Contribution of advanced windows and façades to buildings decarbonization: A comprehensive review

of advanced


INTRODUCTION
Windows and façades are important components of a building that provide natural lighting and ventilation and establish necessary contact with the external environment necessary for healthy indoor ambient, but they also permit entry of undesirable solar heat in hot seasons and permit heat escape from the indoor ambient in cold seasons, which increase the building energy demands to cope with these inefficiencies and augment its contribution to offensive emissions.This scenario led to tremendous increase in research and developments of windows and façades to cope with their deficiencies and increase their capabilities to handle the ever-changing ambient conditions to keep the indoor ambient comfort and healthy.
Windows and façades are glazed areas whose U-values are high in comparison to other components of the building.Figure 1 shows the U-values of some of the commercially available glazing systems (Akram et al., 2023).Irrespective of this deficiency, large, glazed areas are widely used in modern and contemporary architecture.Several studies were dedicated to improving the thermal performance of glazed windows and façades by increasing the thermal resistance of the glazed system by using phase change material (PCM), inert gases and vacuum glazing, etc., and using smart windows to better control indoor heat and luminance.Smart technologies include active dynamic glazing, which modulates the entering amount of visible light, reduce energy demands, and enhance thermal and visual comfort (Casin, 2018).
Figure 2 shows the classification of established glazing technologies (Michael et al., 2023).
In regions of moderate winter and summer single glass windows are still in use mainly because of the cost ability to effectively reduce entering solar radiation, luminance, and glare besides promoting cooling energy savings.Figure 3 shows the energy flow for a single-pane window glass (Chow et al., 2010).
Low-e coatings are almost fully developed and widely applied in many applications.Investigations on electro thermal coatings to convert electricity to heat by the Joule effect are ongoing but limited by the need of power supply, while photo thermal coatings were investigated to improve the glazing performance by absorbing ultraviolet radiation (UV) and infrared radiation (IR).Figure 4 shows the mechanisms of heat transfer and daylight through a double-glazing unit (Mohammad & Ghosh, 2023).
Further improvements were incorporated in double glass windows including forced and induced airflow in the spacing separating the glass sheets, Figure 5 (Chow et al., 2010).
Another improvement was incorporated using liquid flow, especially water to avoid heat flow to the interior ambience.(Chow et al., 2010) Figure 6 shows water cooled double glass window (Chow et al., 2010).The reported results indicated a big reduction of heat gain and enhancement of indoor comfort.
Aerogel is a silica-based, porous material composed of 4.00% silica and 96.00% air prevents convection heat transfer, has a thermal conductivity of about 0.013 W/mK, but allows light transmission.
Studies showed that the inclusion of PCM in glazed units can decrease the building energy demands, enhance the indoor comfort and delay and attenuate temperature but has the drawback of possible leakage and overheating.
Vacuum glazing is widely used in very cold countries because of low heat loss and high visible transmittance.Heat radiation is the only mode of heat transfer through these windows.Figure 7 shows a general layout of the vacuum window.
The above highlights show that the glazed windows and façades received were and still are subject to intense research activities and dedicated developments to make them efficient, safe, affordable, and astatically attractive.
The main objective of this work is to present an updated review of the state of research, development and applications of windows and façades in the building sector.The review covers natural illumination and visual comfort, natural ventilation, thermal comfort, and the influence of window area on natural illumination and visual and thermal comfort.The review also covers the performance enhancements of windows and façades including reflective solar films, vacuum glazing, double glass windows with filling materials such as windows with water flow, window with PCM, window with stagnant inert gas filling, ventilated windows (VWs) and façades, windows with aerogel and smart glazing.
Since commercial programs and computational codes are handy tools for simulation and thermal performance calculations of components and buildings, a section is dedicated to these simulations codes and relevant publications that used these tools in the analysis and simulations.The conclusion section contains the most relevant conclusion of the present review as well as future trends in research and developments in windows and façades.
The novelty of this paper is that besides energy reduction and avoidance of emissions usually treated in most papers and reviews, this paper relates explicitly natural illumination, natural ventilation and window and façade geometry to thermal and visual comfort, stress reduction, and overall health and well-being of the occupants of the building.
The main contributions of this paper to the area include addressing explicitly natural illumination, natural ventilation and their positive impact on attention restoration, stress reduction, and overall health and well-being.As passive strategies they can significantly help increase the liveliness, performance, and visual comfort of the residents, besides reducing the overall electrical energy consumption of a building.The paper also highlights the importance of the appropriate configuration of windows to improve visual and thermal comfort, reducing glare, distributing light, and controlling solar energy gain.Also, the paper provides an updated review on the stage of research, development and applications of windows and façades in the building sector.This review is organized in five sections, as below: 1. Introduction: The actual scenario in window and façades technologies are presented and commented on.
The section ends by presenting the objectives of the review, contribution of the review to the area and finally the basic contents of the review.

Illumination, visual comfort, thermal comfort, & natural ventilation:
In this section natural illumination, visual comfort, natural ventilation, thermal comfort are treated and studies reporting scientific research and results are presented.Impact of Figure 6.Schematic illustration of solar absorbing glazing (Chow et al., 2010) Figure 7. Schematic of double evacuated glazing unit (Michael et al., 2023) the window to wall ratio on visual and thermal comfort are presented.

Performance enhancements of windows & façades:
In this section windows with water flow, window with PCM, window with stagnant inert gas filling, VWs and façades, windows with aerogel and smart glazing.

Commercial programs & codes:
In this section most used commercial codes for simulating the thermal performance of buildings and components are presented and commented on.
5. Conclusions, future trends in research, & developments: In this section the main findings of the review are reported and the gaps for future research and developments are highlighted.

ILLUMINATION, VISUAL COMFORT, THERMAL COMFORT, & NATURAL VENTILATION
Windows provides the contact between the internal space of a building and external surroundings.Internal natural illumination, natural ventilation, and reduction of heat transfer to and from the building can be achieved partially by traditional glazed windows under severe and sudden variations.Hence it is necessary to improve windows and façades to provide quick and adequate response to the momentary changes and to ensure continuously comfortable and healthy ambience for the occupants.

Natural Illumination & Visual Comfort
Daylight is a free energy and cost-effective alternative to artificial lighting and has favorable effects on comfort, health, well-being, and productivity.A fair amount of research work was done to evaluate natural daylight effects on occupants, and work efficiency.Yao (2014) conducted a study to investigate the effects of movable solar shades on energy consumption, indoor thermal and visual comfort.Results showed that movable solar shades provided 30.87% energy savings, improved thermal comfort by 21.00% and visual comfort by 19.90%.Carlucci et al. (2015) provided a review of indices for assessing visual comfort and presented recommendations and suggestions for improvement.Nasrollahi and Shokri (2016) reviewed the basic concepts of daylighting, and the architectural parameters in an urban context.The results indicate that these factors are of high significance.Hoffmann et al. (2016) investigated coplanar shades with different geometry and material.Energy use increased substantially when an additional interior shade was used for glare control.Acosta et al. (2016) conducted a study to investigate the impact of window design on energy savings for lighting and visual comfort in residential spaces.Silva et al. (2016) provided a literature review on the use of PCMs in glazing and shading solutions focused on PCM technologies developed for translucent and transparent building envelopes to improve energy efficiency.Costanzo et al. (2016) investigated the application of thermo-chromic windows to an existing office building for energy savings, daylighting, and thermal comfort.The results showed an overall energy savings from around 5.00% for cold climates to around 20.00% in warm climates.Vlachokostas and Madamopoulos (2017) conducted a study to investigate the thermal performance of a liquid filled prismatic louver façade and concluded that the liquid filled louver system enhanced the indoor natural light by 9.00%, reduced the glare by 20.00% and enhanced absorption of solar radiation by 52.40%.Shen and Tzempelikos (2017) presented details of a simplified model-based shading control.The method was generalized and can be applied to any shading/glazing properties, location, orientation, and room configuration.Makaremi et al. (2018) examined the influence of reflectance of surfaces and lighting strategies on energy consumption and visual comfort.The results show the possibility of electrical energy savings up to 45.00% by increasing surface reflectance properties.Eisazadeh et al. (2019) investigated the influence of glazing characteristics and shading device configuration and concluded that they severely impact energy cost, daylighting, and visual comfort.Gutiérrez et al. (2019) examined the design and daylight performance of a new louver screen for office buildings and concluded that it can provide adequate daylight levels and visual comfort.Dogan and Barch (2019) provided a review on daylighting metrics for residential architecture and introduced a concept for a new climate-based, annual evaluation framework.Zhang et al. (2019) presented the results of a human experiment of the effects of glazing types (color and transmittance) on participants' alertness, mood and working performance.For a high Circadian Stimulus CS level (≥0.3), glazing color and transmittance did not significantly affect human alertness.A low CS level (<0.3) caused significant negative mood to occupants.Day et al. (2019) reported similar results from a large-scale study in the US.Ke et al. (2019) provided a review on recent progress in smart windows including functional materials, device design and performance.Kaasalainen et al. (2020) investigated the direct and combined energy impacts of window related architectural design and concluded that cooling can be a significant factor and that energy efficiency should always be evaluated.Onubogu et al. (2021) presented a review on the existing technologies of daylighting systems covering both passive and active daylighting systems, besides providing recommendation for future research.Khaled and Berardi (2021) presented a review on coating technologies for glazing applications and commented that both static and dynamic technologies contribute to enhance optical and thermal performances.Foroughi et al. (2021) developed an optimization model to identify the optimum window design parameters.The results show that selecting optimum window dimensions and locations can reduce the total building energy consumption.Feng et al. (2021) reviewed and analyzed window design studies to achieve high performance, presented simulation-based optimization methods, and identified potential challenges and future research trends.Eleanor et al. (2022) identified critical needs in research, tools, and technologies to enable more effective use of daylight.Advanced window technologies and integrated design can enable achieving health, comfort, and net zero energy goals.Lee et al. (2022) proposed a technique for improving the efficiency of the light shelf using a solar module and evaluated experimentally its performance.The results showed that the energy consumption of the proposed system is reduced by 3.50%-32.70%.Li et al. (2022) provided an overview of research advances in optical transmittance, thermal resistance, and thermal inertia along with photothermal transmittance in glazing envelopes, with a special focus on the integration of PCMs.Li and Tang (2024) provided a review on PCM window for thermal and light dynamic regulation and identified the potential of these concepts to improve light and thermal regulation.Table 1 presents a summary of some studies on natural illumination and visual comfort in buildings.

Comments
Visual connection to nature has been demonstrated to have a positive impact on attention restoration, stress reduction, and overall health and well-being.Inside buildings, windows are the primary means of providing a connection to the outdoors, and nature views may have similar effects on the occupants.
Glazed envelopes suffer from the defects of high solar transmittance, poor thermal insulation, and low thermal inertia, while traditional windows, as the major source of daylight, have a common problem, which is uneven distribution of daylight in the room besides possible glaring.Day lighting is a passive strategy, which is significant in increasing the liveliness, performance, and visual comfort of the residents.It helps to reduce the overall electrical energy consumption of a building.Since day lighting is essential for the general comfort of the occupants and considering that artificial lighting accounts for a considerable part of electrical energy consumption in buildings, there is a need to design appropriate lighting scenarios for buildings in a way to reduce energy while meeting visual comfort requirements.

Natural Ventilation
Natural ventilation can provide occupants with thermal comfort and a healthy indoor environment, besides being an effective strategy for reducing energy consumption especially for commercial and office buildings.Aflaki et al. (2014) presented a review on ventilation techniques.Results showed that building orientation and apertures size are effective to increase indoor air ratio.Zhang et al. (2014) presented a review on diffuse ceiling ventilation (DCV) and examined thermal comfort, air quality, pressure drop as well as radiant cooling potential.Yu et al. (2015) provided a review on cooling and ventilation in office buildings including natural ventilation, building thermal mass activation, DCV and proposed a system combining the three technologies.Kasima et al. (2016) conducted a computational fluid dynamics (CFD) study to investigate the effect of different opening positions on wind-induced ventilation performance.Salcido et al. (2016) presented a review to analyze the use of mixed mode ventilation systems in office buildings.The authors showed the progress made and future challenges for use in office buildings.Nomura and Hiyama (2017) provided a review on natural ventilation performance of office buildings in Japan and indicated that natural ventilation performance depends considerably on the building design.Omrani et al. (2017) conducted a study to analyze the effects of natural ventilation mode on thermal comfort.Results highlighted better performance of cross ventilation over single-sided ventilation.Elshafei et al. (2017) investigated the effects of window parameters on indoor natural ventilation in a residential building.Modifications in window size, window placement, and shades improved the air temperature and the air velocity.Palme et al. (2017) presented natural ventilation as a mitigation strategy to reduce overheating in buildings.The overheating risk of a small house is evaluated with and without considering urban heat island effect.Results show that an important portion of the indoor heat can be removed.2019) evaluated the possibility of using natural ventilation in school buildings and concluded that the strategy was viable, and that single-side ventilation and cross-ventilation can improve cooling and air quality in school buildings.Chen et al. (2019) examined various types of control including spontaneous, manual, and the fully automatic window/heating, ventilation, and air-conditioning control system and concluded that the fully automatic system was more adequate and showed energy savings of 17.00-80.00%with zero discomfort.Guo et al. (2019) developed an approach integrating sensitivity and parametric simulation analysis and showed that the climatic conditions and night ventilation have strong effect.Wu et al. (2019) presented a review on DCV, highlighted the research findings, and proposed simplified modeling methods for DCV system design tool.Yang et al. (2019) reviewed advanced air distribution methods, limitations and solutions and analyzed measuring and evaluating methods for ventilation and air distribution.Solgi et al. (2019) studied PCM behavior when used with night ventilation and showed that insulated envelopes increase night ventilation efficiency and stabilize PCM transition temperature.Rahnama et al. (2020) evaluated the cooling capacity of DCV system and indicated that the highest cooling capacity is achievable with evenly distributed heat load in the room and active diffuse panels in the ceiling.Hu et al. (2020) experimentally investigated the performance of a phase change material enhanced ventilated window (PCMEVW) and found that the room inlet air temperature was 1.4 °C lower compared to the normal VW and the average energy saving was 1.6 MJ/day.Hu et al. (2020) proposed a PCMEVW system for ventilation preheating/precooling purposes and showed that the proposed system greatly decreases the energy demands for summer and winter applications.Piselli et al. (2020) investigated possible improvements from using natural ventilation on PCM performance in cooling application and showed that PCM inclusion in the building envelope resulted in significant cooling savings.Guo et al. (2020) evaluated the resulting thermal performance from coupling a cool roof with night ventilation and concluded this can result in 27.00% savings in the annual cooling energy.
Hati (2021) presented a review on ventilation systems, variable speed drive and discussed various energy efficiency strategies and artificial intelligence-based models.Zhang et al. (2021) presented a review of combined natural ventilation and commented that the coupling between different natural ventilation systems still requires more future research.Zhong et al. (2022) reviewed research on single-sided natural ventilation and commented that in future investigations, different methodologies should be coupled.Maghrabie et al. (2022) presented a review of natural ventilation of buildings based on solar chimney (SC) and concluded that combined SCbased cooling/heating energy systems can be an effective strategy for energy efficient buildings.Sadeghian et al. (2022) investigated the role of design parameters on the performance of DCV systems and concluded that dispersed configuration had the highest draft rate of 14.00%.Zaniboni et al. (2022) presented a review on natural and mechanical ventilation concepts and concluded that thermo-hygrometric comfort is an important parameter.Mateus et al. (2023) reviewed the methodologies applied to study the natural ventilation of large air masses and techniques used for the validation of CFD models and commented that greater agreements were found in the models´ formulations.Table 2 presents a summary of some studies on natural ventilation in buildings.

Comments
Buildings consume more than 40.00% of global energy use and ventilation is one of the largest sources of energy consumption.Also, the high cost of energy has intensified research interest in passive energy saving strategies for buildings.Night ventilation has been shown to reduce the energy demand for cooling buildings as well as significantly improve thermal comfort.DCV also has great energy saving potential and can handle high cooling loads without inducing thermal discomfort.PCMs have a big potential to be used as passive strategy for improving energy efficiency and occupants' thermal comfort in buildings.However, their performance still needs to be enhanced to have them effectively used.

Thermal Comfort
ASHRAE (2017) defines thermal comfort as "that condition of mind that expresses satisfaction with the thermal environment".The six factors considered in ASHRAE are temperature, thermal radiation, humidity, airspeed, activity level (metabolic rate), and occupant clothing (degree of insulation).Saadatjoo et al. (2019) investigated the effect of porosity distribution pattern on natural ventilation and concluded that porosity could be changed to fulfill most of the building environmental requirements.Hawila et al. (2019) conducted a study to quantify the interactions and optimize building glass façades.The results indicated that the optimized design enhanced thermal comfort and energy-savings.Krstic-Furundzic et al. (2019) presented the estimation of energy performance of different hypothetical models of façade design.Results showed the effects of the various alternatives of shadings on the reduction of environmental pollution and energy demands.Fahmy et al. (2020) simulated an educational building and concluded that shading the roofs and southern façade of building envelope were the most effective.
Abd El-Rahman et al. (2020) commented that a significant part of the building energy is consumed for achieving thermal and optical comfort.The important parameters include building shape, orientation and the window to wall ratio and need to be adequately combined to achieve thermal comfort and energy efficiency.Ko et al. (2020) investigated the influence of a window on the thermal and emotional aspects of the occupants and concluded this strategy is important for their comfort and concentration.Yang et al. (2021) performed sensitivity analysis on the correlations between indoor thermal comfort, energy consumption and design parameters of BIPV/T-double skin façades (DSFs) and concluded that solar heat gain coefficient (SHGC) of the BIPV/T-DSF's external window significantly affects indoor thermal comfort and energy consumption.Bahri et al. (2022) presented a review on tools and techniques used to improve thermal comfort in a double skin façade for residential buildings.Results suggest that simulation is the most accurate in comparison with other methodologies.Shahrzad and Umberto (2022) investigated the optimization of a novel opaque dynamic façade and concluded that the thermal resistance in the façade could be varied as a function of airflow in the façade.Jiang et al. (2022) conducted a study to explore the influence of natural views and daylight on health, thermal perception and energy savings and concluded that visual window improved the occupants' tolerance to the thermal environment.Table 3 presents a summary of some studies on thermal comfort in buildings.

Comments
Natural illumination and natural ventilation can reduce energy demands and improve the thermal comfort.One can also observe the recent concern about visual comfort especially in working areas and impacts human comfort and efficiency.The important parameters of buildings such as building shape, orientation, and the window to wall ratio, need to be adequately combined to achieve thermal comfort and energy efficiency.

Effect of Window to Wall Ratio
Window-wall ratio (WWR) is an important building design parameter that has significant effects on the fashionable appearance of the building as well as on the internal thermal, visual, and acoustic comforts.ASHRAE (2017) has established that WWR of 0.24 is considered ideal for indoor daylight and natural ventilation.Table 4 shows the summary of WWR values and their effects on building performance.Also, the ratio of glass area to floor area is relevant in building design.A value in the range of 20.00-30.00% of the floor area is considered adequate.
Goia ( 2016) conducted a study to seek for the optimal WWR in European climates and concluded that these values occur in the range (0.30<WWR<0.45)and that the total energy use may increase by 5.00-25.00%for the worst WWR configuration.Carlos and Corvacho (2017) assessed the influence of thermal performance and ventilation on human comfort and found that it was possible to minimize the air changes without compromising air quality and enhance thermal performance and comfort.Fang and Cho (2019) proposed a building performance optimization process that can evaluate daylight and energy performance of building design.Troup et al. (2019) conducted a study to evaluate the effects of WWR in actual office buildings and the results showed increased median total energy use intensity with increasing fenestration due to increasing cooling loads.Alwetaishi (2019) assessed the influence of WWR in various microclimate regions and suggested WWR of 10% in hot and dry and hot and humid climates.Ashrafian and Moazzen (2019) focused their study on the impact of different transparency ratios and window combinations on occupants' comfort and the energy demands of a classroom.The results indicated that a WWR of 50.00% can decrease artificial lighting by 15.00% and ensure indoor comfort.Phuong et al. (2019) proposed an integrated approach to determine window to floor ratio to provide the target Daylight Factor, energy efficiency for the tropical climate.The results showed that the recommended window to floor ratio is 15.20% to 18.50% for Daylight Factor of 1.35%.Ozel and Ozel (2020) investigated the effect of WWR on the thermal performance of different wall materials and concluded that for the case of bare wall the wall material affected the glazed area, whereas in the case of the insulated wall, the effect was marginal.Shao et al. (2020) used the Design builder software to determine the optimum value for WWR for rural house design considering other parameters such as building orientation and thermal performance among others.Their recommendation is that the effect of various factors should be considered in the design to determine the WWR of rural houses.Sayadi et al. (2021) investigated different cases in various climatic conditions to obtain the optimal WWR based on the minimum energy use of a building during a complete year.Boutet and Hernández (2021) focused his study on determining an optimized design proposal that achieves thermal and daylighting habitability conditions.The results showed significant thermal and daylighting behavior enhancements with important reductions in the cooling loads.Saber (2021) investigated the effects of the WWR and recycled panels on energy consumption.All panel cases showed that the least energy consumption with lighting included occurred for WWR of 45.00-55.00%.They recommended the use of recycled materials-based panels for the envelope with WWR less than 50%.
ASHRAE has established that window to wall ratio of 0.24 is considered ideal to allow optimum indoor daylight and natural ventilation, which reduces energy costs.The larger the window, the more heat or light will penetrate the room, which causes overheating and glare.An appropriate configuration of windows can effectively improve visual and thermal comfort by reducing glare, distributing light, and controlling solar energy gain.

PERFORMANCE ENHANCEMENTS OF WINDOWS & FAÇADES
Since glazed windows and façades are the weakest elements for heat transfer between the building and exterior ambient, many studies and developments were devoted to enhancing their thermal and optical characteristics to make them not only more efficient, but also dynamic enough to cope with ever-changing climatic conditions.

Reflective Solar Films
Sustainability and energy conservation are essential for buildings management together with satisfactory indoor environment and comfort.Windows and translucent elements are responsible for maintaining comfortable indoor thermal and visual environment (Figure 8).Many window geometries and attachments were developed to improve the performance of windows.Among these possible solutions solar control films (SCFs) outstanding as a cheap and effective solution that controls light and heat penetration, filters out UV and IR, reduces glare and minimizes the use of electric lighting.Figure 9 shows possible arrangements for windows with reflective film for summer and winter operations.Yu and Su (2015) presented a review of methods for indoor daylight assessment, and methods used for predicting energy savings from daylight to make available information for sustainable design and energy management.Hee et al. (2015) reviewed the impacts of window glazing on the energy and daylight performance and the optimization techniques used in choosing the emerging glazing technologies.Moretti and Belloni (2015), in their study to evaluate the effects of SCFs on the thermal, and daylight performance for moderate climate, reported a reduction of 60.00% in the incoming radiation, about two to three o C of the indoor temperature, increase of artificial lighting, decrease of the cooling demand by 29.00% while the heating demand increased by 15.00%.Li et al. (2015) investigated the effect of solar films on building energy consumption and concluded that solar films have good potential for energy saving.Rezaei et al. (2017) presented a review on various types of glass coatings and glazing systems and evaluated the potential of using different window technologies for hot, cool, and temperate climates.Xamán et al. (2017) presented the results of thermal evaluation of a room fitted with a double glass window and SCF.The results showed a reduction of 62.00% of the energy gains for hot climate, and insignificant effect on the indoor temperature in the cold climate.Teixeira et al. (2020) conducted a study to investigate the thermal and visual comfort performance of different types of SCFs and concluded that the highly reflective SCF has the highest thermal and optical performance, while the spectrally selective film showed an annual reduction of 38% of energy consumption.Abundiz-Cisneros et al. ( 2020) investigated alternative materials to substitute silver (Ag) in a low-e filter coated and concluded that an aluminum-based filter has a good cost-benefit performance.2019) evaluated the influence of SCF on natural luminance of a public hospital building in a Mediterranean climate and commented a reduction of 12.20% of the electric consumption of the artificial lighting system, Figure 10.The room with solar film showed a reduction of 3.30% in energy cooling demand.In contrast, heating energy demand increased by 6.50% in comparison with the room with no solar film.
Daylight luminance levels and their spatial distribution are important design parameters to achieve indoor visual comfort and sustainability in buildings.While a proper day lighting scheme increases the efficiency of the building, the excessive use of glazed surfaces can contribute to thermal and visual discomfort.Pereira et al. (2020) analyzed the impact of single glazing with different SCFs on the indoor luminance and its distribution on horizontal plane and concluded that all SCFs reduced the indoor luminance.In another work, Pereira et al. (2022) presented a review of the performance of SCFs applied to glazing systems and identified and discussed interactions of glass-film systems, climatic conditions on energy savings and comfort.

Vacuum Glazing
Vacuum insulated glazing is an effective technology suitable for severe thermal performance requirements, enhanced thermal efficiency, sound insulation, and can achieve U-values as low as 0.7 W/m 2 K. Table 5 shows comparison of vacuum-glazing windows commercially available (Aguilar-Santana et al., 2020).Fang et al. (2006) developed a technique to determine the heat transfer coefficient of the evacuated gap and the comparison between the measured and predicted temperature profiles showed good agreement.Jelle et al. (2012) conducted a market review of the best performing fenestration products including electrochromic vacuum glazing and evacuated aerogel glazing as potential candidates for enhancing thermal and daylight performance of windows.Ghosh et al. (2017) analyzed the variation of vacuum glazing transmittance with clearness index and showed that clearness index below 0.50 offers single value of transmittance.Alam et al. (2017) evaluated savings in space heating due to the installation of fumed silica and glass fiber and vacuum insulation panels (VIPs).The results show that VIP insulation reduced the annual space heating, energy  2019) reported on recent advances in the utilization of vacuum glass in contemporary window construction and highlighted the reduction of the weight of components, reduction of conduction and convection heat transfer are among the benefits of vacuum glazing.Aguilar-Santana et al. (2020) reviewed the performance of available window technologies with a special focus on the U-value.The authors concluded that further research is needed to develop window technologies with multiple functional attributions and characteristics such as high insulating properties, generation of energy, etc. Fang et al. (2020) analyzed the solar thermal performance of two configurations of air-vacuum layered triple glazed window and concluded that allocation of the vacuum gap near the indoor region reduces cooling load and enhances energy efficiency.Aguilar-Santana et al. (2020) experimentally determined the U-value of active insulated windows and compared them with other windows and concluded that vacuum glazing achieved a U-value 78.00% lower than traditional single glazing window units.Uddin et al. (2023) presented a review on PV combined with vacuum glazing, actual progress, and prospects for the design of energy-efficient buildings.

Double Glass Windows with Filling Materials
To reduce the heat gain or loss from double glazed windows and façades and improve the thermal performance adequate filling materials are inserted in the gap to provide the required effect.Varieties of gases and liquids were tried, tested, and evaluated such as stagnant inert gases, flowing air and flowing liquids, aerogels and PCM.Table 6 provides a comparison of window gap fillers.

Windows with water flow
The use of the concept of fluid flow in the double glass window can enhance its thermal performance while the heat gained by the flowing liquid can be used in other applications.Triple and multiple glass panes windows may integrate with the liquid flow some additional functional component such as the air layer, or vacuum gap to enhance the system thermal performance.Figure 11 shows the details of a water-flow double window.
An experimental investigation of a glazed façade had been conducted by Qahtan et al. (2011) utilizing a water film and concluded that the flowing water film on the glazed façade lowered the glazing surface temperature by 7.2 to 14.0 o C and decreased the indoor temperature by 2.2 to 4.1 o C. Chow et al. (2011aChow et al. ( , 2011b) ) connected the cavity of a double pane window to a water-flow circuit to absorb solar heat, reduce room heat gain, enhance thermal performance, and concluded that the proposed system is adequate for warm regions.Adu (2015) characterized the performance of water as gap-filler for double-glazing units and found that the thermal transmittance and SHGC were better than those of the traditional units, maintained lower indoor temperature swings, reduced the incident solar radiation, and maintained  Romero and Hernández (2017) used a net energy balance radiation model to solve the spectral problem and determine the wavelength-averaged absorptances of the different layers of a multilayer water flow glazing system to be able to precisely simulate the performance of this type of windows.Lyu et al. (2019) proposed a triple glazing vacuum-water flow window and showed a heat reduction of about 44.00% due to the combined effects of vacuum gap and water flow.In another work Lyu et al. (2019) suggested using hot water flow to make the gap work as a heat radiator.The simulation results showed that the proposed system was viable.Li et al. (2019) investigated water-flow window for possible use in hospital patient wards with large demand of hot water.The numerical simulations showed reduction of penetrated solar energy, better thermal indoor conditions, and hot water for general use.The water inside fluid glaze façades creates a vertical hydrostatic pressure, which must be supported by the glazing.The simulations showed that pillars can solve this pressure problem (Escoto & Hernández,2019).Chow and Liu (2020) reported that the results of applying the dynamic simulation model of water-filled double-glazing indicated a thermal efficiency in the range 26.00-51.00%.Double-circulation water-flow window is composed of four layers of glass panes and two layers of flowing water, which utilizes solar energy for domestic hot water and regulates heat gain through window (Li et al., 2020).Results showed that the annual solar collection efficiencies were 16.20% and 4.30% for the external and internal water circulation, respectively.
A literature review showed that fire safety and reduction of energy needs is a challenge in buildings with glass façades.Water wall system as a building façade is a possible solution that can allow achieving both objectives (Rathnayake et al., 2020).The transparency of the water wall allows daylight to enter the building and maintains good visual performance, while the water layer acts as a fire safety mechanism when needed.
In a review by Piffer et al. (2021) on windows filled with liquids with spectrally-selective and high thermal capacity properties to enhance optical and thermal performance and commented that these windows can transmit more light than when it is empty, promote solar heat gain or reduce cooling demand.Huang et al. (2021) provided a review on the application of fluids and other materials as fillers for multiglazing windows to enhance thermal performance including airflow, flowing liquids, aerogels, and PCM.They included suggestions for future research and developments.Ghosh (2023) mentioned that diffuse transmission has several advantages like offering uniform daylight and reducing glare.Diffuse transmission windows prepared with aerogel, PCMs, and polymer dispersed liquid crystal had been reviewed and their potential for future buildings applications was discussed.

Window with phase change material
The use of PCMs as gap filler enhances the thermal inertia of windows, increases thermal resistance, and filters out unwanted radiation as UV and IR besides reducing window glare.A general arrangement for including PCM in double glass window is shown in Figure 12 reproduced from (Khetib et al., 2021).
A triple-glazed window can permit filling one of the window cavities with PCM to regulate indoor daylight, while the second provides the required thermal insulation.Two possible PCM-glazing arrangements are presented in Figure13.For summer conditions PCM should be placed near the exterior side, part a in Figure 13, while for winter PCM plays a role of dynamic insulation and should be located as in part b in Figure 13.Ismail and Henriquez (2002) presented the results of a numerical and experimental study on thermally efficient windows.The results of transmittance and reflectance tests indicated large reductions in the infrared and UV while maintaining good visibility.Goia et al. (2013) proposed a prototype of a simple PCM glazing system, compared the results with those of a reference conventional double glass window and showed improvement of thermal comfort.Zhong et al. (2014) investigated the effects of the inclusion of PCM in double glass window on the building thermal performance and reported good agreement between the experimental and numerical results.Li et al. (2016) investigated a configuration of triple-pane window, where PCM was placed in the cavity near the external side.The results showed a reduction of 5.5 • C of the internal surface temperature and a decrease of 28.00% of heat gains.Wieprzkowicz and Heim (2018) investigated the thermal performance of PCM glazed unit under real climatic conditions  2018) reviewed the experimental and numerical methods used to predict the thermal and optical behavior of window including methods that permit the prediction of the combined effects on the thermal, daylight and energy behavior of buildings.Ehms et al. (2019) reviewed the numerical approaches used to formulate the solidification and melting processes and determine the heat exchange and transition velocity between the phases.Li et al. (2019) evaluated the thermal performance of glass window with silica aerogels and PCM as in Figure 14 and showed that the thermal conductivity and thickness of silica aerogel are important parameters and concluded that the concept is viable for cold climates.Yang et al. (2020) investigated two models to determine optical properties of PCM based nanofluids.The results showed that the large concentration of the nanoparticles produced big extinction and scattering coefficients of the paraffin nanofluid.Li et al. (2020) reviewed the optical and thermal performance of glazed units with PCM, presented and discussed the research methods, and indicated future works on PCM glazed units.Liu et al. (2019) developed a simplified method to analyze the thermal performance of multilayer glazing façades and showed good agreement between measurements and numerical predictions.
DSFs are considered as sustainable design elements for reducing energy consumption in buildings.However overheating problems in warm seasons have been reported in various studies (Li et al., 2019).The authors evaluated experimentally and numerically the thermal performance of an integrated PCM blind system for DSF buildings and reported that the system stabilized DSF indoor temperature.Uribe and Vera (2021) analyzed the impact of PCM glazing on buildings' energy performance and thermal and visual comfort and reported reduction of energy consumption and enhancement of the thermal and visual comfort.Khetib et al. (2021) simulated the three modes of heat transfer in an air-filled double-glazed window (DGW) and showed that the highest heat loss occurred for the vertical window.Li et al. (2022) provided a review on optical transmittance, thermal resistance, and thermal inertia in glazed envelopes with PCMs and reported lack of information on acoustic data of glazed system with PCM.Li et al. (2022) investigated a roof based on silica aerogel and PCM glazed systems.The results showed improvement of the thermal performance.Kaushik et al. (2022) examined the heat transfer characteristics of a double-glazing window system having a nano-disbanded phase changing material and indicated a decrease of the indoor glass panel temperature by 8.5 o C and enhanced energy conservation by 4.61%.Liu et al. (2022)

Window with stagnant inert gas filling
Inert gases are used as fillers for double glass windows because of their thermal resistance.It is known that double glass windows filled with Argon can reduce the conductivity of the window by 67.00% in comparison with air-filled windows.Windows filled with Krypton can reduce the overall U-value by 17.20% compared to Argon filled windows, while Xenon-filled gap showed the lowest U-values of 0.28 W/m 2 K but is more expensive to manufacture.Table 7 presents comparative values of triple-glazing heat transfer solar coefficient and transmittance for gas fillers (Aguilar-Santana et al., 2020).Jelle et al. (2012) provided a review on actual fenestration technology at the time and possible future research and development to improve the fenestration products and make them not only efficient providing thermal and visual comfort but also affordable.The review covered U-values of commercially available glazed products such as aerogel, vacuum, low-emissivity coatings, and electro chromic vacuum windows.Arici and Kan (2015) evaluated the thermal performance of double, triple, and quadruple glass windows and presented correlations for predicting the glazing U-value for the cases of air and Argon as fillers.Aguilar et al. (2015) evaluated the thermal performance of a double pane window with three types of commercial glass and recommended the use of reflective film with double pane window in warm and cold climates.Lolli and Andresen (2016) conducted a study to evaluate emissions reduction due to substitution of tripleglazing units with double glass window with argon gas and monolithic or granular aerogel and concluded that the two options were viable.Windows are essential components of buildings, which provide vision, air ventilation, passive solar gain, and daylighting, but also contribute much to the thermal loads of buildings because of their high U-values, Figure 15, (Cuce, 2014).Hence, it is essential to reduce the U-value of windows and façades to improve thermal performance of components.Baek and Kim (2019) developed a hybrid triple glazing that combines vacuum and carbon dioxide (CO2) in gaps and concluded that the performance was comparable to that of the case with Argon gas.In another study Baek and Kim (2021) the authors analyzed the insulating effect and performance of double glazing with CO2 as filler.From comparison with other gas fillers, they confirmed that glazing with CO2 gas performance was like that of Argon gas.A review on thermal insulation materials, and factors that influence the choice for building applications was provided by Imhade et al. (2022).

Ventilated windows and façades
In VWs and façades part or total ventilation air is drawn through the gap separating the pair of glass sheets to be heated by solar radiation and inserted into the building, or alternatively to be induced from the indoor ambient by solar radiation action and removed to the exterior ambient.A dual airflow arrangement can allow preheating the fresh supply air or cooling it according to the season, and hence such window arrangement is suitable for different climates.Another possible solution is using reversible VWs, as shown in Figure16 (Lago et al., 2020).
Figure 17 shows a sketch of VW integrated with PCM (Hu et al., 2020).The ventilated double glass arrangement acts as a passive heating system, where part of the heat loss is returned by the airflow, while part of the incident solar radiation heat is removed by the flowing air to the indoor ambience.Carlos et   2012) analyzed the two functions and concluded that the concept is applicable on any facade.Carlos and Corvacho (2013) studied the performance of ventilated double window for the preheating of the ventilation air and concluded that the proposed system was viable.De Gracia et al. ( 2013) provided a review on DSFs modeling including analytical and lumped models, network models, control volume, and CFD and commented on their limitations and advantages.Barbosa and Ip (2014) reviewed application of DSFs technologies to provide guidelines to optimize designs in naturally ventilated buildings for improving indoor thermal comfort.Skaff and Gosselin (2014) investigated ventilated fenestration on energy performance and their benefits.They proposed a model to determine the total heat reduction provided by ventilated glazing.Carlos and Corvacho (2015) studied numerically and experimentally the variation of SHGC of a ventilated double window with working conditions and types of glass and concluded that the results can be helpful for building energy analysis.Gloriant et al. (2015) proposed simplified models to be used in building simulation codes.Predictions from these simplified models were compared with those from CFD modeling and showed good agreement.Carlos (2017) commented that the double VW can preheat the incoming air, reduce thermal losses through windows and decrease the heating load of the building.Souza et al. (2018) investigated the efficiency of a naturally ventilated DSF and showed that DSF contributes to the decrease of the indoor temperature.Tukel et al. (2019) investigated the effect of air layer thickness, glass coating emissivity and number of panes on the thermal characteristics in glazed roof and showed that the U-value was reduced to 0.77 W/(m 2 K) and energy saving potential achieved about 71.00%.Lago et al. (2019) developed and validated a model for the ventilated double glass window with reflective solar film.The results showed that the solar reflective film can reduce the penetrating solar energy by about 64.70%.Zhang et al. (2019) investigated a triple glazed exhaust-air window (TGEW) and concluded that TGEW can reduce 25.30% and 50.10% of the annual cooling and heating loads, respectively.Choi et al. (2019) analyzed the cooling energy performance of a slim double-skin window and showed that the proposed system can reduce the cooling load and decrease the solar heat gains.Hu et al. (2020) proposed a PCMEVW system for ventilation preheating / precooling applications and showed a building energy saving increase of 62.30% and 9.40% for summer and winter, respectively.Huang et al. (2021) provided review on fluid fillers for multi glass windows covering application technologies, performance analysis methods, and evaluation of different building applications.Khosravi and Mahdavi (2021) investigated the ability of VWs to preheat the incoming ventilation air and showed that taller cavities and a smaller cavity depth can enhance the incoming air temperature.Sadko and Piotrowski (2022) reviewed the investigations of the thermal properties of window systems and glazed buildings partitions.Kumar et al. (2022) reviewed different building parameters to provide a conceptual framework for the building envelope.The proposed framework includes life cycle assessment, occupant's satisfaction, and social benefits.Preet et al. (2022) reviewed the studies on DSF systems and discussed the influence of geometric design parameters on heat transfer and fluid dynamics occurring in DSF system.Figure 18 shows the schematic diagram of DSF system used by the authors.Tao et al. (2023) proposed a new theoretical model for a naturally ventilated double-skin façade to calculate the thermal and ventilation performance under varying environmental conditions.Substantial performance improvements can be achieved by using hybrid nanoparticle-enhanced phase change material (NePCM) in DGWs but can cause local overheat and impacts negatively thermal comfort and natural lighting (Yang et al., 2023).Figure19 shows the model of DGW filled with NePCM used by the authors.

Windows with Aerogel
Glass façades and windows are responsible for thermal comfort, daylighting, natural ventilation, and a big part of the energy demands due to thermal losses.Aerogel, due to its optical transparency and low thermal conductivity if inserted in windows and façades can significantly improve their performance and reduce discomfort and glare.(Yang et al., 2023) shows a window with aerogel insulation, while Table 8 shows datum on aerogel glazing available in literature (Ding,2020).Buratti et al. (2012) investigated innovative glazing systems with silica aerogel for energy saving.The monolithic aerogel glazing showed better performance in comparison with granular systems because of light transmittance, and thermal insulation.Experimental results from Gao et al. (2014) indicated that the optical and thermal properties of aerogel glazed units are dependent of the particle size of the granules and the results (for large granules) showed 58.00% reduction in heat losses and 38.00% reduction in light transmittance compared to a traditional DGW.Cotana et al. (2014) assessed the effects of inserting aerogel in an innovative glazed system and concluded that aerogel reduced energy for heating by 50.00% in winter, increased the acoustic insulation index by three dB and reduced luminance by 10.00%.Ihara et al. (2015) evaluated the energy performance of aerogel granulate glazing systems from an office façade and indicated that the proposed façade can achieve a lower energy demand than a double glazed.In another work, Ihara et al. (2015) confirmed experimentally that convection in the granular cavity does not affect the thermal performance of aerogel granulate glazing systems.
Simulations and comparisons of aerogel window with traditional Argon-filled coated double-glazing showed that the aerogel window provided a low U-value of 0.30 W/m 2 K, the daylight transmission of the aerogel window was lower than that of DGW (Garnier et al. 2015).
The different properties and energy performance of aerogel glazed units including durability can provoke architectural challenges and aesthetic problems (Gao et al., 2016).The authors highlighted the need for guidelines to regulate the use of aerogel glazing and presented suggestions and recommendations.In another work, Gao et al. (2016) analyzed the application perspective of aerogel glazing in energy efficient buildings by evaluating their energy efficiency, process economics, and environmental impact and concluded that it can reduce energy consumption by 21.00% and has a possible payback time of about 4.40 years.Buratti et al. (2017) investigated different glazing systems with different granular aerogel and different glass sheets and reported a reduction of 63.00% in the U-value and about 30.00% in light transmittance.Moretti et al. (2018) compared the thermal performance of air and aerogel-filled PC systems and found that the impact of the aerogel was significant in reducing the U-value by 46.00-68.00%,light transmittance by 0.61 and 0.42 for 16 mm and 40 mm aerogel thickness, respectively.Also, the authors reported significant effect on reflectance while the solar factor was like that of the low-e glazing.Berardi (2019) provided review on the aerogel-enhanced opaque systems including cement-based products, aerogel-enhanced plasters, aerogelenhanced blankets and commented future research and development challenges.Mujeebu (2019) presented a review of aerogel including glazing technologies, production, properties, manufacturing, aerogel windows and application in buildings besides challenges in research and developments.Leung et al. (2020) examined the application of aerogel glazing technology and concluded that it can significantly reduce the heat gains and cooling energy.Almeida et al. (2020) reviewed the alumina-silica-based aerogels, including their fabrication processes and physical and thermal properties and commented that the insertion of the alumina phase makes the aerogels stable at high temperatures and maintain low thermal conductivity.Buratti et al. (2021) presented a review of aerogel glazing systems focusing on the main properties of interest in building applications including the material itself, the assembled glazing systems, thermal and optical properties, reliability, and durability of the aerogel glazed products.Zhang et al. (2021) numerically investigated the energy performance of different glazing configurations including glass windows filled with silica aerogel or PCM and compared with traditional air-filled glass windows.Figure 21 shows the schematic of double and triple glazing windows used in their study.Lamy-Mendes et al. (2021) provided a review on the production process of silica aerogels, and thermal and physical properties panels of blankets, cement, mortars, concrete, glazing systems, among others.Meti et al. (2023) presented a review on the progress in the development of aerogels and their classification into three categories: inorganic, organic and organic-inorganic hybrid materials, Figure 22.Recent achievements in organic, inorganic, and hybrid materials and their outstanding physical properties were discussed focusing on adjusting the properties    2023) improved the building insulation using polycarbonate windows with nanogel.The incorporated nanogel layer reduced emissions and decreased the annual energy consumption by 29.00%.
Jadhav and Sarawade ( 2023) provided a review that demonstrated the significant improvements in the mechanical and thermal properties of the nanocomposites.
Table 9 presents summary of results of some references on thermal enhancement of windows and façades.

Smart Glazing
Functional thin films open new applications fields for smart glazed systems by adding other functionalities such as power generation.Power generation through window coatings is relatively new and can be achieved by using semitransparent solar cells as windows.
Figure 23 shows a comparison of electric lighting energy and cooling energy of different glazing technologies (Granqvist et al., 2009), while Table 10 presents a summary of electro chromic, photo chromic, thermos-chromic and gas chromic windows (Aguilar-Santana et al., 2020).
Selection of window glazing is complicated when energy saving and daylighting are required simultaneously, but optimization techniques can provide a balanced solution for this problem Hee et al. (2015).In their review, Anderson et al. (2016) discussed developments on low emissivity coatings to   (Granqvist et al., 2009) replace indium and provided a perspective for future trends.Rezaei et al. (2017) reviewed glass coatings and glazing systems and showed possible applications in different climatic conditions.Table 11 shows the benefits and drawbacks of static and passive windows Rezaei et al. (2017).Attia et al. (2018) investigated the current trends of adaptive façades, evaluation of their thermal and optical performance and provided current trends and future challenges.
Low heat loss through glazing systems can be achieved by suppression of convection by use of multiple glass panes with aerogels, inert gas, or vacuum between the panes to reduce convective heat transfer (Ghosh & Norton, 2018).Low emissivity coatings are also required to reduce the radiation heat transfer.Oh and Park (2019) analyzed building energy and daylight performance in an office building and concluded that cited parameters were improved in both cases.Aoul et al. (2019) presented a review on electro chromic glazing and concluded that it can reduce electricity demand and provides energy savings for commercial and residential buildings.Ke et al. (2019) reviewed recent progress in smart windows focused on multi-functionality and enhancement of design and performance.
Tällberget al. (2019) conducted a review on thermoschromic, photo chromic and electro chromic smart windows and commented that the electro chromic window showed the best performance in all cases and highlighted the necessity of adequate operational control strategy.Aburas et al. (2019) presented a review of the thermoschromic films, coatings and glazing and commented that thermos-chromic windows reduce heating and cooling loads.Attia et al. (2020) proposed a conceptual framework and technological classification for adaptive façades, where the multi-functionality and performance requirements of façade technologies can be inserted.
Chromogenic can be used in building façades to reduce the global energy consumption of the building energy consumption, improve the indoor visual comfort, and reduce the risks of glare and excessive artificial lighting Cannavale et al. (2020).In another work Cannavale et al. (2020) provided a review on smart electrochromic windows to enhance building energy efficiency and visual comfort from the available devices and concluded that electro chromic windows can enhance energy efficiency in the building sector.Tong et al. (2021) reviewed TRSG technologies, compared their key optical switch response, challenges and potential solutions and commented that TRSGs are key-elements for climate-adaptive envelopes.Yehia et al. ( 2021) reviewed different types of glazing including conventional and advanced technology.The main objective is identifying their potential to enhance thermal and lighting performance.Onubogu et al. (2021) presented a review on the existing technologies of daylighting systems including both passive and active daylighting systems equipped with sun tracking.The authors recommended further research and developments to make daylighting systems less expensive and relatively easy to install in buildings.Wang and Narayan (2021) provided a review focused on the recent advancement of thermos-chromic materials for smart windows including performance and commercialization and indicated possible challenges for future development.Fathi and Kavoosi (2021) evaluated the influence of electro chromic widows, types of glazing and BIPVs on energy consumption of office buildings.The simulations showed a

COMMERCIAL PROGRAMS & CODES
Many simulation codes and computer programs were developed for the calculation of thermal loads, evaluation of thermal comfort conditions and energy performance of buildings.Currently, there are several computational tools to analyze the thermal performance and energy consumption of buildings.According to DOE (2022), the US Department of Energy's Directory of Computer Simulation Tools has more than 408 simulation programs developed in several countries, such as: BLAST (1992), Comis (1990), EnergyPlus (2012, ,2016), DOE-2 (1993DOE-2 ( , 1985)), Sunrel (1975), TRNSYS (2019, 2012, 2022), eQUEST (2010, 1994) Energietechnic GmbH, 2012).TRNbuild ( 2018) is an interface for the geometric, thermal, and optical definition of a specific building, while IDA ICE (Arasteh et al., 1994;Kalamees 2004) is a flexible wholebuilding performance simulation tool and is relatively easy to extend the existing modeling functionality.DesignBuilder ( 2019) provides an easy-to-use interface to develop building designs from concept through to completion.
IES Virtual Environment (VE) (2011) is building performance analysis software that designers can use to test different window options, identify passive solutions, compare low carbon and renewable technologies, and draw conclusions about energy use, CO2, and occupant comfort.
The LT Method (Baker & Steemers, 1996) is an energy design tool that responds to the parameters available at the beginning of the project development.This method provides an annual primary energy output for lighting, heating, cooling, and window ventilation.ASHRAE Toolkit for Building Load Calculations (Pedersen et al., 2003) is written entirely in FORTRAN 90.The load toolkit components provide a valuable resource for making the thermal break-even load calculation procedure more readily available to ASHRAE members.This toolkit helps application developers incorporate the Load Calculations method, which is presented in the 2001 ASHRAE Handbook-Fundamentals (2001) as the preferred method.Over the years, Autodesk has developed software and devices that, using the same calculation mechanisms as EnergyPlus, provide the workflow and the possibility of energy efficiency simulations.The most recent software for this purpose is Autodesk Insight and its predecessor versions: Green Building Studio (2008) and Project Solon, all available in the cloud.
Finally, CFD tools (ANSYS, 2022) and (COMSOL, 2022) are also used to model the heat transfer in windows but, it is worth mentioning that these models are not integrated with the commercial programs.ANSYS is a general-purpose, finiteelement modeling package for numerically solving a wide variety of mechanical problems including heat transfer, and fluid problems.COMSOL Metaphysics allows simulating acoustics, fluid flow, heat transfer, and chemical phenomena in one environment.

CONCLUSIONS, FUTURE TRENDS IN RESEARCH, & DEVELOPMENTS Conclusions
A fair number of studies were dedicated to reducing energy and emissions of buildings by increasing thermal inertia of buildings structures and components and enhance their thermal performance.
Natural illumination and natural ventilation are important since they can reduce energy demands and improve thermal and visual comfort, which significantly impacts human comfort and efficiency in working areas like offices and classrooms.
The review shows tremendous progress in smart window technology and windows with internal insertions and reflective film, which when implemented can significantly improve the thermal performance of the buildings and residences.Additional efforts are needed to reduce the costs of these new products and facilitate their inclusion in old and new buildings.
The review did not show any publications on financial incentives, tax bonus and adequate public policies and awareness programs to enhance incorporation of new technologies on windows and façades in old and new buildings as well as in popular residences.
The review shows that a fair amount of research and development was done to improve the thermal and operational performance of windows and façades by using filling materials such as absorbing gases, PCM and water flow, vacuum, and aerogels.Smart windows and façades received a lot of attention, but their cost is still high and additional efforts are required to provide low-cost solutions for application in old and new buildings.
According to the bibliography consulted, the nature of the analyses is predominantly numerical, while experimental studies are less frequently addressed, Figure 24.Precisely the costs of the technologies, as well as the maturity of the concepts, are factors that condition this behavior.
In addition, the literature does not provide much information on optical and thermal properties, Figure 25.This, in general, limits the potential applications of the technologies under development.In fact, the state-of-the-art analyses showed a limited number of studies considering the combination of different technologies to take advantage of the benefits of each one in the same configuration.
Based on the U-value and G-value collected, it is possible to define the application range for each technology studied.In the case of vacuum glazing, it is possible to observe a reduction in the U-value parameter as the number of layers increases, from a range of 1.0-1.4 for double-glazing, to 0.7-0.9 for tripleglazing.This parameter shows the lowest values for aerogelbased technologies, from 0.4 to 0.72.However, of all the concepts, photochromic technology has the highest U-value range, from 0.53 to 1.58.Regarding the G-value parameter, used to measure the transmittance of solar gain through glazing, gas-filled technologies had the widest range, from 0.34 to 0.61.
A fair number of commercial simulation and calculations codes are available for local and global simulation of thermal performance of buildings.To improve energy thermal performance and increase the use of these tools there is a need to invest in the development of new codes and validate them in real buildings applications.
Considering the importance of reducing energy consumption of the building sector and reduce its emission contribution, it is hoped that this review can throw some light on the research and development opportunities and be of help for developers and young researchers, practicing engineers and general readers interested in the fascinating topic of windows and façades.1. Windows, façades, roofs, and walls are the major contributors to the buildings' heat losses and gains.To make buildings more sustainable and less energy consuming it is necessary to invest in thermally efficient materials and apply new technologies for windows, walls, and façades besides adopting energy efficiency strategies for heating and cooling.
2. Buildings should be designed to be self-sufficient as much as possible benefiting from natural energy resources as solar and wind and minimizing the needs for artificial lighting and ventilation, which leads eventually to having healthy internal ambient with adequate passive thermal and visual comfort.
3. Natural illumination is an important issue in commercial and office buildings, essential to promote well-being, visual comfort and reduces stress, besides reducing the cooling load and hence energy consumption.It is essential to promote the use of natural ventilation concept in buildings and residences when possible.
4. Strategies for natural ventilation systems must be encouraged in buildings to reduce the thermal load and consequently reduce the energy consumption in air conditioning systems.
5. Further research and development must be encouraged to reduce the cost of smart windows to help popularize their use in commercial buildings and residences.
6. Financial incentives, tax bonus and adequate public policies and awareness programs are required to enhance incorporation of new technologies on windows and façades in old and new buildings as well as in popular houses to reduce energy consumption and emissions.
7.More research work and development are required to provide cheap and effective performance enhancement equipment and strategies validated by extensive laboratory and infield tests.
8. Additional efforts should be directed towards characterizing the optical and thermal properties of the technologies under development, to broaden their range of applications.9.It is also important to focus on analyzing new configurations that combine the advantages of the concepts, taking advantage of the properties of the different technologies.
10. Computers and simulation codes are handy tools for the development of new products and local and global simulation of thermal performance.There is a need to invest in the development of new codes and validate them in real applications.

Figure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 5 .
Figure1.U-& g-value of some commercial glazing systems(Akram et al., 2023) Tong et al. (2021) provided a review of transparentreflective switchable glass (TRSGs) technologies for application in building façades, while El-Eshmawy et al. (2021) reviewed the different types of glazing, categorized them into conventional and advanced and showed their thermal and lighting performance.Pereira et al. (2021) investigated different SCFs applied on existing windows of a building.SCF considered as the best retrofitting solution showed an annual carbon footprint of 4,447 MJ/m 2 /40 y and 380 kgCO2eq/m 2 /40 y, respectively.Calama-González et al. (

Figure 11 .
Figure 11.Details of a water-flow double glass window(Chow  et al., 2011)

Figure 23 .
Figure 23.Comparison of electric lighting energy & cooling energy of different technologies(Granqvist et al., 2009)

Figure 24 .
Figure 24.Consulted literature classification by work nature (Source: Authors' own elaboration)

Table 1 .
Summary of some reference results on natural illumination &

Table 2 .
Summary of some reference results on natural ventilation

Table 3 .
Summary of some reference results on thermal comfort To save energy, it is possible to reduce air changes in a room to minimum, enhancing both local thermal performance & comfort.Shading roofs & southern facade of building envelope are most efficient scenarios for passively modified version of building.Ko et al. (2020) Numerical Window Providing a window with a view in a workplace is important for comfort, emotion, & working memory & concentration of occupants.Yang et al. (2021) Numerical Building-integrated photovoltaic/thermal double-skin façade Solar heat gain coefficient of BIPV/T-DSF's external window possessed highest importance affecting indoor thermal comfort & energy consumption.

Table 6 .
(Huang et al., 2021)ane medium(Huang et al., 2021) Michael et al. (2023)t concepts related to PCMs and details of the different types of PCMs according to climate.Xu et al. (2022)prepared a phasechange gel with relatively high melting enthalpy, better leak proof characteristics, better thermal insulation, besides increasing time lag and lowering peak temperature.Arasteh et al. (2023)reviewed energy and thermal performance of PCMincorporated glazing units combined with passive and active techniques including current passive smart glazing technologies, whileMichael et al. (2023)provided a review on established and emerging glazing technologies and the inclusion of multiple functionalities to improve overall building performance.

Table 9 .
Summary of some reference results on thermal enhancement of windows & façades

Table 9 (
Continued).Summary of some reference results on thermal enhancement of windows & façades

Table 12 .
Summary of some reference results on smart glazing