry of the envelope changes, such as wall–roof junctions and projecting balcony slabs (linear) or single penetrations in the thermal envelope (point) [23,24]. Point thermal bridges generally have a negligible effect.
• Airtightness quantifies the flow of air infiltrating buildings and is usually measured at a constant pressure difference of 50 PAs [25]. While it brings fresh air and dilutes pollutants, it also allows heat gains, compromising thermal comfort; therefore, it should be minimized [15]. Reducing airtightness from 0.7 to 0.5 air changes per hour (ACH) in an Omani low-rise residential building resulted in a 6.8% reduction in cooling and a 4.8% reduction in total energy consumption.
• Thermal transmittance (U-value) measures the heat flow through an envelope component per unit area, time, and temperature difference (W/m²K). Reducing the U-value of external walls (from 2.32 to 0.3 W/m²K) and the roof (from 0.6 to 0.2 W/m²K) in a low-rise residential building in Al-Ain, UAE, cuts cooling demand by 19.3% and total electricity use by 15.5% [21].
• Glazed areas, such as windows and skylights, are evaluated based on thermal and optical properties: U-value, solar heat gain coefficient (SHGC), and visible transmittance (Vt). Performance can be static (fixed properties) or dynamic, changing with solar intensity (photochromic), temperature (thermochromic), or electrical charge (electrochromic) [7,11].
• Thermal mass refers to a building’s capacity to absorb, store, and release heat, which is influenced by specific heat capacity, density, and thermal conductivity. High thermal mass materials, such as water and concrete, store more thermal energy, whereas foam insulation stores less thermal energy. Increasing the thermal mass in a low-rise residential building in Al-Ain, UAE, reduced cooling by 13.4% and overall electricity use by 11.9% [21].
• Light-colored and reflective coatings on a building’s exterior reduce heat transfer to interiors [22]. In Qatar, changing the exterior finishing of a low-rise residential building from medium to light color reduced the cooling load by 12% [26]. In Saudi Arabia, replacing a dirty roof with a conventional concrete roof reduced the peak cooling load by 5.68%, and replacing it with a cool roof by 8.23% [27].
• Green walls and roofs lower energy loads through evaporative cooling [22]. A study conducted in several cities worldwide revealed that, in Cairo, buildings with green roofs reduce cooling energy use by at least 27% [28]. However, green surfaces also increase maintenance costs and water consumption [22].
• Ventilation measures enhance energy efficiency and improve indoor air quality and comfort. These include double skin walls (with an air gap between layers), evaporative cooling (using water bodies), a thermal mass for night cooling, and solar chimneys (with water-absorbing sheets) [22,29]. However, evaporative measures are less effective in high-humidity climates.

These passive design elements can be applied individually or in combination, depending on climatic conditions, architectural design, and building requirements.

2.2. Dubai Context

Understanding the local context is crucial for evaluating passive cooling design effectively, as it provides the necessary background for implementing appropriate design strategies and measures. Therefore, this section presents the climatic conditions specific to Dubai, the building design requirements mandated by local regulations, and the Solar Decathlon Middle East (SDME) houses chosen as case studies.

2.2.1. Dubai Climate

Dubai has a subtropical desert climate characterized by high levels of solar radiation throughout the year. There are two main seasons: a mild winter and a long summer, with hot temperatures lasting from April to November. Owing to its proximity to the Arabian Gulf, Dubai’s summer temperatures are somewhat milder than those of other inland cities (Figure 1). However, this closeness to the sea results in higher humidity levels, diminishing outdoor comfort during the summer months. Throughout the year, the temperature ranges from approximately 10 °C at the lowest points to approximately 45 °C, with a daily average temperature that varies between 17 °C and 37 °C. The daily minimum, maximum, and average relative humidities fluctuate from approximately 20% to nearly 100%.

2.2.2. Vernacular Architecture

This section highlights the successful application of the passive design strategies employed in traditional dwellings in the UAE, which rely solely on local, natural resources without the need for active systems. The design of these dwellings is influenced by both the climate and sociocultural aspects of the local lifestyle. Seasonal climate variations dictate settlement patterns, with people residing near coastal areas in winter for fishing and pearl catching and then moving to oasis areas in summer for data collection [31].

Social factors also play a role, particularly in the sturdiness of houses. Bedouins, nomadic Arab tribes, use temporary structures, whereas the rest of the population uses permanent houses. Privacy and gender-based segregation within living spaces are common design elements across all dwellings.

There are three types of traditional UAE dwellings, as illustrated in Figure 2 [31]. One was permanent, whereas the other two were temporary (the tent and the Al Arish):

• Permanent houses are made of coral stones or guss (a mud mixture) and covered with palm tree trunks. They are organized around the interior courtyard and can be used as winter houses, built near coastal areas, or summer houses, built near palm tree farms.
• Al Arish, used by Bedouins during summer, is made of palm tree leaves. They are composed of two separate spaces: the main space for lounging and sleeping and the smaller space for cooking and storing.
• Traditional tents, used by Bedouin during the winter, are made of animal skin and hair. They are easy to fold and carry on camels during relocation. The main tents are divided into two parts based on gender segregation, and secondary, smaller tents are used for cooking and storage.

Although each traditional dwelling type is unique, it follows certain common design elements influenced by the need for passive cooling, local resources, and privacy. These include the inner courtyard, liwan, mashrabiya windows, barjeel, and local construction materials [31]:

• The courtyard, central to the design of traditional dwellings, offers a shaded outdoor space inside the house, toward which the main openings of the house are oriented, whereas, socially, it provides a private area for family activities and gender-segregated socialization [31].

Common design elements also include the following:

• The liwan, a colonnaded porch facing the inner courtyard, shades the walls of the house and serves as an outdoor space for more private family gatherings [32].
• Exterior openings toward the street are minimized for heat protection of the inner spaces and privacy, with the main ventilation and daylight coming from those oriented toward the shaded inner courtyards [31].
• Mashrabiya, an oriel window with carved wooden latticework, a traditional element of Islamic architecture, allows natural ventilation while providing privacy [33,34].
• Majlis is the room designated for male gatherings. It is located near the entrance and is often the only room in a house with street-facing windows [32].
• Barjeel, a wind tower that provides cross-ventilation, starts at approximately two meters above ground, is approximately fifteen meters high, and has a funnel and top openings to catch air from all directions, increasing its velocity and creating a cool area below, typically used for eating and entertaining [31].
• Construction materials come from local natural sources: coral blocks for permanent houses, approximately 60 cm thick, have low thermal conductivity due to their cellular nature, providing a comfortable interior space, and mountain stones for fortresses. Palm trees for Al Arish dwellings provide adequate ventilation in highly humid areas, and Bedouin tents use animal skin, a breathable material which helps moderate the effects of the environmental extremes [31].

In summary, traditional UAE dwellings provide valuable insights into effective passive design techniques that utilize local natural resources without relying on active systems. They are shaped by climatic and social factors, and their context-specific design highlights the fundamentals of thermal comfort and energy efficiency in arid deserts. While vernacular housing does not meet contemporary comfort standards, it often outperforms modern typologies in terms of sustainability, environmental impact, and the use of locally sourced natural materials [35]. Learning from it provides a guide for integrating sustainable and culturally responsive passive design strategies in modern residential buildings, highlighting the value of historical wisdom in contemporary practices. Table 1 presents various studies on vernacular architecture’s cooling strategies, accompanied by quantitative results.

2.2.3. Dubai Building Design Regulations

The building energy efficiency in Dubai is regulated by the Al Sa’fat-Green Building System [36], whereas the overall building design, including energy conservation measurements, is regulated by the Dubai Building Code (DBC) [37]. In 2020, Al Sa’fat replaced the Dubai Green Building Regulations and Specifications issued in 2010 [38]. Late in 2021, the DBC was published, and at the beginning of 2023, Al Sa’fat was revised to align with the DBC.

Al Sa’fat is based on mandatory general requirements and optional provisions that determine the certification levels: silver (base level), gold, and platinum. On the other hand, DBC is based only on mandatory requirements without certification levels. Both regulations apply to private and public buildings, whether new or existing structures are undergoing renovation, reconstruction, or modification.

Building energy efficiency assessments for Al Sa’fat and DBC can be prescriptive-based or performance-based. The prescriptive assessment involves passive and active design requirements, such as thermal transmittance (U-value) for envelope elements and coefficients of performance (COPs) for HVACs. The envelope performance requirement values are summarized in Table 2. Alternatively, the performance-based assessment simulates the building’s annual energy demand and compares it with a reference building that meets all prescriptive requirements.

Table 1. Studies on vernacular architecture cooling strategies with quantitative results.

Strategy	Location/ Climate	Building Type	Characteristics/Dimensions	Age (yrs)	Simulation (S)/Measurements (M)	Method	Temperature/ Thermal Load Reduction/ Comfort	Energy Savings/ CO2 Reductions	Ref
Windcatchers	UAE (BWh-Hot desert climate)	House	2-story, two X-blade windcatchers 15 × 25-m plot	N.S. ≈ 100	(S) Whole-building energy simulations, plus CFD	Two scenarios: natural ventilation with windows only and a combination of windows and wind catchers.	Up to 5 °C lower than the exterior	74–111 kg (CO2e)/mont h reduction.	[39]
Windcatcher + solar chimney + radiative cooling	Yazd, Iran (BWh-Hot desert climate)	Single and two-story buildings	Two designs: one with a separate solar chimney and windcatcher and the other combined. Spaces are 6 m wide and 3 m high.	N/A	(S) CFD	Five cases were analyzed, combining levels and the windcatcher configurations.	3 °C reduction during peak hours	60% (cooling), 80% (ventilation). During peak hours (Best cases).	[40]
Windcatcher	Riyadh, KSA (BWh-Hot desert climate)	University courtyard	Rectangular, open-to-sky courtyard surrounded by buildings (20 m wide, 60 m long, and 16.5 m high)	N/A	(S) CFD and (M)	Analysis of the thermal and flow conditions during peak outdoor summer conditions.	−8 to −12 °C	37 metric tons of CO2 per tower (in the six hot months).	[41]
Courtyard + natural ventilation	Damascus, Syria (BWk-Cold desert climate, with hot, dry summers)	Traditional Courtyard house	Two halls: 1st floor north hall (light mass structure + cross ventilation) and ground floor south hall (heavy mass structure + single ventilation)	N.S.	(S) and (M) Summer period	Analyzed six ventilation mode for each of the two cases.	−3 °C (day) −6 °C (night). Heavy mass: up to 20% (daytime).	Up to 72% (summer energy saving, mixed mode) + peak load reduction.	[42]
Courtyard + natural ventilation	Kashan, Iran (BWh-Hot desert climate)	Courtyard house	Eight zones of the building were studied.	N.S. ≈130	(S) Whole-building energy simulations	Analyzed four scenarios, two to determine the thermal loads and two for the cooling energy demand (with and without natural ventilation).	−2 to −5.6 °C. Add from 21 to 35 days in thermal comfort.	0.6 to 47.3 (kWh/m2-yr) saving.	[43]
Courtyard	Riyadh, KSA (BWh-Hot desert climate)	New house	2-story 303.9 m (net floor area)	2	(S) Whole-building energy simulations; (M) energy consumption	Study how to improve a new house’s performance by adding traditional solutions.		4.08% to 5.15% annual energy saving.	[44]
Courtyard	Seville, Spain (Csa-hot-summer Mediterranean)	Three residential buildings	3 courtyard cases: 2-fl. (9.0 × 9.0 × 8.5 m), 2-fl. (5.6 × 4.0 × 8.5 m), and 4-fl. (7.3 × 6.6 × 14.0 m).	N/A	(S) and (M) in summer months	3 simulations per case: reference, with the shading effect, and with the measured temperatures.	−10.5 to −14.4 °C (peak time ∆T outdoor vs. courtyard).	8–18% cooling. Energy saving.	[45]
Iwan	Yazd, Iran (BWh-Hot desert) Cost of Bandar Abbas, Iran (BWh-Hot desert climate, coast: hot humid) Tabriz, Iran (BSk-Cold semi-arid climate) Sari, Iran (Cfa-Humid subtropical climates)	Room (south oriented)	Rooms L × W × H (4, 4, 3 m) window-to-wall ratio 16.6%	N/A	(S) energy simulations; (M) in Yazd in July	Each room was simulated without shading devices and with four shading solutions at each location. Plus, a study of the optimum shading depth.	-	Average savings with South Iwan: Yazd 29%; Bandar Abass coast 26%; Tabriz 14%; Sari 32%.	[46]
Mmashrabiya	Riyadh, KSA (BWh-Hot desert climate)	New house	2-story 303.9 m (net floor area)	2	(S) Whole-building energy simulations; (M) energy consumption	Study how to improve a new house’s performance by adding traditional solutions.	-	4.40% to 5.06%. Annual energy saving.	[44]
Mashrabiya + mud, coral stone, and wood reinforced walls	Jeddah, KSA (BWh-Hot desert climate)	Historic building	2 identical rooms facing west, located on the first and second floors. Rooms L × W × H (4.0 × 3.6 × 3.9) Mashrabiyas W × H (2.4 × 3.1)	≈170	(M) The two rooms, Mashrabiya, and the adjacent courtyard (from 4 Aug to 1 Sep)	Analyzed the effect of open and closed Mashrabiya, building thermal mass, swing temperature, air movement, and differences between indoor and courtyard temperature.	74–77% reduction in indoor temperature swing. The temperatures at peak times were 6.8° to 8.5 °C, lower than those of the courtyard. Open mashrabiyas enhance air movement and reduce temperature by up to 2.4 °C compared to closed ones.	-	[47]
Mashrabiya	Riyadh, KSA (BWh-Hot desert climate)	House (1950s)	Mashrabiyas on its west and south facades. House area: 315 m2.	≈70	(S) Whole-building energy simulations	Simulations with and without Mashrabiya.	14% reduction in operative temperature; 77.8% (peak solar gain reduction);	5.7% (monthly cooling load reduction.	[48]
Mashrabiya + wet cloth	Jeddah, KSA (BWh-Hot desert climate)	Traditional house	2 identical rooms facing west, located on the first and second floors. Rooms L × W × H (4.0 × 3.6 × 3.9 m) Mashrabiyas W × H (2.4 × 3.1 m)	≈170	(M) The two rooms, Mashrabiya, and the adjacent courtyard (from 30 Jul to 8 Aug)	Two periods (Mashrabiya closed in both rooms, open in one room). Evaporative cooling (pots, water, spray, and cloth).	−5.8 °C (lower than outdoor temp. without wet cloth). But none of the solutions were able to maintain comfort in the summer at midday. −7.8 °C (lower than outdoor temp. without wet cloth).	-	[49]
Shading, local materials, and natural ventilation	Bushehr, Iran (BWh-Hot desert climate)	Traditional dwellings	TB1 (367.5 m2) TB2 (432.3 m2) MB1 (215.6 m2) MB2 (106.4 m2)	N.S.	(S) Whole-building energy simulations; (M) energy, gas, and short-term weather data	Two modes: passive ventilation during moderate periods of the year, and split units for cooling during hot, humid periods.	Reduction in discomfort hours TB1 34% and TB2 12%	Cooling energy TB1 is 26% lower than MB2. TB1 is 46% lower than MB1.	[50]
Rowshan	Jeddah, KSA (BWh-Hot desert climate)	Single-family home	A single room was studied with six Rowshan configurations, each of them in three orientations	N/A	(S) Room energy performance and CFD	Rowshans’ potential to provide comfort and reduce energy use. Optimum size of the opening grid.	With small openings: North 15–22% (reduction in indoor air temp. from Nov. to Apr.); South and Northwest: 2–3 °C (lower than outdoor). 42.3% hours in comfort in a year.	-	[51]

Table 1. Studies on vernacular architecture cooling strategies with quantitative results.

Strategy	Location/ Climate	Building Type	Characteristics/Dimensions	Age (yrs)	Simulation (S)/Measurements (M)	Method	Temperature/ Thermal Load Reduction/ Comfort	Energy Savings/ CO2 Reductions	Ref
Windcatchers	UAE (BWh-Hot desert climate)	House	2-story, two X-blade windcatchers 15 × 25-m plot	N.S. ≈ 100	(S) Whole-building energy simulations, plus CFD	Two scenarios: natural ventilation with windows only and a combination of windows and wind catchers.	Up to 5 °C lower than the exterior	74–111 kg (CO2e)/mont h reduction.	[39]
Windcatcher + solar chimney + radiative cooling	Yazd, Iran (BWh-Hot desert climate)	Single and two-story buildings	Two designs: one with a separate solar chimney and windcatcher and the other combined. Spaces are 6 m wide and 3 m high.	N/A	(S) CFD	Five cases were analyzed, combining levels and the windcatcher configurations.	3 °C reduction during peak hours	60% (cooling), 80% (ventilation). During peak hours (Best cases).	[40]
Windcatcher	Riyadh, KSA (BWh-Hot desert climate)	University courtyard	Rectangular, open-to-sky courtyard surrounded by buildings (20 m wide, 60 m long, and 16.5 m high)	N/A	(S) CFD and (M)	Analysis of the thermal and flow conditions during peak outdoor summer conditions.	−8 to −12 °C	37 metric tons of CO2 per tower (in the six hot months).	[41]
Courtyard + natural ventilation	Damascus, Syria (BWk-Cold desert climate, with hot, dry summers)	Traditional Courtyard house	Two halls: 1st floor north hall (light mass structure + cross ventilation) and ground floor south hall (heavy mass structure + single ventilation)	N.S.	(S) and (M) Summer period	Analyzed six ventilation mode for each of the two cases.	−3 °C (day) −6 °C (night). Heavy mass: up to 20% (daytime).	Up to 72% (summer energy saving, mixed mode) + peak load reduction.	[42]
Courtyard + natural ventilation	Kashan, Iran (BWh-Hot desert climate)	Courtyard house	Eight zones of the building were studied.	N.S. ≈130	(S) Whole-building energy simulations	Analyzed four scenarios, two to determine the thermal loads and two for the cooling energy demand (with and without natural ventilation).	−2 to −5.6 °C. Add from 21 to 35 days in thermal comfort.	0.6 to 47.3 (kWh/m2-yr) saving.	[43]
Courtyard	Riyadh, KSA (BWh-Hot desert climate)	New house	2-story 303.9 m (net floor area)	2	(S) Whole-building energy simulations; (M) energy consumption	Study how to improve a new house’s performance by adding traditional solutions.		4.08% to 5.15% annual energy saving.	[44]
Courtyard	Seville, Spain (Csa-hot-summer Mediterranean)	Three residential buildings	3 courtyard cases: 2-fl. (9.0 × 9.0 × 8.5 m), 2-fl. (5.6 × 4.0 × 8.5 m), and 4-fl. (7.3 × 6.6 × 14.0 m).	N/A	(S) and (M) in summer months	3 simulations per case: reference, with the shading effect, and with the measured temperatures.	−10.5 to −14.4 °C (peak time ∆T outdoor vs. courtyard).	8–18% cooling. Energy saving.	[45]
Iwan	Yazd, Iran (BWh-Hot desert) Cost of Bandar Abbas, Iran (BWh-Hot desert climate, coast: hot humid) Tabriz, Iran (BSk-Cold semi-arid climate) Sari, Iran (Cfa-Humid subtropical climates)	Room (south oriented)	Rooms L × W × H (4, 4, 3 m) window-to-wall ratio 16.6%	N/A	(S) energy simulations; (M) in Yazd in July	Each room was simulated without shading devices and with four shading solutions at each location. Plus, a study of the optimum shading depth.	-	Average savings with South Iwan: Yazd 29%; Bandar Abass coast 26%; Tabriz 14%; Sari 32%.	[46]
Mmashrabiya	Riyadh, KSA (BWh-Hot desert climate)	New house	2-story 303.9 m (net floor area)	2	(S) Whole-building energy simulations; (M) energy consumption	Study how to improve a new house’s performance by adding traditional solutions.	-	4.40% to 5.06%. Annual energy saving.	[44]
Mashrabiya + mud, coral stone, and wood reinforced walls	Jeddah, KSA (BWh-Hot desert climate)	Historic building	2 identical rooms facing west, located on the first and second floors. Rooms L × W × H (4.0 × 3.6 × 3.9) Mashrabiyas W × H (2.4 × 3.1)	≈170	(M) The two rooms, Mashrabiya, and the adjacent courtyard (from 4 Aug to 1 Sep)	Analyzed the effect of open and closed Mashrabiya, building thermal mass, swing temperature, air movement, and differences between indoor and courtyard temperature.	74–77% reduction in indoor temperature swing. The temperatures at peak times were 6.8° to 8.5 °C, lower than those of the courtyard. Open mashrabiyas enhance air movement and reduce temperature by up to 2.4 °C compared to closed ones.	-	[47]
Mashrabiya	Riyadh, KSA (BWh-Hot desert climate)	House (1950s)	Mashrabiyas on its west and south facades. House area: 315 m2.	≈70	(S) Whole-building energy simulations	Simulations with and without Mashrabiya.	14% reduction in operative temperature; 77.8% (peak solar gain reduction);	5.7% (monthly cooling load reduction.	[48]
Mashrabiya + wet cloth	Jeddah, KSA (BWh-Hot desert climate)	Traditional house	2 identical rooms facing west, located on the first and second floors. Rooms L × W × H (4.0 × 3.6 × 3.9 m) Mashrabiyas W × H (2.4 × 3.1 m)	≈170	(M) The two rooms, Mashrabiya, and the adjacent courtyard (from 30 Jul to 8 Aug)	Two periods (Mashrabiya closed in both rooms, open in one room). Evaporative cooling (pots, water, spray, and cloth).	−5.8 °C (lower than outdoor temp. without wet cloth). But none of the solutions were able to maintain comfort in the summer at midday. −7.8 °C (lower than outdoor temp. without wet cloth).	-	[49]
Shading, local materials, and natural ventilation	Bushehr, Iran (BWh-Hot desert climate)	Traditional dwellings	TB1 (367.5 m2) TB2 (432.3 m2) MB1 (215.6 m2) MB2 (106.4 m2)	N.S.	(S) Whole-building energy simulations; (M) energy, gas, and short-term weather data	Two modes: passive ventilation during moderate periods of the year, and split units for cooling during hot, humid periods.	Reduction in discomfort hours TB1 34% and TB2 12%	Cooling energy TB1 is 26% lower than MB2. TB1 is 46% lower than MB1.	[50]
Rowshan	Jeddah, KSA (BWh-Hot desert climate)	Single-family home	A single room was studied with six Rowshan configurations, each of them in three orientations	N/A	(S) Room energy performance and CFD	Rowshans’ potential to provide comfort and reduce energy use. Optimum size of the opening grid.	With small openings: North 15–22% (reduction in indoor air temp. from Nov. to Apr.); South and Northwest: 2–3 °C (lower than outdoor). 42.3% hours in comfort in a year.	-	[51]

Table 2. Al Sa’fat 2023 and Dubai Building Code (DBC) 2021 thermal envelope mandatory and voluntary requirements [36,37].

Criteria	Regulation/Certification		Wall and Floor	Roof	Vertical Glazed Area			Horizontal Glazed Area
					<40%	40–60%	>60%	≤10%	>10%
U-value (W/m2K)	DBC and Al Sa’fat Silver Al Sa’fat Golden and Platinum	Mandatory Voluntary	≤0.57 ≤0.42	≤0.3 ≤0.3	≤2.1 ≤1.9	≤1.9 ≤1.7	≤1.7 ≤1.5	≤1.9 ≤1.9	≤1.9 ≤1.9
Shading coefficient	DBC and Al Sa’fat Silver Al Sa’fat Golden and Platinum	Mandatory Voluntary	- -	- -	≤0.4 ≤0.32	≤0.32 ≤0.25	≤0.25 ≤0.25	≤0.32 ≤0.32	≤0.25 ≤0.25
Light transmittance	DBC and Al Sa’fat Silver Al Sa’fat Golden and Platinum	Mandatory Voluntary	- -	- -	≥0.4 ≥0.32	≥0.32 ≥0.25	≥0.25 ≥0.25	≥0.32 ≥0.32	≥0.25 ≥0.25

Table 2. Al Sa’fat 2023 and Dubai Building Code (DBC) 2021 thermal envelope mandatory and voluntary requirements [36,37].

Criteria	Regulation/Certification		Wall and Floor	Roof	Vertical Glazed Area			Horizontal Glazed Area
					<40%	40–60%	>60%	≤10%	>10%
U-value (W/m2K)	DBC and Al Sa’fat Silver Al Sa’fat Golden and Platinum	Mandatory Voluntary	≤0.57 ≤0.42	≤0.3 ≤0.3	≤2.1 ≤1.9	≤1.9 ≤1.7	≤1.7 ≤1.5	≤1.9 ≤1.9	≤1.9 ≤1.9
Shading coefficient	DBC and Al Sa’fat Silver Al Sa’fat Golden and Platinum	Mandatory Voluntary	- -	- -	≤0.4 ≤0.32	≤0.32 ≤0.25	≤0.25 ≤0.25	≤0.32 ≤0.32	≤0.25 ≤0.25
Light transmittance	DBC and Al Sa’fat Silver Al Sa’fat Golden and Platinum	Mandatory Voluntary	- -	- -	≥0.4 ≥0.32	≥0.32 ≥0.25	≥0.25 ≥0.25	≥0.32 ≥0.32	≥0.25 ≥0.25

2.3. Case Studies Solar Decathlon Middle East (SDME)

Solar Decathlon (SD), initiated by the United States (US) Department of Energy (DOE), gathers university students to design, build, and operate sustainable and high-energy-efficient, grid-connected, solar-powered houses. In the final stage of the competition, teams showcase their houses in an exhibition open to the public while undergoing the ten contests of the competition, hence the name Decathlon.

Following this model, the SDME was established in June 2015 through an agreement signed between the Dubai Electricity and Water Authority (DEWA) and the US Department of Energy (DOE) to organize two competitions for the Middle East, one in 2018 and one in 2020, which was eventually postponed to 2021. SDME showcases live demonstrations of innovations in design, materials, and technology aimed at achieving zero energy buildings (ZEBs) suitable for the region.

The author initially aimed to analyze all the houses participating in the SDME. However, after reviewing all available documents and drawings for each house, they discovered that only eight from the 2018 edition had the necessary information for the analysis planned for the present study. On the other hand, all eight houses that reached the final phase of the 2021 edition have all the required information. As a result, the study includes sixteen houses, eight from each of the two SDME editions, as shown in Figure 3.

Table A1. SDME teams’ abbreviations, names, and universities.

#	SDME	Abbrev.	Team Name	Universities
1	2018	BU	EFdeN	Ion Mincu University of Architecture and Urbanism, Romania; University Politehnica of Bucharest, Romania; Technical University of Civil Engineering Bucharest, Romania; and Birla Institute of Technology and Science Pilani Dubai Campus, UAE.
2	2018	BX	BaityKool	University of Bordeaux, France; Amity University, UAE; and An-Najah National University of Palestine.
3	2018	NCT	TDIS	National Chiao Tung University, Taiwan.
4	2018	SUR	Sapienza	Sapienza University of Rome, Italy.
5	2018	TUE	VIRTUe	Eindhoven University of Technology, The Netherlands.
6	2018	UOS	Know-Howse	University of Sharjah, UAE; and University of Ferrara, Italy.
7	2018	UOW	UOW	University of Wollongong-Australia, Australia; and University of Wollongong Dubai, UAE.
8	2018	VT	FutureHAUS	Virginia Tech University, USA.
9	2021	HWU	ESTEEM	Heriot Watt University, UK and UAE.
10	2021	KFU	KU	Khalifa University, UAE.
11	2021	MPU	TAWAZUN	Manipal Academy of Higher Education, UAE.
12	2021	SCU	SCUT	South China University of Technology, China.
13	2021	BUD	HARMONY	The British University in Dubai, UAE.
14	2021	UOB	Go Smart	University of Bahrain, Bahrain.
15	2021	UOS	Sharjah	University Of Sharjah, UAE.
16	2021	UOL	Desert Phoenix	University of Louisville, USA. American University in Dubai, UAE. American University in Sharjah, UAE. Higher Colleges of Technology, UAE.

in the Appendix A shows the names of the universities responsible for each of these projects. These houses contribute to the identification of best practices and innovative passive design solutions that can serve as references for new construction and build onto retrofitting efforts in similar climates. During the competition, the ten contests are evaluated individually via one of three methods: jury evaluation, where a jury of experts is selected for each specific context; monitored performance, which is based on performance measurements taken during the context period; or task completion, which checks whether certain tasks have been completed.

While the SDME did not include an isolated passive monitoring period, such as for Solar Decathlon Europe, the passive design choices of the houses can still be evaluated, as they directly influence five contests: energy efficiency, sustainability, energy management, comfort conditions, and house functioning.

While the active design of SDME houses has been quantitatively assessed in a previous study on their HVAC and photovoltaic systems [52,53,54] and two earlier studies explored aspects of passive design [55,56], this research focused exclusively on the role of passive design, providing a more comprehensive and comparative evaluation of its impact across the selected houses.

2.3.1. SDME Rules

Specific rules ensure a regulated setting for the competition, including limitations on the dimensions of the houses, comfort conditions, house functionality, and the occupancy rate:

• Dimensional limitations ensure equal sun access, defining house volume as a truncated pyramid. For SDME 2018, the base is 20 m × 20 m, the top is 10 m × 10 m, and the height is 7 m. For SDME 2021, the base is 20 m × 20 m, the top is 13.2 m × 13.2 m, and the height is 7.5 m, as shown in Figure 4.

• Other rules include the following:
• The architectural footprint should not exceed 150 m², excluding roofless areas, with a measurable area of 45–90 m² for single-story House 1950s and up to 110 m² for multistory houses, with no floor over 70 m². Minor exceptions may be approved after verifying that they do not adversely affect the neighbors and do not confer any competitive advantage.
• Comfort conditions evaluate a house’s ability to provide interior comfort by controlling temperature, humidity, daylight, indoor air quality, and acoustic performance. The most relevant passive design strategies are related to temperature, relative humidity, and lighting levels, as summarized in

  Table 3. Comfort conditions required by the SDME rules.

  	Temperature	Relative Humidity	Air CO2 Content	Lighting Level	Façade Airborne Sound Insulation	HVAC Acoustic Values
  Full points	23 °C ≤ T ≤ 25 °C	35% < RH < 60%	CO2 < 800 ppm	LL > 300 lx	SI > 42 dB	AV < 25 dB
  No points	27 °C ≤ T ≤ 21 °C	70%< RH < 25%	CO2 > 1200 ppm	LL < 100 lx	SI < 30 dB	AV > 35 dB

  .
• House functioning assesses task completion for ordinary household activities such as baking, washing dishes, preparing dinner, using electronics, and laundering.
• Occupancy rules require 2–6 occupants during contests, excluding jurors and visitors. During public tours, comfort condition measurements are temporarily suspended.

These rules constrain all participants equally, ensuring that SDME provides a chance to study real building energy performance in a controlled environment [9].

2.3.2. SDME Monitored Data

Regarding weather conditions during the SDME contests, both competition periods—17–27 November 2018 and 13–24 November 2021—coincided with the onset of winter in Dubai, characterized by mild weather. The temperatures ranged from 15 °C to 34 °C, with humidity levels between 10% and 85%, and solar radiation levels below 800 W/m². The average temperature was similar for both years: 24.2 °C in 2018 and 24.5 °C in 2021. The average relative humidity was 57.1% in 2018 and 50.1% in 2021, showing a slight decrease. On the other hand, the solar radiation increased from 182 W/m² in 2018 to 212 W/m² in 2021.

3. Materials and Methods

The methodology evaluates passive design strategies and measures in SDME competitions through prescriptive-based design assessment against Dubai’s standards, modeled energy performance simulations, and real-time monitored data on indoor conditions and energy use. By combining these methodologies, a comprehensive view of the energy performance of the SDME houses is provided, ensuring robust findings for both theoretical and practical design. The analysis is based on SDME participating team-submitted reports, architectural models, drawings, and data from direct measurements and evaluations during the competition.

3.1. Prescriptive-Based Assessment

A prescriptive-based assessment is used to evaluate the passive design measures that affect a building’s energy performance. This includes design elements such as the building’s shape and compactness to minimize the exposed surface area, window-to-wall ratios for optimal lighting and heat control, orientations for wind and solar benefits, and shading elements to reduce solar radiation [57,58]. Additionally, the thermal transmittance of the houses’ envelopes is assessed against Dubai’s regulatory requirements.

3.2. Modeled-Based Energy Performance Assessment

For the modeled-based energy performance assessment, Building Energy Models (BEMs) are used to create detailed energy models and simulate the performance of houses. The method consists of five stages: data gathering, energy modeling (as illustrated in Figure 5) parameters variation, energy simulation, and results and analysis. Simulating the passive performance of houses provides valuable insights into both monthly and peak energy performance, helping to identify the most effective design strategies.

3.2.1. Data Gathering

First, data for the BEM are collected and processed from in-person visits, team-submitted reports, architectural modules, drawings, and competition jury reports.

3.2.2. Energy Modeling

Next, energy models are created to replicate the passive performance of the houses using Rhinoceros 3D (version 7.0) and its plug-in, Grasshopper. Rhinoceros 3D is used for geometry, whereas Grasshopper, with its components Honeybee (version 1.5) and Ladybug (version 1.5), handles weather data visualization, daylight simulations, and energy modeling. Similar frameworks have been used in the literature for arid desert residential buildings [59], and with other types of buildings and climatic conditions [60,61,62]. The house volumes, apertures, and shading devices are modeled. Zone adjacencies are then solved, and construction sets are applied for both opaque and translucent elements.

The models are then converted to OpenStudio models (OSMs) and translated to input data files (IDFs) for simulation in EnergyPlus (version 22.2), the energy simulation engine. Building programs are assigned, primarily using the ‘Apartment’ subcategory. Since these models aim to replicate the passive performance of dwellings, a full HVAC system is not added; instead, an ideal system is used to calculate thermal loads and report energy use for district heating and district cooling. For more information on the model’s parameters and boundary conditions, please refer to

Table A2. Building Energy Modeling input parameters, specifying whether they are customized or default to the modeling software’s library.

Parameter	Value Unit	Description
Building Program	Midrise Apartment
People	default 0.028 [people/m2]	Object that describes the occupancy of the program.
Lighting	default 6.5 [W/m2]	Object that describes the lighting usage of the program.
Electric Equipment	default 6.7 [W/m2]	Object that describes the electric usage within the program.
Gas Equipment	N/A	Object that describes the gas equipment usage within the program.
Service Hot Water	default 0.15 [L/h-m2]	Object that describes the service hot water of the program.
Infiltration	default 0.00057 [m3/m2 of façade]	Object that describes the outdoor air leakage of the program.
Ventilation	default 0.35 [ACH]	Object that describes the outdoor air requirement of the program.
Cooling Setpoint	customized 24 °C	Object that describes the cooling setpoint temperature of the program.
Heating Setpoint	customized 0 °C	Object that describes the heating setpoint temperature of the program.
Opaque Elements
R-value	customized [m2-K/W]	Thermal resistance of material.
Roughness	default “medium rough”	Very rough; rough; medium rough; medium smooth; smooth; very smooth.
Thermal Absorption	default 0.9	A number between 0 and 1 for the fraction of incident long-wavelength radiation that is absorbed by the material.
Solar Absorption	default 0.7	A number between 0 and 1 for the fraction of incident solar radiation that is absorbed by the material.
Visual Absorption	default 0.7	A number between 0 and 1 for the fraction of incident visible wavelength radiation that is absorbed by the material.
Transparent Elements
U-factor	customized [W/m2K]	Thermal transmittance of the glazing system.
SHGC	customized	Value 0–1, heat gain coefficient of the glazing system. Includes both directly transmitted heat as well as solar heat that is absorbed by the glazing system and conducted toward the interior.
Tvis	default 0.6/customized	Value 0–1, visual transmittance of the glazing system.

in Appendix A.

Since the passive performance of the buildings was not directly monitored, no data were available to validate or calibrate the models. However, any resulting errors are expected to have minimal impact on the findings, as the analysis focuses on the relative percentage reduction in energy demand rather than on absolute values.

3.2.3. Parameter Variations

Parameter variations are applied to the SDME houses to evaluate the impact of geometric parameters as passive design strategies. There are no variations in the geometric parameters (volumes, glazing areas, orientations, etc.), as they are modeled with the exact shapes and sizes of the built houses. First, all the houses are simulated, using the DBC’s minimum thermal properties (Table 2) to isolate the effect of geometry and orientation. This is considered the baseline case. Second, the houses are modeled with their actual thermal transmittance values, as designed and built. To determine the influence of the envelopes on the thermal performance, the cooling intensities of the models with the minimum DBC values are compared with those of the modules having their specific thermal transmittance values. Finally, the houses are modeled with DBC values but without shading elements, and the results are compared with those with shading, providing insight into the influence of shading on cooling effectiveness.

3.2.4. Results and Energy Performance Analysis

The analysis covers monthly energy performance, sensitivity analysis, overall energy usage, intensity, and peak consumption at various intervals. A comparative study of modeled and monitored data is conducted to assess energy performance, considering design parameters and mandatory requirements. The findings are aggregated to provide a comprehensive view of energy performance.

3.3. Monitored Performance-Based Assessment

For the monitored performance-based assessment, real-time data on indoor comfort parameters and energy consumption during the SDME competitions (17–27 November 2018 and 13–23 November 2021) are used. These measures, part of the energy management and comfort conditions contests, along with airtightness test results, are used to evaluate the actual performance under real-world conditions, validating the effectiveness of the passive design strategies implemented.

3.3.1. Energy Consumption

The energy management contest is used to assess each house’s energy self-sufficiency, consumption, and management system, including load consumption per surface area, net electrical balance, temporary generation-consumption profile correlation, and demand response, considering exterior temperature and humidity levels.

3.3.2. Indoor Comfort Conditions

The comfort conditions contest evaluates temperature, humidity, and daylight. The interior temperature is monitored with globe-type thermometers (BS EN ISO 52003-1) [18] in two thermal zones per house, alongside relative humidity sensors. Daylighting is assessed based on illumination levels at a height of 0.90 m in two zones. The minimum requirements for these parameters are summarized in Section 2.2.3.

3.3.3. Airtightness

The envelope airtightness is evaluated by mechanically pressurizing the building and measuring airflow rates at specific indoor-outdoor static pressure differences using Retrotec blower door equipment and adhering to ASTM E779-19 standards [63]. After all doors and windows are closed and airflow dampers are sealed, the blower door assembly is set up at the main entrance. The fan creates pressures from 10 to 50 Pa in 10 Pa increments, with measurements taken at each stage during both pressurization and depressurization. The DBC specifies that only new air-conditioned buildings with a cooling load of 1 MW or greater must meet the airtightness requirement of no more than 10 m³/h/m² at a pressure difference of 50 Pascals (Pa). Although there are no requirements for small buildings or houses in Dubai, the organizer of the SDME conducted airtightness tests in 2021 using Retrotec blower door equipment and adhered to ASTM E779-19 [63]. This test is performed for five SDME 2021 houses: HWU, KFU, SCU, UOB, and UOS.

4. Results and Discussion

The passive design assessment is presented and discussed in this section, including both prescriptive-based and performance-based evaluations that incorporate modeled and monitored data. The effectiveness of various passive design strategies is analyzed, design predictions are compared with actual performance, and insights are provided into the practical implications of implementing these measures in SDME houses.

4.1. Prescriptive-Based Assessment

This section presents the passive design measures applied to the SDME houses.

Table 4. SDME houses’ characteristics and geometric measures.

	BU	BX	NCT	SUR	TUE 4	UOS	UOW	VT	BUD	HWU	KFU	MPU	SCU 5	UOB	UOL	UOS	Mean
Num of Floors	1	1	2	1	1	2	1	1	1	1	2	1	1	1	2	1	-
Floor Area [m2]	92.6	98.7	70.3	103.2	73	93.9	125.5	91.2	95.1	117.6	132.5	100.6	138.6	100.5	141.2	99.8	116
Compactness 1	0.8	1.4	1.4	1.2	1.0	0.7	0.9	1.0	0.9	1.1	1.2	1.1	1.0	1.0	0.7	0.9	1.0
Form Factor 2	3.9	4.7	4.1	4.0	4.1	2.9	3.5	3.4	3.4	4.0	4.0	3.8	3.7	3.5	2.8	3.3	3.7
WW Ratio	13%	14%	21%	12%	11%	16%	12%	11%	27%	12%	18%	18%	16%	16%	6%	23%	17%
North 3 WWR	25%	3%	19%	36%	38%	31%	50%	13%	66%	62%	47%	49%	40%	41%	15%	41%	34%
East 3 WWR	61%	47%	33%	7%	14%	25%	18%	13%	11%	14%	11%	16%	14%	0%	12%	14%	19%
South 3 WWR	7%	3%	34%	35%	48%	36%	26%	61%	9%	18%	35%	17%	32%	21%	62%	23%	29%
West 3 WW	7%	47%	14%	22%	0%	9%	6%	13%	13%	6%	7%	18%	14%	38%	12%	21%	17%

¹ Compactness—the ratio between the thermal envelope area and the building volume (A/V). ² Form Factor—the ratio of the thermal envelope area and the building conditioned floor area (A/AF). ³ Orientations: North (315°–45°); East (45°–135°); South (135°–225°); West (225°–315°). ⁴ TUE shape factor indicated is the one “as assembled.” As an upper-floor apartment, its form factor is 2.7. ⁵ SCU also has a skylight-to-roof ratio of 7.1%.

summarizes the geometric parameters of the SDME houses, including the building area, volume, and envelope area, as well as the compactness, shape factor, window-to-wall ratio (WWR), and orientation.

Table 5. SDME houses thermal envelope transmittances (U-values) [W/m²K] with overall averages and DBC benchmark values.

Exterior Envelope	BU	BX	UOW	VT	KFU 1	SCU	UOB	UOS	Mean	DBC
U-value walls [W/m2K]	0.13	0.12	0.16	0.20	0.15	0.14	0.26	0.22	0.17	≤0.57
U-value roof [W/m2K]	0.13	0.15	0.19	0.10	0.10	0.16	0.33	0.20	0.17	≤0.30
U-value exposed floor [W/m2K]	0.19	0.17	0.16	0.14	0.19	0.53	0.26	0.24	0.24	≤0.57
U-value glazing 2 [W/m2K]	0.50	1.10	1.40	0.33	1.30	0.70	1.52	1.67	1.06	≤2.10

¹ KFU also has a floor in contact with the ground with an F-Factor equal to 1.263 W/m·K. ² Other DBC requirements for glazing: shading coefficient of 0.32 and light transmittance of 0.25.

details metrics related to the house envelope, such as the thermal transmittance (U-value) of walls, roofs, floors, and glazing. Finally, distinctive passive design elements, derived from both traditional techniques and modern innovations, are discussed.

4.1.1. Buildings’ Geometric Parameters and Envelope Metrics

Buildings’ geometric parameters and envelope metrics are as follows:

• Compactness, defined as the ratio between a building’s thermal envelope area and its volume (A/V), also known as the “surface-to-volume ratio”, significantly impacts energy efficiency in extreme climates. In this regard, among the projects studied, UOS and UOL are identified as the most efficient designs with a surface-to-volume ratio of 0.7. Both houses are characterized by a high degree of compactness, featuring floor areas comparable to those of other houses across two levels, resulting in a nearly cubic shape. The A/V ratio is used for preliminary comparisons of buildings with similar heights, but it becomes unreliable when the heights vary. Increasing the interior height increases the HVAC energy demand while lowering the A/V ratio, falsely suggesting greater compactness. Thus, it is considered insufficient to assess energy performance in such cases. Therefore, it is recommended to consider this ratio along with other ratios, such as the form factor or the relative compactness [57,58].
• Form factor, which represents the ratio of a building’s thermal envelope area to its conditioned (habitable) floor area (A/AF), is considered crucial for energy efficiency, particularly in arid desert climates where lower values are more favorable. Two-story houses UOL (2.8) and UOS (2.9) are found to have the most adequate shape factors among all the houses. The average shape factor across all projects is 3.7.
• Several projects, including BX and SUR, demonstrate design trade-offs by including shaded courtyards, but this results in “U-shaped” or “O-shaped” floor plans with reduced compactness and elevated shape factors. In contrast, apartment-style units benefit from lower shape factors due to reduced exposure to external environments. This is the case with TUE, which was designed as part of a multistory apartment complex. As an apartment, it would achieve a form factor of 2.7 or less, making it one of the most efficient geometries. The compactness and form factor ratios of the KU project are not as low as those of the other two-story projects (UOS and UOL) since this team prioritized their shading and ventilation strategies, placing most of the upper volume offset from the lower volume.
• The window-to-wall ratio (WWR) values range from a minimum of 6% (UOL) to a maximum of 27% (BUD). The average WWR value was 17%, which is considered a low value as required in extreme climates. A lower WWR can lead to better thermal efficiency in hot climates. Therefore, UOL has the lowest and most favorable ratio, whereas BUD has the highest and least favorable ratio.
• The WWR per orientation is analyzed as follows. The north-facing WWRs range from 3% (BX) to 66% (BUD), with an average of 34%. South-facing windows range from 3% (BX) to 62% (UOL), with an average of 29%. East-facing windows range from 0% (UOB) to 61% (BU), with an average of 19%. West-facing windows range from 0% (TUE) to 47% (BX), with an average of 17%. North-facing windows have the highest average WWR, which is considered advantageous in areas with latitudes similar to Dubai’s, as they increase daylight harvesting with reduced direct solar gain. South-facing windows have a moderate average, being a good daylight source, and the direct solar gains in this orientation can be effectively mitigated using small overhangs or other types of horizontal shading devices. East and west windows are shown to have the lowest averages, which is considered an ideal solution for hot climates since the solar angles in these orientations are the lowest and the hardest to block. Considering the opening orientation, houses with high WWR are advised to include adequate shading to minimize the potential of increasing the cooling loads. A low WWR in all facades, with the lowest WWR in the east and west, and adequate shading are considered essential to balance natural light, direct solar heat gain, and thermal efficiency in arid desert climates.
• The thermal transmittance values, often referred to as the U-value (W/m²K) values for walls, roofs, floors, and glazing across different SDME houses, including the overall U-value average and the Dubai Building Code (DBC) minimum requirement, are shown in Table 5. This table is color-coded to highlight the efficiency of the envelope components, with green indicating greater thermal insulation. The wall U-values range from 0.12 (BX) to 0.26 (UOB), which are 79% to 54% lower than the DBC requirement (≤0.57 W/m²K). The roof U-values range from 0.10 (VT and KU) to 0.33 (UOB), with UOB slightly exceeding the DBC benchmark of ≤0.3 W/m²K.
• Owing to the temporary nature of onsite assembly, most SDME houses feature floors elevated above ground level. The U-values of these exposed floors range from 0.14 W/m²K (VT) to 0.53 W/m²K (SCU), all of which are below the maximum limit of 0.57 W/m²K set by the code. The KU house stands out as the only two-story unit, with part of the upper floor exposed and the lower floor slab directly in contact with the ground. Its exposed floors have a U-value of 0.19 W/m²K, and the ground-contact slab includes perimeter insulation with an F-factor of 1.263 W/m·K, in accordance with ASHRAE Standard 90.1 [64], and the DBC, which requires one meter of insulation around the perimeter of the ground slabs in contact with the ground [37].
• The U-values range from 0.33 (VT) to 1.67 (UOS), all of which comply with the DBC benchmark of ≤2.1 W/m²K. All the houses are found to exceed the DBC requirements for envelope thermal transmittance, except for UOB, whose value is close to the maximum roof requirements. Therefore, the thermal losses are minimized in all the houses, especially in VT, BU, and BX. Overall, the data emphasize the importance of optimizing the compactness, shape factor, and window-to-wall ratios to improve the thermal efficiency and energy performance of buildings. These metrics serve as benchmarks for identifying areas where specific houses excel or need further enhancement.
• The shading elements adopted by the SDME houses vary in shape, form, size, and orientation. Several houses, including 2021 BUD, HWU, KFU, MPU, SCU, and UOB, opted for horizontal shading on the southern façades due to the high radiation in Dubai. For example, the 2021 SCU designed shading to cover the entire southern façade throughout peak summer days, whereas the KFU used an upper volume shift to create a shaded courtyard. Other shading configurations included the 2021 UOS, which shaded the entire courtyard with vertical fencing, and the 2021 BUD, which used a double-shell façade for wind circulation and shading. The 2021 MPU project implemented a unique solution inspired by traditional Islamic architecture, using a mashrabiya, a type of oriel window enclosed in carved wooden panels [33].

4.1.2. Distinctive Passive Design Elements from Traditional Techniques and Modern Innovations

A variety of distinctive passive design elements are integrated into the architectural designs of SDME houses to increase energy efficiency, thermal comfort, and sustainability. These design elements are drawn from both traditional techniques and modern innovations, focusing on blocking solar radiation, optimizing natural ventilation, improving insulation, and leveraging thermal mass to regulate indoor temperatures. The design elements include the use of double-shell systems inspired by traditional wind towers, inner courtyards, advanced window technologies, and the strategic use of materials and volumes for thermal inertia, among others (Figure 6).

Distinctive passive design elements from traditional techniques and modern innovations include the following:

• A double shell system, inspired by the Barjeel (traditional wind tower), is featured in the BUD house, providing extra insulation and reducing heat gain. Similarly, a roof-based system is used in the SCU house to enhance natural ventilation. A traditional wind tower is incorporated into the HWU house to direct cooler air into its courtyard, creating a natural cooling effect. The Green Moss Wall, included in the BUD house, increases its insulation level and improves its air quality by filtering pollutants.
• Advanced window technologies are employed in several houses. Double-row windows are used in the HWU house to reduce thermal bridging, whereas interior windows are integrated into the KFU for wall shading and natural cooling. Mashrabia, a traditional carved wooden panel, is adapted by the MPU to allow air circulation while providing shade. Vacuum windows, which provide superior insulation, are used by the MPU and UOS, whereas building-integrated PV windows are featured in the VT house.
• Ventilation strategies, skylight, and ventilated façades are applied in the SCU house to enable free cooling, daylighting, and minimize thermal gains through the walls. The UOB and HWU utilize hollow spaces under the house for cooling via air circulation. BUD, SCU, and UOS use movable interior partitions to offer flexible space arrangements and, in the case of UOS, provide acoustic insulation. The UOS and SCU employ ventilated façades to improve airflow and cooling.
• Shaded courtyards are incorporated into several houses. An inner courtyard, covered with a skylight, is featured in the SCU, which is designed to function as either an open or enclosed space.
• Phase Change Materials (PCM) are used in the HWU house to regulate indoor temperatures, whereas volumes for thermal inertia are strategically placed in the KFU house design to act as buffers and provide self-shading.

These distinct designs showcase the integration of both traditional and modern passive design elements to enhance energy efficiency, thermal comfort, and sustainability in contemporary housing. Each element contributes uniquely to the overall performance of the building, leveraging natural elements and innovative materials to create comfortable living environments.

4.2. Modeled Performance-Based Assessment

This section presents the results of the model-based energy performance assessment. Building Energy Models (BEMs) are created to simulate the performance of each SDME house, providing valuable insights into both monthly and peak energy performance. These insights are used to identify the most effective design strategies. The three-dimensional (3D) renderings of these BEMs are shown in Figure 7.

4.2.1. Geometric Parameters Combined

The cooling intensity results (kWh/m²) for the SDME houses, modeled with DBC thermal transmittance minimum requirements, are presented in Figure 8 using box-and-whisker plots. This figure provides quantitative insights into the effectiveness of geometric parameters (Table 4) as passive cooling design measures.

Figure 8a shows the cooling load intensity (kWh/m²) for all the houses aggregated on a monthly basis. The cooling intensity values range from nearly 0 to 85 kWh/m². The overall median is calculated to be 23.5 kWh/m². During the summer months (July to September), when solar heat gains are greater, the spreads are wider, with cooling intensities ranging from 22 to 85 kWh/m² and a median of 60 kWh/m². In contrast, during the cooler months of winter (December to February), the spreads are narrow, with a median cooling intensity of 2.5 kWh/m², due to the low solar heat gains and mild ambient temperature. The primary heat gains during this period are from internal heat sources, including lighting, occupants, and equipment.

Figure 8b illustrates the cooling load intensity (kWh/m²) using box-and-whisker plots to show the spread of cooling intensity values for each house. The values range from 0.5 to 85 kWh/m² for some teams, such as the 2018 NCT, and from 0.5 to 12 kWh/m² for others, such as the 2021 UOL.

A larger spread in the plots indicates greater variability in cooling intensity throughout the year, possibly influenced by building compactness, the window-to-wall ratio, and orientation. In contrast, smaller spreads reflect more consistent cooling demands, suggesting more effective passive cooling designs and strategies. Overall, the median values range between 12 and 34 kWh/m², with an overall median of 23.5 kWh/m². Notably, teams from 2018 tended to have higher median cooling intensities than those from 2021 did, suggesting that these teams had greater cooling demands, likely due to differences in building design and geometric parameters.

This analysis identifies the houses with the most efficient combination of geometric measures: UOL, UOW, VT, and BUD. These houses have the lowest window-to-wall ratios (WWRs) and small window areas facing east and west. The UOL house, which has the lowest WWR ratio at 6%, also presents the lowest cooling intensity value. The UOW and VT houses, with very low WWRs of 12% and 11%, respectively, also recorded very low cooling intensity values. Although the WWR of the BUD house is the highest at 27%, it performs well, as 66% of its windows face north, with less than 13% oriented toward the east and west. This finding underscores the importance of reducing direct solar gains and minimizing glazing areas, particularly in the most unfavorable orientations, such as the east and west, in Dubai. Finally, the results indicate that geometric measures can greatly reduce the cooling energy intensity in arid desert climates; in this case, their reduction can reach 60% (UOL vs. NCT and BU).

4.2.2. Thermal Transmittance

Figure 9a,b show the cooling intensity for houses modeled with the respective thermal transmittance utilized in the houses (Table 5) compared with the corresponding DBC values, which are again presented in two graphs. These figures illustrate the improvement in reducing the required cooling intensity by reducing the thermal transmittance of the houses’ envelopes.

Figure 9a shows year-round cooling intensity, with values ranging from 0 to 70 kWh/m² and a median of 25 kWh/m², peaking from July to September, and reaching values ranging from 27 to 69 kWh/m², with a median of 50.5 kWh/m². In the cooler months, the median decreases to 3 kWh/m². Figure 9b shows team-specific variations, with cooling intensities ranging from 0.5 to 78 kWh/m² for some teams, such as 2018 BX, and from 1 to 33 kWh/m² for others, such as 2021 WOB. The median values range from 17 to 32 kWh/m², with an overall median of 22 kWh/m².

Compared with the cooling intensity of the DBC model (baseline), there is a 14% reduction in the overall median, with variations ranging from 18% during the coldest months to 12% during the hottest months. The findings of this section highlight a significant opportunity to enhance energy behavior in Dubai and similar cities by strengthening regulatory requirements for thermal transmittance. These results align with those reported by Al-Tamimi in his study in the Arabian Desert, where the increase in thermal insulation led to a reduction in energy demand of at least 14.7% [65].

Together, these graphs highlight that, while cooling intensity peaks during summer for all teams, the efficiency in managing these demands varies significantly, underscoring the critical role of design choices in seasonal cooling performance. The graph also highlights the differences in cooling intensity between various houses or teams, with larger spreads indicating greater variability in cooling needs, possibly due to less effective cooling systems or more variable environmental conditions. Smaller spreads suggest more consistent cooling demands, potentially due to more stable internal and external conditions or better-performing building designs.

As shown in Table 5, all the houses have lower thermal transmittances than the maximum required by the DBC. This is why they all show important improvement; even UOB, which has the highest U-values, experienced a reduction of 11%. On the other hand, the BU House, which far exceeded the DBC’s requirement, showed the most significant improvement, 19%.

4.2.3. Shading Elements

Figure 10 shows the percentage reduction in total annual cooling load intensity (kWh/m²) and annual peak cooling load (W) for each of the sixteen houses after applying shading elements, compared with the base case without shading. All the houses are modeled using DBC thermal transmittance values. The reduction in total annual cooling load intensity ranges from approximately 2% to 13%, whereas the reduction in annual cooling peak load ranges from approximately 3% to 11%, with both showing an average reduction of approximately 6.5%. A previously cited study conducted in Saudi Arabia reported similar results, with the use of shading devices resulting in a 6.6% reduction in energy consumption [65].

The most significant reductions in the annual cooling load due to the use of shading elements are observed in the 2018 BU and 2018 NCT houses. The 2018 BU house achieved the most significant reductions in both total energy (13.1%) and peak energy (11.2%). Despite having a slightly below-average window-to-wall ratio (WWR), it has the highest east-facing WWR (69%), which is fully shaded by horizontal and vertical shading screens, thereby significantly reducing heat gains. The 2018 NCT has the second-highest WWR, with most of its openings in the East and South. Similar to the BU house, these openings are shaded with optimized horizontal and vertical shading screens, which significantly reduce the cooling demand.

Conversely, in 2018, TUE and 2018 UOW fell below average, with UOW at 2.1% total and 2.7% peak, and TUE at 2.9% and 4.7%, respectively. The 2018 TUE house has a minimal total WWR of 11%, with 48% of its openings facing south. However, these openings are unshaded, resulting in minimal reductions in cooling intensity compared with unshaded conditions. Similarly, the 2018 UOW has insufficient shading, leading to lower energy reduction outcomes.

There is great variability in the results shown in Figure 10. The highest and lowest reductions are observed in the SDME 2018 houses, whereas the SDME 2021 houses show reductions in total and peak loads closer to the average values. The highest impacts in 2021 are observed in the 2021 BUD, HWU, and UOS houses. On the other hand, the 2018 BX, 2021 HWU, and 2021 UOB presented greater peak reductions than the others. In other cases, the 2018 UOW and 2021 SCUT houses presented the least energy reduction, despite being ranked second and first in their respective competitions. Overall, the graph highlights the importance of shading in reducing the cooling load intensity and peak load, with variations across houses underscoring the need to integrate shading for increased energy efficiency and improved cooling management.

4.3. Monitored Performance-Based Assessment

This section presents the results of monitored performance-based assessments, with a focus on indoor comfort conditions, energy performance, and airtightness. This analysis is crucial for understanding the performance of passive design strategies under real-world conditions. The collection of real-time data during the SDME 2018 and 2021 competitions allows for the evaluation of the effectiveness of these strategies in maintaining comfort and energy efficiency.

4.3.1. Indoor Comfort Conditions

Figure 11 shows the average monitored temperatures in the living room and bedroom for the studied SDME 2018 and SDME 2021 houses, alongside the outdoor temperature (ambient temperature) over the competition period, from 17 November to 27 November for SDME 2018 and from 13 November to 23 November for SDME 2021.

The outdoor temperature ranged from approximately 15 °C to 35 °C for both years, peaking at 30–35 °C during the day and decreasing to approximately 18 °C during the night. The indoor temperatures (bedroom and living room) remain relatively stable, fluctuating within a narrow range of 22 °C to 25 °C throughout the contest period, indicating effective control of indoor conditions despite outdoor temperature extremes. There is a slight increase in temperature during the daytime, corresponding to the outdoor temperature peaks, but the indoor temperature remains fairly consistent and comfortable compared with that of large outdoor swings.

The graphs clearly show that, while outdoor temperatures exhibit large variations, the indoor temperatures in both 2018 and 2021 remain much more stable, indicating the ability of the houses to maintain thermal comfort regardless of external conditions. Houses from both competitions maintain temperatures within the ideal comfort range (20–25 °C). The outdoor temperature peaks every day around noon to mid-afternoon (12:00 p.m. to 3:00 p.m.), but these peaks have a minimal effect on the indoor temperature, further demonstrating the effectiveness of both passive and active cooling measures in these houses.

In conclusion, both the 2018 and 2021 houses successfully regulated indoor temperatures despite significant diurnal fluctuations in outdoor temperatures. Compared with the 2018 houses, the 2021 houses appear to have better thermal performance, maintaining more consistent indoor conditions, likely due to improved passive design strategies and/or more efficient HVAC systems.

4.3.2. Airtightness

Airtightness plays a critical role in optimizing energy efficiency and sustainability for mechanically conditioned buildings, especially in regions with extreme climates. However, the accelerated construction timelines typical of Solar Decathlon competition sites often hinder participating houses from achieving the desired airtightness performance.

The 2021 airtightness tests reveal an average rate of 14.5 m³/h/m² across five competing houses, setting a comparative benchmark. The SCU emerged as the top performer with an 8.3 m³/h/m² leakage rate, whereas the KFU had the lowest airtightness, 29.8 m³/h/m². HWU’s result of 16 m³/h/m² slightly exceeded the average, whereas UOB and UOS obtained below-average leakage rates of 10 m³/h/m² each. These findings highlight the significant differences in airtightness among competitors.

4.3.3. Electricity Consumption

Figure 12 presents the monitored hourly variation in electricity consumed for HVAC (kWh) during the competition periods of SDME 2018 (17–28 November) and SDME 2021 (13–24 November) for all days when each team was connected to the grid. The red lines represent the average of the daily electricity consumption for each case.

The X-axis represents the hours of the day, whereas the Y-axis represents the variation in electricity consumed by HVAC systems each hour. The consumption varies from 0 to 8 kWh across different houses. The red line represents the mean line of HVAC energy consumption over time.

Most houses exhibit fluctuations in energy consumption throughout the day, with HVAC energy use peaking from midday to the end of the afternoon, likely due to higher external temperatures, and decreasing at night. Houses, such as 2021 KFU, exhibit larger fluctuations and higher peaks than others do, with spikes exceeding 6 kWh. In the case of KU, two potential reasons for its high HVAC consumption and peaks were issues with air conditioning control and the house’s low airtightness. Conversely, the winning houses from the 2018 and 2021 competitions, VT and SCU, exhibited lower HVAC consumption along with minimal fluctuations and peaks. These houses report no problems with their air conditioning system and are airtight, especially considering their rapid construction.

5. Conclusions

This study demonstrates the significant potential of passive design strategies in reducing cooling energy demand and enhancing indoor comfort in residential buildings situated in arid desert climates, using the Solar Decathlon Middle East competition as a real-world, full-scale experimental case. The standardized yet context-specific framework of this competition provides a unique opportunity to evaluate and compare diverse passive design approaches under identical environmental constraints, offering insights that are otherwise difficult to obtain in conventional research settings.

This study employs prescriptive and performance-based approaches, incorporating both modeled and monitored data to assess passive design strategies for residential buildings in arid desert climates. Energy performance is evaluated using both the mandatory thresholds of the Dubai Building Code and house-specific design parameters, providing a comprehensive understanding of how passive measures perform under extreme conditions.

The prescriptive assessment examines design features including geometry, thermal envelope properties, and shading configurations. Most houses adopt compact forms, with envelope-to-volume and envelope-to-floor area ratios averaging 1 and 3.7, respectively. The average window-to-wall ratio of houses is 17%, with openings oriented predominantly to the north to optimize daylight and minimize heat gain. Shading is used strategically, with horizontal devices on southern façades and vertical ones on the east and west. The U-value of the house’s thermal envelope exceeds the Dubai Building Code requirements, with values up to 80% lower than the required minimum. Many designs integrate vernacular elements, such as courtyards, ventilated façades, and mashrabiyas, not only as aesthetic references but also for their proven climatic functionality.

The modeled performance assessment uses Building Energy Models to simulate the effects of these passive design features on the cooling demand. Three scenarios were modeled: first, all the houses use the minimum mandatory thermal properties to isolate the effects of geometry and orientation (baseline). Second, the shading is removed to assess its individual impact; and third, actual thermal values are applied. The results show that geometry alone accounts for up to 60% of the variation in cooling intensity across houses. Shading contributes to an average 6.5% reduction in total and peak cooling loads, and improved envelopes lower the cooling demand by 14%, highlighting the potential for regulatory enhancements.

The monitored performance assessment draws on real-time data collected during the Solar Decathlon Middle East contest period. Despite outdoor temperatures ranging from 15 °C to 35 °C, the indoor temperatures of both the 2018 and 2021 houses remained consistently between 22 °C and 25 °C, confirming the effectiveness of passive strategies. Energy use data show that well-designed and tightly sealed houses consume less cooling energy and experience fewer load peaks. However, the airtightness results vary widely, averaging 14.5 m³/h/m² across the 2021 houses. Notably, one of the best-designed houses had one of the poorest airtightness scores, underscoring the critical role of construction quality in realizing performance potential.

Together, these findings demonstrate that passive design is a powerful and essential foundation for energy-efficient buildings in desert climates, with substantial potential to support progress toward net-zero targets.

By combining prescriptive, modeled, and monitored assessments, this study presents a robust methodology for evaluating passive design strategies for residential low-rise buildings in arid desert climates. Passive strategies are not only low-cost and low-maintenance solutions but also culturally and climatically appropriate approaches for mitigating energy demand. The results offer valuable guidance for architects, engineers, and policymakers seeking to implement climate-responsive designs.

6. Limitations and Future Research Work

While this study offers a comprehensive framework for evaluating passive design strategies in arid desert climates, several limitations should be acknowledged. Additionally, optimized occupant behavior during the contest periods does not reflect real-world usage patterns, which can significantly impact energy demand.

Future research could explore broader building types, evaluate the economic feasibility of envelope upgrades through life cycle cost analysis, and incorporate longer-term monitoring across all seasons. Passive-only monitoring phases and real-world occupant behavior studies could further enhance model validation and provide practical design guidance for Dubai and similar regions, aiming to offer concrete, data-driven recommendations for policymakers and regulators.