1 Introduction

Recently, numerous nanomaterials and nanotechnology applications have been used in construction (Abdelmonteleb Mohammed Aly, 2020; Abdelrady et al., 2021). Due to its significant commercial value, nanotechnology is receiving more financing and economic aid (Ge & Gao, 2008). Nowadays, nanomaterial-based refurbishment technologies have been developed significantly. Designing all of the necessary conditions for accuracy at the molecular and atomic levels in materials engineering has resulted in the production of materials with many special characteristics, which have provided novel and effective solutions to many problems, including decreasing the impact of heat gain in the building's outer envelope, transmittance, fire resistance, waste recycling, emission reduction, and increasing the internal environments (Lalbakhsh, 2011). Nanomaterial technologies, such as Aerogel glazing, could help architects create more indoor and outdoor architectural designs by allowing them to construct functions and forms to meet customer demands. Aerogel is currently employed in window glass systems, concrete, plaster, and building insulation blankets (Abdelhafez et al., 2023; Mohamed & Gomaa, 2023). Thus, nanoparticles incorporated in building envelopes are beneficial economic choices that save a lot of money while improving the physical environment and addressing environmental issues (Berardi, 2018; Lalbakhsh, 2011).

School buildings account for a significant portion of non-industrial energy consumption in the United Kingdom (Barbhuiya & Barbhuiya, 2013). In Egypt, school buildings with a high student density are an important part of the public building stock (Ayman Ragab Mahmoud, 2022a). According to the statistics of the Ministry of Education, there are approximately 58,807 existing school buildings in Egypt that accommodated approximately 25.06 million students in 2021/2022 (Ministry of Education, 2022). Energy use for lighting and cooling in schools is seen as an important area for study. Most of the child's time is spent in school. Classrooms are different from other places because they have more people in them. Also, children in the classroom became important sources of sense and latent heat that must be dealt with in the summer and could contribute to reducing energy use in the winter (Yael Valerie Perez & Isaac Guedi Capeluto, 2009). Several studies (Heschong, 2002; Kort & Smolders, 2010; Seyedehzahra Mirrahimi et al., 2013) have shown that the appropriate temperature, lighting, humidity, and room air quality help people learn.

A study of 3,766 kids in 27 schools found seven important design factors that explain 16% of the differences in how well children do in school (Barrett et al., 2015). J. Rucińska and A. Trzski investigated the impact of daylight penetration through windows on the placement of two models within a classroom. The study involved measurements and simulations to assess the illuminance levels in the room. The results revealed significant differences between the calculated illuminance levels and the measurements. While the general characteristics of the illuminance distribution were similar, there were notable deviations in illuminance levels across the room. These differences were found to be dependent on both external illuminance conditions and the room's location. The study also examined the effects of modifying the types of glazing on the warmth, air quality, and lighting of the room. In the specific classroom under investigation, artificial lighting accounted for a substantial portion of total energy consumption (76%) and primary energy usage (48%). Evaluations were conducted to assess the potential energy savings in lighting and cooling by implementing a lighting management system that considered the time of day and natural light availability. The results indicated a significant opportunity for energy conservation (Rucińska & Trząski, 2020).

To achieve energy efficiency in terms of cooling and lighting energy demand, this paper presents an integration between the aerogel glazing system and building orientations for the design of school classrooms in Aswan, Egypt, considering local climate conditions. In a broader sense, the purpose of this study is to serve as an advisory to architects and designers that keeping the thermal and visual comfort of building occupants, as well as lowering energy usage, are fundamental elements of sustainable design that can be beneficial for both people and the surrounding environment.

2 Literature review and background

It is widely acknowledged that natural illumination enhances children's comfort, health, and activity level, particularly in shared spaces. In this context, classroom lighting design should concentrate on required illumination levels, available natural light, and illuminance value uniformity. Thus, a comprehensive evaluation of the space is crucial to assure proper visual task performance due to natural light and, during the design phases, to reduce energy consumption from the use of artificial light (Galatioto & Beccali, 2016). Consequently, lately, numerous attempts have been made to develop windows with insulation capabilities in order to attain the aforementioned objective.

2.1 Development of insulated window-based aerogel glazing systems

The illumination in a school building should enable users to read, see others with whom they are conversing, and perform other visual duties associated with learning, teaching, and school administration for improved performance, efficiency, and productivity. Daylighting has been associated with improved mood and motivation, less fatigue, and less eye distress (Nancy Ruck & Aschehoug, & Sirri Aydinli, 2000). Numerous studies indicate that the quality of lighting can enhance worker performance and productivity in workplaces (Nancy Ruck & Aschehoug, & Sirri Aydinli, 2000), industrial settings, and retail environments. With features such as:

  • visualizing the luminous environment of a given daylighting design.

  • estimation of daylight-related factors in a room with diffuse daylight.

  • identifying potential sources of radiation and assessing visual comfort indices.

  • estimation of prospective energy savings resulting from daylighting.

  • control of sun ray penetrability and visualization of the dynamic behaviour of sunlight.

To ensure that a high-performance building envelope functions properly, it should be meticulously designed to include acceptable levels of thermal insulation, prevent air penetration, provide adequate solar control, and eradicate thermal bridges to the greatest extent possible(Hannah et al., 2022; Rehman, 2023). Kalwall systems incorporate thermal break technologies which can result in isolated areas of greater heat transfer (Kallwall hight performance translucent building & "kalwall facade brochure", 2020). Figure 1 depicts an old, single-pane window glazing that produced excessive glare and inhibited the learning process in the classroom. One of the companies developing daylighting systems replaced with an energy efficient system based on the aerogel, which creates a comfortable and productive classroom and produces non-glare daylighting while decreasing both energy demands and maintenance requirements.

Fig. 1
figure 1

Developed system based on the Aerogel daylighting system (Kallwall hight performance translucent building & "kalwall facade brochure", 2020)

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Baloch et al. (Baloch et al., 2020) examined several daylighting characteristics and logical test results in 2670 primary school pupils from 12 European nations. The statistics indicate that daylighting-related classroom factors affect schoolchildren's performance and may account for more than 20% of performance test scores. The window-to-wall area ratio seems to have the largest impact on the classroom, suggesting that more windows are better, perhaps due to indirect sunlight. However, in hot, dry locations, larger windows need more cooling energy.

Several studies have investigated the effect of nano-glazing on the energy efficiency of windows in various climates. For instance, a study by Aburas et al. (2021) present the advancement of thermochromic glazing technology intended for windows, which has the ability to regulate indoor penetration of solar irradiation. The researchers employed building simulation techniques for desert, Mediterranean, and temperate climates, resulting in energy savings ranging from 7.1 to 46.4%. The technology involves the direct application of VO2 nanoparticles onto glazing material, which effectively achieves a balance between luminous transmittance and solar modulation.

A transparent nano-insulation is evaluated by Aliakbari et al. (2021) for its effect on thermal comfort and energy consumption in five climate zones in Iran. To assess how insulation impacts factors like thermal comfort and energy use, a dynamic model has been developed. When applied to a standard building, nano-insulation could reduce energy use by more than 7.61%. Another study was conducted by Buratti et al. in Iran (2022). They investigated the impact of double-glazing with aerogel insulation on solar gain and cooling load in primary schools situated in hot-dry, hot-humid, and cold climates in Iran. Numerical simulations of a two-story school were carried out to assess the influence of the aerogel glazing system. The findings indicate that the utilization of aerogel glazing can lower solar gain by up to 73% and cooling load by up to 33% when compared to a simple glazing window in hot-dry climates.

Sadooghi (2022) investigates the energy-saving potential of a novel switchable window compared to conventional glazing systems in a typical Canadian house located in Toronto. The study shows that the proposed switchable glazing system significantly reduces building heating and cooling energy consumption, resulting in a decrease in HVAC annual energy bills by 47 and 33% compared to conventional clear single and double pane windows, respectively.

Alwetaishi (2019) investigates the impact of glazing to wall ratio on energy performance in different microclimate regions in Saudi Arabia. The research identifies the optimal glazing ratio based on computer modelling and field monitoring studies, with glazing-to-wall ratios of 10% recommended in hot and dry and hot and humid climates. The study also highlights the negative impact of south and east-facing glazing on heat gain in all locations.

Other studies attempted to develop aerogel glazing by integrating phase-change materials (Radi, 2020). However, systematic research on phase transition and photothermal transmission in glazing envelopes with nanoparticle-containing phase change materials is lacking ("Incorporating phase change materials into glazing units for building applications, 2022). Buildings consume a lot of energy to keep residents comfortable. Transparent envelope components such as glazing windows, curtain walls, and roofs contribute to 40% of building energy usage ("Incorporating phase change materials into glazing units for building applications, 2022; Mirrahimi, 2016). Solar film coatings ("Energy savings potential of reversible photothermal windows with near infrared-selective plasmonic nanofilms", 2022), multilayer glazing pane systems (Zhang et al., 2022), and heat insulation materials (Huang et al., 2022) have been studied to increase glazing envelopes' solar photothermal transmission. However, phase-change materials were used to fill the glass envelope to increase thermal performance.

Qu et al. (Huang et al., 2022) examined building envelope PCM type, thickness, and configuration for thermal comfort and energy savings. Energy savings varied from 4.8 to 34.8% in the local climate, and thermal comfort lasted longer than in a standard residence. Zhang et al. (2021) examined the energy performance of 10 glazing designs filled with PCM or silica aerogel in a severe cold environment compared to a standard air-filled glass window. Wieprzkowicz and Heim (2020) examined the optical property differences generated by the phase change process on a triple-glazed PCM window with five different paraffin fillings compared to a regular triple-glazed window. The best option was a mix of PCMs with varied phase change temperatures, as none of them worked well alone. The phase change technique controlled daytime heat storage and nocturnal heat release, enabling solar energy utilization and thermal energy savings ("The evolution of building energy retrofit via double-skin & responsive façades: A review", 2021). Cascone et al. (2018) examined PCMs for office building energy retrofitting. GE systems utilize organic and inorganic solid-to-liquid PCMs (Guo & Zhang, 2021). Paraffin is a cheap organic PCM with high latent heat and a large phase transition temperature range ("The evolution of building energy retrofit via double-skin & responsive façades: A review", 2021).

Dong Li et al. (2020) examine the thermal performance of glass windows containing glass, silica aerogel, and phase change material (PCM), which store and restore heat, offer super thermal insulation, and let in the daylight. They found that silica aerogel's thermal conductivity and thickness have a significant impact on the glazing unit's thermal performance, whereas density and specific heat have no effect. Yuxin Ma et al. (2022) suggested a revolutionary glazing window combined with solid–solid phase change material and silica aerogel and performed a parametric analysis to assess its implementation potential in China's severe cold zone. In 10% of property fluctuations, changes in phase characteristics of materials such as melting temperature, latent heat, absorption coefficient, and refractive index are very important to building energy performance.

2.2 Effectiveness of daylight design and the dynamic performance indices

Metrics of the amount and quality of daylight are crucial for determining whether a building delivers adequate (or even acceptable) daylighting for the advantages of human well-being, visual comfort, and energy efficiency. Academics and designers currently only use a few daylight measures. Most of them are really simple, illuminance-based, and have a limited history of use or investigation (Howlett et al., 2007; Veitch, 2001).

Several established and widely recognized methodologies for assessing and certifying the sustainability of buildings, such as LEED (Leadership in Energy and Environmental Design), BREEAM (Building Research Establishment Environmental Assessment Methodology), and DGNB (Deutsche Gesellschaft für nachhaltiges Bauen), incorporate guidelines for daylight within their evaluation frameworks. In general, the majority of practitioners rely on the daylight factor as their principal metric, though they employ diverse computation methodologies and normative frameworks. In addition to utilizing the daylight factor as an indicator, visual comfort is often described based on criteria such as glare control, illuminance levels, and a view of the outside.

According to BREEAM standards, a minimum of 80% of the floor area in occupied spaces should have an average daylight factor of 2% or higher. According to the regulations, residential buildings have to ensure that living rooms, dining rooms, and study areas achieve a minimum average daylight factor of 1.5% and that 80% of the working surface receives direct light from the sky (Camelia, 2022).

Daylight is a significant variable considered in the assessment of a building's LEED credit rating. According to the USGBC's 2005 statement, the LEED credit 8.1 standard mandates that 75% of the area set aside for essential visual tasks achieves a minimum Daylight Factor of 2%, with direct sunlight penetration excluded. The LEED regulations promote the implementation of diverse shading methods that are considered optimal, indicating that it is reasonable to expect the occurrence of direct sunlight intermittently (Esmailian et al., 2021; Mardaljevic, 2006; Vaisi & Kharvari, 2019).

Solar altitude, climate, and other time-dependent environmental factors have a substantial impact on the effectiveness of the daylighting design. However, very few existing tools provide the user with a comprehension of the annual performance of a daylighting design, and similarly, few lighting metrics emphasize this temporal aspect of light measurement. The two time-based metrics currently in use are Daylight Autonomy (DA) (Reinhart & Herkel, 2000; Reinhart & Walkenhorst, 2001), which provides the percentage of annual work hours where daylight is sufficient to exceed a benchmark illuminance, and Useful Daylight Illuminance (UDI), which is similar to DA except that the benchmark is replaced by the illuminance range of 100–2000 lx (Mardaljevic, 2004; Nabil & Mardaljevic, 2016).

Rogers's experiments determine the annual distribution of "daylight saturation," which is a measure of daylight uniformity in each area. Rogers discovered that rooms with uniform daylight saturation provide sufficient DA for at least 60% of the workplace. Z. Rogers and D. Goldman (2006) developed novel dynamic performance indices based on climate-based daylight modelling (CBDM). The Rogers-suggested variables for the Daylight factor are used to compute the minimum daylight autonomy (DAm), along with measurements of daylight uniformity and any worst-case scenarios. The determination of daylight levels is based on the target illuminance, which sets the threshold for illuminance. Latitude and longitude are considered along with weather information for the yearly simulation in location and climate, respectively (Reinhart et al., 2006). The continuous Daylight Autonomy (cDA), a new parameter created by Rogers, further specifies thresholds for visual tasks (e.g., below 500 lx). When the illuminance values fall below the minimum, cDA attempts to assign time step credits in accordance with the DA definitions. Rogers observed that good uniform daylight saturation results in rooms that achieve sufficient DA for at least 60% of the workplace (Kleindienst et al., 2008; Reinhart et al., 2006; Rogers & Goldman, 2006).

Natural light demands different evaluation methodologies than artificial light. Useful daylight illuminance (UDI) is a significant variable in artificial lighting, especially for designs that seek to provide general illumination at a specified objective illuminance. The geometric relationship between the room and its diminutions results in a less uniform illumination. Another cause is both complex and seasonal: the daylight sources of solar radiation and clouds differ in position and luminous flux throughout the day and year (Lm, 2013).

IESNA has recently published a new method for analyzing the natural illumination potential of buildings and spaces. This method is based on the new metric of spatial daylight autonomy (sDA), which characterizes the proportion of floor area with an illumination level greater than or equal to 300 lx for at least 50% of the annual occupied hours (IES Daylight Metrics Committee, 2012). This concept does not refer to a specific location because it investigates the use of daylight across an entire surface. Several researchers (Kazanasmaz et al., 2016; Lo et al., 2017) have defended their use of this metric in their findings. The daylight autonomy minimum (DAm) is computed using the variables established for the daylight factor (DF) (Reinhart et al., 2006).

3 Material and methodology

3.1 Study area and modeling

According to Köppen–Geiger climate classification ("World map of the Köppen-Geiger climate classification updated", 2006; Rubel et al., 2018). Aswan is a hot and dry southern governorate in Egypt (24.0889° N, 32.8998° E) and has a governorate area of 34,608.00 km2. It is 194 m above sea level. Aswan's climate is hot in the summer and warm in the winter. It has a continental climate with increasing maximum and minimum temperature differences throughout the day, regardless of the season. The case study is The Egyptian Japanese School, an existing school building in Aswan as in Fig. 2, which is considered the study investigation location. The building was created as 2D in AUTOCAD software Fig. 3, then exported for modeling in Design-Builder (DB) software licensed version (V.5.0.0.105).

Fig. 2
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The location of the school building

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Fig. 3
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The case study characteristics; a The case study architectural plan, b The building model in the simulation software (Design Builder) (Ltd & "Design Builder", 2019)

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The school building has a ground floor plan and three typical floors, is categorized into two blocks connected by an interior corridor as in Fig. 3. a, and each block has two stairs and services. One of these blocks has about six classrooms, while the other block includes laboratories, the library, and other service spaces. The simulation focused on one of these blocks which has student classrooms.

3.2 Simulation and proposed scenarios

The simulation engine described the modeling investigated shown in Fig. 4. It is a simulation of an existing educational building (case study) located in Aswan, carried out by the Design Builder software version (V.5.0.0.105). The cross-sections of investigated windows have been presented in Fig. 5.

Fig. 4
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The Study workflow

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Fig. 5
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The investigated windows structure. (1) Single clear glazing, (2) Double aerogel glazing

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Table 1 lists the scenarios under consideration. The orientation of the base-case (BC) scenario was north. As depicted in Fig. 6, Sc1, Sc2, Sc3, and Sc4 represent the north, east, south, and west orientations of the buildings. The General Authority for Educational Buildings in Egypt (GAEB) executed a uniform architectural blueprint across the nation, regardless of variations in geographical location and orientation. However, the orientation of a building is contingent upon the dimensions and attributes of its site. To address this, the present study concentrates on building orientations to aid stakeholders in identifying the most favorable orientations for new schools with respect to daylight and heat gain through windows. Moreover, the study provides recommendations for GAEB to enhance thermal and illuminance efficiency of their educational building envelopes through the incorporation of suggested aerogel glazing systems.

Table 1 Investigated scenarios in the simulation process
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Fig. 6
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The proposed scenarios

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All these scenarios were tested using 6 mm-thick double-paned windows filled with aerogel granules ranging in size from 0.7 to 4.0 mm. While the current base-case scenario (BC) for Egyptian schools is a single pane of clear glass 3 mm thick. Table 2 shows the specifications of the analyzed window scenarios. Design-Builder software was fed additional input data containing alternative hypotheses while running simulations. Table 3 shows the specifications for these hypothetical situations, as well as the characteristics of aerogel granules, acquired from the product datasheet. (https://www.cabotcorp.com/solutions/products-plus/aerogel/particles).

Table 2 Specifications of the school building model
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Table 3 Simulation hypothesis input
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Several new daylighting indicators have been proposed in recent decades to address the incapability of older metrics to assess these conditions. Building and location data, weather data, and activity schedules are all required input variables for these metrics. Two metrics allowed to evaluate of a natural daylight space as in previous studies (Kazanasmaz et al., 2016; Lm, 2013; Turan et al., 2020) for one year using two different performance parameters. The first parameter is Spatial Daylight Autonomy (sDA), which reports a percentage of surface area that exceeds a specified illuminance level. The second parameter is Annual Sunlight Exposure (ASE), which adds a new dimension to daylight analysis by focusing on one viable source of visual discomfort: direct natural light.

Both parameters resulted from hourly illumination and daylighting area using the same building information and simulation methodology in the Design-Builder software. To evaluate the effect of aerogel glazing on daylighting in classroom building designs, both should be reported together to an understanding of aerogel’s expected effect.

Considering the variety of software inputs as in Tables 2 and 3, the variations in the methodology used to generate these parameters can be significant. For example, the data collection process was conducted before the simulation process. From the as-built drawings for the building, the exterior walls, interior walls, and roofs of the model are simulated using the same materials that were used in the construction process. Table 3 shows the most common temperatures for cooling and heating setpoints according to the Egyptian lifestyle (Attia et al., 2012), so it's an accurate representation of the real situation. The Egyptian code for natural lighting specifies that classrooms should be illuminated at a value of 300 lx (Ministry of Housing Utilities & the Urban Development, 2006). The school administration has also been asked for additional information, including occupancy and building schedules.

3.3 Model validation

The process of validating simulation results ensures that they are accurate. Hence, a comparison was drawn between the simulated data and those derived from actual observations of air temperature and light intensity. Field measurements of the selective classroom were taken from September through May, while the space was in use by students. On September 22, the first-floor classroom's air temperature and light intensity were compared to ensure the base case model was validated. The Hobo U12 data logger (Onset Computer Corporation, Bourne, MA, USA) has been used to keep track of the temperatures, humidity, and light intensity of the classroom under study. The typical workday began at 8:00 AM and ended at 15:00 PM. The difference between the calculated and observed air temperature in the studied classroom ranged from 0.08 to 4.61% over the course of an entire workday. While the difference between the observed and simulated light intensity ranged between 0.6 and 2.03% over the course of the same entire workday. It is allowed if the difference is less than 5% (Rahman et al., 2008). Figure 7 depicts the model validation.

Fig. 7
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Validation of the school building model: a air temperature validation and b light intensity validation

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4 Results and discussion

This section describes the results of the classroom performance evaluation inside the existing educational building (The Egyptian Japanese School building) in terms of two categories:

  1. (1)

    Daylight availability

  2. (2)

    Window heat gain and thermal comfort evaluation.

This study focuses on investigating the impact of Aerogel glazing on both daylighting performance and heat gain in relation to the primary orientations of the building. It should be noted that the window-to-wall ratio is not considered in this specific analysis, as it is recognized as a limitation of the study.

4.1 Daylight analysis

In addition to analyzing the windows' heat gain performance in the educational building, this section tries to determine the relative impact of classroom orientation on daylighting levels in existing educational buildings. It aims to examine the heat gain and daylighting aspects that affect the design decision for classroom orientation and then assess the applicability of the local government guideline that requires classrooms to be oriented towards the most efficient orientation in terms of improving the energy efficiency of buildings located in the hot arid region (Housing & Building National Research Center HBRC, 2005).

As mentioned previously according to the material datasheet, light transmission for granules is > 90% per cm. while the light transmission for double glazing filled with granules is 0.671 as assessed in Design Builder software for the case study. Specifically, the material datasheet states granules have a light transmission of greater than 90% per cm. This means that if a beam of light passes through one centimetre of granules, more than 90% of the light will be transmitted through the material. Otherwise, the light transmission for double glazing filled with granules was assessed to be 0.671 using Design Builder software for the case study. This means that when light passes through double glazing filled with granules, only 67.1% of the light will be transmitted through the material. In conclusion, granules and double-glazed granules transmit light differently. Granules transmit more light than double-glazed granules.

A classroom located on the first floor, as depicted in Fig. 8, was selected as the site for extracting daylight measurements and heat gain from both the exterior and interior windows. Because of its characteristics, it was chosen. Even though the classroom is the same size as the others, it has two walls that depict building façades. This implies that the classroom can become warmer than others due to the amount of heat transferred into the building.

Fig. 8
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The simulated classroom

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By obtaining the illuminance and annual daylighting results and focusing on these indicators for all classroom orientations cases (Sc1, Sc2, Sc3, Sc4):

  • Spatial Daylight Autonomy (sDA)area in range.

  • Annual Sunlight Exposure (ASE) area in range.

  • Useful Daylight Illuminance (UDI) area in range.

  • Max Illuminance in Area (lux)

  • Average Daylight Factor (DF) (%)

  • Uniformity ratio

Our concern was that aerogel glazing would lower the sDA ratio for the area, and it may not comply with LEED v4 guidelines (Giarma et al., 2017), which advocate using daylight instead of electric lighting in spaces with at least 55% sDA. Table 4 demonstrates that the sDA area factor of the four orientation scenarios exceeded the minimal criteria (300 Lux) for the 50% minimum of the classroom space.

Table 4 The annual daylight simulation results for the simulated classroom
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The sDA percentage for Sc2 and Sc4 (east and west orientations, respectively) exceeded 80% of the total area, indicating that these orientations provide optimal lighting for occupants. While Sc3 (south orientation) surpassed its rivals by more than 90% in terms of sDA. The north orientation of the remaining orientation Sc1 is within the acceptable sDA range (69.5%). SC3 generally achieved the highest points in sDA after the BC, in addition, it was the second lowest value in the ASE test. That means a good amount of lighting with glare less than the BC.

According to Table 4, the ASE values fell for most orientations in all scenarios except Sc1. This implies that the glare effect is reduced in Sc2, 3, and 4, but never in Sc1, which has the same orientation as BC. In addition, the east orientation of Sc2 had the lowest ASE value (51.4%).

Sc1 (north direction) had a UDI factor of 89.6%, which was higher than BC (87.89%) and Sc4 (85.75%). Sc3 (south direction) has the lowest orientation rate at 78.37%. The northern orientation typically receives the least amount of direct sunlight throughout the day, which reduces the potential for glare and excessive heat gain. However, it still receives a relatively high amount of indirect sunlight, which provides ample daylight and illuminance. This indirect sunlight is typically more evenly distributed throughout the day, which results in a more consistent and balanced lighting environment. Therefore, buildings with a northern orientation tend to have a higher UDI compared to other orientations.

The DF is a measure of the amount of natural light that enters a building and is typically expressed as a percentage of the external illuminance. A DF of 2% is generally considered to be the minimum acceptable level of natural light for indoor spaces. It can be shown from Table 5 that all orientations of the four scenarios succeeded in exceeding the minimum standards for DF (BRE, 2016; Giarma et al., 2017), which states that at least 80% of the net floor area in occupied spaces must have enough DF (2%). This means that a significant amount of natural light could enter the building during daylight hours. Results also mention that maximum DF values were measured to be 14.90% higher in the north and south orientations of Sc1 and Sc3. This suggests that buildings with a north and south orientation may be able to receive more natural light than those with an east and west orientation. The DF value was also found to be 13.49% in the westward direction of Sc4, indicating that west-facing windows may receive significant natural light during certain times of the day. Overall, the study found that the minimum value of the DF was consistently above 2.6% across all investigated cases. This indicates that the buildings were able to receive a sufficient amount of natural light during daylight hours. However, it is worth noting that the optimal DF value may vary depending on the specific requirements of the indoor space, such as the type of activity being performed, the desired level of visual comfort, and the type of lighting system being used.

Table 5 Illuminance simulation results for the simulated classroom
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Additionally, the maximum illuminance, as shown in Table 5 and the diagrams in Fig. 9, is higher for Sc1 and Sc3 than the illuminance in the other directions, while Sc4 achieves the lowest maximum illuminance value. In terms of minimum illuminance values, Sc4 achieves the highest value compared to other investigated scenarios.

Fig. 9
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DF and illuminance diagram for the investigated zone in terms of building orientations scenarios

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4.2 Window heat gain and thermal comfort evaluation

The investigation of how much heat is gained through the building's windows constitutes the second part. The findings from the simulations were used to plot the average monthly heat gain for each of the several scenarios that were investigated. Several classroom orientation scenarios have been taken into consideration (Sc1, Sc2, Sc3, Sc4). To evaluate the impact that aerogel glazing has on the building's energy performance, the heat gain was measured coming in via the building's exterior and interior windows together in a variety of building orientations. The results depicted in Figs. 10, 11, and 12 indicate that the implementation of aerogel glazing in a south-facing position resulted in a decrease in the amount of heat gained through the building's external windows throughout the Sc3 scenario. Specifically, this option resulted in a reduction of approximately 25% in annual heat gain from the exterior windows as well as a 73% decrease in solar heat gain from the interior windows. These outcomes were achieved in conjunction, resulting in an overall 26.88% reduction in windows (both exterior and interior) during the Sc3 scenario. The observed phenomenon can be attributed to the increased thermal load of the building, which is caused by the solar radiation and heat gain on the southern facade, which is more exposed to solar radiation.

Fig. 10
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Solar heat gains for interior windows

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Fig. 11
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Solar heat gains for exterior windows

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Fig. 12
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Annual solar heat gains saving ratio

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Approximately a 6% reduction in solar heat gain could be achieved through the utilization of exterior windows at Sc1. Additionally, interior windows contribute 48% of the total annual heat gain. Sc2 and Sc4 exhibit the highest annual heat gain during hot period from April to October, despite their greater capacity to warm the environment in comparison to Sc1, Sc3, and BC during winter. While Sc2 and Sc4 failed to reduce the amount of heat coming in through the exterior and interior windows during the summer, Sc1 and Sc3 were effective. This is because the southern facade receives more direct sunlight throughout the day than the northern facade. This means that the southern facade is more exposed to solar radiation and heat gain, which can increase the thermal load of the building. Aerogel glazing has a high insulation value and can reduce the amount of heat transferred through windows while still allowing sufficient natural light into the building. Therefore, applying aerogel glazing on the southern facade can significantly reduce heat gain and improve energy efficiency during the warmer months. In contrast, the northern facade is typically shaded from direct sunlight, and therefore, receives less solar radiation and heat gain. Figure 10 and Fig. 11 depict the effects of aerogel glazing windows on the solar heat gain through the interior and exterior windows respectively.

As a consequence of the thermal gain coefficient of the windows, Sc3 facing south will achieve the greatest cooling savings ratio less than BC, especially during the summer months, improving the building's energy efficiency and making it the most promising. It also saves more energy than the BC during the winter months. These findings are consistent with previous research indicating that the main glass surface should face south for maximum energy savings (Ayman Ragab Mahmoud, 2022b; Shaviv, 1981).

Winter heating savings ratios are higher for Sc2 and Sc4, however, Fig. 12 shows that all scenarios and BC have a negative impact on annual heat gain. These findings indicate that building orientations facing east, and west exhibit lower efficiency compared to other orientations. The observed outcome concerning the western orientation can be attributed to the extended period of direct sunlight exposure and a lowered angle of solar altitude during the afternoon hours. The outcome was a significant influx of thermal energy via the west-facing exterior windows and east-facing interior windows. Furthermore, the utilization of aerogel glazing in school buildings exhibits low thermal conductivity, thereby enabling it to preserve warmth throughout the night without dissipation. Consequently, a significant amount of internal heat remains for an extended duration in all orientations, especially in buildings oriented towards the east, owing to the additional heat generated by direct radiation on this facade during the first few hours of the day.

This study was expanded to investigate the impact of using aerogel glazing windows on the internal thermal comfort of classroom buildings with four main orientations. The study utilized monthly average predicted mean vote (PMV) as a metric to evaluate thermal comfort (Cheung et al., 2019). PMV is a widely used statistic in thermal environmental modeling, design, assessment, and management that incorporates parameters such as air temperature, humidity, clothing insulation, and metabolic rate. PMV results were obtained from Design Builder simulation software in accordance with ASHRAE standards (Ragab et al., 2023). The study findings, as depicted in Fig. 13, demonstrate a potential improvement in PMV values for the proposed scenarios compared to the baseline BC scenario. Specifically, the results indicate that three months fall within the acceptable range of PMV between − 0.5 and 0.5 for all investigated scenarios. Sc2 and Sc4 exhibited high PMV values exceeding 1.3 for 6 months, whereas sc1 and Sc3, representing the north and south directions, demonstrated acceptable thermal performance compared to Sc4. These results are consistent with the heat gain characteristics of the investigated classroom building orientations. The study also found that the use of aerogel glazing windows resulted in a 14.66 and 19.68% improvement in PMV values for Sc1 and Sc3, respectively, during the hottest month.

Fig. 13
figure 13

Monthly thermal comfort values using the PMV index

Full size image

5 Conclusion

This study evaluates the thermal and illumination performance of aerogel glazing as a promising building insulation technology for classrooms located in hot regions. The investigation involves replacing conventional windows with aerogel glazing in four different building orientations and analyzing parameters such as heat gain, average daylight factor (DF), spatial daytime autonomy (sDA), and uniformity ratio using Design Builder software. The key findings of this study could be summarized as follows:

  • All building orientations experienced lighting performance improvements with aerogel glazing.

  • The northern orientation (Sc1) is the most recommended orientation, as it could reduce solar heat gain by 7.46% and has an average daylight factor (DF) of 2.65%.

  • The northern orientation (Sc1) has the highest useful daylight illuminance (UDI) of 89.6% and spatial daytime autonomy (sDA) of 69.52%, which is compliant with the standards for LEED V4, i.e., ≥ 55% sDA.

The results of this investigation could guide architects and building engineers in selecting passive design strategies, such as aerogel glazing, for classroom buildings in hot desert climates. The findings could also have implications for energy-efficient building design. Further future studies could be conducted to explore the economic feasibility of implementing aerogel glazing solutions in classroom buildings and the potential impacts on the building life cycle. Therefore, the study concludes that aerogel glazing is an effective building insulation technology that can balance classroom window-specific heat gain with daylight in hot locations.