Sino-American Building Energy Standards Comparison and Recommendations towards Zero Energy Building

Building energy conservation has gained tremendous interest since the 1970s energy crisis. Building energy standards have been established as prescribed guidelines for energy savings in buildings worldwide, among which those from China and the United States of America (USA) are representative of their advanced concept, comprehensive content and prospective guidance. This paper collected and generalized the main building energy standards in China (GB50189, JGJ26, JGJ134 and JGJ75) and the USA (ASHRAE 90.1), in terms of updating history, current status, energy saving potential and future development directions. Furthermore, a qualitative and quantitative comparison of the selected standards was performed. The results show that China has a more intact and comprehensive building energy standard system, better implementation, higher improvement in energy saving rates, and a more perspicacious upgrade towards zero-energy target, which results in effective energy savings in buildings. The ASHRAE standards have more fixed chapter framework, integrity and independence between versions, more detailed classification of building envelope and HVAC systems but less effective energy-saving effect and relatively poor implementation. The actual efficiency of standards in building energy saving is synthetically determined by the standard content, efficient implementation and explicit guidelines for future development, which is achieved through four main procedures. Based on the results, recommendations have been proposed for the future development of building energy standards with the ultimate goal toward zero energy buildings.


Introduction
Due to global concern of climate change and CO 2 emissions, building energy conservation is of pivotal significance as building energy consumption accounts for approximately 30% of world total energy consumption [1,2]. Numerous countries have taken building energy conservation as priority for GHG emissions reduction and climate change mitigation.
Ambitious goals have been set for building energy-savings in different countries and regions [3]. The United Nations Framework Convention on Climate Change has set the goal to maintain the global temperature rise within 2 • C above the pre-industrial level towards 2050 through the reductions in carbon emissions [4]. Energy Performance of Buildings Directive in Europe has set the goal to build net zero energy buildings and to reach a neutral energy situation in the whole stock by 2050 [5]. Furthermore, Canada aims to reduce 50% of greenhouse gas emissions through building energy savings and green roof applications by the end of 2030 [6]. In order to respond to these policies and regulations related to zero-energy building, many studies have been conducted to estimate the effect of zero energy building constructions to the whole building stock and global climatic conditions [7][8][9]. Salem investigated the impact of zero energy building constructions on the built environment through simulations of zero-energy building standards to existing UK commercial and residential buildings [8]. They also adopted the zero energy building standards in a certain UK hotel and presented the energy performance and economic analysis of the retrofitted hotel [9]. Relevant studies all show promising results for zero energy buildings as a useful tool of climate change mitigation and building energy performance improvement.
In pursuit of these proposed targets, building energy standards have been compiled and implemented as prescriptive guidance for realization of building energy savings worldwide. The British government defined a mandatory building energy efficiency standard named "Part L", which is revised every 4-5 years. In this standard, energy efficiency of different buildings is stipulated, as well as parameter limits of building envelope and HVAC systems [10]. Canada established its national building energy standard "Model National Energy Code of Canada for Houses" (MNEECH) in 1997. This standard contains three approaches for compliance verification: prescriptive path, trade-off path, and performance path [11]. France also defined "RT 2000" standard for building energy saving goals of: (1) reducing greenhouse gas and energy use in buildings, (2) simplifying building regulations applications and (3) enhancing manufacturers competition and preparing transition to European regulations and norms [11]. America has long led the building energy standard development with two renowned standards and codes: ASHRAE 90.1 and IECC [12,13]. Both standards have substantial nation-wide influence and have been referred by other countries in establishment of their own national building energy standards. Therefore, America has always predominated the building energy saving domain until the rise of China in the 1980s, when China established its first building energy standard "Design standard for energy efficiency of civil buildings (heating residential buildings) JGJ26-86". Afterwards, China put great strength and efforts in the compilation and improvement of building energy standards [14]. Building energy standards for residential buildings in cold and severely cold regions have been defined and completed through the "3-step energy efficiency improvement" of 30%, 50% and 65% [15]. As for residential buildings in hot summer/cold winter and hot summer/warm winter regions, together with public buildings in different climate zones, corresponding building energy standards have been continuously improved according to the overall national energy-saving goals [16].
Technical exchange should be conducted between countries about building energy standards in terms of establishing purpose, standards system, chapters and content, key parameters, implementation status and future developing trends in order to enhance the scientific basis for future development with clear improving targets and perspicuous directions [10,11,17]. Zhang Shicong performed a comparative study of public energy codes and standards between foreign countries and China, with focus on outline, development history and current situation. Based on previous results, future targets of building energy codes and technology roadmaps have been proposed in different countries [15]. Lin Gu also conducted a comparative study of building energy efficiency standards between China and several developed countries, in which discrepancies in code coverage and stringency, and improvements to high level objectives of building energy efficiency standards have been identified. Furthermore, recommendations for future building standards improvements in China have been noticed [18]. Yan conducted a comparison of representative building energy standards between China and USA in 2015, mainly on commercial buildings in terms of standard framework, content and parameters. Suggestions for future development of public building energy standard have been proposed [19]. Lan also compared contemporary building energy standards between China and USA in 2015 based on logic deduction and simulation calculus from different aspects in order to provide new prospect and direction for future China building energy standards establishment [20]. However, these comparative studies were conducted based on contemporary situations which cannot reflect the latest developing trend of building energy standards in China and USA. Furthermore, these studies only compared basic standards' content between countries that overlooked the actual efficacy discrepancy of building energy saving and underlying causality between energy standards and actual building energy conservation realization. Especially under the 2060 carbon neutralization goal in China, precise positioning China among all countries dedicated in building energy conservation field through latest compar- ison with USA building energy standards has great significance for future building energy saving development as pioneering paradigm toward zero energy buildings.
Therefore, this paper collected and introduced the latest versions of main building energy standards of two representative countries: China (GB50189, JGJ26, JGJ134 and JGJ75) and USA (ASHRAE 90.1) and obtained insights from comparative study for future building energy standard improvements. Standards generalization in terms of updating history, current status and developing directions are presented in Section 2, followed by detailed introduction of building energy standards in China and USA in Sections 3 and 4 respectively. Pivotal comparative study has been conducted through qualitative (standards system, chapter framework) and quantitative analysis (building envelope, and HVAC system efficiency) in Section 5. Based on obtained results, standards disparities have been analyzed, together with underlying relationship between building energy standards and actual energy-saving efficacy which is discussed in terms of four main procedures in Section 6, leading to corresponding recommendations for future building energy standards development. Finally, in Section 7, under consideration of China's dual target of carbon peak and neutralization in 2030 and 2060, zero energy buildings have been set as the ultimate goal for future building energy standards developing direction with scientific and constructive conclusions.

Update and Improvement Process
China's building energy standards and ASHRAE 90.1 series have undergone several updates since the 1980s (Figure 1). Updating years and corresponding energy saving rate improvements for each revised version compared to the 1980s baseline can be noticed [21,22].

Higher Development Goals and Related Work
China and America have put great efforts in technical standards concerning the design, operation, construction and evaluation of nearly zero and zero energy buildings in order to answer to future mainstream research trend in building energy conservation. Figure 2 shows the comparison between these two countries on the efforts they have made in guidance and instructions of nearly zero and zero energy buildings construction.
ASHRAE has established the Advanced Energy Design Guides (AEDG Series) as fundamental technical support for further building energy conservation achievements. Three guides are defined: zero energy, 50% energy and 30% energy in order to realize zero energy, 50% energy savings compared to buildings that meet the minimum energy saving requirement of ASHRAE90.1-2004 and 30% energy savings compared to buildings that   [23,24].
Similarly, China's building energy standards have also achieved energy-saving rate improvements for each updating version compared to the 1980s baseline. The JGJ26 series have accomplished the so-called "three-step" energy efficiency improvement through promotion of energy-saving rate requirements by 30%, 50% and 65% in 1986, 1995 and 2010 versions, respectively. The newest JGJ26-2018 version issued a higher energy-saving rate requirement of 75% as a mandatory standard. The initial energy-saving rate of 30% compared to the 1980s baseline was set for the JGJ134 (2001) and the JGJ75 series (2003), and then revised with higher energy-saving rate requirement of 50% in 2010 and 2012, respectively [25,26].
In general, China's building energy standards have accomplished more ambitious energy-saving rate improvement than the ASHRAE 90.1 series for public buildings, with the ultimate energy-saving rate requirement of 75% in the JGJ26-2018 compared to the 62.4% in the ASHRAE 90.1-2019, indicating a leading role in building energy field.

Higher Development Goals and Related Work
China and America have put great efforts in technical standards concerning the design, operation, construction and evaluation of nearly zero and zero energy buildings in order to answer to future mainstream research trend in building energy conservation. Figure 2 shows the comparison between these two countries on the efforts they have made in guidance and instructions of nearly zero and zero energy buildings construction.

Higher Development Goals and Related Work
China and America have put great efforts in technical standards concerning the de sign, operation, construction and evaluation of nearly zero and zero energy buildings i order to answer to future mainstream research trend in building energy conservation. Fig  ure 2 shows the comparison between these two countries on the efforts they have mad in guidance and instructions of nearly zero and zero energy buildings construction.
ASHRAE has established the Advanced Energy Design Guides (AEDG Series) as fun damental technical support for further building energy conservation achievements. Thre guides are defined: zero energy, 50% energy and 30% energy in order to realize zero en ergy, 50% energy savings compared to buildings that meet the minimum energy savin requirement of ASHRAE90.1-2004 and 30% energy savings compared to buildings tha meet the minimum energy saving requirement of ASHRAE90.1-1999, respectively [27 Furthermore, each version of all three guides addresses a special building type. ASHRA AEDG Series are voluntary standards for designers and builders to realize high energ efficient building design, construction and operation with two zero energy guides, fiv 50% guides and six 30% guides that include parameters, energy efficiency, renewable en ergy utilization and HVAC technology requirements for corresponding building types.  ASHRAE has established the Advanced Energy Design Guides (AEDG Series) as fundamental technical support for further building energy conservation achievements. Three guides are defined: zero energy, 50% energy and 30% energy in order to realize zero energy, 50% energy savings compared to buildings that meet the minimum energy saving requirement of ASHRAE 90.1-2004 and 30% energy savings compared to buildings that meet the minimum energy saving requirement of ASHRAE 90.1-1999, respectively [27]. Furthermore, each version of all three guides addresses a special building type. ASHRAE AEDG Series are voluntary standards for designers and builders to realize high energy efficient building design, construction and operation with two zero energy guides, five 50% guides and six 30% guides that include parameters, energy efficiency, renewable energy utilization and HVAC technology requirements for corresponding building types.
While ahead of America, China has already launched and issued the technical standard for nearly zero energy building as the first national standards in 2019 for advancing energysaving in buildings. This standard covers the definition of ultra-low, nearly zero and zero buildings with specific energy saving rate of 50%, 60-75% and 100%, respectively. Building energy consumption intensity and renewable energy utilization rate have been added to the standard as compulsory requirements in the qualification evaluation, together with more stringent parameter requirements for building envelope and HVAC equipment or systems. In technical terms, China has more perspicuous future developing direction toward zero energy buildings and has already taken actions to establish and issue related standard as a solid foundation for future promotion and ultimate popularization. On the other hand, America is still compiling the nearly zero energy building standards with no officially issued national codes for design, construction, operation, or evaluation towards nearly zero or zero energy buildings.
In general, China's contemporary building energy standards and the ASHRAE 90.1 series are the most complete and representative building energy standards globally. Therefore, it is of great importance to conduct a comparative study on standards between these two countries for mutual development of future building energy standards.

Energy-Saving Milestones in China
China started building energy conservation in the 1980s, with remarkable accomplishment witnessed through the "three-step" energy saving strategy for residential buildings in severe cold and cold regions and processive modification of building energy standards in other climate zones [16].
The first building energy standard: design standard for energy efficiency of civil buildings (heating residential buildings) JGJ26-86 was established in 1986. This standard set the goal of 30% energy-saving rate compared to the HVAC energy consumption baseline for buildings built in 1981. It was subsequently revised in 1995 by improving the energy saving rate to 50% compared to the baseline consumption. In 2010, this standard was renamed the design standard for energy efficiency of residential buildings in severe cold and cold zones JGJ26-2010, with a final improvement of the energy saving rate of 65% to complete the "three step strategy". The latest version of this standard with a new energy-saving target of 75% was established in 2018.  Figure 3 shows the current stratified system of building energy standards in China. It can be defined as a "1 + 1 + 3 + 1" mode, i.e., one enforced energy saving standard in buildings, one energy standard in public buildings, three energy standards for residential building and one standard for nearly zero energy buildings. The first standard is a mandatory building energy standard covering all types of buildings in different climate zones, with threshold requirements for key parameters and energy efficiency, together with an auxiliary requirement for the use of renewable energy  The first standard is a mandatory building energy standard covering all types of buildings in different climate zones, with threshold requirements for key parameters and energy efficiency, together with an auxiliary requirement for the use of renewable energy for basic energy conservation in buildings. The enforced, public and nearly-zero building standards need to be adopted nationwide, while building energy standards for residential buildings are adopted by different climate zones in the country. For example, the JGJ26 series are only applicable to regions where central heating is required (severe cold and cold regions in Figure 3. The JGJ134 series should be adopted in the hot summer and cold winter regions in the middle and lower reaches of the Yangtze River and surrounding areas. Finally, the JGJ75 series are mainly adopted in hot summer and warm winter regions in the coastal areas of southern China. Moreover, building energy standards for temperate zones are being developed.

Implementation and Efficiency of Energy Savings
After being approved and issued by the government, local administrations will establish corresponding laws to stipulate implementation. Accordingly, construction unit must carry out the project by following the requirements of building energy saving standards and under the supervision of relevant departments. Figure 4 shows that implementation of building energy standards in design stage for new buildings reached 100% in 2010, with the construction stage implementation accomplished 95% in new buildings. At present, latest building energy standards are fully implemented in design stage nationwide. While construction implementation rate is nearly completed in most first-ties cities with prominent economic status [20]. The first standard is a mandatory building energy standard covering all types of buildings in different climate zones, with threshold requirements for key parameters and energy efficiency, together with an auxiliary requirement for the use of renewable energy for basic energy conservation in buildings. The enforced, public and nearly-zero building standards need to be adopted nationwide, while building energy standards for residential buildings are adopted by different climate zones in the country. For example, the JGJ26 series are only applicable to regions where central heating is required (severe cold and cold regions in Figure 3. The JGJ134 series should be adopted in the hot summer and cold winter regions in the middle and lower reaches of the Yangtze River and surrounding areas. Finally, the JGJ75 series are mainly adopted in hot summer and warm winter regions in the coastal areas of southern China. Moreover, building energy standards for temperate zones are being developed.

Implementation and Efficiency of Energy Savings
After being approved and issued by the government, local administrations will establish corresponding laws to stipulate implementation. Accordingly, construction unit must carry out the project by following the requirements of building energy saving standards and under the supervision of relevant departments. Figure 4 shows that implementation of building energy standards in design stage for new buildings reached 100% in 2010, with the construction stage implementation accomplished 95% in new buildings. At present, latest building energy standards are fully implemented in design stage nationwide. While construction implementation rate is nearly completed in most first-ties cities with prominent economic status [20].  Better implementation ensures more energy efficient design and construction of buildings, which in turn leads to lower building energy consumption and energy intensity. As can be seen from Figure 5 incited from the Annual Report on China Building Energy Efficiency, under strict implementation of building energy standards both in the design and construction stage, the energy intensity of public buildings in China shows a noticeable declining trend from 2012 to 2017, especially in 2017 when the drastic reduction rate of 16.59% compared to the previous year was realized through the latest version of the Design standard for energy efficiency of public buildings 2015 issued and implemented in 2015, which contributed to the total building energy intensity reduction rate of 24.86% from 2012 to 2017. According to the current energy saving status of buildings, a new energy saving goal for public and residential buildings in the next 10 years has been set to update the new version of standards.
noticeable declining trend from 2012 to 2017, especially in 2017 when the drastic reduction rate of 16.59% compared to the previous year was realized through the latest version of the Design standard for energy efficiency of public buildings 2015 issued and implemented in 2015, which contributed to the total building energy intensity reduction rate of 24.86% from 2012 to 2017. According to the current energy saving status of buildings, a new energy saving goal for public and residential buildings in the next 10 years has been set to update the new version of standards.

ASHRAE and ANSI
Building energy conservation in America started in 1973, after the severe strike of energy crisis. Afterwards, numerous organizations, standardization groups and professional institutes have taken charge of the formulation of building energy standards. With the first building energy standard ASHRAE 90.75 being published, the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) has become one of the most authoritative organizations specialized in building energy standard formulation. From then on, the Federal Energy Administration has announced that all states should adopt ASHRAE90.75 as the minimum building energy-saving requirement [15]. In 1989, ASHRAE has changed its standard number into the new name of energy standard for buildings except low-rise residential buildings 90.1, which became the uniform national standard.
Another representative building energy standards organization in America is the American National Standards Institute (ANSI). ANSI is a non-profit organization for standardization set up in 1918. This organization can approve the standard to become the national standard of the United States, yet it does not establish the standard itself. However, standards are formulated by the corresponding standardization groups and submitted to ANSI for approval. At the same time, ANSI plays a coordinating role between the federal government and the non-governmental standard systems in order to guide the national standardization activities.

Main Building Energy Standards and Codes
Among all building energy standards in America, the two most influential are the ASHRAE 90 and the IECC series [13,28].
ASHRAE 90 series are recommended standards that consist of two parts. First part is the ASHRAE90.1 for commercial and residential buildings with more than three stories,

ASHRAE and ANSI
Building energy conservation in America started in 1973, after the severe strike of energy crisis. Afterwards, numerous organizations, standardization groups and professional institutes have taken charge of the formulation of building energy standards. With the first building energy standard ASHRAE 90.75 being published, the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) has become one of the most authoritative organizations specialized in building energy standard formulation. From then on, the Federal Energy Administration has announced that all states should adopt ASHRAE90.75 as the minimum building energy-saving requirement [15]. In 1989, ASHRAE has changed its standard number into the new name of energy standard for buildings except low-rise residential buildings 90.1, which became the uniform national standard.
Another representative building energy standards organization in America is the American National Standards Institute (ANSI). ANSI is a non-profit organization for standardization set up in 1918. This organization can approve the standard to become the national standard of the United States, yet it does not establish the standard itself. However, standards are formulated by the corresponding standardization groups and submitted to ANSI for approval. At the same time, ANSI plays a coordinating role between the federal government and the non-governmental standard systems in order to guide the national standardization activities.

Main Building Energy Standards and Codes
Among all building energy standards in America, the two most influential are the ASHRAE 90 and the IECC series [13,28].
ASHRAE 90 series are recommended standards that consist of two parts. First part is the ASHRAE 90.1 for commercial and residential buildings with more than three stories, and the second is the ASHRAE 90.2 for low-rise residential buildings. However, a newer version of the ASHRAE90.2 from 2010 has not been approved due to the failure of reaching a consensus and therefore ASHRAE 90.2 is no longer adopted. On the other hand, the ASHRAE 90.1 has been adopted nation-wide ever since 1975.
The IECC series are voluntary codes established in 1998. They cover all building types and HVAC systems ever since it replaced ASHRAE 90.2 and Model Energy Code (MEC) and became the main building energy conservation guidance for low-rise residential buildings in America. The IECC series contain two independent codes: (1) the IECC residential building code mainly used for residential buildings below three floors, multiple independent residences, and R-2, R-3 and R-4 grade buildings below three floors; and (2) the IECC commercial building code mainly used for buildings not included in the scope of residential buildings [29].  Figure 6 shows that by the end of December 2019, the ASHRAE 90.1-2007 and the ASHRAE 90.1-2013 are still the most adopted standards for commercial buildings within 11 and 10 states, respectively. In addition, seven states still implement the ASHRAE 90.1-2010. ASHRAE 90.1-2016 is only adopted by five states with prosperous economic development, while ASHRAE 90.1-2019 has not been adopted by any states at all. Finally, six states do not have any certain statewide building energy standards, while five states adopted standards that are less stringent than ASHRAE 90. 1-2007 [30]. This is mainly due to the economic factors among different states, as well as the governmental attentions towards building energy saving. residential building code mainly used for residential buildings below three floor ple independent residences, and R-2, R-3 and R-4 grade buildings below three flo (2) the IECC commercial building code mainly used for buildings not include scope of residential buildings [29].

ASHRAE90.1 Implementation
ASHRAE90.1 series do not feature effective implementation after establishm ure 6 shows that by the end of December 2019, the ASHRAE 90.1-2007 and the A 90.1-2013 are still the most adopted standards for commercial buildings within 1 states, respectively. In addition, seven states still implement the ASHRAE 9 ASHRAE 90.1-2016 is only adopted by five states with prosperous economic ment, while ASHRAE90.1-2019 has not been adopted by any states at all. Finally, do not have any certain statewide building energy standards, while five states standards that are less stringent than ASHRAE 90. 1-2007 [30]. This is mainly du economic factors among different states, as well as the governmental attentions building energy saving.

Standard Systems and Characteristics
The American building energy standard system is mainly composed ASHRAE90.1 and the IECC series. The former is applicable to commercial or re buildings with more than three floors, while the latter is applicable to residential b with three floors or below. China's building energy standard system includes forced building energy standard (to be issued), one public building energy stand 50189), three residential building energy standards for cold and severe cold zones for hot summer and cold winter zone (JGJ134) and for hot summer and warm win

Standard Systems and Characteristics
The American building energy standard system is mainly composed of the ASHRAE 90.1 and the IECC series. The former is applicable to commercial or residential buildings with more than three floors, while the latter is applicable to residential buildings with three floors or below. China's building energy standard system includes one enforced building energy standard (to be issued), one public building energy standard (GB 50189), three residential building energy standards for cold and severe cold zones (JGJ26), for hot summer and cold winter zone (JGJ134) and for hot summer and warm winter zone (JGJ75), and one technical standard for nearly zero energy buildings. Comparison between China's and the United States' building energy standard systems is shown in Figure 7.
We can see that Chinese building energy standards have a more intact standard system with a stratified upgrading path towards enforced standards, national standards and zero energy building standard, while the American building energy system is focused on only two lateral standards.
As most of the residential buildings in China are higher than three floors, this paper mainly compares the versions of ASHRAE 90.1-2010 to 2019 with the latest standards of residential and public buildings in all climate zones in China. The comparison is performed qualitatively and quantitatively and includes the chapters framework, implementation, building envelope parameters, HVAC system parameters, energy-saving goals and developing trends.  We can see that Chinese building energy standards have a more intact standard tem with a stratified upgrading path towards enforced standards, national standards zero energy building standard, while the American building energy system is focused only two lateral standards.
As most of the residential buildings in China are higher than three floors, this p mainly compares the versions of ASHRAE 90.1-2010 to 2019 with the latest standard residential and public buildings in all climate zones in China. The comparison is formed qualitatively and quantitatively and includes the chapters framework, implem tation, building envelope parameters, HVAC system parameters, energy-saving goals developing trends. For the first category of basic knowledge, the ASHRAE 90.1 has four chapters rel to purpose, scope, abbreviations, definitions and acronyms and administration and forcement. On the contrary, China's building energy standards contain only two chap related to the general provisions and terms. For the main technical index category building standards contain chapters related to building envelope and HVAC system both countries. For additional technical index category, the ASHRAE 90.1 has conten lated to the service water heating and power, while China's building energy stand contain different technical indices for different climate zones and building types. In GB50189-2015, water supply and drainage, electric and renewable energy applica have been regulated. In the JGJ26-2018 standard, climate zone and energy consump have been stipulated in addition to the two aforementioned indices. The JGJ 134-2010 the JGJ75-2012 contain chapter related to the calculation index for indoor thermal e ronmental design, while the latter also regulates the comprehensive evaluation for bu ing energy efficiency design (Figure 8).   For the first category of basic knowledge, the ASHRAE 90.1 has four chapters related to purpose, scope, abbreviations, definitions and acronyms and administration and enforcement. On the contrary, China's building energy standards contain only two chapters related to the general provisions and terms. For the main technical index category, all building standards contain chapters related to building envelope and HVAC systems in both countries. For additional technical index category, the ASHRAE 90.1 has content related to the service water heating and power, while China's building energy standards contain different technical indices for different climate zones and building types. In the GB50189-2015, water supply and drainage, electric and renewable energy application have been regulated. In the JGJ26-2018 standard, climate zone and energy consumption have been stipulated in addition to the two aforementioned indices. The JGJ 134-2010 and the JGJ75-2012 contain chapter related to the calculation index for indoor thermal environmental design, while the latter also regulates the comprehensive evaluation for building energy efficiency design (Figure 8).

Building Envelope Requirements
China and America building standards both set requirements for building envelope performance parameters [19,31]. Figure 9 presents different classifications of building envelopes in building energy standards in these two countries [24,32,33].

Building Envelope Requirements
China and America building standards both set requirements for building envelope performance parameters [19,31]. Figure 9 presents different classifications of building envelopes in building energy standards in these two countries [24,32,33]. ASHRAE 90.1 contains of seven types of building envelopes: roofs, walls above grade, below grade wall, floors, overhanging floor, slab-on-grade floors and opaque doors. Roofs, floors and walls above the ground are all divided into three or more categories, while overhanging floor, slab-on-grade floors and opaque doors are all divided in two different categories. China's building standards contain less building envelope categories and feature different classification method. In this method, roofs, walls above and below ground, slab-on-grade floors and overhanging floors and doors contain only one specific type for each standard, while floors are divided into several types in the GB 50189-2015 and the JGJ26-2018. These two standards also contain categories that are related to the deformation joint and core plate under balcony door, which do not exist in the ASHRAE 90.1 contains of seven types of building envelopes: roofs, walls above grade, below grade wall, floors, overhanging floor, slab-on-grade floors and opaque doors. Roofs, floors and walls above the ground are all divided into three or more categories, while overhanging floor, slab-on-grade floors and opaque doors are all divided in two different categories. China's building standards contain less building envelope categories and feature different classification method. In this method, roofs, walls above and below ground, slab-on-grade floors and overhanging floors and doors contain only one specific type for each standard, while floors are divided into several types in the GB 50189-2015 and the JGJ26-2018. These two standards also contain categories that are related to the deformation joint and core plate under balcony door, which do not exist in the ASHRAE 90.1. The JGJ134-2010 and the JGJ75-2012. That is because they have more simplified building envelope classification due to climatic characteristics that emphasize cooling more than heating, thus building envelope requirements are less stringent.
Based on the performed review and analysis, we selected corresponding building envelope types from the ASHRAE 90.1 and China's building energy standards and compared their parameters requirements after converting them to the same unit. Figure 10 shows the building envelope parameters requirements from the ASHRAE 90.1 series and the latest China's building energy standards for public and residential buildings in terms of heat transfer coefficient (U value, W/m 2 K) of roofs, walls above ground and mass floors, together with the thermal resistance (R value, m 2 K/W) of walls below ground. For public buildings, GB50189 included all related building envelope parameters for different climate zones, while for residential buildings we selected the corresponding Chinese standards in all four climate zones for comparison: (1) In terms of U value of roof, both countries gradually decreased the U value from south to north, and restrictions in China's cold regions are decreased by 40-77.8% compared to those in hot summer and warm winter regions; ASHRAE 90.1-2019 is 33-50% lower than the limit of the 2010 version. The limit of the Chinese joint construction energy saving standard is higher than ASHRAE 90.1, and the limit of the energy saving standard in residential buildings in hot summer and warm winter, hot summer and cold winter and cold areas is also higher than ASHRAE 90.1-2019; (2) As for the U value of wall above the ground, the limit value has also decreased significantly from south to north (except for residential buildings located in hot summer and cold winter climate in China). ASHRAE 90.1 decreased the U value of wall by about 42.8-55%, while China decreased it by about 50%. The limit requirements of ASHRAE version have few changes and the limits of energy efficiency standards for public buildings in Chinese climate regions and residential buildings in severe cold regions are lower than ASHRAE 90.1-2019; (3) As for the R value of the below ground wall, energy standards in both countries set only limit requirements for cold and severe cold areas, which are gradually increasing from south to north. The limit value of severe cold regions in the USA increased by about 30.7-50%, and that of China increased by about 33% compared to the values of cold regions. ASHRAE 90.1-2019 is about 38.4-50% higher than the 2016 version limit. The limits in Chinese standards are lower than ASHRAE 90.1-2019 for public buildings; but higher than ASHRAE 90.1-2019 for residential buildings in sever cold regions. (4) As for the U value of floor, ASHRAE sets limits for all climate zones, while Chinese standards set limits only for public buildings in cold and severe cold regions, and for residential buildings in hot summer and cold winter regions, and cold and severe cold regions. The limit values in ASHRAE decreased by 40.1% and 52.4% for public and residential buildings, respectively, from the 2010 to 2019 version, while those in China decreased by 80% for residential buildings from the hot summer and cold winter region to the severe cold region. China's standard limits on public buildings are similar to ASHRAE, but those on residential buildings are much higher (from 73.3% to 80.3%) than ASHRAE.
Compared to the latest ASHRAE 90.1-2019 standard, China building energy standards have less stringent requirements for U values of roofs and floors and for R values of belowground walls, while they have more demanding requirements for U values of above-ground walls.

HVAC System Energy Efficiency Limits
The HVAC systems represent one of the most important components of the building energy saving standards. Tables 1 and 2 introduce main HVAC equipment and systems covered by the ASHRAE 90.1 and China's building energy standards. Sustainability 2021, 13, x FOR PEER REVIEW 12 of 20 Figure 10. Comparison of requirements for building envelope parameters between ASHRAE 90.1 and Chinese building standards.

HVAC System Energy Efficiency Limits
The HVAC systems represent one of the most important components of the building energy saving standards. Tables 1 and 2 introduce main HVAC equipment and systems covered by the ASHRAE 90.1 and China's building energy standards. ASHRAE 90.1 series contains 17 types of HVAC equipment and systems. Each revised version since 2013 added two, three and one HVAC equipment and system category, respectively. This indicates that a more comprehensive and advanced building energy performance evaluating mechanism with new devices indicators and system parameters has been created with the update of the ASHRAE 90.1. Compared to the ASHRAE 90.1, China's building energy standards contain only 10 HVAC equipment and systems, and have less abundant performance parameters that include COP, EER or IPLV. Both the ASHRAE 90.1 and China's standards stipulate the minimum efficiency of water chillers, unitary air conditioners, decentralized room air conditioners, multi-connected air-conditioning (heat pump) units, boilers and other frequently used equipment in building HVAC systems. Electrically operated unitary and applied heat pumps 6.8.1C Water chilling packages 6.8.1D Electrically operated packaged terminal air conditioners, packaged terminal heat pumps, single-package vertical air conditioners, single-package vertical heat pumps, room air conditioners and room air conditioner heat pumps 6.8.1E Warm air furnaces and combination warm air furnaces/air-conditioning units, warm air duct furnaces and unit heaters 6.8.1F Gas and oil-fired boilers 6.8.1G Heat rejection equipment 6.8.1H Heat transfer equipment 6.8.1I Variable flow air conditioners We have selected two representative HVAC systems in the version of ASHRAE 90.1-2010 to 2019 and compared their minimum energy efficiency limits as shown in Figure 11. It can be noticed that the ASHRAE 90.1 series made evident energy efficiency improvement for air conditioners (ICOP) values from 2010 version towards 2019 version. ICOP values of air-cooled air conditioners have been improved by approximately 30%, while ICOP values of water-cooled air conditioners have been improved from 9% to 13%. ergy performance evaluating mechanism with new devices indicators and system parameters has been created with the update of the ASHRAE90.1. Compared to the ASHRAE 90.1, China's building energy standards contain only 10 HVAC equipment and systems, and have less abundant performance parameters that include COP, EER or IPLV. Both the ASHRAE 90.1 and China's standards stipulate the minimum efficiency of water chillers, unitary air conditioners, decentralized room air conditioners, multi-connected air-conditioning (heat pump) units, boilers and other frequently used equipment in building HVAC systems.
We have selected two representative HVAC systems in the version of ASHRAE 90.1-2010 to 2019 and compared their minimum energy efficiency limits as shown in Figure 11. It can be noticed that the ASHRAE 90.1 series made evident energy efficiency improvement for air conditioners (ICOP) values from 2010 version towards 2019 version. ICOP values of air-cooled air conditioners have been improved by approximately 30%, while ICOP values of water-cooled air conditioners have been improved from 9% to 13%.  Figure 12 shows the comparison between revised corresponding minimum energy efficiency limits for air conditioners in China's standards and those in the ASHRAE 90.1-2019. It can be noticed that the energy efficiency limits in the ASHRAE 90.1-2019 are slightly higher than those of GB-50189 in China. Minimum energy efficiency limits gap between corresponding equipment in two standards varies from 0% to 15.56%, which indicates a relatively even level of HVAC system energy performance requirements between building energy standards in two countries.  Figure 12 shows the comparison between revised corresponding minimum energy efficiency limits for air conditioners in China's standards and those in the ASHRAE 90.1-2019. It can be noticed that the energy efficiency limits in the ASHRAE 90.1-2019 are slightly higher than those of GB-50189 in China. Minimum energy efficiency limits gap between corresponding equipment in two standards varies from 0% to 15.56%, which indicates a relatively even level of HVAC system energy performance requirements between building energy standards in two countries.

Comparative Analysis of Content
Based on the previous comparative study, the following conclusions can be drawn with a comprehensive understanding of the disparities in building energy standards between China and the USA from both a qualitative and quantitative perspective: (1) China and the USA have made great progress in building energy standards with a gradual improvement in the energy saving goals of 75% and 62.4%, respectively, compared to building energy consumption baseline in 1980; (2) Building envelope parameters in ASHRAE 90.1-2019 have more stringent regulations than those of China building energy standards except for the wall above ground, while HVAC efficiency parameters requirements are close. (3) China features more effective actual implementation across the nation, while ASHRAE90.1 has not been adopted in any states in the USA by the end of 2020, most

Comparative Analysis of Content
Based on the previous comparative study, the following conclusions can be drawn with a comprehensive understanding of the disparities in building energy standards between China and the USA from both a qualitative and quantitative perspective: (1) China and the USA have made great progress in building energy standards with a gradual improvement in the energy saving goals of 75% and 62.4%, respectively, compared to building energy consumption baseline in 1980; (2) Building envelope parameters in ASHRAE 90.1-2019 have more stringent regulations than those of China building energy standards except for the wall above ground, while HVAC efficiency parameters requirements are close. (3) China features more effective actual implementation across the nation, while ASHRAE 90.1 has not been adopted in any states in the USA by the end of 2020, most states still adopt ASHRAE 90.1-2009 and 2013 versions, which are much older than the latest version. (4) China has already taken actions to establish and issue a mandatory technical standard for nearly zero and zero energy buildings, covering definitions, key envelope and energy-related parameter requirements, energy-saving goal and device efficiency regulations, while the USA has established only voluntary instruction of AEDG series for zero energy buildings mainly focused on initial design.
However, merely comparing the current standards is not enough. Building energy standards are only the first step of building energy conservation realization. Actual energy saving effect, namely, the real energy saving efficiency still needs further efforts being made subsequently which form the crucial factors of real energy saving efficiency realization.

Crucial Factors for Real Energy Saving Efficiency
Compilation alone is not enough to provide a complete picture of the potential effect of building energy standards in pragmatic energy conservation in buildings. The final realization of energy saving efficiency in real life should be through four main processes that are of crucial significance for building energy standards. As shown in Figure 13, the underlying causality between the actual building energy-saving effect and building energy standards consists of four key procedures extrapolated below, each with a phased target: (1) Implementation of the design stage: establishing the goal of energy saving Implementation in the building design stage guarantees a construction permission of building projects. Building energy standards regulate the energy saving goal achieved through the assigned energy saving targets for the building envelope and the HVAC system efficiency for planned buildings. The individual energy saving targets are further stipulated by the corresponding demand parameters of the building envelope and the HVAC system efficiency; (2) Implementation of the construction stage: energy saving potential This process realizes great energy saving potential in the building stock through rigorous implementation during the entire construction stage. Building energy standards strictly adhere to the construction field, which leads to the accumulation of new buildings that have a substantial reduction in energy consumption and intensity in the following years; (3) Building energy consumption statistics: actual energy-saving efficacy Building energy standards efficacy may be assessed through calculated building energy reduction results through newly built building area and corresponding gradually reduced building energy intensity realized through building energy standards regulations. Building energy consumption increase may slow down and gradually reach peak in certain time and then decrease according to latest research [34]; (4) Standards upgrade: a new energy saving goal Based on the calculated results of energy savings in buildings, the impact of building energy standards on the future building stock can be estimated by the trend of energy consumption marked by the corresponding peak value and occurrence time. A new energy saving goal can be set for a new version of the energy standards of building upgrading according to the peak value and occurrence time for the zero-energy goal achieved as soon as possible.
(4) Standards upgrade: a new energy saving goal Based on the calculated results of energy savings in buildings, the impact of building energy standards on the future building stock can be estimated by the trend of energy consumption marked by the corresponding peak value and occurrence time. A new energy saving goal can be set for a new version of the energy standards of building upgrading according to the peak value and occurrence time for the zero-energy goal achieved as soon as possible. Figure 13. Actual realization of energy saving efficiency through building energy standards.

Recommendations for Future Development
Based on the obtained results, innovative recommendations for the future development of China's building energy standards can be proposed from the perspectives of improving the standards, implementation process and the future upgrade direction: (1) Standards improvement (a) China's building energy standard system should be reformed into a more concise pack of "1 Mandatory + 1 Higher purpose". This would stipulate building energy saving requirements for all building types in different climate zones in a Figure 13. Actual realization of energy saving efficiency through building energy standards.

Recommendations for Future Development
Based on the obtained results, innovative recommendations for the future development of China's building energy standards can be proposed from the perspectives of improving the standards, implementation process and the future upgrade direction: China's building energy standard system should be reformed into a more concise pack of "1 Mandatory + 1 Higher purpose". This would stipulate building energy saving requirements for all building types in different climate zones in a single standard, while boost the development of zero energy buildings through technical standards for nearly zero and zero energy buildings; (b) Requirements of key parameters and energy saving goals should be improved according to the future building energy conservation trends. Technical strategies, such as passive building design, active energy technology and the use of renewable energy, should be in focus; and (c) Requirements for the use of renewable energy should be itemized and regulated, including solar, wind, hydrogen and geothermal energy. Related renewable energy technologies such as energy storage, PV generation, distributed energy system and multi-energy complementation must be stipulated by key parameters requirements and technical paths.
(2) Implementation of design and construction stages (d) Strict regulations on the implementation of standards should be guaranteed by potent governmental legislation on energy saving laws and regulations and provision of technical support and service in the building energy saving research and application; and (e) By establishing an effective supervision mechanism during the actual construction process, ineligible building projects that do not comply with the requirements of energy standards in buildings should be immediately rectified or halted if necessary. Buildings that fall out of the energy standards stipulation should never be built.
(3) Direction of standards upgrade (f) Zero energy building should be the ultimate goal of energy conservation in buildings under the urgent dual targets of carbon peak and neutralization. Therefore, the technical standard of ultra-low, nearly zero and zero energy buildings must be imperatively promoted throughout the country and gradually enforced according to the building energy consumption trend estimated for the institutional guarantee of carbon peak in 2030 and neutralization in 2060.

Conclusions
Building energy standards represent prescriptive guidance and technical instruction for scientifically developing the energy-saving process that can form a solid foundation in the future development of building industry. Numerous countries compiled and implemented building energy standards, such as the American building energy standard ASHRAE 90.1 series that can be considered as the representative and authoritative standard to draw lessons from. Therefore, the Sino-American building energy standards comparative study has been conducted between the standards of GB50189, JGJ26, JGJ134 and JGJ75 in China and the ASHRAE 90.1 standards in America. Based on the qualitative and quantitative analysis of building energy standards system, chapter framework, building envelope, efficiency requirements of parameters in the HVAC system and developing trends, we have drawn several conclusions that clearly demonstrate the strengths and weaknesses of building energy standards in the two countries, along with future improvements and directions: (1) China and the USA have made great progress in building energy standards with a gradual improvement in the energy saving goals of 75% and 62.4%, respectively, compared to building energy consumption baseline in 1980; (2) At present, China's building energy standards have a greater improvement in energy savings, along with a more intact standard system and a stratified upgrading pathway towards enforced standards, national standards and zero energy building standards. National standards comprehensively match different climate zones for different building types; (3) More efficient implementation of building energy standards in China has realized a greater energy saving potential by lowering building energy intensity, and thus a substantial reduction in building energy consumption, while in the USA most states still adopt ASHRAE 90.1-2009 and 2013 versions, which are much older than the latest version in 2019. However, by the end of 2020, ASHRAE 90.1-2019 has not yet been adopted; (4) China has already taken actions to establish and issue a mandatory technical standard for nearly zero and zero energy buildings, while America only established a voluntary instruction of AEDG series for zero energy building; (5) In the future, China should add new technical content to the building energy standards aimed at regulating the use of renewable energy based on key parameters and related technology, while reinforcing efficient implementation during the construction stage trough strong governmental legislation and a supervision mechanism; and (6) Future energy saving goals in building energy standards must be set with the ultimate purpose of China's carbon peak and neutralization target in 2030 and 2060, respectively, according to the actual trend of building energy consumption under the real influence of the implementation of building energy standards. Zero energy buildings will be the final goal in the building energy conservation industry that can be realized through the gradually applied building standards for ultra-low, nearly zero, and zero energy buildings in the next 30 years.
Author Contributions: Y.F. and S.Z., led the design of the research and manuscript preparation. Y.F. conducted methodology, data analysis and conclusions generalization. S.Z. further discussed the results and guided manuscript revision. W.X. supervised all research works and X.C. commented the manuscript at all stages for final manuscript preparation. All authors have read and agreed to the published version of the manuscript.