Global Prospects, Advance Technologies and Policies of Energy-Saving and Sustainable Building Systems: A Review

: Automation, modernization, economic development and global progress depends on efﬁcient extraction and utilization of energy. Power generation by burning fossil fuels makes various adverse impacts on the environment. Additionally, the worldwide fossil fuel reserve is limited and depleting very fast. Hence, efﬁcient energy usage and savings are crucial to address the environmental issues to ensure sustainable development. Buildings, both commercial and residential, represent a major energy consumption sector. Approximately 40% of the total energy is reportedly consumed in the building sector. Worldwide building energy consumption, performance measuring systems and best practices, energy-saving techniques and policies are reviewed and summarized in this article. Underﬂoor air distribution, double-glazed windows, use of highly efﬁcient electric motors and variable speed drives may play a great role in reducing building energy consumption. In the UK, the application of double-glazed windows in commercial buildings can save 39–53% energy. The proper maintenance of a building’s central heating system can save up to 11% energy. The automatic HVAC control system can reduce up to 20% of the building’s total heating load. Proper utilization of a VSD system in motor and building performance optimization by an ANOVA tool also proved instrumental in saving energy. Apart from this, the building codes of different countries also help to improve building performance by reducing energy consumption. This study will help building researchers and policymakers to make a framework for sustainable, green building.


Introduction
All over the world, energy consumption is growing very quickly because it is essential for modernization, economic growth, automation and social development. From 1994 to 2014, the growth rate of primary energy consumption and CO 2 emission was 49 and 43%, respectively, and the annual average growth rate was 2 and 1.8%, respectively [1]. In 2017, the growth rate of annual energy consumption was 3.2% in developing countries [1,2], while developed countries saw a 1.1% growth in the same year [1]. According to the International Energy Outlook report, energy consumption worldwide will grow by 56% from 2010 to 2040. It is considered that the building sector will be the largest energy-consuming sector by that time [3]. According to the IEA, buildings consumed 32% of total energy and contributed 33% of GHG emissions in 2012 [1,4]. In 2010 in the USA, 38.9% of total energy was consumed by the building sector, wherein 34.8% was consumed in cooling, heating, air conditioning and ventilation systems [1,[4][5][6][7]. Moreover, the Hong Kong building sector consumed 60% of total energy and over 90% of electricity [4]. Globally, energy usage in the residential, commercial, industrial and transportation sector is 22, 19, 31 and 28% of total energy, respectively [8]. In the developed world, residential and commercial buildings' energy demand is 30-40% of the total energy [9]. It has been projected for China that building energy consumption will be 35% of total energy by 2020 [3]. A study has described how the domestic sector of buildings alone consumed 24 to 27% of total buildings' energy use [6]. Apart from the high percentage of total energy consumption, buildings contribute 19% of all GHG emissions, 33% of black carbon emissions and 51% of global electricity consumption per year [2]. In the last decade, the annual average building energy consumption growth rate all over the world was 2.2% from the year 2005 to 2015. [2]. Ma et al. [8] revealed buildings' energy use in different regions of the world in proportion to the total energy as shown in Table 1.  [3] The heating and cooling of space are two major factors in building energy consumption. At present, space heating and cooling consume 20% of the building's total energy; if this trend continues it will be 50% by 2050 [5]. In developing countries, the average energy consumption propagation rate is 3.2%. However, in developed countries this annual average growth rate is 1.1%. For China, in particular, this growth rate is 3.7% [6]. Table 2 shows the residential and commercial buildings' energy consumption worldwide. Buildings' energy consumption growth rate in the UK is 0.5% per annum, which is way below the European figure (1.5%). The annual building energy consumption growth rate in Spain is 4.2%, which is much higher than the EU (1.5%) and North America (1.9%). EU buildings consume almost 37% of total energy, which is higher than transport (32%) and industry (28%). In the UK, 39% of total energy is consumed by buildings, which is slightly above the European figure. Conversely, Spanish buildings consume 23% of total energy, which is 14 points below the European figure [6]. Table 2. Worldwide commercial and residential building energy consumption percentage [10,11]. Commercial  18  11  11  8  7  Residential  22  28  26  15  16  Total  40  39 37 23 24 In the USA and Europe, CO 2 emission rate related to building energy use is 38 and 36%, respectively [7]. It is assumed that buildings consume 32% of the total energy wherein 24% is consumed by residential buildings and the remaining 8% is consumed by commercial buildings. Space cooling and heating dominates the residential buildings' energy use, which is 32% of total building energy, followed by cooking, water heating, appliances, lighting and the cooling energy consumption being 29,24,9,4 and 2%, respectively. On the other hand, space heating and cooling of commercial buildings consume 33% of the total building energy, followed by lighting, water heating, cooling and other equipment being 16, 12, 7 and 32%, respectively [8]. The rate of building energy consumption would be doubled by 2050 [8]. In the USA, 47% of the total building energy is consumed by 95% of the small and medium-sized commercial buildings. In European countries such as Finland, Bulgaria and Spain, building energy consumption per square meter is 320, 150 and 220 kWh, respectively. It is assumed that building energy consumption in developed countries such as the USA and Europe will be reduced by the appropriate use of building energy policies [4]. Still, in the USA per capita building energy consumption is far greater than in China because of its higher energy intensity and per capita building floor area. Nowadays, per capita building energy use rate is rapidly increasing in China due to the quick growth of the Chinese economy. The energy intensity ratio of the Chinese building to the USA building has rapidly expanded from 7 to 12%. Building is the largest end-use sector in E.U. and it consumes about 40% of the total energy as well as 55% of electricity [9]. Most of the building's energy is consumed in space cooling, space heating, residential appliances, lighting and other requirements. The percentage of building sector energy demand growth up to 2050 is shown in Figure 1. The energy consumption growth rate in space cooling is higher.  Figure 2 shows the building energy use breakdown in the residential and service sector of the different regions in the world. In the residential sector, the EU used the largest amount of energy for space heating. On the other hand, China used the highest amount of energy in the service sector for space heating. For space cooling, in both the residential and service sector the USA uses the highest amount of energy compared to China and EU. However, for water heating in both residential and service sectors, China used a larger amount of energy compared to the other two regions. Lighting used a lower amount of energy in the residential sector compared to the service sector. Both in the residential and service sector, for appliances and other equipment, the USA used the largest amount of energy compared to China and the EU. However, building energy consumption depends upon some factors which affect the buildings' energy needs. These factors are classified into two categories, namely "physical environmental factors" and "artificial designing parameters". The "physical environmental factors" include the amount of solar radiation, outdoor temperature and wind speed, etc., and "artificial designing parameters" include building form factor and orientation, transparency ratio, thermo-physical as well as optical properties of building material and the distance between buildings [10]. Dependence on energy sources or the type of fuel used to generate power for building consumption is shown in Figure 3. The USA use the largest amount of electricity in building (50.3%), followed by natural gas (38.4%), oil (7.7%) biomass and waste energy (2.6%) and a very small amount of coal as well as other renewable energy sources. China produces the maximum amount of building energy by utilizing biomass and waste resources (47.1%), followed by electricity (15.2%), coal (14.3%), oil (11.6%), natural gas (7.4%) and commercial heat and other renewables are 4.1 and 0.4% correspondingly. Lastly, the EU utilizes the highest amount of natural gas (36.7%) to generate building energy. Electricity and oil use 31.1 and 13.6%, respectively, and some little amounts of other resources. Both the USA and EU give attention to electricity and natural gas to generate building power. However, China gives attention to biomass and waste energy sources which are quite good. The increasing use of energy creates concerns regarding its early exhaustion along with heavy environmental impact such as global warming, increasing GHG emission, ozone layer depletion and climate change [1,2,8]. Additionally, concern grows about the utilization of fossil fuels and their implications for the environment [11]. It has been estimated that building energy consumption produced 33% of global greenhouse gas emissions in 2010. The amount of GHG emission produced by building energy consumption was 10 GtCO 2 eq/year in 2010 [8]. Since the building sector consumes a major part of overall energy consumption and produces GHG which has negative impacts on the environment, research related to the energy savings in the building sector is urgent. Analyzing the situation, it is highly needed to investigate the building energy consumption characteristics and to make it efficient.

Final Energy Consumption (%) USA UK EU Spain World
Ma et al. simulated the residential building sector carbon roadmap for China, where it has been found that the emission mitigation from 2000 to 2015 is 1.817 billion tons of carbon dioxide (BtCO 2 ). They also predict that the peak value of carbon emission in the residential building sector will be 1.419 BtCO 2 around 2037. They claim that the strategies used in this study for emission analysis are more feasible and accurate, which would be very helpful for decision-making of residential building carbon mitigation [12]. In another study, Chen et al. used the carbon Kuznets curve (CKC) model to analyze the residential and commercial building's carbon emission changes across 30 provinces in China. They found that most of the samples fit the CKC model, thus it is considered as a robust model of emission analysis. The CKC model can be used for carbon emission analysis for different countries as well as for different types of buildings [13].
From the above information, there is a lack of comprehensive information about the technologies of building energy savings and energy use policies. The main aim of this study is to review building energy consumption scenarios, different designs as well as operating parameters of energy consumption, as well as energy-saving techniques with related policies for energy-efficient buildings. In this review, the current technologies, policies and regulations are comprehensively summarized to make a pathway for building researchers to form correct decisions and choose appropriate building energy-consumption-related technologies, legislations and policies.

Energy-Saving Technologies in Building System
Generally, energy used in a building can be accrued in various ways and a statistical process can be used for studying the building's overall performance and minimizing the energy requirement of the building. Different statistical models are used to interpret the real-world data in terms of individual theory to develop energy-efficient buildings. However, the main objective of the present discussion is to detect the equipment which used a major portion of the energy and to apply energy-efficient options for that equipment. This part of the paper mainly identifies the components responsible for consuming most of the energy along with the energy-efficient options/alternatives for these components. Figure 4 presents an overview of energy-saving techniques in buildings.
There are different ways to make a building a net-zero energy consuming one. Figure 5 shows the connection between energy grids and buildings (it does not show the energy balance). The system boundary of a building is characterized by a certain load and some sort of energy generation source. The load includes the net energy demand and the proficiency of technical installations. On-site available renewable energy sources are used actively (e.g., ground source heat pump) and passively (e.g., solar gains through windows) to satisfy the building's load partially. Depending upon the temporal matching between generation, load and available storage possibilities the on-site renewable energies are also used to generate electricity which partly covers the load and is partly fed into the grid.

Underfloor Air Distribution (UFAD)
Underfloor air distribution (UFAD) is an advanced system that provides conditioned air in a building through an elevated floor plenum. Recently, this approach has become popular in commercial and office buildings. Brauman et al. reported that construction firms anticipate more than 35% of newly built office buildings prefer raised floors and one-half of those buildings might be using UFAD by 2004 [18]. Practically, the UFAD system has been practised even though the codes, standards, guidelines and design tools have not been published yet. A recent investigation has reported that the UFAD technology is more energy-saving, effective as well as better than other conventional types [19]. The growing use of UFAD in North America gives an advantage compared with CBAD (ceiling-based air distribution), which does not have a way of recycling fresh air from the supply to the return path. This allows maximum utilization of fresh air, which improves the quality of indoor air. Floor grilles are also used to control the volume of air by changing the position of the damper. Another application of UFAD is to increase the pliability of space subdivision. Generally, this type of air distribution system is more widely used for heating purposes, i.e., for warming the occupant's feet. A conceptual view of a UFAD system is shown in Figure 6. The advantages of UFAD systems in the air distribution of a building can be summarized as below [20]: • Local environment can be controlled to increase the thermal comfort for individual occupants.

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Efficiency of ventilation and quality of indoor air can be improved by supplying fresh air at the zone of floor level or nearby the occupant. • Energy use in the building can be saved by controlling the temperature of supply air between 17 and 20 • C for the UFAD system, or at 13 • C for overhead systems, to improve COP of HVAC. • Energy savings from the fan are related to the lower atmospheric static pressure ranging from 12.5 to 5.0 Pascal. Depending on the design strategy it is necessary to reduce the central fan energy use compared to the overhead air distribution system because of extremely low operational statistic pressures (pressures are typically 0.1 in H 2 O (25 Pa) or less) in the underfloor air distribution system. • Height between two floors for newly developed construction can be decreased so that the average service plenum height would be reduced. About 5-10% decrease in floor-to-floor heights could be reduced in a building which uses UFAD compared to CBAD. This can be achieved by minimizing the service plenum's overall height and/or by using a concrete (flat slab) structural approach.

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Health and productivity can be improved. • Expenditure related to inhabitant churn, varying interior and remodeling can be reduced [21].
However, despite many benefits of the UFAD system there are several drawbacks, too. One such example is that the temperature of supply air generally needs to be maintained above 18 • C or 65 • F to avoid the occupants feeling cold, otherwise UFAD would have to be designed in such a way that it would be 1 m from the occupant working station [22]. Furthermore, discharge of air should be calculated carefully and spread to the occupants' area to avoid noise. An elevated floor is needed for the air plenum, which increases the cost value and becomes essential for structural, architectural and service management. Some main problems that have hindered the widespread use of the UFAD system in the present days include [19,23]:

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Unfamiliar new technology that includes design of the entire building, construction of the building and its operation process.

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The lack of understanding of some new but fundamental elements, such as stratification of indoor room air, plenum performance of underfloor airflow and leakage phenomena considerations, as well as the performance of the entire building.

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Higher installation costs compared with the overhead system. • Mold growth and condensation might take place in concrete slabs when the temperature of supply air is lower than 17 • C or 63 • F. • Accumulation of mold and dirt, if a timely cleaning process is not conducted. • Difficulty to maintain necessary plenum pressure, as all surfaces must be leak-proof and air leakage into the occupied space is wasted energy [24].
Jing et al. [25] suggested some special techniques for saving energy in buildings which are as follows:

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The wall should be heat-insulating. • Architectural doors and windows for lower energy consumption. This is because the thermal characteristics of windows, as well as doors, give a major impact on air conditioning energy use which is the main thermal layout of winter and summer buildings.

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Energy-saving glass should be used.

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Composite doors or windows devoted to materials should be applied or improved.

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The door, curtain, wall and windows installation system should be improved.

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The roof should be heat insulating.

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Building energy preservation technology should be developed effectively.

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Building an energy management system should be implemented.

Heating, Cooling and Window Systems
Generally, heating, cooling, lighting and household appliances are the main energy consumers in the building systems where the air conditioning system consumes a major part [27]. Many countries give a relatively high priority to building energy saving [28]. It was identified through long limited studies that attention should be given to non-domestic building stock [29]. Improvement of physical ventilation in public and office building design can reduce energy consumption [30]. Considerable energy from air conditioning can be saved if using capacity is reviewed regularly and the unit which consumes excessive energy is replaced [31,32]. Investigation in [33] pointed out that the indoor environment thermal comfort can have an important influence on building energy requirements. Tsagarakis et al. [28] suggested specific measurements that can be consigned at the manufacturing and retrofitting stage by considering cost analysis for appropriate equipment selection. This paper emphasizes the use of double-glazed windows, air conditioning as well as heating for saving energy in a commercial building.

Heating and Air Conditioning
The heating process consumes major energy in the buildings system, which takes up approximately 40% of total energy use in a building [14]. All previous regulations related to energy savings in building have been summarized in a recent directive, i.e., 2002/91/EC on "Energy Performance of Buildings" [34]. The utilization of automatic controllers can conserve a huge amount of energy at the timekeeping comfortable environment for the occupants [35]. Proper maintenance of central heating equipment, approximately 11% of total energy use per annum can be saved [36,37]. On the other hand, energy required for cooling purposes could be saved by improving the building envelope, applying different methods for cooling and using more efficient AC equipment in office buildings [38]. Mortimer et al. [39] stated that use of HVAC equipment with advanced design and automatic control techniques could effectively save about 20% of the total load. By adjusting the set point of temperature [40] and using optimal control techniques [41], a great quantity of energy could be saved from the HVAC system. Therefore, experts are emphasizing the outcome of research regarding the use of HVAC equipment, the related cognizance quality and the consent to pay for consolidation of technologies about high energy savings, could be a vital part of fundamental law for designing and applying campaigns about energy cognizance and objected attention regarding the improvement and utilization of technologies of high energy efficiency. Technical professionals and local or regional authorities have planned and organized awareness campaigns regarding energy efficiency for the public at the same time objected to interventions about office buildings, considering the special socio-cultural body of the "targeted audience". The above-mentioned assessment could be adequately incorporated in the energy-saving design of the generic building which complies simultaneously with the European Directive 2002/91/EC about buildings energy performance [34]. Such steps can ensure acquirement of energy-saving about 20% and reduction of greenhouse gases by about 20% within 2020, in the zone of European Union.

Double Glazing Windows
Buildings with good insulation can reduce their energy consumption because there is a lower loss in cooling or heating systems [42]. It was found that most of the heat loss, or heat gain for the case of cooling, occurs through the windows of a building [43]. The use of double glazed windows can play a vital role in enhancing energy efficiency of lighting, building heating and cooling systems [44], along with the improvement of temperature and acoustic comfort environment of that building in the indoor condition [45]. Figure 7 shows the heat transfer mechanism of double-glazed windows. The heat transfer coefficient of double glazed windows is about half of that of the single glazed window [14]. For these reasons, in the present time, the double-glazed window has become a widely adopted standard for newly developed housing or commercial buildings. In recent years, normally double-glazed windows have been used in the construction of new and commercial buildings. This led to a change in building regulation which inspired reduction of heat loss through building construction materials. Double glazing can considerably lower the overall heat transfer coefficient of the glazed area of a building and it has become trendy [46]. Research on the energy saving of commercial buildings in the UK discovered that about 39 to 53% of energy could be saved by substituting single-glazed windows with double-glazed windows [39]. Effective replacement of window frames could also save energy in the building. The substitution of old windows frames with new ones improves energy efficiency but at a high cost [28].

Triple Glazing and Super-Insulated Windows
Gorantla et al. (2021) investigated triple-glazed window design strategies such as spectral characteristics, daylight factors, cost savings by the air conditioning system as well as payback periods. They found that the most energy-efficient glazing is the TWG35 glass unit with S-E orientation which saves 16.72 USD/m 2 .year air conditioning cost compared to the other window glass units. The payback period of TWG35 was 2.2 years. However, the lowest payback period is 2.1 years revealed by the TWG33 window glass unit with a net cost recovery of 16.55 USD/m 2 .year [48]. A multi-pane glazing unit reduces the heat transfer through it compared to a single-pane unit. Figure 8 shows a triple glazing window unit with 5 mm thick reflective glasses. The total thickness of the triple-glazed window unit is 35 mm including a 10 mm air gap between two glass units [48]. .9% heat gain in summer, as well as 74 and 46.8% heat loss in winter, respectively, compared to the conventional double-glazed as well as triple-glazed windows [49]. Figure 9 shows the schematic diagram of the switchable exhaust air windows. The main parts of the SEA window are three glass panes, two movable Venetian blinds and two air cavities. Different air cavities host different Venetian blinds in different seasons. The two air cavities act as an exhaust air ventilation channel to remove indoor air from the outside environment. The switchable components help to flow exhaust air in different cavities. The upper and lower switchable components are shown in Figure 10. In winter, the Venetian blind is on the indoor side, but, in summer it is on the outdoor side shown in Figure 9 [49].   2021) tested the thermal performance in the cooling operation of the triple-glazed window with water flowing through it. Surface temperature fluctuation of the inner glazing of the window was narrowed down. Compared to the IGU (insulated glazing unit) insulated water flow window (TWFW), the performance of VG (vacuum glazing) insulated water flow window (VWFW) is more favorable. The thermal performance of a triple-glazed water flow window is also promising. The thermal efficiency of TWFW and VWFW ranges from 21.9 to 36.13% and 24.69 to 45.95% correspondingly [52]. Larsson et al. (1999) studied super-insulated windows through numerical and experimental thermal analysis. A triple-glazed window with low emissivity in which the inert gas krypton filled the two closed spaces acted as the super-insulated window. One pane in each space was coated with oxidized metal with a low emissivity factor. The numerical and experimental investigation of windows has been operated for various winter cases. Due to good resistance to heat transfer, the window provides comparatively higher surface temperature in the inner pane [53]. Garnier et al. (2015) investigated the thermal performance of super-insulated aerogel windows and compared them with double-glazed argon-filled windows. The results showed that the heat loss index and daylight transmission of the aerogel window is significantly lower than the double-glazed windows [54].

Energy Preservation by Using the Electrical Motor of High-Efficiency
Demand-side management is a significant idea to increase efficiency by reducing load growth. This concept helps the motor manufacturing company to seek new technology for increasing the efficiency of motors, which leads to manufacturing energy-saving electric motors. Many renowned motor manufacturing companies, especially in Europe and the USA, have been producing energy-saving electric motors [55]. Saidur [56] suggested that a high-performance electric motor should use low-loss materials, that the energy loss due to copper or core losses can be reduced. It has been seen that the primary objective of a manufacturing company is to lessen the manufacturing cost, but the efficiency of the motor can be increased by developing its design which leads to conserving energy by using a motor. The energy-efficient design may include a bigger rotor conductor, more copper used in filling slot, using of ferrosilicon alloys plate in magnetic cores, and upgrading airgaps in core heads, bearings as well as fans. Dimensional parameters also should be improved for better design. Nevertheless, the cost can be a limiting factor to the widespread use of the high-efficiency motor, as it was reported that the cost of a high-performance motor was approximately 10 to 15% higher than a typical one [57]. The power consumption of the motor depends on the rated speed. The relationship between percentage power consumption and the rated speed of the motor is shown in Figure 8. The capacity variation of a motor by matching actual load requirements can improve the efficiency of such systems. As the power requirement in the motor varies with the cube of the speed, a large amount of power can be minimized by reducing a small amount of speed which is shown in Figure 11. The speed of the motors, pumps, compressors and fans can be modulated by variable speed drives (VSDs). It provides continuous control, matching motor speed depending on the specific requirement, which may reduce the cost of energy [16,58].

Motor Energy Savings Utilizing VSDs
Instead of improving the performance of the motor itself, the overall efficiency of a motor-based application can be improved as well via the use of a variable speed drive (VSD). Most of the buildings are designed considering the maximum demand of the load. However, in most cases, different systems of buildings need to operate at maximum load for a very short time in a day. Sometimes few systems are running for a long time without any reason, causing unnecessary energy loss. Such inefficient operation is particularly common in the AC system of a building. In this case, VSD can be used to control the speed of the motor for the pump or fan according to their load demand. Since the power requirement of a fan or pump is proportionate to the cube of their speed, and energy can be saved by fitting the speed of the motor to the requirements of load using VSD to improve the overall system efficiency [58]. The advantages of using VSD in fan and pump applications include [59]: • Savings of energy. • Improvement of efficiency over increasing of power factor of the system. • Simplification of pipe systems (removal of by-pass lines and control valves). • Normal starting and stopping procedure.

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Dynamic response leads to better control compared to DC drives.
It was proven that for a fixed load that is lower than the rated load, the electrical motor with lower-rated power can be used rather than VSD in giving good cost-saving [59]. The basic mechanism used in the variable speed drive is shown in Figure 12 while Figure 13 reveals the basic components of VSD.  Energy savings by utilizing VSD have been predicted by mathematical formulations. Many approaches are used to predict energy savings by utilizing VSD in industrial motors. The energy consumption by fans, pumps, motors, etc., varies with the third power of changing the speed, which is why a small change in speed will cause a huge change in energy usage. VSD is utilized to fit the load requirements to save energy. Typically, the appropriate design of VSD systems can save 20 to 70% of energy use compared to the conventional motor. By using VSD, the energy saving by a motor can be predicted as follows [16,58,61,62]: where AES VSD is the annual energy saving using VSD (kWh), n is the number of motors, P is motor power (kW), H avg_usage is annual average usage hours and S SR is the percentage of energy savings using speed reduction. Based on the annual energy savings by Equation (1), the yearly bill savings (BS) can be measured by the following equation [16,58]: A mathematical formula has been generated to predict the energy savings by utilizing high-efficiency motors. The annual energy savings by replacing the standard efficient motor with the highly efficient motor can be calculated as follows [16,61,62]: where AES is the annual energy saving in kWh, W is the rated power of the motor in Watt, L is the load factor in percentage, hr is the number of hours in yearly usage, E std is standard motor efficiency in percentage, and E ee is the energy-efficient motor efficiency in percentage.

Cluster Analysis
Cluster analysis, also known as clustering, is a common statistical analysis tool that can be used to identify the groups of data, which are intimately related to one another but distinctly different from others, from a pool of statistical data [63]. In the case of numerical data, cluster analysis can separate the data into clusters in which individual data are very close to the average of all data in that cluster. The main objective is to minimize the summation of the differences of individual data to their nearest mean. This procedure of clustering is the prototype-based or centre-based procedure. Based on a statistical model, clustering analysis also can be developed which is known as model-based clustering [64]. In this way, one can find the best data for designing the statistical model. For building sector energy assessment, cluster analysis can be used for the classification of energy performance phenomena in cumulative frequency distribution [15,65]. A cluster analysis tool was used by Petcharat et al. [66] to evaluate the possible energy savings from lighting systems in buildings. It was reported that cluster analysis gives a set of selected data that can be used for calculating the total energy conservation gained from lightning equipment of commercial buildings, which are not like the single desired value of the normal averaging method. Unlike the normal averaging method, which delivers a single sample value, cluster analysis provides a group of typical values for calculating the total efficient energy preservation that could be achieved from commercial buildings lighting equipment, if a certain model of lighting power density, i.e., LPD value is mentioned in the code of building energy. In simulation investigation, the savings of desired energy can find out from clustering outcomings that have lower errors than the general approach from all general case studies. Besides, the clustering analysis method has also been used for categorizing meteorological data to find out the average characteristics in the planning of renewable energy usage schemes [67].
Another special type of statistical model which is very similar to clustering is known as the mixture model [68]. A mixture model is formed by many distributed statistical data, where every distribution is related to one cluster whose parameters can give detailed information of the respective cluster by its centre and its distribution zone [69]. The application of another statistical technique, known as the Expectation-Maximization (EM) procedure [70], allows the characteristic value of any data to be determined from hitherto intractable ML (maximum likelihood) estimation or complicated ML cluster. There are some other applications of statistical techniques such as in dairy science, AIDS epidemiology and medical imaging.

Performance Enhance by Using the Optimization Approach
Building energy system design follows a series of decision-making processes which depend on various choices of solutions. The design variables are usually selected based on certain requirements, such as to increase energy saving. Generally, the problem is complex to explain and also difficult to solve, so designers normally have to depend on simple thermal design rules or rely on their experience [71]. The dependency on personal experience might give inaccurate or incomplete results, so there is a need for proper methods that can help the designers in designing energy-efficient buildings. In the last several decades, a huge amount of research has been carried out for selecting the suitable conceptual primary architectural design stage of a building, such that the building will be compliant to building code, energy-efficient and environmentally friendly [72]. The use of optimization technology in architectural design is comparatively new. Wang demonstrated a method to minimize the shape of the building floor by applying a genetic algorithm and approach to determine the life cycle effect on the environment and lifetime cost [73]. Caldas developed a productive tool applying a genetic algorithm to minimize the sizing and placing of windows for official buildings to evaluate their environmental impact [74]. In general, the optimization technique is complex; thus, it is required advanced mathematics and related tools [75]. A novel mathematical model, termed Analysis of Variance (ANOVA), was presented by H. E. Mechri et al. [72]. This solution is solved based on gradual alternative solutions to design choices. The motive of this model is to analyze the required energy for incertitude of cooling or heating based on some design factors. ANOVA is a statistical method where the variability of results is divided depending on the various input variables. The benefits of this assessing method are that every design-related variable can play a significant role in the energy output of a building. Thus, the ANOVA method is used to determine the output variable and its effects on input variables and it gives the gist of the conclusion depending on individual variables [72].

Performance Indicator of Buildings
Environmental evaluation of a building is complex, proceeding with adapted objective and systematic assessments on the performance of the building. These procedures start by designing, constructing and operating the buildings related to sustainable development characteristics. Building environmental evaluation is the tool of control, but also a method of durable building design. About 19 years ago there were incredible improvements in the environmental evaluation of buildings all over the world. Buildings adopting environmental evaluation methods or systems have increased since the introduction of BREEAM (UK). The other important methods developed in different countries include LEED (USA), Green Globes (Canada), NABERS (Australian), SBTool (international), CASBEE (Japan), France HQE (France), HK-BEAM (Hong Kong), E-audit (Poland), LEnSE (EU), Protocollo ITACA (Italy). The evaluation processes are built based on the life cycle of the buildings: existing buildings, predesign, renovation and new buildings. In earlier times, the demand for environmental suitability, safety and responsibility of buildings were enhanced. The factors of durability are included in the environmental evaluation process of building used in various countries for evaluating durability and environmental performance [76]. Indicator assessment of energy performance tools and planning are designed principally fulfilling the two requirements: classification of energy and diagnosis of energy utilization performance. Energy classification gives a stable or authorized tool to create a communication with the efficiency of relative energy used in buildings and creating concern about carbon emissions to the owners and general people and preservation gains. Energy performance diagnosis aims to identify the faults and detect the reason behind the poor performance of energy utilization in building and take the necessary steps to increase energy performance [77].

Energy Performance Measuring System
The term energy performance is an indicator related to the building quality in case of proper energy utilization [78]. Energy performance indicators (EPI) are quantitative means to predict the performance of energy. The most common EPI tool that is used in many buildings is the energy use intensities (EUI) with a measuring unit of kWh/m 2 . There are six factors mentioned in IEA Annex 53 project [79] which determine building energy efficiency. These factors are the envelope of the building, climate, building maintenance and operation, energy systems and building services, quality of indoor environment condition, and lastly, behavior and activities of building occupants. The energy performance evaluation used in building establishments can be divided into two types, i.e., performance-based as well as feature-specific. By utilizing performance-based tools, evaluations are performed by correlating data from the performance indicators (e.g., CO 2 emission or EUI) with approved benchmarks. When someone uses the feature-specific approaches, all credits can be achieved if they meet the definite features. The final result will be rewarded according to the achievement of the final destination that assessed the previous period [80]. Evaluating the building energy performance using performance-based tools is more precise and sometimes preferable as it acts based on quantifiable performance detectors. However, the improvement of the evaluation procedures and methods is difficult when analyzing the performance of desired buildings. Recently, a large number of applications and research on energy performance assessment systems related to this type of quantitative performancebased method have been reported [77]. Arno Schlueter et al. presented energy analysis by using the thermo-economic concept of exergy in regulating energy performance buildings. Exergy analysis could be considered as energy quality, which is matching between building form, construction materials as well as technical systems of the building. In the design process, data storage and parameter access building information are used. A different prototypical tool such as Design Performance Viewer (DPV) can be used for building information modeling where the energy and exergy are precisely calculated. These types of tools can estimate energy as well as exergy performance very fast for specific design and can facilitate necessary input parameters by utilizing non-expert decision characteristics. Graphical presentation and a few selected key performance indexes are used to visualize the experimental result [81].

Overall Thermal Transfer Value (OTTV) Concept
The overall thermal transfer value (OTTV) concept is utilized to calculate the heat gain or heat loss through the building envelope. The method of OTTV was introduced by the American Society of Heating, Refrigerating and Air conditioning Engineers (ASHRAE), and is used for evaluating the thermal performance of a building to see if it is compliant with the standard energy efficiency or not. Yik and Wan (2005) mentioned that the OTTV is a proper performance measuring the index of building envelope energy used in the regulatory control system. To standardize energy use, the investigators are researching many parameters. Heat transfer of buildings from outside to inside was measured by the OTTV concept. Lam (2000) revealed that there are mainly three factors which are needed to be considered for the appropriate calculation of heat transfer of building from outside to inside. These three factors are solar radiation through the glass, conduction through the glass and conduction through the opaque surface [82]. OTTV has good performance indices compared to the thermal transmittance; this is because the effect of direct solar radiation onto the envelope of the mechanically cooled building was considered. Pantong et al. [83], reported that the OTTV is applicable for calculating the heat received by building outer structure envelope per annum, which is obtained through the building's air conditioning system's cooling coil sensitivity. From the above concept, the energy use per year of the entire building can be related to the performance of air conditioning, lighting and wall. The lowest precondition of the building envelope was approved based on the principle of the lifetime costing procedure. Lifetime costing includes capital cost, maintenance and operating cost with labor energy cost [83]. Integrating solar as well as thermal radiation with convective heat transfer in a building's opaque wall surface may cause a net conduction heat flow through the wall material. Hui (1997) revealed that it is necessary to calculate OTTV for individual building walls for the same orientation, then the weighted average values are considered for further calculation. The method of OTTV calculations for both roof and wall are similar. Lam et al. (2005) represented that the OTTV calculation for the roof is simpler compared to the wall because it does not contain a large amount of glazing as the wall surface. Yik and Wan (2005) tried to find the compliance criteria for calculating OTTV of walls and roofs and measured the constant value of OTTV as 26.8 Wm −2 . OTTV is affected by the three parameters which are related to the outdoor environmental temperature as well as solar radiation, i.e., equivalent temperature difference (TD eq ), solar factor (SF) as well as the temperature difference between indoor and outdoor conditions. The better performance of the product is obtained at the lower value of the solar factor. During calculating equivalent temperature difference, both the conduction heat gain due to temperature difference between the indoor and outdoor environment and the effect of solar radiation on an opaque surface should be considered. Most of the developed countries practice OTTV equations to build building energy codes and improve thermal performance. There are different building design parameters considered during the design of different types of buildings. The main parameters are the window to wall ratio (WWR), thermal transmittance of building materials, shading coefficient (SC) as well as solar absorption (α). Saidur et al. [82] found that a lower value of WWR will reduce the OTTV. The WWR value for residential buildings is already very low, hence it does not have a significant impact on the OTTV. Saidur et al. also stated that OTTV is highly affected by the shading coefficient. Around 15% of reduction in OTTV regards shading coefficient 0.3. Applying a coating on window glass can easily minimize the shading coefficient of the window. Heat conduction through the wall can be reduced by minimizing the U-value of the wall. Up to 50%, OTTV can be minimized by reducing the U-value of the opaque wall to 1.0 Wm −2 K −1 . Saidur et al. also indicated that the value of OTTV can be lowered by reducing the value of wall absorption, such that a 50% reduction in OTTV can be observed when wall absorption is 0.3.
Chow and Yu [84] studied the technique for determining OTTV. Using only one index of energy implementation, such as OTTV, might be insufficient to produce a power-saving as well as cost-efficient building design [85]. In the case of building energy management along with other energy-efficient options, one of the criteria such as HVAC equipment and systems should always be considered. Yik and Wan studied the performance of OTTV as an energy-saving index for an air-conditioned building, where they identified several principle drawbacks of the OTTV concept and conducted simulations to investigate the effect of these drawbacks [86]. Envelope Thermal Transfer Value (ETTV), with a unit of W/m 2 , is a numerical index that calculates the average heat energy received by a building via its entire structure. This is a prescient renovation from the main equation of OTTV, in which relevant elements do not match perfectly with the equation of heat received [87]. The concept of using better building components for determining the three elements of heat received via an outer wall of a building was first discussed by Chou and Lee [88] and Chou and Chang [89]. The elements are (a) conduction of heat via windows; (b) conduction of heat via turbid wall and (c) radiation of sunlight via windows and ventilation. ETTV is specially researched for buildings in the tropical region where there is a huge temperature difference between the interior as well as the exterior environment of the building and quotidian temperature variation is quite small. Several principle factors influence the measurement of the ETTV value of any building [90]. These factors are the ratio between windows and wall, coefficient of shading, absorbance capacity of turbid wall, and lastly, the overall thermal coefficient of windows and wall.

Energy Intensity and Specific Energy Consumption
In some studies [91][92][93], the energy efficiency is calculated by using energy intensity indices such as the specific energy consumption (SEC) [94], which depends on the energy expenditure to the product generated specific system. To apply this calculation, physical, monetary and thermal-based indices could be used. The physical-based indices are normally used for a field and they are related to the total energy use (C) along with an activity index (A) that is calculated in physical terms such as the building constructed area.
The money-based indices are generally used to measure the efficiency of energy uses in a highly summarized means such as a country-based scheme, they depend on the energy utilization as well as activity indexes that are calculated in terms of money value: Among the three indices, the thermal indices are less used because these indices are more suitable for a device such as a boiler.
Generally, efficiency measurement is the opposite of its relative intensity measurement. Due to the different economic structures of different countries, the SEC metrics cannot be applied in measuring building energy efficiency [91]. Therefore, it is essential to use an energy efficiency index (EEI) for the true assessment of energy intensity with reference one.
This value can be gained either from past data series or the energy proficiency produced from the best practice analysis. Energy intensity (EI) is an energy performance indicator of a building that can be determined as follows: where ∑ n i AEC is the sum of energy used by different equipment i to n, TFA is the total floor area (m 2 ). Figure 14 shows commercial buildings' energy intensity in different countries. In Malaysia, the energy consumption of hospital buildings is higher compared to commercial buildings because of larger energy savings in commercial buildings. In 2005, the Ministry of Energy, and Ministry of Green Technology and Water in Malaysia has constructed a new energy-saving building that consumes energy intensity of approximately 114 kWh/m 2 [95]. Malaysian Energy Centre was registered as a not-for-profit company by 12 May 1998 that is being administered by the Ministry of Energy, Communications and Multimedia (MECM), Malaysia. They played important roles to accomplish the requirement for the national energy research centre which coordinates different activities specifically for planning, research, development and demonstration (R, D & D) in energy sectors. To develop energy saving in buildings, the Malaysian Energy Centre recently developed some low-energyconsumption buildings.

Energy Policies for Building
For building energy efficiency there are three aspects of policy proposals which are support and information programs, economic and financial incentives, and control and regulatory mechanisms. For control and regulatory mechanisms policy, it was categorized as the dominant policy instruments which are characterized by laws, standards and codes [97].
For developing energy efficiency as well as minimizing energy demand it is necessary to invest greater effort to achieve the effectiveness of the energy policy of buildings as suggested by Clift [98,99]. Rapid growth of the economic sector in China, was leading the country to be the biggest energy consumer and producing GHG emissions in the world. Due to these global issues, improvement of energy efficiency should be realized especially in the building sector, which contributes to the highest energy use. This resulted in the development of a legal system framework in endorsing building energy effectiveness in China. In 1994, the Ministry of Housing and Urban-Rural Development (MOHURD) begin to implement the relevant policies by building energy-efficient offices and energy-saving coordination groups. By 1997 in China, building energy-saving laws were declared by the Government of the People's Republic. The cities of Shanghai, Beijing, Tianjin and Shenzhen were selected by MOHURD for demonstration in better application of building energy effectiveness [97]. Shen et al. presented the contemporary progress of BEE (Bureau of Energy Efficiency) policy mechanisms in seven particular countries and areas by investigating their practices on this policy [98]. The governments play an important role in ensuring environment protection issues and encourage building energy efficiency development by introducing and practising BEE (Bureau of Energy Efficiency) policy [98,100]. In 1997, Denmark becomes the primary country in applying for the requisite building energy efficiency program. In this program, all Danish residential and commercial buildings are necessary to be assessed and the rates of their energy efficiency by legislation during the buildings are on sale by showing of assessment and rating results [98,101]. In the United States (U.S), Energy Policy Act 2005 (EPAct-2005) has been introduced. This policy offers tax incentives to the residential and commercial sector of buildings in giving support for energy-proficient building as well as market sharing of products [98,102].
In Japan and China, policy instruments for these two countries are analyzed at both national as well as regional levels. In these countries, buildings' energy savings have been improved by using Building Energy Saving (BES) policy [103]. At the national level, general BES policies were demonstrated while at the regional level, specific BES policies were presented, such as carbon trading programs [103]. In Japan, the rules and regulations on building energy proficiency were implemented under the "Energy Conservation Law" which was first selected in 1979. The focus of implementing these laws is to promote the industrial sector's energy proficiency. These laws are the foundation of Japanese energy policies and were amended several times [103].

Building Codes and Standards
Building development regulatory procedures are administrated by building codes, standards, rules and regulations. The main objective of implementing building codes and standards is to attain energy proficiency and minimize energy use in residential as well as commercial building sectors. Indian standard building codes are prepared as a handbook named "Central Building Research Institute" which was adopted by Roorkee [104]. The handbook presents information related to climatology, insulation of heat, ventilation, lighting and planning for functional design of non-residential buildings according to the Indian standards. This handbook provided general calculation procedures and fundamentals of numerous conceptual. However, for residential or any other buildings, this book did not provide any specific energy efficiency guidelines [105].
Stephen Berry reviewed that in the modification of Building Codes of Australia and other countries which are subjected to Regulation Impact Statement (RIS), that it is mainly focused on a net present value (NPV) of costs and advantages obtained through private, public well as social influences. Calculations of NPV for building energy regulatory comprises various factors such as:

•
It is necessary to save energy by applying advanced machinery as well as resources to meet the higher standard; • To change the level of standard, the construction, as well as maintenance cost, also fluctuates; • The costs associated with assessing the standard are specifically government administrative and industry development costs; • Asset value changing for improving energy performance; • Changing energy infrastructure because of improving energy performance.
According to the Building Code of Australia, the technical and economic evidence from energy models can be utilized to regulate the energy efficiency of buildings. The tests are mainly considered foreseen in increasing construction and maintenance costs as well as direct energy savings, which are related to projected higher performance rather than the broader societal impact of conversion to reduce energy consumption and low carbon impacts [106]. Salvalai et al. presented that most of the energy usage in the building sector could merely be avoided by presenting obligatory energy standards, which is efficient in reducing GHG emissions as well as slow minimization of non-renewable energy sources. There are a substantial number of directives as well as related laws to improve energy proficiency in the European Union. Two mechanisms analyzed which participate in energy evaluation in the building sector are energy regulation and certification [107,108]. Under EPBD a building's energy certification is required in the member state, which has become a significant part of energy-saving [108,109]. In the mid-1970s use of regulatory codes for maintaining energy consumption in buildings became apparent [108]. To attain the EU climate and energy objectives, protecting the transition and low carbon economy acts as a key which can be obtained by developing the energy efficiency of buildings. The factors which can help to develop the energy proficiency of EU buildings are specifically that it is required to reduce 20% GHG emissions with 20% energy recovery by 2020, and use of renewable energy share by 20% in the EU gross final consumption [108].
In Hong Kong, more than 80 commercial buildings showed that their building energy code upgraded energy effectiveness and reduction of air pollution [110]. In China, the national building standard code helped to drive 62% energy saving in the residential public building sector; on the other hand, the United Kingdom building code exposed energy savings up to 75% [111,112]. National Council of the United States published the USA Model Code for Energy Conservation regarding the first oil crisis in 1977 [106,113], while in 1978, New Zealand introduced mandatory insulation requirements [106].
Building standard and codes have changed over time, and the construction market of building materials continuously changed over time. The new products always replace the existing product, which would be more cost-effective. According to the building codes, the minimum energy requirement is set which reduces the required energy price for heating and cooling. The new products are more energy efficient, which reduces the energy cost over time [114]. The requirement of energy-efficient building codes develops intelligent solutions and improves products. All the member states of the European Directive on Energy Performance in Buildings set building standards and are regularly reviewed and updated. The review time is not longer than five years. The International Energy Conservation Code (IECC) and the ASHRAE are also updated their building codes and standards regularly [114].
The European Building Codes describe the standards for the design and construction of EU buildings. The national standards of building codes can be issued by predefined legal status. The processes of approving the standard building codes at the national level are as follows [115]: (i) At first, the draft standard code has to be established by a group of experts or organizations with verified skills in the corresponding field. (ii) Then the draft is required to be submitted to a broad audience for public discussion.
(iii) After that, the draft is required to be presented to the national building authority for public consultation with a limited schedule. Then a final draft is required to be submitted to the national standard authority for ratification. (iv) After ratification, the final draft of standard building codes is recommended for implementation. (v) The building standard is published and comes into force.
In the USA, regional standard bodies are responsible for local building codes. The State Building Authorities must obey the standard building codes to approve the design of new building construction [115].

Building Codes for Energy Efficiency
Building energy codes are required when undergoing major renovations of existing and new buildings to obtain minimization of energy effectiveness requisite. Well-designed, implemented and enforced codes can guide elimination of inefficient construction technologies without increasing project costs. Generally, the building codes specify the requirements of "thermal resistance" of building envelope and windows, air leakage reduction and minimization of cooling and heating equipment effectiveness. Due to these simple procedures, energy usage could be reduced by 30% or more, resulting in cost savings for consumers and businesses. Building energy codes also reduce air pollution, greenhouse gas emissions and peak energy demand. Based on these benefits, most of the states implemented building energy codes in commercial as well as residential building construction [116].
In the 1970s, building energy regulations-in other words, building energy codescome up as the main tool for minimizing energy usage and developing energy proficiency in buildings. Depending on the scope, energy quality as well as quantity of requirements, and way of acquiescence as well as enforcement method, the building energy principles have appeared and have been continuously altered from the 1970s onwards. Lombard et al. reviewed Heating, Ventilation and Air conditioning (HVAC) techniques of energy effectiveness in various buildings especially for non-residential, i.e., commercial, institutional and hospital buildings. In building energy-related facilities, the HVAC system is the greater energy usage device that consumes 10-20% energy in an advanced country. Producing minimum energy efficiency in HVAC systems will lead to profit to the building energy codes. There are different types of HVAC systems in several energy regulations which have different significance and approximations depending on climate effect as well as building construction habits [117].
In the US, the energy efficiency building codes were developed by the 1970s because of their national concern such as global warming and energy crisis [118]. The residential building energy consumption could be reduced by implementing building insulators which were developed by the Council of American Building Officials in 1983 [119]. After the execution of the Federal Energy Policy and Conservation Act (EPCA), this energy efficiency building code, i.e., Model Energy Code (MED) became an obligatory policy in the residential sector. After that, the model energy code was edited and updated for a few periods [118]. The International Energy Conservation Code (IECC) was widely used as a mandatory energy policy in various states for the residential building sector in the USA Laitner et al. revealed that implementing appropriate building codes means improving the efficiencies of different devices usually used in buildings such as heating, ventilation and cooling system equipment; proper insulation of floors, ceilings and walls and appropriate position of doors and windows [120]. Approximately 40 to 50 billion barrels of oil could be saved through energy-efficiency measurement by the end of 2030, which is equal to around 12 years saving energy of residential consumption in the USA as reported by Laitner [121]. Koirala et al. revealed that appropriate application of building codes reduces the energy consumption of households by 1.8% of electricity, 2.8% of heating oil, 1.3% of natural gas and 7.54 million tons of CO 2 per year [122].

Discussion
In this era of energy poverty, building energy use is the most important issue that needs to be mitigated. In most countries, buildings are considered as the maximum energy use sector which exceeds other energy consumption sectors, i.e., industry and transport. Building energy consumption reduction by efficient technologies will lead to energy conservation, as well as energy savings which alleviate the energy shortage. Numerous factors affect building energy use within which efficient use of energy technologies and policies are crucial. A comprehensive review has been made of building energy-efficient technologies and policies. Efficient building energy technologies such as UFAD, an effective HVAC system, double-glazed windows, efficient motor use and optimization of energy use extensively reduce building energy consumption. Worldwide recognized building energy policies are also helpful to minimize energy usage in buildings. The technologies and policies described in this study can be utilized to minimize building energy use for new or existing buildings and therefore to mitigate energy poverty in their own country as well as all over the world. This review study will guide future researchers and building designers to further update the technologies and policies and to design energy-efficient buildings.

Conclusions
Based on the comprehensive review, it was found that there are numerous promising technologies for efficient energy usages and energy saving in the building sector. As buildings consume approximately 40% of total energy worldwide, the appropriate utilization of building energy-saving techniques has a good impact to construct a building that is more energy proficient. The important findings are as follows: 1.
Using double-glazed windows instead of single-glazed windows discovered that about 39 to 53% of energy could be saved for commercial buildings in the UK.

2.
Approximately 11% of total building energy could be saved by proper maintenance of central heating equipment. 3.
Using advanced design and automatic control techniques of HVAC equipment could effectively save about 20% of the total building heating load. 4.
Using efficient motors can also save building energy. The application of VSD makes the motor more efficient and reduces 20-70% energy consumption compared to the conventional motor. 5.
Using advanced mathematics and optimization tools such as ANOVA improves building performance and reduces energy consumption. 6.
Energy policies, control and regulatory mechanisms are very important to improve energy consumption scenarios and efficient energy use in buildings. Private entrepreneurs, industries, governments and different agencies are needed to come forward to implement these energy policies. Chinese national building standard code reduces 62% energy consumption in the public building sector. The United Kingdom's national building code helps to save up to 75% building energy.
The appropriate use of building energy-saving techniques can save a huge amount of energy which would be appreciated to minimize the environmental impact and improve building sustainability. All necessary information has been collected and summarized for different types of buildings, locations and weather situations based on the available literature. The numerical values may vary depending on applications, locations and weather situations in real case scenarios.  Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable, this is review paper.