Towards Zero: A Review on Strategies in Achieving Net-Zero-Energy and Net-Zero-Carbon Buildings

Abstract

The establishment of net-zero-energy and net-zero-carbon buildings can offer significant opportunities to reduce environmental impact in the building sector. Several successful net-zero-energy buildings highlight the feasibility of reducing energy consumption via energy-efficient strategies and the use of renewable energy technologies. To comprehend the existing innovatory designs, techniques, and practices employed to achieve net-zero-energy buildings, this research aims to review the up-to-date advancements in net-zero-energy building practices. The utilization of embodied carbon assessments to achieve the net-zero status of buildings is explored. The findings indicate an escalating global interest and participation in the field of study, and reveal three major areas related to net-zero-energy buildings: multidisciplinary approaches, energy systems, and guidance, which together cover thirteen subfields. The role of life cycle assessment in buildings is emphasized, offering insights into the role of embodied emissions relative to operational emissions over the entire life cycle of a building. In the end, possible future study directions are outlined, including balancing energy efficiency with sustainability, and assessing the impact of design on emissions and economic outcomes. These areas collectively contribute to transforming sustainable building concepts into reality.

1. Introduction

The concept of net-zero-energy buildings (nZEBs) and net-zero-carbon buildings (nZCBs) has gained significant attention in recent years as a crucial concept to tackle climate change and diminish greenhouse gas emissions. CO₂ emissions worldwide are heavily influenced by the building sector [1], emphasizing the need for appropriate strategies. The environmental impact of buildings is assessed using methods of energy balance analysis, emission calculations, and comparative assessments. nZEBs refers to buildings that require minimal energy, wholly powered by renewable energy sources (RESs). On the other hand, nZCBs account for both the operational energy use of a building and the carbon emissions embedded in its materials, along with construction processes across its entire life cycle, including disposal. Both nZEBs and nZCBs aim for high energy efficiency and utilize renewable sources of energy, either on-site or off-site, to achieve a net-zero energy or net-zero carbon balance [2,3,4].

Consequently, many research efforts have targeted net-zero energy achievement through innovatory technologies and strategies [5,6,7,8]. The studies suggest that nZEBs can be achieved by reducing energy usage through measures that boost energy efficiency and by implementing renewable energy solutions to fulfill the remaining energy requirements [9]. Upon establishing a net-zero-energy building, an extensive strategy offered for energy saving, environmental protection, and CO₂ emission reduction is provided. However, these buildings also rely on renewable energy sources as well as sophisticated building service systems, necessitating comprehensive evaluation methods to support their development [10].

Taking it a step further, the achievement of nZCBs is realized when embodied carbon, comprising the carbon emissions from producing and transporting construction materials, is also offset. For example, Monahan and Powell [11] conducted a life cycle assessment of modern construction methods in housing. The results from this specific study showed that when considering embodied carbon, concrete accounted for the largest share, representing 36% of the material-related total. Moreover, Akbarnezhad and Xiao [12] reviewed approaches to lower the embodied carbon in buildings and ways to measure the embodied carbon. They highlighted the importance of considering not only operating carbon but also the embodied carbon in the life cycle of a building.

Given the rapid advancements in nZEBs and nZCBs, we would like to conduct a detailed, systematic, and up-to-date analysis of contributions within this discipline. By specifically considering embodied carbon in building life cycle assessments, this analysis bridges the gap between nZEBs and nZCBs, utilizing data from January 2013 to April 2024. This study reviews case studies and existing research on the progress towards establishing both nZEBs and nZCBs, aiming to evaluate the transferability of current approaches and findings and suggest future research directions by identifying gaps and priorities. To guide the study, two research questions were presented as follows:

• What are the current innovative strategies, building design approaches, and advanced materials driving global trends toward achieving nZEBs, emerging since 2013?
• When considering the impact of embodied emissions, how can the assessment strategically quantify and compare the total carbon footprint of nZEBs and nearly nZCBs, thereby evaluating the potential to facilitate the development of nZCBs?

In this paper, the Section 2 outlines the methodological steps that were undertaken to select the papers for review. The Section 3 provides a literature content analysis of the reviewed papers. From the Section 4, Section 5 and Section 6, key findings from the literature are sorted, summarized, and discussed. Finally, the Section 7 presents the conclusion.

2. Research Methodology

To examine the development of research in the domain of nZEBs and nZCBs, a systematic literature review of a time period of 12 years was carried out. This review was organized into three key stages: gathering and retrieving literature, analyzing the content, and discussing domain knowledge. The literature search utilized the database of Scopus. The content analysis involved keyword co-occurrence analysis, paper publication statistics, and qualitative content analysis to map out the main application areas, existing challenges, and future research potentials. A discussion on domain knowledge was proposed from the gathered information. Figure 1 presents a workflow for the literature review process, with corresponding details for each step.

The first step involved retrieving articles from the Scopus online database, selected for its broad access to academic resources, with increasing use from various countries and knowledge domains [13]. The search utilized a specific set of keywords within titles, abstracts, and keywords of the papers. The following set of keywords was used: (TITLE-ABS-KEY (“Net-Zero Carbon Building” OR “Net Zero Carbon Building” OR “Net-Zero Energy Building” OR “Net Zero Energy Building” OR “NZCB” OR “NZEB” OR “NZE” OR “ZEB” OR “ZCB”) AND TITLE-ABS-KEY (“embodied carbon” OR “embodied emissions” OR “embodied energy”)). This initial search yielded 110 papers from Scopus. Subsequently, English-language papers were reviewed for their relevance to the research theme and category of literature, the time period, and the subject matter. The literature search commenced in January 2013, including only “articles” published until April 2024, and covered subject areas in “Engineering”, “Energy”, “Environmental Science”, “Social Sciences”, “Materials Science”, “Computer Science”, and “Mathematics”. Applying these filters, a total of 64 papers were exported into a spreadsheet for further bibliometric analyses. In the next step, the bibliometric analyses aimed to assess the connection of the identified papers to the research topic using keyword co-occurrence mapping, year-on-year publication trends, top research topics, and region-wise publications.

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) scheme was followed during the detailed content analysis (Supplementary Materials). A PRISMA flow diagram outlining the literature filtering process is presented in Figure 2. Three papers were excluded due to their lack of relevance or insufficient information, failing to address the research questions adequately. Consequently, 61 papers were thoroughly reviewed to meet the research objectives. These papers, along with three papers recommended by experts, were classified into three groups categorized by their research fields and subfields within the discipline. This classification will be discussed in Section 4, Section 5 and Section 6, respectively. Relevant information from these papers was extracted to facilitate a discussion on domain knowledge.

3. Literature Content Analysis

3.1. Keywords Occurrence and Co-Occurrence Analysis

To effectively highlight the key terms and enhance comprehension of the core concepts, a word cloud, as shown in Figure 3, is created based on index keywords and their occurrences. In this visual representation, the relative size of each word corresponds to its frequency of occurrence, allowing more common terms to appear in larger fonts. Dominating the visualization, “Life cycle” emerges as the most recurrent term with 34 mentions, followed by “Zero energy buildings”, which appears 24 times, and “Buildings” is noted 23 times. Additionally, terms like “Energy efficiency”, “Architectural design”, “Environmental impact”, and “Embodied energy” are also significantly featured due to their frequent mentions.

On the other hand, keyword co-occurrence mapping provides valuable insights into the scope and relevance of the selected papers within this field. Utilizing VOSviewer software v1.6.20, this analysis includes 51 of the 649 index and author keywords that appear more than five times each in the co-occurrence map. The keywords are organized into four clusters based on their interconnections.

Table 1. The occurrence frequencies of keywords.

Cluster	Keywords	Occurrences
Cluster 1 (14 items)	Architectural design	22
	Energy utilization	13
	Carbon	12
	Intelligent buildings	11
	Building	10
	Sustainable development	9
	Construction	8
	Emission control	8
	Embodied carbons	7
	Energy use	7
	Carbon emission	5
	Carbon emissions	5
	Construction industry	5
	Ecodesign	5
Cluster 2 (14 items)	Life cycle	35
	Zero energy buildings	24
	Energy efficiency	23
	Embodied energy	17
	Life cycle assessment (LCA)	9
	Zero energy building (ZEB)	9
	Life cycle energies	8
	Net-zero energy buildings	8
	Global warming	7
	Renewable energy resources	7
	Energy performance	5
	Life cycle energy	5
	Operational energy	5
	Renewable energies	5
Cluster 3 (13 items)	Buildings	23
	Environmental impact	18
	Greenhouse gases	15
	Embodied emissions	12
	Gas emissions	12
	Energy conservation	11
	Structural design	9
	Carbon dioxide	8
	Building materials	7
	Photovoltaic cells	7
	Zero emission building	7
	Design	6
	Norway	5
Cluster 4 (10 items)	Life cycle assessment	15
	Climate change	10
	Housing	10
	Life cycle analysis	7
	Residential building	7
	Carbon footprint	5
	Cost benefit analysis	5
	Decision making	5
	Investments	5
	Thermal insulation	5

displays the occurrence frequencies of these keywords. Figure 4 visualizes the co-occurrence map. In the co-occurrence map, the size of each bubble indicates the frequency of the keyword, with larger bubbles representing a higher frequency. The width of the links indicates the strength of associations, with thicker links representing stronger associations.

3.2. Research Trends

Table 2. Trends in research output over time.

Year	Number of Publications
2013	2
2014	3
2015	3
2016	9
2017	4
2018	9
2019	3
2020	4
2021	5
2022	9
2023	7
2024 (until April)	6

and Figure 5 display the distribution of publications by year within the research area of interest for the papers retrieved from the database, showcasing the trends in research output over time. There are overall fluctuations in the number of publications from year to year, with some years showing similar numbers. On the other hand, these data reveal increases in research activities, notably in the years 2016, 2018, 2022, and 2023, indicating periods of heightened interest and activity in the field.

Additionally, to analyze the trends in research topics, the study employs bi-grams instead of single words within the publication titles. In this context of study, a bi-gram is defined as a pair of consecutive words derived from a sequence of tokens.

Table 3. The occurrence frequencies of bi-gram words.

Term	Frequency
Zero energy	23
Life cycle	18
Net zero	18
Energy buildings	13
Nearly zero	10
Energy building	9
Case study	7
Cycle energy	7
Zero emission	6
GHG emissions	5
Cycle assessment	4
Zero carbon	4
Building design	3
Embodied energy	3
Embodied GHG	3
Energy efficiency	3
Environmental impact	3
Analysis life	2
Building envelope	2
Buildings case	2

and Figure 6 summarize the occurrence of bi-gram words, drawing attention to “zero energy”, “life cycle”, and “net zero” as the three most frequently mentioned topics. This approach provides an understanding of the themes and areas of focus in the research.

3.3. Region-Wise Publications

To examine the geographical origins of the authors, country names are extracted from the papers’ affiliation information. This information provides details about the likely geographical locations of the authors. The data are organized to quantify the scholarly contributions by country based on the number of papers produced. In the dataset of publications, a total of 28 distinct regions are present. This analysis reveals that Italy and Norway lead in contributions, with a total of 13 papers and 12 papers, respectively. They are followed by the United States, with a total of seven, Australia, with a total of six, and Switzerland, with four. Notably, Canada, Peru, and Portugal have three papers each, while Belgium, China, India, Iran, Spain, Sweden, Turkey, and the United Kingdom have a count of two papers each. The results are presented in

Table 4. Number of publications by country.

Country	Number of Papers
Italy	13
Norway	12
United States	7
Australia	6
Switzerland	4
Canada	3
Peru	3
Portugal	3
Belgium	2
China	2
India	2
Iran	2
Spain	2
Sweden	2
Turkey	2
United Kingdom	2
Brazil	1
Chile	1
Croatia	1
Czech Republic	1
Ecuador	1
Estonia	1
Germany	1
Greece	1
Malaysia	1
Netherlands	1
New Zealand	1
South Korea	1

, listing the countries and their respective paper counts. Additionally, Figure 7 offers a visual representation of the distribution.

3.4. In-Depth Review of Content

The papers filtered through the PRISMA scheme (Figure 2) are classified into three groups based on their research fields and, more specifically, into their respective subgroups. The topics can be classified into three major fields and thirteen subfields. Consequently, the paper categorizes the literature into the following sections:

• Section 4: Advancing Towards nZEBs, which focuses on sustainable building practices for nZEBs. This includes discussions on approaches, design optimization, and retrofitting existing buildings.
• Section 5: Energy System, which explores photovoltaic technologies, the environmental impact of renewables, and choosing appropriate energy systems for nZEBs.
• Section 6: Governance, which examines definitions and frameworks used to evaluate building performance in relation to nZEBs, along with a critical analysis of these methods.

The analysis of these articles reveals a clear emphasis on three key areas: sustainable building practices, energy technologies, and governance frameworks. A summary of the topics and the number of publications is presented in

Table 5. Summary of the topics and the number of articles.

Section		Topics		Sub-Topics		No. of Articles
Section 4	Advancing towards nZEBs	Section 4.1	Establishment of nZEBs	Section 4.1.1	Approaches, common characteristics or strategies	8
				Section 4.1.2	Emission evaluation	3
		Section 4.2	Optimization in Building Design	Section 4.2.1	Material innovation and calculation methods	7
				Section 4.2.2	Analytical and parametric modeling	8
				Section 4.2.3	Optimal design for climate and cost	6
		Section 4.3	Retrofitting Buildings	Section 4.3.1	Operation scenarios	1
				Section 4.3.2	Balancing between operational energy and embodied energy	3
				Section 4.3.3	From net-zero energy to net-zero carbon	3
Section 5	Energy System	Section 5.1	PV Technologies			3
		Section 5.2	Environmental Impact of Renewables			6
		Section 5.3	Choice of Energy Systems			4
Section 6	Governance and Others	Section 6.1	Definitions and Frameworks for Evaluating Building Performance			5
		Section 6.2	Critical Analysis			10

.

4. Results and Discussion: Advancing towards nZEBs

This section analyzes the practicalities of achieving nZEBs, focusing on three key areas of discussion: approaches, design optimization, and retrofitting existing buildings.

4.1. Establishment of nZEB

Section 4.1 explores various approaches for establishing nZEBs, analyzing common characteristics and successful strategies employed in existing nZEB projects. Methods for evaluating emissions associated with nZEBs are also examined to provide an understanding of their environmental impact.

4.1.1. Approaches, Common Characteristics, and Strategies

In this study, the term ‘nZEBs’ is used with an expanded scope that includes both the traditional focus on achieving net-zero energy consumption through design and operation, and the broader goal of zero carbon emissions. The feasibility of energy-efficient designs has been proven to achieve the goal of nZEBs, with a number of references to nZEBs already established around the world. They were set up for research purposes, and many of their designs can be adopted in other buildings to help decrease carbon footprints. For example, one of the successful cases is the Construction Industry Council (CIC) Zero-Carbon Building, a net-zero-carbon building intended for the hot and humid environment of subtropical Hong Kong [14]. In terms of design, one of the key considerations adopted in this case was the ability to respond to climate. For instance, so as to cope with the hot and humid weather during summer, a special building shape and a large open space were included in the layout so as to enhance ventilation. Furthermore, another crucial point for a ‘Zero-Carbon’ building was balancing energy demand and supply. In this aspect, a full life cycle assessment of the building, incorporating material manufacturing, construction, and a 50-year operational period, was conducted. Renewable energy facilities corresponding to consumption requirements were strategically planned on-site to attain carbon neutrality. Specifically, the two main on-site renewable systems were PV panels and a small-scale biodiesel combined cooling, heating, and power (CCHP) plant.

Current knowledge can facilitate the promotion and establishment of nZEBs. Rafiei and Adeli [15] performed a literature review of key concepts in sustainable design and their applications in high-rise buildings. The paper divides these concepts into categories including passive solar and envelope environment design, renewable energy resources, embodied energy reduction, and innovative structural systems, among others. In a similar manner, the potential for substantial reductions in CO₂ emissions through the strategic selection and combination of materials in steel-reinforced concrete (SRC) composite structures was evaluated [16]. Variations in design in terms of material selection, building height, and geometry, which are key structural elements, significantly influence the embodied carbon of residential buildings [17].

In general, energy performance and demand level were concerns in nZEBs. Various approaches are suggested and tailored in prior research to accommodate diverse situations. Taking into account the impact of various retrofit scenarios on both embodied and operational carbon emissions, the feasibility of refurbishing buildings in historic preservation zones to enhance energy performance, cost optimality, and carbon footprint reduction has been showcased [18]. A previous study demonstrated the assessment of building performance through the integration of multiple methods [19]. On a nearly zero-energy combined living and office space, energy balance was quantified, involving both dynamic simulation models and on-site data monitoring. In this way, scenarios that could optimize energy performance can be identified. This case study in Switzerland showed that using approaches such as automated blind control and night cooling can decrease cooling loads and create a more comfortable environment for occupants. Furthermore, the outcomes of simulations and empirical analysis showed coherence.

On the other hand, in the study carried out by Muñoz et al. [20], the overall energy consumption was assessed in a new school building in a time scope of two years. Based on the life cycle energy perspective, the pre-use phase, the use phase, and the expected post-use phase were all taken into consideration. So that the true impact of the building across the life cycle could be analyzed, the consumption of energy only took a portion of 56% out of the total demand. Similarly, several case study buildings located in Ireland were investigated from a life cycle viewpoint, aiming to highlight the impact of changes environmentally and economically [21]. Fitting into a temperate oceanic climate in this case, high thermal and air tightness performance can be a useful strategy to meet an nZEB standard when being compared to on-site-generated renewable energy sources. The approach can lower the life cycle economic costs of buildings and, at the same time, achieve an nZEB status.

4.1.2. Emission Evaluation

Another important subject of study in nZEBs is the comparative analysis of the two main contributors to emissions: building operation emissions (OEs) and embodied emissions (EEs) in building materials. There have been research efforts focusing on life cycle emissions evaluation [22,23]. For instance, Kristjansdottir et al. conducted a comparative life cycle assessment of eight houses focusing on CO_2e emissions over their life cycle. Within the context of this study, the authors highlighted that construction materials accounted for a significant portion of the total emissions.

A subsequent study by Hu [24] involving the life cycle scale was conducted. Various net-zero building metrics—Life Cycle Energy (LCE), Life Cycle Carbon Emissions (LCCEs), Life Cycle Embodied Carbon (LCEC), and Life Cycle Cost (LCC)—were studied. In this study, the definitions are as follows: LCE quantifies the total primary energy use, both operational and embodied, throughout all life cycle stages of a building; LCCE measures the total carbon dioxide equivalents emitted over the full life cycle of a building, including both operational and embodied emissions; LCEC refers to the total amount of carbon emissions associated with the materials and processes involved in the construction, maintenance, and disposal of a building, excluding operational emissions; and LCC encompasses all costs tied to the life cycle of a building, covering construction, operation, maintenance, and disposal. As a result, the study highlighted that life stage B6, the operational energy use, is the major contributor to LCE and LCCE, but it has a relatively smaller impact on LCC. On the other hand, life stage A3, which involves the manufacturing of products for building construction—possibly including both construction materials and the building itself—contributes the most to the LCCE metric.

4.2. Establishment of nZEB

Section 4.2 presents the design optimization techniques for nZEBs. This section will discuss the role of innovative materials and calculation methods in minimizing a building’s environmental footprint. Analytical and parametric modeling techniques used to optimize building design for both climate and cost-effectiveness are summarized.

4.2.1. Material Innovation and Calculation Methods

As innovative materials emerge, understanding the environmental effects of building materials becomes key to reducing the carbon footprint of building construction, operation, and end-of-life treatment by comprehensive life cycle assessment (LCA). In the study by Thiel et al. [25], results showed that the production of building materials which involved concrete, structural steel, photovoltaic (PV) panels, inverters, and gravel contributed the most significant environmental impacts. The choice of materials is crucial when considering the carbon emissions associated with construction. Due to the significance of materials in carbon emissions, many of the researchers also focused on discovering the feasibility of innovative materials, such as recycled aggregate concrete [26], natural materials of straw bale and earth [27], advanced building components [28], and timber [29], with the aim to decrease embodied energy in the process of building construction, operation, and end-of-life treatment when compared to conventional materials.

In addition, thermal insulation materials also have a dominating effect on the achievement of nZEBs. Five materials—extruded polystyrene (XPS), expanded polystyrene (EPS), rockwool (RW), glass wool (GW), and cellular glass (CG)—were evaluated by Yilmaz et al. [30] at various thicknesses to determine the optimal choice, with multiple scenarios generated. This assessment considered several factors, including the building envelope’s thermo-physical performance, energy consumption, cost across the life cycle, environmental effects, and thermal comfort. It can aid decision-makers in material selection during the design process.

Given that uncertainties inevitably exist during the process of examining the carbon footprints of buildings due to the considerable quantity of materials, which in turn influence the precision of the overall assessment of carbon emissions, Cornaro et al. [31] presented a study that measured the deviations in lifetime CO_2e emissions estimations for a net-zero educational building case study, particularly concerning operational energy. The study examined 19 building materials that exerted a considerable impact on the building’s total carbon emissions, including transportation and recurring embodied carbon. A distribution model was developed to represent the range of outcomes with uncertain parameters such as the building’s life expectancy, embodied CO_2e, and transport distance during its life cycle.

4.2.2. Analytical and Parametric Modeling

Analytical models concerning building features have been developed to provide an understanding of various interactions for simulation and optimization purposes. When designing a building prototype, Zalamea-León et al. [32] proposed a method to integrate a Building Energy Model (BEM) into the Building Information Modeling (BIM) process. This approach not only enabled the use of a renewable energy simulator, but also facilitated a cradle-to-grave life cycle assessment (LCA).

Considering the connection between the energy demand for the entire life cycle and the energy used for operational purposes in an autonomous building, the study by Grazieschi et al. [33] explored the transfer in energy demand from the operational phase to the embodied and end-of-life phases due to the integration of renewable energy. Additionally, it identified the optimal level at which the minimum life cycle energy demand was achieved. On the other hand, Fosas et al. [34] presented a simple-to-use model, ZEBRA, aiming to scope zero-carbon buildings in the beginning stage of design. With 33 inputs from the user, the model yields results for both embodied and operational carbon. These outputs can be useful for early-stage calculations, serving as preliminary results and for educational purposes.

Energy simulation software has been implemented to predict the energy and environmental (2E) performances of the buildings so as to optimize building design. In addition to simulation, parameters have been considered for evaluation such that the optimal design can be identified. For instance, a general approach of parametric computational evaluation considering the whole life cycle in different climates was carried out [35], aiming to reduce thermal loss and increase solar gains. In the study, thermal performance, embodied energy, and cost were investigated with different envelop technologies, such that the case with the highest energy efficiency and economic feasibility of buildings was identified.

On the other hand, Kamel et al. [36] demonstrated the feasibility of using parametric modeling with Grasshopper to design nZCBs. The research identified key parameters including geometry, construction technology, envelope thermal resistance, and on-site renewable energy production. It aimed to minimize operational carbon and embodied carbon. Furthermore, the study incorporated future climate data and 2099 weather conditions to account for potential climate changes.

A parametric design was adopted in a nZEB located in both Nordic and Mediterranean Climate Zones [37,38], specifically engineered to be climate-responsive. This approach aims to minimize the embodied GHG emissions while optimizing solar irradiation. The findings show that the proposed designs in the study, obtained through evolutionary solvers of Galapagos and Octopus, could establish optimized solar irradiation in the Mediterranean and the Nordic regions, with the consideration of model orientation, facade exposure, and the envelope surface area. In terms of vernacular houses, simulation-based, multi-objective optimization methods enhance passive cooling features [39]. Within this specific study, it was found that the best energy and environmental performance was observed with passive cooling elements like an external wall bonded with paraffin, a cemented ground floor false roof, double blue 6 mm air glazing, a 1.5 m overhang, a window-to-wall ratio of 32%, and a site orientation of 167 degrees.

4.2.3. Optimal Design for Climate and Cost

Studies on nearly nZEBs underscore the critical need to consider both embodied and operational carbon footprints, aiming to develop cost-optimal, climate-conscious designs. For instance, in cold climates, whether to increase insulation thickness to compensate for less renewable energy demand, as well as the ideal thickness for thermal insulation, are investigated [40,41]. Also, the impact of embodied carbon exerted as a whole life cycle aspect was studied; as a matter of fact, the embodied impact is often neglected when compared to the operational part by the current assessment framework [42]. In contrast, studies on vernacular houses in North-East India, which experience warm and humid climates, focused on optimizing passive cooling features for energy efficiency and environmental friendliness [39].

In terms of structural elements, Miller et al. [43] conducted a study on structural design, incorporating beam and slab, flat slab, and flat plates, while simultaneously evaluating conventionally reinforced and post-tensioned (PT) construction methods. The application of PT methods can substantially decrease the concrete volume and, crucially, the steel mass needed for a structure regarding embodied emissions. Additionally, the reduction in overall building weight allowed for further material savings. Similarly, Volf et al. [44] evaluated the environmental impact of curtain walls. A new curtain wall system using natural materials was designed. This developed system was proven by experimental testing to meet standard technical requirements, indicating its potential for reducing embodied environmental impact. The reduction of environmental burdens significantly relies on well-designed and efficient systems.

Last but not least, with the rising ecological concerns about preserving biodiversity, a regenerative design has been proposed. In the study conducted by Attia [45], the gap between negative impact reduction architecture and positive impact regenerative architecture was identified using a detailed life cycle analysis (LCA). The design, construction, and operation of the two nZEBs studied demonstrated a paradigm shift towards more sustainable and impactful architectural practices.

4.3. Retrofitting Buildings

Section 4.3 addresses the challenge of retrofitting existing buildings to comply with nZEB standards. This section explores operation scenarios, the balance between operational and embodied energy, and the transition from net-zero energy to net-zero carbon for nZEB retrofits.

4.3.1. Operational Scenarios

Besides focusing on buildings still in the design phase, studies have also concentrated on the effectiveness of zero-energy renovation strategies, highlighting methods for reducing operational energy demand and environmental impacts. Measures such as preventing heat loss with insulation and the addition of renewable energy systems have been implemented to decrease the net operational energy demand. Comparative studies have analyzed four different zero-energy renovation scenarios for two Dutch dwelling types [46], including the use of insulation packages and installation of PV modules, primary energy consumption (PEC), and environmental impacts measured in terms of embodied energy and land. When transitioning to nZEBs, the importance of integrating energy generation and conservation measures in achieving nZEB objectives was highlighted.

4.3.2. Balancing between Operational Energy and Embodied Energy

On the other hand, the balance between operational efficiency and embodied emissions cannot be ignored. Reducing operational energy might result in an increase in embodied energy due to the materials used during building retrofitting. In the cold climate of Oslo, Norway, a study on the energy and environmental impact of adding insulation layers for the energy retrofitting of an apartment building was presented [47]. The study examined mineral wool, aerogel, and vacuum insulation panels (VIPs) as insulation materials, with varying insulation levels (0.18 W/m²K, 0.15 W/m²K, and 0.10 W/m²K) used to attain the respective U-values. The study showed that advanced materials such as VIPs and aerogel produced higher levels of embodied emissions, thereby impacting the electricity-to-emissions conversion factors. Therefore, to achieve a balance between operational and embodied energy during renovation, a case study was carried out aiming for an energy-efficient retrofit project while reducing environmental impact [48]. The approach, which involved thermodynamic simulations and finite element analysis, can serve as a reference for future works. Additionally, an approach that integrates a life cycle assessment (LCA) with a life cycle cost assessment (LCC) has been implemented in another renovation project [49]. Cost efficiency was also a critical aspect in the development of solutions.

4.3.3. From Net-Zero Energy to Net-Zero Carbon

From a broader perspective, research has shifted towards achieving a balance of life cycle energy and material emissions (ZEB-OM) by exploring design and material strategies through comprehensive life cycle assessments [50]. The redesign of an earlier concept for a single-family Zero-Greenhouse-Gas-Emission Building was investigated. The study evaluates various design parameters, including area efficiency, embodied emissions, the thickness of insulation, types of heating systems, and roof designs, aiming to reduce greenhouse gas emissions. Quantification processes from a life cycle perspective are crucial for understanding the transition from a low-energy building to an nZEB. Berggren et al. [51] provided a detailed life cycle analysis through the evaluation of eleven case studies, facilitating comparisons between net energy savings and the increase in embodied energy during the transition. Likewise, the study by Besana et al. [52] analyzed reuse and retrofitting strategies for an abandoned office building in Milan with the goal of achieving nZCBs. It implemented the Whole Life Carbon Assessment (WLCA), which considered both embodied and operational carbon impacts. The investigation covered retrofit strategies, materials, and their carbon emissions. By utilizing bio-based materials and efficient design strategies, the embodied carbon was reduced by 91% compared to standard new construction, thus verifying the potential for significant carbon emission reduction. Beyond analyzing strategies, to offer numerical insights, the life cycle emissions across the entire lifespan of a net-zero energy building in India—including construction, use, end-of-life, and recycling phases—were quantified. The purpose is to identify the difference between net-zero energy and net-zero carbon via life cycle assessment (LCA).

5. Results and Discussion: Energy System

Section 5 explores three key aspects of nZEB energy systems: photovoltaic (PV) technologies, the environmental impact of renewable energy sources, and selecting the most appropriate energy system for each building.

5.1. PV Technologies

The pursuit of nZEBs has heightened interest in renewable energy sources, particularly photovoltaic (PV) technologies and system designs. Good et al. [53] presented research taking into account various PV technologies (Si-mono, poly-Si, CIS, and high-efficiency Si-mono) and installation methods for flat roofs. The analysis focused on the energy yield, embodied emissions, and the greenhouse gas emission payback time (GPBT). Subsequently, the net emission balance of the building as a whole was examined. The results of this specific study suggest that the best lifetime emission balance is obtained by the system with the largest area of high-efficiency Si-mono modules, and the highest greenhouse gas return on investment is seen with optimally oriented CIS modules.

While focusing on energy generation, it is equally crucial to consider the environmental impact of the PV panels themselves. Kristjansdottir et al. [54] particularly highlighted the potential for significant variations in GHG emissions and payback times based on system design choices. The analysis focused on the greenhouse gas emissions of photovoltaic (PV) systems in Norwegian Zero Emission Pilot Buildings, examining their greenhouse gas emission payback times (GPBT) and their overall contribution to the buildings’ emissions. The findings indicate that total embodied emissions per square meter of module area range from 150 to 350 kg CO₂ eq/m², with emissions from mounting systems varying based on the materials used.

The Overall Renewable Energy Fraction (OREF) has been introduced alongside the traditional parameter used to assess a building’s proximity to achieve net-ZEB, which is the non-renewable primary energy balance [55]. Additionally, the indicator On-Site Energy Fraction (OEF) was extended so that off-site renewable energy was included. The study demonstrates that the OREF can serve as an effective indicator of a building’s autonomy from fossil fuels and its contribution to minimizing the environmental impact associated with its energy consumption.

5.2. Environmental Impact of Renewables

In buildings, enhancements in energy efficiency often led to a parallel increase in environmental impacts. Houlihan et al. [56] attempted to confirm the net-zero-emission concept by investigating whether nZEBs could be achieved by balancing operational and material embodied emissions with on-site renewable energy generation. The results from this specific study indicated that although the PV system could produce more than the operational needs, embodied emissions from materials and systems are also significant contributors to the overall CO_2eq emissions, with the largest emissions coming from the PV panels, external walls, and foundations. In addition, Kneifel et al. [57] employed a life-cycle assessment (LCA) approach, studying the relative impacts of cost-optimal energy efficiency. The research revealed that achieving net-zero energy performance in this case led to more than a 40% increase in embodied flows, potentially offsetting some reductions in operational flows.

A comparative analysis has been carried out of the environmental impacts of renewable energy sources (RESs) on a net-zero-energy building, specifically focusing on both the embodied and operational greenhouse gas (GHG) emissions [58]. An LCA was conducted, focusing on the construction materials and operational energy use of buildings. The study examined six scenarios of building-integrated RES technologies, which encompassed photovoltaic panels, solar thermal collectors, and ground-source heat pumps. In addition, the study by Cusenza et al. [59] analyzed four design scenarios involving two types of thermal insulation materials, extruded expanded polystyrene (XPS) and cellulose fibers (CF), and two configurations of a building-integrated photovoltaic system, with and without battery storage. The objective is to assess the building’s energy needs, primary energy consumption, and various environmental impacts associated with each scenario.

As much of the research suggested, the analysis outcomes were significantly influenced by the selected approach. For example, uncertainties in operational energy accountancy exist as different life cycle inventory databases were used [60]. Moreover, as presented in the study [59], the system boundary for designing nZEBs from a life cycle perspective might lead to very distinct results.

5.3. Choice of Energy Systems

To determine which of the energy systems should be implemented, economic and energy viability can be examined; Zhou et al. [61] presented a case study on the solar energy system of a multi-story apartment in a temperate climate. The researchers compared “Life Cycle Zero-Energy Buildings” (LCZEBs)—which offset the total energy used over a building’s lifespan, including construction and demolition—with nZEBs, which balance energy use through on-site renewable sources and grid interaction. Economic viability was determined through the net present value (NPV) and solar energy utilization.

On the other hand, in exploring alternative energy system designs in buildings, the study by Tumminia et al. [62] examines the current use of solely PV systems in a net-zero energy building prototype against various designs that include batteries and fuel cell systems for future applications. It highlights that while PV systems alone can meet energy demands, integrating them with energy storage or fuel cells can significantly lessen grid dependency and greenhouse gas emissions. For instance, within the analysis scenario described in the paper, the storage capacity of 20 kWh paired with a 4.56 kW PV system could reduce greenhouse gas emissions by 50.4% compared to the base case. Fuel cells offer a good load match with a high energy efficiency, but their specific CO_2eq emissions are higher than the base case. Therefore, the effectiveness of these integrated systems hinges on finding the optimal balance between storage capacity, system size, and the inclusion of fuel cells.

In the case specifically of grid-connected nZEBs, the stability and efficiency of renewable energy sources are critical, necessitating effective energy management programs. The research by Saberbari et al. [63] focused on the effective integration of renewable energy. It explored the potential of solar and wind energy at the site, assessed the average energy demand of households, and conducted design and cost analyses for various system configurations. In addition, during the management of HVAC and energy storage systems, the use of a robust predictive control (MPC) model was presented to ensure occupants’ thermal comfort [64]. Uncertainties in electricity pricing, operational constraints of the power grid, and the physical components of the buildings are accounted for to minimize costs in grid-connected energy management systems.

6. Results and Discussion: Governance and Others

Section 6 explores the governance aspect of nZEBs, focusing on definitions, evaluation frameworks, and a critical analysis of these methods.

6.1. Definitions and Frameworks for Evaluating Building Performance

Just as the choice of inventory approaches is crucial, the evaluation frameworks play a vital role in determining if a building meets the criteria, especially when considering both operational and embodied energy. In a case study conducted in Italy [65], existing tailored nZEBs were verified. The case study building was monitored for one year, assessing energy consumption and the on-site renewable energy generation. When the nZEB’s methodological framework was extended, in other words, both the operational and embodied energy were also considered, the processes of accounting for the building’s energy balance became more complicated, and yet more assumptions had to be made. Deficits existed in the original neutral situation, making it just a nearly nZEBs. The research suggested the need for the development of comprehensive frameworks to guide the implementation and assessment processes.

Efforts by Taherahmadi et al. [66] were made to review and synthesize existing definitions of nZEBs such that a comprehensive definition proposed can standardize communication among energy planners and policymakers. Indeed, an in-depth examination of the definition was presented by Torcellini et al. [2], coming up with four main perspectives depending on the metric and boundary.

• Net-zero site energy: it suggests that an nZEB produces at least as much energy at the site as it consumes annually.
• Net-zero source energy: it suggests that an nZEB consumes only as much energy as is calculated from the energy source, which also includes the generation and delivery processes of energy to the site. The calculation often involves site-to-source conversion multipliers between the imported and exported energy.
• Net-zero energy costs: it suggests that a ZEB’s annual energy costs are neutral; the money earned from exporting energy to the grid equals the cost of the energy consumed from the grid over the year.
• Net-zero energy emissions: it suggests that a ZEB’s renewable energy production offsets the emissions from non-renewable energy sources.

Furthermore, a standardized approach to system boundary definition in Life Cycle Energy Assessment (LCEA) was proposed [67]. The transparency and reliability of LCEA results were hoped to be improved, particularly within the context of building energy efficiency regulations (BEERs). The framework was crafted based on studies, which include 243 case studies across more than 15 countries. It is structured around six key dimensions: temporal, physical, methodological, hypothetical, spatial, and functional, which together cover 15 components.

With identified frameworks, it became possible to recommend best practices to facilitate the large-scale adoption of nZEB standards. In the study by Li et al. [68], four scenarios of residential energy use were proposed, aiming to predict future energy use. The scenarios of high carbon, business-as-usual, accelerated policy, and net-zero emissions represent different energy-saving strategies in inner Melbourne. Effective strategies include enhancing building envelope efficiency, upgrading heating and hot water systems, and leveraging renewable energy to support policy-making.

6.2. Critical Analysis

While the concept of nZEBs was proposed, there have been discussions over the current nZEB design and evaluation approaches:

• Data selection during greenhouse gas (GHG) emissions assessment of buildings:
  In the study by Loli et al. [69], two approaches were used to calculate the embodied GHG emissions for the construction of the “Zero-Emission Building (ZEB) Laboratory” in Trondheim, Norway. In this building, net-zero greenhouse gas (GHG) emissions are achieved by offsetting emissions from material production, construction, operation, and end-of-life phases with an equivalent amount of on-site renewable energy production. The analysis revealed a substantial difference between the specific data derived primarily from Environmental Product Declarations (EPDs) and the generic data from the ecoinvent database.
• Balance between energy efficiency and environmental sustainability:
  For a nearly zero-energy building design, De Masi et al. [70] examined the environmental impact of increased insulation and the integration of photovoltaic systems. Results indicate that the most energy-efficient solutions do not always coincide with optimal environmental outcomes, highlighting the need for case-by-case detailed assessments.
• Decisions about building design:
  Grazieschi et al. [33] showed that low-energy design solutions performed better than net-zero-energy building solutions from a life cycle energy perspective. For instance, increasing insulation and maximizing installed PV power without battery storage effectively reduced the life cycle energy demand in the studied autonomous building. Meanwhile, in terms of economic feasibility, the nZEBs showed superior economic benefits compared to Life Cycle Zero-Energy Buildings (LCZEBs), which offset the total energy used over a building’s lifespan, including construction and demolition [61].
• Climate-dependent behavioral variations:
  Moazzen et al. [71] conducted a comprehensive analysis in the mild–humid, hot–humid, and cold climatic zones of Turkey, showing that the energy consumption, environmental impact, and cost-effectiveness of design solutions for school buildings could be influenced across different climates. Again, the need for a tailored assessment was highlighted.
• Evaluation and certification for buildings:
  The current approach of Green Building Codes (GBCs) was critiqued [72]. With the implementation of the Occupancy Correction Factor (OCF), the energy ratings of buildings could be altered. The recommendation to account for occupancy and actual usage patterns was also crucial for achieving broader environmental and social sustainability goals. Similarly, the building energy rating system in Australia is intended to decrease operational energy but does not take into account the embodied energy of the building and its systems. For example, when a standard 6.0-star building is transformed into a more energy-efficient 8.7-star building according to the standard, the proportion of embodied emissions (EEs) significantly increases from 20–40% to 50–75%. It has been suggested that the proportion of EEs should be regulated as well [73].
• Disparity in the development of nZEBs between developed and developing nations:
  From a case study by Li et al. [74], the nZEBs approaches in the top three carbon-emitting countries—China, India, and the United States—were investigated. The study identified that the United States had an earlier start in this area and collaborations between different stakeholders are involved. China and India started their development a bit later, and nZEBs are developed only at the government’s initiative. This development gap highlighted the need for enhanced collaboration, broader policy frameworks, and technology sharing.
• Attitudes of stakeholders:
  Yet, as an examination of social engagement, the attitudes and opinions of key stakeholders in the industry have been studied [75]. On the other hand, Davidson [76] conducted a study to examine the correlation between cultural influences, occupant behavior, and the effectiveness of energy efficiency strategies in Canadian homes. The study proposed that for existing homes, the most effective initial reduction in energy use should come from changing occupant behavior, followed by improving existing building envelopes. The social acceptability of stakeholders is indeed vital for the sustainable goal.

7. Conclusions

This paper identifies the research progress in the field of sustainable architecture and construction, with a specific focus on nZEBs and nZCBs. It analyzes up-to-date research published between January 2013 and April 2024 through bibliometric methods and in-depth review. The analysis reveals a global involvement in research activity, concentrating on three major areas covering thirteen subfields. Additionally, this paper includes a detailed analysis of practical implementation strategies, such as design optimization techniques, retrofitting existing buildings, the strategic selection of energy systems to fulfill energy requirements, and governance frameworks that facilitate the promotion and establishment of nZEBs. Regarding a building’s environmental impact, the critical role of embodied carbon in building life cycle assessments is emphasized. At the end, key directions for future research are highlighted.

The bibliometric analysis focused on three main areas: keyword analysis, research trends, and geographical contributions. Tools such as VOSviewer were utilized to investigate the relationships between keywords, identifying three major clusters in keyword co-occurrence. Notably, terms like ‘Life cycle’, ‘Buildings’, and ‘Energy efficiency’ emerge as significant, highlighting their central roles and the interconnectedness within the discourse. The examination of research trends revealed a fluctuating number of publications, with notable increases in research activity in 2016, 2018, and 2022, indicating periods where interest was triggered. The geographical analysis of publications further demonstrates the extensive global engagement in sustainable building practices. Overall, there has been widespread global participation and growing academic interest in sustainable building practices in addressing environmental challenges and advancing towards more sustainable, energy-efficient solutions.

From the content review, three major fields and thirteen subfields can be deduced as focus areas of study. Current multidisciplinary innovative approaches, involving global participation, aim to achieve nZEBs by focusing on not only the design of the buildings themselves but also the energy systems within and appropriate guidance. Studies on technological advancements toward establishing nZEBs, including approaches, common characteristics, strategies, and emissions evaluation, played a crucial role in the transition to net-zero-energy buildings. On the other hand, to optimize buildings in the early design phase, there is a growing interest in adopting innovative materials and developing modeling techniques to achieve cost-effective and climate-responsive building designs. For meeting energy and carbon reduction goals in existing buildings, some research examines the trade-offs between operational energy and embodied energy, as well as the shift from net-zero energy to net-zero carbon in retrofitting buildings for improved energy performance. In the energy domain, the selection of energy systems, the exploration of photovoltaic technologies, and the environmental impact of renewable energy sources are also noted. Furthermore, the development of definitions and frameworks for evaluating building performance and critical analysis of the existing governance guidelines were also topics of exploration. They addressed the research progress and challenges in transforming sustainable building concepts into real-life practices.

The assessment of embodied carbon throughout the whole life cycle grants an understanding of the environmental impact of buildings. Section 4.1.2 highlights the substantial impact of embodied emissions (EEs) from construction materials by comparing them with operational emissions (OEs) from building use. This comparison highlights the significance of EEs in the overall emissions profile. Additionally, to further enhance a building from net-zero energy to net-zero carbon, a comprehensive life cycle assessment (LCA) was essential to identify the gap between net-zero energy and carbon targets. As discussed in Section 4.3.3, applicable design strategies such as area efficiency and insulation can be adopted, along with consideration of the carbon emissions from building materials. In this way, the establishment of nZEBs and nZCBs can be effectively fostered.

Notably, this paper discusses nZEBs and nZCBs by specifically focusing on the strategic aspects of achieving net-zero energy and net-zero carbon goals, respectively. Section 4 provides practical strategies for nZEB implementation, analyzing real-life approaches and design optimization techniques for cost-effective, climate-responsive buildings. Section 5 explores the strategic selection of energy systems, considering both photovoltaic technologies and their environmental impact. Finally, Section 6 examines the governance frameworks of nZEBs and nZCBs. These insights allow various stakeholders to design, implement, and govern nZEBs and nZCBs for the establishment of a sustainable built environment.

The review yields several intriguing findings, particularly regarding the importance of embodied carbon in achieving net-zero carbon buildings. Various nZEB research publications have focused on operational energy. This paper highlights the significance and role of embodied carbon. For achieving true net-zero carbon buildings, embodied carbon—the emissions associated with materials, construction, and demolition—is crucial due to its significant environmental impact.

The research topic can be further extended to explore how strategies may interfere with each other. This interference can occur when only operational carbon is considered, as well as when both operational and embodied carbon are taken into account. For instance, if the concept of circularity [77] is considered during material selection, a material with high embodied carbon might be permitted in the recycling loop. This mechanism reuses product outputs as inputs, fostering continuous material use throughout the production cycle. The overall environmental impact varies from case to case. Therefore, it is important to note that strategies are not absolute—alternative aspects must be considered; materials with the lowest embodied carbon might not always be the most appropriate choice.

In addition, to achieve net-zero energy status, offsetting the building’s energy demand with renewable energy systems becomes essential. However, even solar PV panels have embodied emissions. Furthermore, on-site energy generation has limitations in achieving autonomy, such as the need for energy storage technologies, difficulties in achieving economies of scale, and social acceptance [78,79,80]. In the case of grid-connected nZEBs, effective energy management programs are significant challenges to address. Ensuring occupants’ thermal comfort is necessary while providing stable electricity, even with the unpredictable nature of renewable solar and wind energy. The above highlights the outcome of a multidisciplinary approach in this area, incorporating insights from various fields.

All in all, the findings of this paper revealed the future direction of the study. The insights gained from current approaches, common characteristics, and strategies can facilitate the promotion and establishment of nZEBs and nZCBs. Alongside the expanding interest and progress in nZEB and nZCB research, as outlined in Section 6.2, there were still controversies regarding design and evaluation approaches. These included the inconsistencies between specific and generic data in greenhouse gas (GHG) emissions assessments, the balance between energy efficiency and environmental sustainability, the impact of design decisions on life cycle energy and economic outcomes, the variation in climate-dependent design, and the examination of current energy rating systems. Therefore, beyond detailed and case-specific assessments that consider both operational and embodied emissions, future studies might explore these aspects to achieve environmental and sustainability goals.