Economic Evaluation of Small Public Office Buildings with Class 1 of Zero Energy Building (ZEB) in Korea by Reflecting Life Cycle Assessment (LCA)

Abstract

This study summarizes the technology level and economic feasibility of two small 500 m² public office buildings that achieved a first-class Zero Energy Buildings (ZEB) certification for the first time in Korea. In addition, a Life Cycle Assessment (LCA) reflecting the ZEB performance in the operation stage was analyzed, and the LCA factors considering the characteristics of small buildings were reviewed. Moreover, an economic analysis was performed through a Net Present Value (NPV) by reflecting the ZEB and LCA, and the results showed little economic feasibility. However, adding the environmental costs calculated through the LCA to the existing energy-saving costs could offset an additional 11.6–11.7%. Therefore, including the environmental impact and cost through these LCA evaluation techniques could be a positive step toward increasing the effectiveness of ZEB applications. This study is significant in that it conducted a comprehensive analysis through ZEB and LCA in terms of carbon reduction for small buildings that achieved a first-class ZEB certification, but it is limited to only two cases.

1. Introduction

1.1. Background and Purpose

As the severity of global climate change increases, domestic and foreign government agencies are implementing policies to curb greenhouse gas emissions and resource consumption across industries. For example, in Korea, policies for recycling greenhouse gases and resources generated by buildings are being enacted and strengthened. Accordingly, the 2nd Basic Plan for Green Buildings (2020–2024) announced the expansion of Zero Energy Buildings (ZEB) and revitalization of Green Remodeling (GR) as key strategies to achieve national greenhouse gas reduction goals [1,2].

In particular, as shown in Figure 1, the ZEB certification system announced a policy to mandate ZEB certification for the public sector in 2020, the private sector in 2025, and even for small buildings with a total floor area of 500 m² or more in 2030. Moreover, the Seoul Metropolitan Government recently announced its 2050 private ZEB roadmap, making it mandatory for new private buildings to start in 2023, ahead of the national goal [3]. Since the implementation of the ZEB certification in January 2017, 2360 design and 351 construction certifications have been completed as of November 2022 [4].

Considering the construction volume in Korea, where approximately 250,000 new buildings are built annually, the proportion of ZEB-certified buildings is insignificant at approximately 1%, based on design certification. Furthermore, over 74% of the 2360 ZEB-certified buildings were classified as Class 5. In addition, only 32 small buildings (less than 661 m²) achieved ZEB certification, accounting for only 1.36% of the total acquisitions. Therefore, the low number of ZEB certifications and certification Classes can be attributed to low economic feasibility, such as high construction costs and relatively low energy prices for ZEB [5,6].

A previous study conducted a survey to assess the technology and economic feasibility required to stabilize the mandatory ZEB policy. Although the buildings were completed and operated by size in the residential and non-residential sectors, the technical difficulties were small. However, completing small buildings in the non-residential sector has not been adequately addressed and requires further supplementation.

Regarding the economy, an analysis was conducted to determine the ratio of additional construction costs required to achieve economic feasibility for non-residential facilities, with a few ZEB analysis cases (41,708.6 m², B7 to 30F). However, there have been no ZEB analyses for non-residential buildings within a small range (less than 661 m²).

In another study, a simulation analysis was performed to set comprehensive envelope thermal performance conditions to achieve a Building Energy Efficiency (BEE) 1++ Class and optimize the energy consumption for small buildings. However, there are insufficient details on how to achieve ZEB [7,8].

This study distinguishes itself from existing research by analyzing the technology and economic feasibility of small office buildings that have achieved ZEB certification using prior research methods [7]. With recent advances in energy-saving design standards and the widespread adoption of passive houses and ZEB technologies, there are limitations in reducing greenhouse gas emissions through energy reduction in the operational stage alone [9]. Therefore, reducing the environmental impact of the entire life cycle of a building through a Life Cycle Assessment (LCA) can contribute to achieving carbon neutrality goals.

As of 2017, small buildings (less than 661 m²) in Korea accounted for 84% of the total number of buildings, and the number of permits has been increasing annually [10]. According to Figure 2, which presents a survey of the number of small building constructions from 2016 to 2020, they represented 88.4–88.9% of all buildings. Therefore, achieving ZEB certification for small buildings is important to achieve the government’s goal of reducing greenhouse gas emissions. In addition, research and analysis of ZEB’s technical aspects and economic feasibility for non-residential sectors of small buildings that have not been previously studied are necessary.

Although small buildings comprise a large portion of the domestic market, they are often neglected in terms of the quality of the living environment compared to medium- and large-scale buildings [11]. This is because small businesses (fewer than five employees), which account for more than 70% of all businesses, operate mainly in small construction industries. Therefore, policy support measures are required to improve the quality of the small construction market [12]. Therefore, obtaining a domestic ZEB certification can improve the quality of small buildings. Additionally, LCA strategies in connection with ZEB can be used to evaluate the economic and environmental impacts of carbon reduction measures.

1.2. Procedures and Methods

This study quantitatively analyzed office buildings within a small non-residential architectural sector that lacked domestic ZEB analysis cases. To achieve this, two small non-residential buildings (public office buildings) with different structures (reinforced concrete and heavy timber structures) were selected to compare the major technologies used to achieve ZEB. Furthermore, the added construction cost and the Net Present Value (NPV) were also analyzed. Additionally, LCA was performed to evaluate the environmental impact, and the economic feasibility of the target building (small building) was evaluated by combining it with an NPV analysis.

The procedure of this study is as follows:

(1)

Prior research review: Prior research on ZEB and LCA conducted mainly in Korea was investigated, and the differences between existing research methods and those used in this study were analyzed.

(2)

Selection of analysis targets: Two small non-residential buildings that have achieved ZEB design and construction certification in Korea, specifically public office buildings that have completed both design and construction, were selected as analysis targets.

(3)

Comparison of application technologies: The major ZEB application technologies for the two cases and representative cases were selected, and the application technologies for buildings before and after the ZEB application were compared and analyzed.

(4)

Energy evaluation and additional construction cost analysis: Energy analysis was conducted using ECO-2, a domestic ZEB certification evaluation program, and additional construction costs were analyzed.

(5)

LCA analysis: In addition to economic feasibility, an LCA based on ISO 14000s and ISO 21931-1 was conducted to evaluate the environmental impact of improvements.

(6)

Based on the results of (4) and (5), the NPV was analyzed considering the annual energy savings and environmental impact reduction costs of buildings that achieved ZEB certification compared to general buildings.

(7)

Based on the analysis results, the current status of the ZEB certification for small buildings and future improvement directions are presented from an economic evaluation perspective.

2. Literature Review

Previous studies have investigated and analyzed ZEB and LCA, mainly in Korea. However, ZEB was considered regarding technical and economic applications based on building use (residential and non-residential) and size. In contrast, LCA was considered regarding analysis targets, methods and programs, and economic feasibility.

2.1. Consideration of Previous Studies Related to ZEB

The information in Figure 1 indicates that the government has implemented mandatory ZEB certification for public buildings sized 1000 m² or more since January 2020. Furthermore, by 2025, ZEB certification will be compulsory for public buildings of 500 m² or more and private buildings of 1000 m² or more. A 2030 ZEB certification will be mandatory for all buildings of 500 m² or more. The Seoul Metropolitan Government has also taken steps to enforce ZEB certification by announcing its plan to mandate ZEB certification for new private buildings starting in 2023, ahead of the national goal. This plan is outlined in the “2nd Seoul Green Building Creation Plan” [13].

As of November 2022, out of the 2711 completed design certification projects, 2360 were certified. Of these, only 351 projects completed their construction certification. Among these, ZEB certification of private buildings accounts for only approximately 50 cases, most belonging to building owners, local governments, and public institutions. More than 98% of the certified projects were public buildings, and the remaining 2% were private buildings. This suggests that ZEB certification is generally not achieved except in cases where it is mandatory.

When analyzing the certification Classes in

Table 1. ZEB certification status by Class.

Category	Class 5	Class 4	Class 3	Class 2	Class 1
Number of certifications	1753	609	205	67	77

, it was found that out of the 2711 projects that completed design certification as of November 2022, 1753 were certified as ZEB Class 5, and 958 were certified as other ZEB Classes 1 to 4; this means that 64.7% of ZEB certifications are in Class 5.

Table 2. ZEB certification status by purpose.

Category	Education	Office	Elderly	Cultural	Etc.
Number of certifications	935	485	210	149	932

and

Table 3. ZEB certification status by size.

Category	~1000 m2	1000 m2~3000 m2	3000 m2~10,000 m2	10,000 m2~100,000 m2	100,000 m2~
Number of Certifications	81	1303	944	365	14

present the certification status by use and size, respectively. Educational facilities (935), offices (485), elderly facilities (210), and cultural facilities (149) were the most common types of buildings used to achieve ZEB certification. Medium-sized buildings with more than 1000 m² to less than 10,000 m² certifications had the highest number of ZEB certifications (82.9%), followed by buildings with more than 3000 m² to less than 10,000 m² certifications (76.5%). Among the total ZEB certifications achieved, only 14 (based on identification) were for small buildings (fewer than 661 m²), which is not within the mandatory scope of ZEB certification (public: 1000 m² or more). This suggests that ZEB certification is not commonly pursued in small buildings other than for specific purposes or at the discretion of the building owner. Overall, the analysis of ZEB certification cases implies no significant technical difficulties in achieving ZEB certification for buildings of various uses and sizes.

Table 4. Survey of previous studies related to ZEB.

ZEB Mandatory Settlement Requirements (Choe, 2017) [14]	Residential Sector		Non-Residential Sector
	Detached House/Multi-Family Housing	Apartment House/ Multi-Unit Dwelling	Small Buildings	Medium and Large Buildings
Technical aspect	(Vincent, 2009) [15] (Jeong et al., 2015) [16] (Choe, 2017) [14]	(Lim, Son & Lee, 2016) [17] (Lee, 2016) [18] (Jo, 2017) [19] (Choe, 2017) [14] (Lim, 2018) [20] (Kim, Kim, & Hong, 2018) [21]	(Park, Jung, & Ahn, 2021) [8] * (Kim et al., 2021) [22] **	(Jo, 2010) [23] (Choe, 2017) [14] (Kim et al., 2017) [24] (Won, 2018) [25] (Jeon et al., 2018) [26] (Kim, 2020) [7]
Economic aspect	(Choe, 2017) [14]	(Lee, 2016) [18] (Jo, 2017) [19] (Lim, 2018) [20] (Kim, Kim, & Hong, 2018) [21]	-	(Kim, 2020) [7]

* Comprehensive thermal performance conditions were established to achieve the Building Energy Efficiency 1++ Class, a prerequisite for ZEB certification. ** The design process of green remodeling (existing buildings) was reviewed for senior citizen centers in Seoul, and the energy performance of Building Energy Efficiency 1++ or 1+++ Class was analyzed.

presents the ZEB performance cases studied in Korea, classified according to the technical and economic aspects necessary for ZEB certification. The cases were further analyzed according to the purpose of the building (residential or non-residential) and its size (small, medium, or large). Previous research has adequately studied the technical and economic aspects of ZEB certification for residential buildings of different sizes, and related technologies have been commonly applied. Similarly, medium and large non-residential buildings have also been sufficiently studied. However, research on small non-residential buildings is lacking because of their exclusion from mandatory ZEB certification, and there are fewer cases of ZEB certification for small buildings than for medium and large buildings. To address this gap, this study provides a unique analysis of the technology and economy of two small 500 m² office buildings that recently achieved ZEB certification.

2.2. Consideration of Previous Studies Related to LCA

The LCA evaluates the environmental impacts emitted during a building’s life cycle, including all materials, construction, operation, and disposal stages, following ISO 14040s and ISO 21931-1. The LCA, which is centered on buildings in Korea, is conducted through domestic the Green Building Certification (G-SEED) and provides the results of analyzing emissions through six major environmental impact assessments and influencing factors using material details to enter the type and weight of materials injected into the building after the design is completed [27,28] Figure 3.

The LCA performance cases conducted mainly in Korea were classified into analysis targets, results, building life cycle stages, analysis methods, programs, and whether economic feasibility was reflected, as shown in

Table 5. Survey of previous studies related to LCA.

Authors	Analysis Classification						Analysis Method/ Analysis Program	Weight **/Cost/Economics
	Analysis Target/Results (CO2 or Six Major Environmental Impacts *)		Building Life Cycle
			Production	Construction	Operation	Disposal
(Park & Jeong, 2015) [29]	Buildings	CO2	-	-	○	-	Conversion of energy consumption	-
(Jeong, 2015) [30]	Buildings	CO2	-	-	○	-	Conversion of energy consumption	-
(Kim et al., 2015) [31]	Building materials	All	○	-	-	-	G-CEAT	-
(Kim et al., 2016) [32]	Building materials	All	○	-	-	-	G-CEAT	-
(Tae, 2015) [33]	Building materials	All	○	-	-	-	G-CEAT	-
(Roh et al., 2012) [34]	Buildings	CO2	○	○	○	○	LOCAS	-
(Keum et al., 2012) [35]	Buildings	CO2	○	○	○	○	Carbon Expert	-
(Roh, Tae, & Sin, 2014) [36]	Buildings	CO2	○	○	○	○	BEGAS 2.0	-
(Lee et al., 2012) [37]	Buildings	CO2	○	○	○	○	ECO-DM	-
(Hong et al., 2012) [38]	Buildings	CO2	○	○	○	○	PBECAS	○
(Lee et al., 2008) [39]	Facilities	All	○	○	○	○	TOTAL (ver 3.0)	○
(Lee, Noh, & Park, 1999) [40]	Products	All	○	-	-	-	ISO 14000	○
(Choi, Lee, & Cho, 2012) [41]	Construction industry	All	-	-	○	-	ISO 14000	○
(Hong, Ji, & Jeong, 2012) [42]	Buildings	All	-	○	-	-	ISO 14000	○
(Cho, Chun, & Choi, 2012) [43]	Construction industry	All	-	-	○	-	ISO 14000	○
(Kwon, 2008) [44]	Facilities	All	○	○	○	○	ISO 14000	○
Performance for this study	Buildings	All	○	○	○	○	Self-development program	○

* GWP, ADP, AP, EP, ODP, POCP. ** A study was conducted on weighting to represent the entire process evaluation as a single numerical value. The six major environmental impacts were converted by weighting around CO₂.

. The analysis targets were often focused on buildings, and most of them were analyzed by including CO₂ as an essential component and adding environmental impact factors. Furthermore, all analyses were conducted independently by developing ISO-based methods or programs, and additional economic feasibility studies were conducted. Therefore, this study aims to analyze LCA characteristics specialized in small buildings based on previous studies, find ways to reduce additional carbon, such as materials, construction, and disposal, and improve economic feasibility in connection with energy savings through the operational stage of ZEB.

3. Selection and Analysis of ZEB Certification Targets

3.1. Selection and Analysis Target

Before June 2018, the construction of small buildings in Korea allowed for the direct construction of residential buildings under 661 m² and general buildings under 495 m². However, owing to the revision of the Framework Act on the Construction Industry in June 2018, direct construction by owners was changed to allow for buildings of 200 m² or less, with institutional improvements made to only allow companies with construction licenses to exceed this limit. Furthermore, in 2016, a separation system for small building design and supervision was introduced following Articles 25 of the Building Act and 19 of the Building Act Enforcement Rules. As a result, a small building supervision system is being implemented, in which the admitter directly designates a supervisor to construct a building of 661 m² or less (as per references [45,46,47]). Based on this size limit, 22 small buildings in Korea have obtained ZEB certification, reflecting the current reality that they are not subject to mandatory applications under current laws.

Of the 22 buildings, only non-residential structures that achieved ZEB design certification in 2021 and construction certification in 2022 were selected, excluding residential buildings such as daycare and senior citizen centers. Both selected buildings were public office buildings of 661 m² or less, with Case 1 having reinforced concrete structures and Case 2 having heavy timber structures. The building outlines are listed in

Table 6. Building overview of Cases 1 and 2.

Category	Case 1		Case 2
	Contents	Bird’s-Eye View	Contents	Bird’s-Eye View
Location	Gwangju Metropolitan City		Yangyang County, Gangwon-do
Purpose	Public office		Public office
Lot Area	9557 m2		2021 m2
Building Area	324.38 m2		319.94 m2
Gloss Floor Area	459.24 m2		442.06 m2
Scale	2nd Floor		2nd Floor
Structure	Reinforced concrete		Heavy timber structure
Total Cost	$1,266,013		$1,650,339
ZEB Rating	Class 1		Class 1

below. The total floor area of the two buildings is 459.24 m² and 442.06 m², respectively, which is 10% smaller than the 500 m² standard for mandatory ZEB certification for public buildings implemented since 2025. However, no similar case has been tailored to this area.

3.2. Overview of Analysis Program and ZEB Certification

The ECO-2 program was used for analysis to evaluate the energy efficiency ratings of domestic buildings and ZEB certification. Based on ISO 13790 and DIN V 18599, ECO-2 calculates the monthly energy demand of buildings using monthly average weather data. It can also predict the monthly energy consumption according to system performance, including heating, cooling, hot water supply, lighting, and ventilation. The program calculates the primary energy consumption for each energy type.

The ECO-2 uses a standard profile for importing meteorological data from the ECO-2 central server, which allows the selection of average data for 66 regions in Korea. The weather data in ECO-2 were based on monthly averages calculated from typical meteorological year data (TMY), including the monthly average air temperature and solar irradiance for each azimuth angle as Korea experiences four distinct seasons—spring, summer, autumn, and winter—the TMY data consider the variations in weather patterns throughout the year.

For domestic ZEB certification, three basic requirements must be satisfied: (1) Building Energy Efficiency (BEE) 1++ or higher, (2) energy independence rate of 20% or higher, and (3) installation of a Building Energy Management System (BEMS) or remote digital meter [48]. Detailed descriptions are provided in

Table 7. Standard for ZEB certification system.

Category	Key Contents
Standard (1)	ECO-2 evaluation	Primary Energy Requirement (kWh/m2·y) = ∑ Energy consumption by use × Primary energy conversion coefficient
	Residential: Less than 90 kWh/m2·y
	Non-Residential: Less than 140 kWh/m2·y
Standard (2)	ECO-2 evaluation	Energy independence rate (%) = Primary energy production per unit area ÷ Primary energy consumption per unit area
	The proportion of renewable energy production among energy consumed by buildings
Standard (3)	Evaluation of applicability with a checklist	(BEMS) Evaluation of nine items, including data collection and display, information monitoring, and control system interworking
	A system that measures and manages energy consumption in real-time	(Remote meter reading) Evaluate six categories, including data collection and display, instrument management, and data management

and

Table 8. ZEB certification rating.

Rating	Energy Independence Rate (%)
Class 1	Energy independence rate above 100%
Class 2	Energy independence rate of 80% or more to less than 100%
Class 3	Energy independence rate of 60% or more to less than 80%
Class 4	Energy independence rate of 40% or more to less than 60%
Class 5	Energy independence rate of 20% or more to less than 40%

.

3.3. Target Project Analysis

3.3.1. ZEB Application Technology for Target Buildings

Table 9. ZEB technology applied to Case 1 and Case 2.

Category		Application Techniques of ZEB in Case 1	Application Techniques of ZEB in Case 2
Architecture	Wall (U-value)	Glass wool 32 K 180 mm (U-value = 0.190 W/m2·K)	Bead method insulation plate 2 type No. 3 250 mm (U-value = 0.126 W/m2·K)
	Roof (U-value)	Extrusion method insulation Plate No. 1 180 mm (U-value = 0.150 W/m2·K)	Glass wool 25 K 40 mm + Cellulose 285 mm (U-value = 0.106 W/m2·K)
	1st Floor (U-value)	Extrusion method insulation plate No. 1 150 mm (U-value = 0.167 W/m2·K)	Extrusion method insulation plate No. 1 150 mm (U-value = 0.169 W/m2·K)
	Window (U-value)	T47 Low-e triple glass (U-value = 0.786 W/m2·K)	T47 Low-e triple glass (U-value = 0.786 W/m2·K)
	Shading	External electric blind (South, west, east)	External electric blind (South, west, east)
	Thermal bridge	Apply thermal bridge-blocking frame	Apply thermal bridge-blocking frame
Mechanical facilities	Heating and cooling equipment	EHP (COP, Heating/Cooling = 4.52/3.90)	EHP (COP, Heating/Cooling = 4.204/3.652)
		Gas boiler (Gas meter)	Gas boiler (Gas meter)
	Hot water	Electric water heater/Gas boiler (Remote meter reading, flowmeter)	Electric water heater (100%, Remote meter reading)
	Ventilation	Heat recovery ventilation system (Efficiency, Heating/Cooling = 76%/67%)	Heat recovery ventilation system (Efficiency, Heating/Cooling = 76%/56%)
Electricity facilities	Light density	5.0 W/m2	6.3 W/m2
	Power trunk lines	Building energy management system (BEMS)	Remote meter electronic meter
	Renewable energy	Photovoltaic 30.1 kWp (Efficiency, 20%)	Photovoltaic 39.6 kWp (Efficiency, 20%)
Energy requirement		−7 kWh/m2·y	−11.1 kWh/m2·y
Energy independence rate		106.5%	110.2%

lists the ZEB technologies applied to the two target buildings. Following Korea’s Energy Saving Design Standards for Buildings, Case 1 corresponds to the southern (normal) region. In contrast, Case 2 corresponds to the central 1 (very cold) region with different insulation performance requirements. In addition, Case 2 is in the cold northern region of Korea, where the insulation performance standards are 20–30% more stringent than those in Case 1, which is in the southern region [49].

In the architectural sector, the insulation performance of the exterior walls, roof, and floor insulation in Cases 1 and 2 was strengthened by more than 15% compared with the domestic legal standards. Likewise, the insulation performance of windows and doors was strengthened by more than 40–60%. In addition, passive design techniques were also adopted, including outdoor electric blinds and thermal-break frames.

The techniques adopted for Cases 1 and 2 were the same as those in the mechanical equipment sector. A high-efficiency electric heat pump (EHP) system was used for heating and cooling, and a gas boiler was used for the hot water supply. In addition, electric water heaters were used in Case 1. A high-efficiency total heat exchanger was used for ventilation in both cases.

In the electrical equipment sector, the techniques adopted for Cases 1 and 2 are the same, which include minimizing the light intensity using high-efficiency LED, using a BEMS or a remote digital meter, and implementing a renewable energy system (photovoltaic system). However, Case 1 applied a higher level of the BEMS than Case 2. Furthermore, Case 1 had a photovoltaic system implementation rate of 30 kWp, whereas Case 2 had a rate of 39 kWp. Therefore, the optimal capacity for achieving the highest level of ZEB certification was applied in both cases.

The ZEB application technologies used in the two target buildings exhibit similar characteristics. The main features of this technology are passive design techniques, including reinforced insulation in the architectural sector, with high-performance windows and doors, high-efficiency facilities, including heating and cooling, hot water supply, and ventilation in the mechanical equipment sector, and high-efficiency lighting, BEMS, and photovoltaic systems in the electrical equipment sector. There were no differences in the application technology when comparing medium and large buildings. However, in the case of mechanical equipment, individual equipment is preferred to centralized equipment. Photovoltaic systems are more commonly used in the electrical equipment sector than in the geothermal heating and cooling systems. Fuel cells are widely used in medium and large domestic buildings; however, they are unsuitable for small buildings because of their high installation and maintenance costs. Geothermal heating and cooling systems are inefficient for small buildings.

In addition to the ZEB certification, Cases 1 and 2 acquired the ‘Passive House Certification Standard’ organized by the Passive House Institute Korea (PHIKO)’ [50]. Compared to the case of achieving only a ZEB certification, there was no significant difference in the level of application for ventilation in the architectural and mechanical equipment sectors (Table 9). External electric blinds and thermal cross-sectional frames have been added; however, their impact on the total construction cost is insignificant and can be commonly used in energy-saving technologies for ZEB certification.

In particular, the non-residential sector does not require a building’s sealing performance when certifying the ZEB and BEE. However, the ‘Passive House Certification Standard’ requires an average of 1.0 h⁻¹ or less pressure and decompression and performs the Blower Door Test (BDT). Therefore, to achieve this performance, efforts must be made to combine the design stages, detail intersections, and use construction-stage building sealing technology. Additionally, when both ZEB and passive-house certification are carried out in small buildings, the difference in cost is small based on the total construction cost and certification fee. Therefore, achieving sealing performance can improve the small buildings’ quality by replacing the specifications’ role in reviewing and confirming various related technical elements in the design and construction stages.

The analysis of the two target buildings showed that ZEB application technology in small buildings is not as complex as in medium and large buildings [28,51] and exhibits similar characteristics. Furthermore, the additional construction costs were similar, with Case 1 costing $233,609 and Case 2 costing $226,074. Therefore, it can be inferred that Case 2, which had the highest energy savings, is more economically feasible.

3.3.2. Comparison and Analysis of Baseline and ZEB Models of Target Buildings

Case 1 was originally designed as a standard building without ZEB certification during the basic design stage. The architectural, mechanical, and electrical sectors were planned based on domestic legal standards, and the installation of a renewable energy system (such as a photovoltaic system) was not considered. Therefore, Case 1 was used as the base model for the analysis in this study. The basic design of the ZEB model is modified to include passive design techniques in the architectural sector, high-efficiency ventilation systems in the mechanical sector, and BEMS and photovoltaic systems in the electrical sector. A photovoltaic system with a capacity of at least 10 kWp was required to achieve the highest ZEB certification level. However, a capacity of 30 kWp was applied in Case 1 to achieve the highest ZEB certification level.

On the other hand, Case 2 was designed for ZEB certification from the planning stage, and the base model was planned by referring to the application technology used in Case 1.

Table 10. Base model technology applied to Case 1 and Case 2.

Category		Application Techniques of Base Model in Case 1	Application Techniques of Base Model in Case 2
Architecture	Wall (U-value)	Extrusion method insulation plate 100 mm (U-value = 0.288 W/m2·K)	Bead method insulation plate 2 type No. 3 130 mm (U-value = 0.234 W/m2·K)
	Roof (U-value)	Extrusion method insulation plate 180 mm (U-value = 0.166 W/m2·K)	Glass wool 25 K 40 mm + Cellulose 185 mm (U-value = 0.149 W/m2·K)
	1st Floor (U-value)	Extrusion method insulation plate 100 mm (U-value = 0.285 W/m2·K)	Extrusion method insulation plate No. 1 90 mm (U-value = 0.266 W/m2·K)
	Window (U-value)	T24 Low-e double glass (U-value = 1.800 W/m2·K)	T24 Low-e double glass (U-value = 1.500 W/m2·K)
	Shading	-	-
	Thermal bridge	-	-
Mechanical facilities	Heating and cooling equipment	EHP (COP, Heating/Cooling = 4.52/3.90)	EHP (COP, Heating/Cooling = 4.204/3.652)
		Gas boiler (Gas meter)	Gas boiler (Gas meter)
	Hot water	Electric water heater/Gas boiler	Electric water heater (100%)
	Ventilation	-	-
Electricity facilities	Light density	5.0 W/m2	6.3 W/m2
	Power trunk lines	-	-
	Renewable energy	-	-
Energy requirement		104.6 kWh/m2·y	110.7 kWh/m2·y

summarizes the application technologies used in the base models for Case 1 and Case 2.

Table 11. Comparison of energy simulation results between base and ZEB models.

Category	Results of Case 1			Results of Case 2
	Energy Requirement (kWh/m2·y) = (a)	Used Area (Total Area) (m2) (b)	Annual Energy Use (kWh/m2·y) = (a) × (b)	Energy Requirement (kWh/m2·y) (a)	Used Area (Total Area) (m2) (b)	Annual Energy Use (kWh/m2·y) = (a) × (b)
ZEB model	−5.7	459.24	−2617.67
Base model	104.6	459.24	48,036.50
Savings (kWh/m2·y)	Differences in annual energy use between ZEB and base models		51,251.18	Differences in annual energy use between ZEB and base models		53,842.91
Savings rate (%)			106.69			110.03
Savings cost ($)	Apply $0.07 Power Charge (40 years of building life)		3598 (66,216)	Apply $0.07 Power Charge (40 years of building life)		3764 (69,273)

compares the annual energy usage of the base and ZEB models. It is important to note that energy usage results may vary depending on the usage profile, actual building usage time, pattern, equipment efficiency, and other factors [52]. However, ECO-2 is Korea’s only officially recognized building energy performance evaluation tool. It is commonly used to estimate building energy performance and usage in the building planning stage and related research fields. Therefore, this study used the annual energy usage comparison values for the base model and the ZEB model in Table 11 for economic evaluation.

3.3.3. Comparison and Analysis for LCA of Baseline and ZEB Models of Target Buildings

The LCA is a technique used to analyze the environmental impacts of human economic activities, products, and services. It considers the use of raw materials and energy throughout the life cycle of a product, as well as environmental loads, such as pollutants and emissions. The LCA is a research method that identifies the environmental aspects and potential impacts throughout a product’s life cycle, from raw material acquisition to processing, use, and disposal.

For LCA analysis, a self-developed LCA program was used, which was developed in compliance with ISO-14040s and ISO-21931 standards [53]. Figure 4 shows the LCA analysis range. The material production stage includes the process of producing materials to be used in buildings, including the consumption of resources and energy required for production, such as the collection and processing of raw materials, and the manufacturing of products [54]. The construction stage includes the energy required in the process of transporting materials to the construction site, and various mechanical equipment required during construction [55]. In addition, in the operation stage, the energy consumed for building operation, and the amount of material and energy consumed in the process of repair and maintenance, are included [56]. Lastly, the dismantling/disposal stage includes the processes of dismantling the building, and the transportation, recycling, incineration, and landfill of the building waste materials generated [57]. Figure 4 shows the self-developed SAMOOCM Life Cycle Assessment (S−LCA) that includes material production, construction, operation, and disposal stages.

The scope for evaluating the environmental impact of small buildings is shown in Figure 4, and details are listed in

Table 12. Scope of LCA evaluation in this study.

Scope	Unit Process	Number	Description
Manufacturing	Building material production	A1~3	The process of producing building materials to be injected into buildings by consuming resources and energy necessary for production, such as the collection and processing of raw materials and the manufacture of products
Construction stage	Building material transportation	A4	The process of transporting materials to be injected into a building from the place of purchase and storage to the construction site
	Building construction	A5	The construction process of building materials transported to the site using various construction equipment
Operation stage	Building maintenance	B4	The process of repairing and replacing aging buildings over time to keep them similar to the initial situation (reflecting dismantling and disposal of replacement materials)
	Building use	B6	The process of living while keeping the indoor environment comfortable by using various equipment machines while residents occupy the building
Disposal and demolition stage	Demolition of a building	C1	The process of dismantling a building using construction equipment
	Transportation of waste materials	C2	The process of transporting waste materials generated after dismantling to the relevant disposal site according to the treatment method
	Disposal and recycling	C4	The process of reclaiming, incinerating, or recycling waste materials

. In this study, B1, B2, B3, B5, B6, and C3, which currently do not have standards or are difficult to calculate in Korea, are excluded from Figure 4.

This study evaluated LCA according to the G-SEED 2016, ISO 14000 environmental management, and ISO 21931-1 standards [27,28]. In Case 1, exterior wall insulation was applied using extrusion method insulation plates at the time of the basic design; however, for the ZEB model, it was changed to glass wool for external insulation using the dry method. Additionally, the urethane coating on the rooftop, applied during the basic design, was changed to high-performance composite waterproofing for the ZEB model; this was found to significantly impact the material sector in the LCA results of the ZEB model compared to the base model. Case 2 was planned at a legal level related to ZEB by referring to Case 1 without any design change; therefore, there was no significant impact other than an increase in material input in the materials sector. The analysis results for Cases 1 and 2 are summarized in

Table 13. LCA results in base and ZEB models of Case 1.

Classification	Unit Process	Number	Global Warming Potential (GWP)		Ozone Depletion Potential (ODP)		Abiotic Depletion Potential (ADP)		Acidification Potential (AP)		Eutrophication Potential (EP)		Photochemical Ozone Creation Potential (POCP)
			(tCO2eq)		(tCFC11eq)		(tSbeq)		(tSO2eq)		(tPO43-eq)		(tC2H4eq)
			Base	ZEB	Base	ZEB	Base	ZEB	Base	ZEB	Base	ZEB	Base	ZEB
Manufacturing		A1~3	3.314E+02	1.149E+03	1.658E-05	1.561E-04	4.739E+00	2.008E+01	6.518E-01	8.501E+00	8.198E-02	1.187E+00	6.881E-01	2.579E+00
Construction stage	Building materials transportation	A4	1.339E+01	1.994E+01	2.720E-06	5.242E-06	9.028E-02	1.348E-01	8.125E-02	1.127E-01	1.424E-02	1.932E-02	3.574E-02	5.116E-02
	Building construction	A5	5.225E+00	5.225E+00	1.883E-09	1.883E-09	3.405E-02	3.405E-02	2.005E+00	2.005E+00	1.759E-03	1.759E-03	2.601E-04	2.601E-04
Operation stage	Replacement	B4	9.613E+02	1.974E+01	3.387E-04	9.805E-07	1.023E+01	4.584E-01	2.621E+00	7.164E-02	2.601E-01	1.001E-02	2.226E+00	2.560E-02
	Operational energy use	B6	1.090E+03	−2.072E+02	3.013E-08	−5.722E-09	6.895E+00	−1.309E+00	1.844E+00	−3.502E-01	3.432E-01	−6.518E-02	7.766E-03	−1.475E-03
Disposal and demolition stage	Building demolition	C1	1.697E+01	2.102E+01	6.659E-10	8.245E-10	1.135E-01	1.406E-01	1.374E+01	1.701E+01	5.918E-03	7.328E-03	1.225E-03	1.517E-03
	Waste transportation	C2	2.620E+01	3.244E+01	9.796E-06	1.213E-05	1.781E-01	2.205E-01	1.276E-01	1.580E-01	2.078E-02	2.573E-02	6.215E-02	7.698E-02
	Landfill/ Incineration	C3	2.495E+01	2.940E+01	4.265E-06	5.297E-06	1.878E-01	2.333E-01	6.410E-01	7.973E-01	4.187E-02	4.965E-02	4.403E-02	5.439E-02
Total			2.469E+03	1.070E+03	3.721E-04	1.797E-04	2.247E+01	1.999E+01	2.171E+01	2.831E+01	7.698E-01	1.236E+00	3.065E+00	2.787E+00

and

Table 14. LCA results in base and ZEB models of Case 2.

Classification	Unit process	Number	Global Warming Potential (GWP)		Ozone Depletion Potential (ODP)		Abiotic Depletion Potential (ADP)		Acidification Potential (AP)		Eutrophication Potential (EP)		Photochemical Ozone Creation Potential (POCP)
			(tCO2eq)		(tCFC11eq)		(tSbeq)		(tSO2eq)		(tPO43-eq)		(tC2H4eq)
			Base	ZEB	Base	ZEB	Base	ZEB	Base	ZEB	Base	ZEB	Base	ZEB
Manufacturing		A1~3	−6.348E+01	−5.439E+01	5.386E-05	5.399E-05	9.747E+00	9.749E+00	3.675E+00	3.691E+00	6.110E-01	6.131E-01	1.129E+00	1.141E+00
Construction stage	Building materials transportation	A4	1.846E+01	1.855E+01	5.393E-06	5.426E-06	1.250E-01	1.256E-01	1.005E-01	1.009E-01	1.703E-02	1.710E-02	4.640E-02	4.662E-02
	Building construction	A5	5.028E+00	5.028E+00	1.812E-09	1.812E-09	3.277E-02	3.277E-02	1.929E+00	1.929E+00	1.693E-03	1.693E-03	2.503E-04	2.503E-04
Operation stage	Replacement	B4	1.706E+03	1.706E+03	6.119E-04	6.119E-04	1.825E+01	1.825E+01	4.633E+00	4.633E+00	4.535E-01	4.535E-01	3.993E+00	3.993E+00
	Operational energy use	B6	1.215E+03	−1.198E+02	3.357E-08	−3.309E-09	7.681E+00	−7.572E-01	2.055E+00	−2.025E-01	3.823E-01	−3.769E-02	8.653E-03	−8.530E-04
Disposal and demolition stage	Building demolition	C1	1.745E+01	1.750E+01	6.844E-10	6.866E-10	1.167E-01	1.171E-01	1.412E+01	1.417E+01	6.083E-03	6.103E-03	1.259E-03	1.263E-03
	Waste transportation	C2	2.699E+01	2.708E+01	1.009E-05	1.013E-05	1.835E-01	1.841E-01	1.315E-01	1.319E-01	2.141E-02	2.148E-02	6.403E-02	6.425E-02
	Landfill/ Incineration	C3	2.458E+01	2.466E+01	3.926E-06	3.941E-06	1.775E-01	1.782E-01	6.385E-01	6.409E-01	1.769E-01	1.770E-01	7.244E-02	7.259E-02
Total			2.950E+03	1.625E+03	6.852E-04	6.854E-04	3.631E+01	2.788E+01	2.728E+01	2.509E+01	1.670E+00	1.252E+00	5.315E+00	5.318E+00

.

In Case 1, achieving the ZEB model significantly improved the energy independence rate in the operating stage (B6) and significantly reduced the environmental impact load. However, the ZEB model showed an increased environmental impact load in the materials sector compared with the base model. Specifically, there was an increase in the material input of 420 tons, which increased the environmental impact load from A1 to A4. Among them, a change from extrusion method insulation plates to glass wool insulation significantly impacted the environmental impact load of the ZEB model for A1–A4. However, A5 did not affect the total area. Additionally, the environmental impact load of the ZEB model was significantly reduced in B4 owing to the exclusion of materials with frequent repair cycles and high environmental impact loads (change from urethane-coated waterproofing to high-performance composite waterproofing). Furthermore, C1–C3 showed an effect similar to that of A1–A4 by adding the same volume of material input.

Similarly, in Case 2, the ZEB model was achieved, resulting in a significant reduction in the environmental impact load, owing to the significantly improved energy independence rate at the operational stage (B6). However, in the materials sector, the environmental impact loads of A1–A4 and C1–C3 increased because of the change in insulation, whereas A5 was not affected because there was no change in the total floor area.

Based on the analysis results, the carbon reduction strategy from an LCA perspective can be summarized as follows: first, according to the analysis results of Cases 1 and 2, achieving ZEB Class 1 and having a high energy-independence rate greatly improved the energy savings at the operating stage, which significantly contributed to reducing the environmental impact load through LCA. Therefore, all strategies related to energy saving should be considered to reduce the environmental impact load at the operational stage, and the most effective technology in small buildings should be applied as a priority.

Second, in the insulation sector of the LCI DB, the global warming potential (GWP) values (tCO₂eq) of glass wool, bead method insulation plate (EPS), and extrusion method insulation plate (XPS) were 1.8970E+02, 1.9580E+00, and 3.0106E+00, respectively. In particular, there was a difference of more than 100 times between the glass wool and EPS. The use of insulation is limited from the perspective of LCA because various factors, such as usage, fire, and the required performance of each part of the building, should be considered. However, EPS and XPS should be prioritized without these considerations.

Third, in Case 1, the reinforced concrete structures had a weight equivalent to 70% of the concrete itself, and the application of low-carbon concrete alone can be expected to sufficiently reduce the environmental impact loads. However, in Case 2, heavy timber structures weighed only 30% and were considered to have no environmental impact because wood can store carbon. However, the overall environmental impact load value for Case 2 was slightly higher than that of Case 1. Therefore, the final value of the LCA analysis was calculated by considering the LCI DB and the weight of each material used throughout the building. In particular, small buildings have a greater impact on the weight of each material than medium or large buildings.

Table 15. Case 1: LCA savings of ZEB and calculation of environmental cost.

Category	Global Warming Potential (GWP)	Ozone Depletion Potential (ODP)	Abiotic Depletion Potential (ADP)	Acidification Potential (AP)	Eutrophication Potential (EP)	Photochemical Ozone Creation Potential (POCP)
	(tCO2eq)	(tCFC11eq)	(tSbeq)	(tSO2eq)	(tPO43-eq)	(tC2H4eq)
ZEB savings	1.399E+03	1.924E-04	2.480E+00	−6.600E+00	−4.662E-01	2.780E-01
ZEB savings rate	−56.66%	−51.71%	−11.04%	30.40%	60.56%	−9.07%
Impact ratio converted to CO2	1.0000E+00	1.3762E+05	1.7812E+02	1.7351E+01	5.5662E+01	1.2111E+02
Total (19.59$/tCO2)	$34,492

and

Table 16. Case 2: LCA savings of ZEB and calculation of environmental cost.

Category	Global Warming Potential (GWP)	Ozone Depletion Potential (ODP)	Abiotic Depletion Potential (ADP)	Acidification Potential (AP)	Eutrophication Potential (EP)	Photochemical Ozone Creation Potential (POCP)
	(tCO2eq)	(tCFC11eq)	(tSbeq)	(tSO2eq)	(tPO43-eq)	(tC2H4eq)
ZEB savings	1.325E+03	−2.000E-07	8.430E+00	2.190E+00	4.180E-01	−3.000E-03
ZEB savings rate	−44.92%	0.03%	−23.22%	−8.03%	−25.03%	0.06%
Impact ratio converted to CO2	1.0000E+00	1.3762E+05	1.7812E+02	1.7351E+01	5.5662E+01	1.2111E+02
Total (19.59$/tCO2)	$56,573

present the CO₂ savings and costs of the ZEB model compared to the base model in prior research [42,43]. The Building Total Cost Assessment (BTCA) method used in previous studies assesses the total cost of a building by converting the environmental load generated during its life cycle into social opportunity costs and integrating and evaluating them with life cycle costs such as building acquisition and operation costs. This method shows the economic value of investing in green building planning elements throughout a building’s life cycle. It evaluates the economic feasibility of environmental investing using a clear cost criterion. In this study, environmental costs were calculated based on the LCA results of the ZEB model using this method. The entire environmental impact category was converted into carbon using an average of $19.59 per tCO₂, which was the amount of carbon emission rights (KAU19-22) on the Korea Exchange between 2019 and 2022. The low reduction was converted into a cost.

4. Economic Evaluation Reflecting ZEB and LCA

In the design stage, the additional construction costs of applying the ZEB technology in the architectural, mechanical, and electrical equipment sectors to achieve ZEB certification were calculated as a percentage of the total construction costs. Case 1 showed that the additional construction cost of the ZEB model compared to the base model was $234,940 (a 22.8% increase from the existing construction cost), and Case 2 was $224,108 (a 15.7% increase from the existing construction cost).

Energy reduction costs through ZEB and carbon reduction through LCA were considered to evaluate the economic benefits. The NPV method was used for the economic analysis because it presents the present value of future benefits and can be used for other analyses while considering the analyzed NPV [58]. The social interest rate was calculated to be 4.5%, and the analysis period was set to 40 years, which is the lifespan of reinforced concrete buildings, based on the residual price rate by year of the 2021 Standard Market Value Adjustment Standard [59]. Equations (1) and (2) represent the equations for economic analysis. If the NPV analysis results in Cases 1 and 2 show a value greater than “0,” they are considered economical.

Table 17. Description of symbols in Equations (1) and (2).

Description of Symbols		Application Criteria
Bt	Present value of benefits	Cost of annual energy savings	Cost of annual energy savings($/year) = annual energy savings(kWh/year) × Power unit price($/kWh)
		Power unit price, $0.7/kWh
Ct	Present value of cost	Additional construction costs for achieving ZEB certification
r	Interest rate	Social discount rate, 4.5%	(Lee et al., 2018) [58]
n	Year of analysis year	Life of reinforced concrete structures, 40 years	(Lee, 2020) [59]
R	Cost of environmental impact reduction	Environmental impact cost converted from LCA results

describes the symbols in Equations (1) and (2) and the NPV analysis results are presented in

Table 18. Case 1 economic evaluation results.

Total Cost [Ct] (Additional Construction Costs for Achieving ZEB Certification)	Total Benefits
	Cost of Annual Energy Savings [Bt]	Cost of Environmental Impact Reduction [R]
$234,940	$66,216 (28.2% Reduction)	$34,492 (11.7% Reduction)
	Sum of total benefits = $100,708 (39.9% Reduction)
NPV analysis results	−$134,232 (<0)

and

Table 19. Case 2 economic evaluation results.

Total Cost [Ct] (Additional Construction Costs for Achieving ZEB Certification)	Total Benefits
	Cost of Annual Energy Savings [Bt]	Cost of Environmental Impact Reduction [R]
$224,108	$69,273 (30.9% Reduction)	$56,573 (11.6% Reduction)
	Sum of total benefits = $125,846 (42.5% Reduction)
NPV analysis results	−$98,262 (<0)

.

$$\mathrm{Net} \mathrm{Present} \mathrm{Value} \left(\mathrm{NPV}\right)={\displaystyle \sum }_{\mathrm{t}=0}^{\mathrm{n}}\frac{\mathrm{Bt}}{{\left(1+\mathrm{r}\right)}^{\mathrm{t}}}-{\displaystyle \sum }_{\mathrm{t}=0}^{\mathrm{n}}\frac{\mathrm{Ct}}{{\left(1+\mathrm{r}\right)}^{\mathrm{t}}}+R$$

(1)

$$\mathrm{Modified} \mathrm{Net} \mathrm{Present} \mathrm{Value} \left(\mathrm{MNPV}\right)={\displaystyle \sum }_{\mathrm{t}=0}^{\mathrm{n}}\frac{\mathrm{Bt}}{{\left(1+\mathrm{r}\right)}^{\mathrm{t}}}-{\displaystyle \sum }_{\mathrm{t}=0}^{\mathrm{n}}\frac{\mathrm{Ct}}{{\left(1+\mathrm{r}\right)}^{\mathrm{t}}}+R$$

(2)

As a result of the analysis, it was found that both Case 1 and Case 2 incurred costs for energy savings through ZEB and carbon reduction through the LCA, resulting in a net present value of less than ‘0’, indicating that they were not economical. However, when environmental costs converted based on LCA were added to the existing energy-saving costs, the NPV could be offset by 11.7% and 11.6% in addition to the existing 28.2% and 30.9% savings in Cases 1 and 2, respectively. Therefore, incorporating these LCA evaluation techniques can increase the effect of applying ZEB by including the environmental impact and cost.

5. Discussion

In this study, two cases of 500 m² office buildings that achieved first-class ZEB certification for the first time in Korea were analyzed before ZEB certification became mandatory for small buildings by 2025. As a result, the ZEB application technology for small buildings was not as complex as that for medium and large buildings, and the ZEB application technology for small buildings showed similar characteristics in both target buildings. Furthermore, there was no significant difference in the application technology of the architectural sector compared to medium and large buildings; however, in the case of the mechanical equipment sector, individual facilities were mainly applied rather than the central types. In terms of achieving an energy-independent rate through ZEB in small buildings, a photovoltaic system is the most preferred renewable energy system. However, geothermal heating and cooling systems are inefficient in small buildings, and fuel cells are unsuitable for small buildings owing to their high installation and maintenance costs.

The additional construction costs for ZEB Class 1 were similar at $234,940 for Case 1 and $224,108 for Case 2, of which more than 30% were attributable to the photovoltaic system. The ratios of the additional construction costs for ZEB Class 1 to the total construction cost were 22.8% for Case 1 and 15.7% for Case 2. In a previous ZEB case study [7], the ratio of additional construction costs for ZEB Class 5 was reported as 17–38%. In this study, the 15.7–22.8% ratio for ZEB Class 1 suggests that it may be more advantageous than in the previous case. However, previous studies analyzing non-residential buildings, including office buildings, have suggested that if the additional construction cost of ZEB is more than 5%, it is not economical in terms of a cost–benefit analysis, and policy support for the mandatory revitalization of ZEB is needed [7]. The additional construction cost analyzed in this study was greater than 5%, which was uneconomical in the NPV analysis.

This study analyzed the LCA characteristics of small buildings. It investigated ways to reduce additional carbon and improve economic efficiency through materials, construction, and disposal while focusing on energy savings through ZEB at the operational stage. The results showed that the ZEB model significantly improved the energy independence rate during the operational stage and significantly reduced the environmental impact load. However, changes in the material input and insulation from extrusion method insulation plates to glass wool insulation had negative effects, reducing the environmental impact load. Overall, the environmental impact load of the LCA was found to be the greatest during the operational stage of energy use.

Furthermore, the environmental impact coefficient through the LCI DB and the weight of each material significantly impacted the materials. In particular, the weight of each material can have a greater impact on small buildings than on medium and large buildings. Therefore, from an LCA perspective, selecting and applying materials with a high potential for reducing material weight and environmental impact load is crucial.

In the economic analysis, the benefits were calculated based on the cost of energy savings through ZEB and the cost of carbon reduction through the LCA. The analysis results show a negative NPV, indicating that the project is not economically feasible. However, adding the environmental costs calculated through LCA to the existing energy-saving costs could offset an additional 11.6–11.7%. Therefore, including the environmental impact and cost through these LCA evaluation techniques could be a positive step toward increasing the effectiveness of ZEB applications.

The case studied In this research achieved a Class 1 ZEB certification with an energy independence rate of over 100%. The construction included external insulation dry walls, external electric blinds, and other passive construction technologies, which increased construction costs, but improved building quality in actual use. However, if the goal is to obtain ZEB certification, it is possible to aim for the lowest ZEB certification, Class 5. In such a case, installing a 10-kW solar panel system in the same building can satisfy more than 20% of the energy independence rate. Furthermore, suppose the application technology is modified by replacing the external insulation dry walls with internal insulation and excluding external electric blinds. In that case, the additional construction cost will be within $75,000, representing approximately 5% of the total construction cost and be considered cost-effective.

In conclusion, this study analyzed the economic feasibility of achieving the first class of ZEB certification, considering energy savings and carbon reduction costs through the LCA. The findings indicate that achieving ZEB certification is not economically feasible. However, by introducing the LCA evaluation technique, the positive impact of the ZEB application can be increased by including the environmental impact and cost. Moreover, it should be noted that an economic analysis that relies on a simulation analysis to predict energy savings has limitations, as it may not fully reflect the quality of passive methods and materials that correspond to their price.

6. Conclusions

6.1. Main Findings

As a result of this study, the ZEB technologies applied to both target buildings include passive technologies, high-efficiency facilities, lighting, BEMS, and photovoltaic systems. Compared to general buildings, Cases 1 and 2 achieved energy requirement savings of −111.6 kWh/m²·y and −121.8 kWh/m²·y, respectively, with savings ratios of 106.5% and 110.2%.

Regarding the LCA, both Cases 1 and 2 achieved the ZEB model, significantly improving the energy independence rate and reducing the environmental impact load. Based on the GWP, the LCA of Cases 1 and 2 were reduced by −56.66% and −44.92%, respectively.

6.2. Theoretical and Practical Implications

Regarding materials, the environmental impact coefficient through the LCI DB and the weight of each material had a significant influence. In particular, small buildings can have a greater impact on the weight of each material than medium- and large-scale buildings. Therefore, from an LCA point of view, selecting and applying materials that have a greater effect on reducing material weight and environmental impact load is important.

In the economic analysis, the additional construction cost for achieving ZEB certification was included in the total cost. In contrast, the energy-saving cost through ZEB and the carbon reduction cost through LCA were considered benefits. As a result, the analysis showed a net presence of less than zero, indicating that it was not economical. However, when considering the existing energy savings and the environmental costs converted based on the LCA, an additional offset of 11.6% to 11.7% was found. Therefore, including environmental impacts and cost values by introducing LCA evaluation techniques can increase the effectiveness of ZEB applications.

6.3. Limitations and Future Directions

This study is significant in analyzing the main application technologies and status of ZEB certification for small buildings, which will become mandatory by 2025, and identifying ways to enhance them. However, ZEB certification for small buildings is still nonmandatory; hence, there are a few certification cases for such buildings because of the lack of awareness regarding quality and performance improvements through ZEB as well as the associated increase in construction costs. Consequently, a shortage of ZEB certification cases for small buildings is a limitation of this study. In the future, as the mandatory ZEB certification requirements will be updated, additional cases will be included. Based on this, we aim to investigate the optimal performance combination and improvement plan that is economically feasible and present detailed findings.