Life Cycle Environmental and Cost Performance of Prefabricated Buildings

Abstract

Global greenhouse gas (GHG) emissions from the construction industry continue to increase at an annual rate of 1.5%. It is particularly important to understand the characteristics of the building life cycle to reduce its environmental impact. This paper aims to assess the environmental impact of prefabricated buildings and traditional cast-in-situ buildings over the building life cycle using a hybrid model. A case study of a building with a 40% assembly rate in Japan was employed for evaluation. It concluded that the total energy consumption, and carbon emissions of the prefabricated building was 7.54%, and 7.17%, respectively, less than that of the traditional cast-in-situ building throughout the whole life cycle. The carbon emissions reduction in the operation phase reached a peak of 4.05 kg CO2/year∙m2. The prefabricated building was found to cost less than the traditional cast-in-situ building, reducing the price per square meter by 10.62%. The prefabricated building has advantages in terms of reducing global warming, acid rain, and health damage by 15% reduction. With the addition of the assembly rate, the carbon emissions and cost dropped, bottoming out when the assembly rate was 60%. After that, an upward trend was shown with the assembly rate increasing. Additionally, this study outlined that the prefabricated pile foundations is not applicable due to its high construction cost and environmental impact.

Full text

Sustainability 2020, 12, 2609; doi:10.3390/su12072609 www.mdpi.com/journal/sustainability
Article
Life Cycle Environmental and Cost Performance of
Prefabricated Buildings
He Wang
1
, Yinqi Zhang
1,
*, Weijun Gao
1,2,
* and Soichiro Kuroki
1
1
School of Environmental Engineering, the University of Kitakyushu, Kitakyushu 8080135, Japan;
wangheawork@gmail.com (H.W.); kuroki@kitakyu-u.ac.jp (S.K.)
2
iSMART, Qingdao University of Technology, Qingdao 266033, China
* Correspondence: gaoweijun@me.com (W.G.); yinqiz90@gmail.com (Y.Z.)
Received: 5 March 2020; Accepted: 18 March 2020; Published: 25 March 2020
Abstract: Global greenhouse gas (GHG) emissions from the construction industry continue to
increase at an annual rate of 1.5%. It is particularly important to understand the characteristics of
the building life cycle to reduce its environmental impact. This paper aims to assess the
environmental impact of prefabricated buildings and traditional cast-in-situ buildings over the
building life cycle using a hybrid model. A case study of a building with a 40% assembly rate in
Japan was employed for evaluation. It concluded that the total energy consumption, and carbon
emissions of the prefabricated building was 7.54%, and 7.17%, respectively, less than that of the
traditional cast-in-situ building throughout the whole life cycle. The carbon emissions reduction in
the operation phase reached a peak of 4.05 kg CO2/year∙m
2
. The prefabricated building was found
to cost less than the traditional cast-in-situ building, reducing the price per square meter by 10.62%.
The prefabricated building has advantages in terms of reducing global warming, acid rain, and
health damage by 15% reduction. With the addition of the assembly rate, the carbon emissions and
cost dropped, bottoming out when the assembly rate was 60%. After that, an upward trend was
shown with the assembly rate increasing. Additionally, this study outlined that the prefabricated
pile foundations is not applicable due to its high construction cost and environmental impact.
Keywords: prefabricated building; traditional building; life cycle; environmental impact
1. Introduction
The Intergovernmental Panel on Climate Change (IPCC) [1] indicated that if the current growth
rate of greenhouse gas emissions is maintained, the global average temperature will increase by 1.5
°C from 2030 to 2052, which will cause serious damage to the ecological environment. The United
Nations Environment Programme (UNEP) points out that the construction industry consumes 40%
of the energy, 30% of the raw materials, and 25% of the solid waste available globally and produces
36% of the total greenhouse gas (GHG) emissions [2]. The global GHG emissions from the
construction industry continue to grow at an annual rate of 1.5% [3]. In developed countries, the
construction industry also accounts for a high proportion of carbon emissions. Carbon emissions
generated by building operation and construction account for 50%, 33%, and over 40% of the total
social output in the UK, Japan, and the US, respectively [4]. Therefore, it is particularly important to
understand the characteristics of in the process of construction, operation, and replacement during
the life cycle of a building to reduce the environmental impact. Prefabricated buildings are buildings
that use prefabricated components and prefabricated construction technology, combined with new
energy-saving technology, which improves the building quality, reduces energy consumption,
shortens the construction period, and saves money in the building life cycle [5–8].

Sustainability 2020, 12, 2609 2 of 18
Prefabricated buildings have been developed rapidly since World War II and are widely used
all over the world [9]. The term used to describe “prefabricated buildings” is slightly different in
various countries and regions, for example, “prefabrication”, “pre-assembly”, “modularization”, and
“off-site manufacturing” [10]. “Modular housing” is used in America [11], “prefabricated housing”
in Japan and mainland China [12,13], “prefabricated buildings” in the Australia [8]; “prefabrication”
in Hong Kong and Singapore [14,15], and “off-site production” in European countries [16], which
refers not only to prefabs but also to elements like reinforcement structures (e.g., cages for columns)
that are manufactured offside and mounted on site. The environmental impacts of prefabricated
buildings and traditional cast-in-situ buildings have been compared using process models. The
results showed that the GHG emissions of prefabricated buildings in the construction stage were less
than those of traditional cast-in-situ buildings [17]. A similar comparative study on the consumption
of materials and energy in the construction process was conducted. It showed that, compared with
traditional buildings, prefabricated buildings have less wood and water usage while causing less
damage to the environment and health [18]. Case studies have shown that the embodied energy
content of a typical concrete frame and block construction accounts for 66% of typical concrete frame
structures [19]. The thermal insulation optimization of prefabricated buildings can effectively reduce
the energy consumption for heating and cooling. The durability is also better [20]. By adopting a
mature recycling system, concrete waste generated from the demolition of buildings can be efficiently
recycled, thereby reducing recycling costs and environmental impact [21]. The recovery rate of
metallic materials and concrete is higher than that of other non-metallic materials. Non-recycled, non-
metallic materials are usually shipped to landfills as waste [22]. Analysis of the carbon footprint and
energy footprint of these two types of buildings showed that prefabricated buildings have reduced
carbon emissions and energy consumption [23].
In the practical and theoretical fields, research on prefabricated buildings has mainly focused on
the performance of building components, economic benefits, and the impact of a single stage
specifically on the environment, but studies from the perspective of the whole life cycle of
prefabricated buildings have been rare [24–26]. It is necessary to extend the prefabricated building
study boundaries to the whole life cycle period. Prefabricated buildings have developed completely
in Japan. From 1970 until now, it has developed to the fourth generation, forming a complete
industrial chain from design and construction to construction operation and demolition [27]. In
addition, the basic data on buildings and environment in Japan are complete, thereby providing a
stable basis for the research and exploration to investigate the impact of prefabricated buildings on
the environment during their life cycle [28]. Moreover, Japan has a high urbanization rate and a high
level of building industrialization [29]. Therefore, the use of Japanese prefabricated buildings as the
research object has significance for the development of the construction industry in other countries.
As a widely recognized environmental impact analysis tool, life cycle assessment (LCA) can be
divided into two types according to the differences in the calculation process and research purposes:
attribution and consequence [30,31]. The attribution LCA model is suitable for relevant studies on the
impact of the building environment [15,32]. There are three kinds of mathematical model for the
attribution LCA—the input-output model (I-O model), the process-based model, and the hybrid
model [32,33]. The I-O model is based on the economic input and output table of a country or region,
which can measure the impact on resource consumption and environment in various ranges, taking
into account the sectoral dependence of related sectors of construction [34]. However, this model has
homogeneity, data timeliness, and uncertainty [35]. The process-based analysis model quantifies the
energy and resource usage of buildings at different stages and improves the accuracy of the results
[36]. However, the data collection of this model is complex and requires high data accuracy.
Meanwhile, the definition of the model’s scope may also cause errors [37]. The hybrid model
combines the characteristics of the process model with those of the I-O model to reduce errors [38].
The common definition of a life cycle is the entire process from the cradle to the grave, which
corresponds to the scope of this paper from the design to the demolition stage. The life cycle of a
prefabricated building is divided into three phases in this paper: construction, use, and demolition.
The life cycle of a prefabricated building is divided into three phases—construction, use, and

Sustainability 2020, 12, 2609 3 of 18
demolition in this paper. The purpose of this study is to analyze the energy consumption and carbon
emissions of Japanese prefabricated buildings across the life cycle from the macro perspective. In
view of the characteristics and limitations of the process-based model and the I-O model, the hybrid
model is selected as the calculation method. In Section 2, the mixed model is used to respectively
explain the supply chain activities and production processes of prefabricated buildings during
different processes. In addition, the calculation methods are introduced. In Sections 3 and 4, the
energy consumption and environmental impact characteristics of prefabricated buildings are
analyzed under different working conditions through data collection and processing, and the
advantages of prefabricated buildings are analyzed through a compassion with traditional buildings.
Section 5 provides the conclusions and further describes possible measures to conserve energy and
reduce environmental impacts.
2. Methods
2.1. Research Scope
The environmental impact (EI) during the life cycle of a building can be divided into three
phases: the construction phase (including the design, material production, and site construction
stages), the use phase (including the operation and replacement stages), and the demolition phase
[39]. It can be expressed by Equation (1):
𝐸𝐼 𝐸𝐼
 𝐸𝐼
=𝐸𝐼. (1)
where EI,EI , and EI stand for the EI from the construction phase, the use phase,
and the demolition phase, respectively.
The hybrid model 𝐸 can be presented as
𝐸 𝐸
𝐸
. (2)
where 𝐸 is the I-O model, and 𝐸 represents the process-based model. The I-O model is used for
the analysis of production processes. The process-based model is for other processes. The two data
sources are different, one is macroeconomic data for the I-O analysis, and the other is physical data
for the process analysis. The I-O analysis method converts the monetary value of all relevant sectors
into the final emissions at production. The process-based model converts the material usage and the
corresponding environmental load into emissions for the operation and demolition phases. Figure 1
shows the calculation system for the LCA.
Figure 1. The calculation system for the life cycle assessment (LCA).

Sustainability 2020, 12, 2609 4 of 18
2.2. Input–Output Model
The input–output table is an important tool to analyze the economic and technical relationship
between production sectors, which can be traced to the embodied environmental impact of a specific
sector of various materials and services. In terms of the activities of various industrial sectors of the
national economy, different industries are interrelated through the supply and demand of products.
The development of each industry needs other industrial sectors to provide production factors for it,
and the output of each industrial sector may be the input of other industrial sectors [40].
Simultaneously, a complete consumption relationship among sectors can be quantified [41]. Table 1
describes the correlations among different sectors, which can be used to calculate the intermediate
consumption among sectors. The original input-output table was expanded (Table 1) to directly
represent the material inputs of the sector, where 𝑋, 𝑌
, 𝑋, and 𝑉
 stand for the total output, final
demand, intermediate use, and value added, respectively. 𝐹 and 𝑁
 are the direct energy or carbon
input of sector i because of the intermediate use and final use of sector j, respectively. 𝐹
, is the total
amount of direct energy or carbon input of sector i.
Table 1. Extended input–output table.
Output sectors Indirect use Final demand Total
output
Input sectors Sector
1
Sector
2 … Sector
n Consume Accumulating
capital Total
Indirect
inputs
Sector
1 X11 X12 … X1n
II
Y1 X1
Sector
2 X21 X22 … X2n Y2 X2
… I … …
Sector
n Xn1 Xn2 … Xnn Yn Xn
Value
added V
1 V2 … Vn -
Material
inputs
Sector
1 F11 F12 … F1n N1 Ff,1
Sector
2 F21 F22 … F2n N2 Ff,2
… IV … …
Sector
n Fn1 Fn2 … Fnn Nn Ff,n
The method has become the mainstream method used to study the environmental impact from
the macro perspective (country, region, sector, etc.) [42]. Based on the “input = output” equilibrium
theory, the Leontief matrix is used to represent the relationship between the total input and total
output [40], which can be described as shown in Equation (3):
Y
=E−A﹒X=L﹒X. (3)
where X=(𝑋,𝑋⋯𝑋
) and Y=(𝑌
,𝑌
⋯𝑌
) are the total input of the sector and the final
demand of the sector, respectively; E is the identity matrix; A=𝑋 𝑋
⁄× is the direct
consumption coefficient matrix representing the value of unit i consumed by unit j; L=(𝐿)× is
the Leontief matrix.
Generally, rough division will reduce the measurement accuracy, but division that is too
detailed increase the complexity of the calculation. The division in the I-O model can adopt the
method of equal proportion division without changing the direct consumption coefficient of other
sectors [30]. Expanding the original input-output table can directly show the material inputs of the
sector. In Equation (4), 𝑋 and 𝑌
 are the total output and innovation value of sector I, respectively.
𝑋 is the usage amount provided by sector i to sector j, and the final demand of supply of each sector

Sustainability 2020, 12, 2609 5 of 18
is 𝑌
. 𝐹
, i represents all material emissions of sector i (including direct and indirect material
emissions).
According to Table 1, the balanced equation of material consumption can be expressed as follows:
𝐹
,=∑
𝑓
 +𝑁=𝜀(∑𝑋 +𝑌


 )

 . (4)
where 𝜀=𝜀× is a diagonal matrix that includes the emission coefficients for all sectors.
Combined with Equation (3), the rewritten equation is as follows [43]:
D=𝜀∙𝐿
 ∙𝑌. (5)
The relationship between the sectoral final material emissions and sectoral final demand is
clearly expressed by Equation (5). Therefore, 𝜀∙𝐿
 can be simplified by the coefficient η which
shows the relationship between the final material emissions and the final demand. Taking sector j for
example, the relation between the total material emissions of relevant sectors in sector j and the final
demand can be expressed in vertical coefficients, i.e., [38]
η=∑𝜀∙𝐿


, . (6)
Therefore, the total substance emissions 𝐷 related to sector j are expressed by the following
equation [44,45]:
𝐷 =η∙𝑌
 . (7)
2.3. Process-Based Model
The operation phase can be divided into two parts: application and modification. In the process-
based model, the EI of the building can be expressed as follows:
𝐶𝐸 =∑𝑀∙𝐸𝐹
,

 . (8)
where i is a kind of building material, 𝑀 is the amount of building materials (electricity, sewage,
maintenance materials, etc.) used, and 𝑀∙𝐸𝐹
, is the environmental load factor of the building
materials per unit of production. It should be noted that the time background and economic
background should be considered in order to select the appropriate environmental load factors. The
factors of materials can be found in relevant data [46].
The environmental impact during the demolishing phase is mainly related to transportation and
material disposal. According to the relevant literature [36], transportation can be calculated according
to the following equation:
𝐶𝐸 =∑𝑀∙𝐷
∙𝐸𝐹
,

 . (9)
where 𝐷（km）is the distance from the demolition site i to the recycling company, 𝑀 represents
the quantity of materials, and 𝐸𝐹, represents the environmental load factor of the building
materials of different modes of transport.
3. Data Collection
This study took a real-life prefabricated building case, which is located in Kitakyushu, Japan,
and investigated its environmental performance. The construction life is 80 years—more information
is detailed in Table 2. Three observation spots—including the site of the case prefabricate building,
the data center that stores architectural drawings and related engineering documentation, and the
prefabrication factory—were selected with the purpose of containing all related data in detail.
Various measuring methods were employed for data collection. The process for each research point
comprised a content evaluation of drawings and documentation of case building, for instance, the
construction schedule and plan, the bill of quantities, the inventories of prefabricated components,
and construction technology specifications, etc., were analyzed in detail. All of the drawings and
documentation were verified and validated by all participants and experts to guarantee the data
quality. Research data from the building were collected from the construction diary, records, and

Sustainability 2020, 12, 2609 6 of 18
calculation reports (Table 3) [47]. The quantity and monetary value of major materials were recorded.
In the operation and replacement phase, the energy consumption data were based on the actual
monitoring data, which came from the operation records of related equipment. The software AIJ-
LCA&LCW ver.4.04 (Architectural Institute of Japan-Life Cycle Assessment and Life Cycle Waste,
2006, Japan), developed by Architectural Institute of Japan, was used to perform data calculations.
The emission factors and I-O databases for all applications were unique and consistent with the
location of the building.
Table 2. Basic information about the prefabricated building case.
Application
Levels
above
Ground
Floor
Area
(m2)
Building
Structure
Total
Project
Cost (JPY)
Project
Cost
(JPY/m2)
Foundation
Type
Prefabrication
Rate of
Structure
Public
buildings 5 33500 CF 847.7x107 20.53x105 Pile 0.4
Table 3. Material consumption of the prefabricated building case (unit building area).
Materials Units Quantity Materials Units Quantity
Reinforcing bar kg/m2 77.53 SBS waterproof roll m2/m2 0.34
Other steel kg/m2 1.61 PVC downpipe m/m2 0.05
Shaped steel kg/m2 50.21 Timber formwork t/m2 6.17
Aluminum t/m2 0.54 Gypsum board t/m2 12.18
Precast
column/beam m3/m2 0.18 Carpet m2/m2 0.35
Precast slab m2/m2 0.82 Vinyl tile m2/m2 0.65
Premixed mortar m3/m2 0.02 Wallpaper m2/m2 0.78
Concrete block m3/m2 0.04 Door and window m2/m2 0.29
Premixed
concrete m3/m2 0.26 Wood product m3/m2 0.02
Cement t/m2 47.67 Glass fiber
membrane m2/m2 0.08
Polystyrene
board (EPS) m2/m2 0.63 - - -
In this study, “prefabricated public building” (PPB) stands for the public building using
prefabrication construction investigated in this case study, and “traditional public building” (TPB)
refers to the assumed public building using cast-in-situ construction.
To maintain consistency, the relevant data from different stages in the life cycle of conventional
buildings were assumed based on prefabricated building materials and energy consumption. The
assumption of the amount of building materials used in the traditional construction method was
based on existing research findings, as shown in Table 4 [6,7,15,18]. PPBs use steel templates, which
can be reused to produce prefabricated components. However, the wooden templates used in TPBs
are disposable. In the operation stage, PPBs adopts factory prefabricated built-in thermal insulation
technology with better thermal performance. According to the conclusions of Takeuchi, the energy
consumption of air conditioning in prefabricated buildings can be reduced by 25% [20]. Service life
is equal to the life span of the building structure. TPBs adopts the on-site construction insulation layer
operation method, which has a service life of 25 years. In the demolition and recovery stage, different
definitions are made for the components of the two kinds of buildings. The demolition and recovery
rate of the building components and internal products of PPBs is higher than that of TPBs [6,48]. The
recovery rates of different components of the two kinds of buildings were respectively assumed.

Sustainability 2020, 12, 2609 7 of 18
Table 4. Assumption of traditional public building (TPB) material consumption.
Percentage
of
Material
Saving
Steel Concrete Timber Mortar Heat
Insulation
Other
Decoration
Materials
Energy
Consumption
PPB 1 1 1 1 1 1 1
TPB 1.3 1.2 1.7 1.2 1.25 1.15 1.25
4. Results and Discussion
Based on the scope and data defined above, the environmental impacts of the two buildings’ life
cycles were compared and evaluated. The results were used to compare the differences between the
PPB and TPB. The characteristics of the environmental impact of the two kinds of buildings at
different stages of the life cycle were evaluated in detail. In addition, the PPB with different assembly
rates and with prefabricated foundation were calculated separately. The effects of the assembly rate
and prefabricated foundation on the carbon emissions of the building throughout the life cycle were
analyzed.
4.1. Material Consumption
In order to facilitate the comparison between them, the total consumption of building materials
was converted into the resource unit demand (kg/m
2
). Figure 2 illustrates the amount of input
resources for the PPB and TPB. The resource consumption of PPB was found to be 3728 kg/m
2
, 9.32%
lower than that of TPB. Resources were saved by 9.59% in the construction process. The main reason
for this is that PPBs use prefabricated components, which can effectively reduce the consumption of
concrete, steel, and wood. Fabricated members use steel templets in the process of being produced,
avoiding the use of wood templets. Thermal insulation is located between the layers of the PPB
concrete structure without the need for mortar as a bonding material, thereby reducing mortar
consumption. Additionally, building components were produced in a factory with highly accurate
control, which effectively reduced the waste of concrete and steel. The resource inputs for the
maintenance and replacement of the PPB components maintaining and replacement were 7.01% and
9.72% less than those of on-situ production, respectively, due to the longer product life of
prefabricated components and lower material change rate in the life cycle of the PPB.
Figure 2. Comparison of input resource between a prefabricated public building (PPB) and a TPB.
Construction waste is produced during construction, repair, and modification and demolition.
Figure 3 illustrates the amount of waste generated by the two types of buildings. The most waste was
generated in the process of material replacement and demolition. During these two processes,
building components such as doors, windows, and partition walls cannot be reused because of their
inevitable destruction. The total solid waste from the PPB was found to be 2257 kg/m
2
, 15.90% less
than that of the TPB. The TPB generated 330 kg/m
2
and 289 kg/m
2
of solid waste during the processes
of construction and replacement, which is 12.49% and 5.76% more than PPB, respectively.

Sustainability 2020, 12, 2609 8 of 18
Figure 3. Comparison of solid waste between the PPB and TPB.
The qualified rate of building components produced in a factory is higher than that of on-site
production, which reduces the generation of waste at the source. The factory is a relatively closed
and stable environment with little external interference, which can reduce the loss and interference
of the natural environment and human factors on materials. At the same time, the large-scale
application of industrial machinery is conducive to the stable construction, thus improving the
quality of products. In addition, some prefabricated components with special technologies were
produced in the factory, such as insulation panels sandwiched between two layers of concrete,
effectively improving the service life and reducing the renewal cycle of building components. On the
contrary, due to the limitations of on-site construction, the thermal insulation layer is attached to the
outer surface of the wall. Due to the poor durability, this will increase the replacement frequency of
components, leading to extra construction waste. Furthermore, the organizational structure of the
factory is relatively simple. On the contrary, the construction site is composed of many construction
departments; the organizational structure is more complex. This also leads to the reduction of the
recycling efficiency and recovery rate. Moreover, in the field investigation of this research, the factory
classified and recycled most of the building materials. However, some construction materials in the
construction site were disordered, making it difficult to effectively recycle some construction
materials.
4.2. LCA-Based EI Assessment Results and Discussion
The whole process from the design to the demolition of a building will influence the
environment, thus the negative impact should be reduced out at every stage of the life cycle. Using
the data collected, including the quantity of building materials, energy consumption, and recovery
rates of different building materials, the building impact on the environment, carbon emissions and
cost during their life cycle can be calculated.
4.2.1. Comparison of Energy Consumption Between the PPB and TPB during their Respective Life
Cycles
There is great energy saving potential in the operation phase where the most energy is
consumed. Following that, energy consumption in the building material production phase is the
second greatest during the life cycle. The site construction and demolition phases account for less
energy than others due to their shorter durations. However, from a macro perspective, there are a
huge number of construction projects every year. Correspondingly, the sum of energy consumption
in these two phases will rise dramatically. The energy saving potential during these two phases
should be considered as well.

Sustainability 2020, 12, 2609 9 of 18
Figure 4 summarizes the energy consumption of the two kinds of building during their life
cycles. The total energy consumption of the PPB was found to be 7.54% less than that of the TPB. The
PPB was shown to use less energy than the TPB at every stage. The energy saving effect in the
operation stage was the most significant, reducing by 66.62 MJ/year∙m
2
. The PPB can reduce energy
consumption by 10.93% and 7.1% in the construction phase and replacement stage, respectively. The
demolition phase was shown to consume the least energy with 1.823 MJ/year∙m
2
, but the energy
saving ratio was as high as 11.29%. The energy consumption reduction in the operation stage mainly
comes from two aspects. First, energy consumption due to air-conditioning usage is lower in the PPB
than in the TPB because of higher thermal insulation of the PPB. Moreover, prefabricated components
have higher durability, which reduces the replacement of building components in the operation
stage.
Figure 4. Comparison of energy consumption between the PPB and TPB during their respective life
cycles.
Energy saving in the design stage is not obvious in the perspective of the building life cycle.
However, during this stage, the PPB can still save 10.28% more energy than the TPB. The modular
design applied, which uses fixed building modules and components and recycles components after
building demolition, dramatically saves design time and money. It guides and standardizes the
demolition and recycling process in the following stage. At the same time, modular design and
construction improve the efficiency of supervision work, so that energy consumption is reduced in
the design stage.
The energy savings of the PPB in the site construction process are mainly realized through two
aspects: one is the reduction of energy consumption brought by material savings; the other is energy
saving due to the improvement in equipment efficiency. The energy consumption in the site
construction process mainly comes from the application of field machinery. The PPB reduces the
mechanical consumption in the field construction and improves the efficiency. During the site
construction process of the PPB, lifting equipment is used to lift complete building components, such
as prefabricated beams, walls, floors, stairs, and so on. On the contrary, the equipment is often used
to lift single building materials or building accessories, such as steel bars or formwork, in the site
construction of the TPB. Therefore, the efficiency of equipment in the site construction of the PPB is
improved obviously. Although the industrial production of prefabricated components increases the
consumption of fuel and electricity compared with TPB, their application can avoid the installation
of some building materials on the construction site, including the insulation layer, concrete, steel bars,
etc. It can eliminate the requirement for concrete pump trucks and lifting machinery, thereby
reducing the consumption of fuel and electricity, achieving energy saving.

Sustainability 2020, 12, 2609 10 of 18
4.2.2. Comparison of Carbon Emissions Between the PPB and TPB during their Respective Life
Cycles
At every stage of the life cycle, the carbon emissions of the PPB were found to be less than those
produced by the TPB (Figure 5). More precisely, the total carbon emissions of the PPB were 81.08 kg
CO
2
/year∙m
2
, 6.26 kg CO
2
/year∙m
2
(7.17%) less than the TPB. During the design, material production,
and site construction phase, the emissions of the PPB were 12.623 kg∙CO
2
/year∙m
2
, 8.29% lower than
the TPB. The carbon emissions of the PPB during the operation phase were reduced by the greatest
amount: 4.05 kg CO
2
/year∙m
2
. In contrast, in the replacement and demolition phase, the emissions
only reduced by 1.069 kg CO2/year∙m
2
in the TPB. In the process of building material production and
building site construction in the PPB, carbon emissions decreased with less usage of wood formwork
and fuel conservation by construction machinery.
Figure 5. Comparison of carbon emissions produced during the construction of the PPB versus TPB
during their respective life cycles.
The thermal insulation panels used in the PPB reduce the energy consumption from air
conditioning. Consequently, the carbon emissions were found to be reduced during the use phase
(including the operation and replacement stages). Therefore, the thermal insulation performance
optimization of the PPB is an efficient way to achieve energy saving carbon emission reduction
during the life cycle. Increasing the thickness of the insulation layer is a general method to improve
the thermal insulation performance. However, this will also lead to an increase in carbon emissions
in the production stage of building materials. Therefore, when the sum of the two influencing factors
reaches the minimum value, the optimal insulation thickness can be obtained to reduce carbon
emissions. Different thermal climate zones have varying optimal insulation thicknesses. It is
suggested that, in prefabricated production, different thicknesses of thermal insulation should be
specified based on the thermal climate zone to reduce the carbon emissions throughout the life cycle
of the building. Factory-made insulation walls have a long service life and low maintenance
frequency. Correspondingly, from a building life cycle perspective, carbon emissions from the
maintenance of prefabricated buildings are reduced.
4.2.3. Comparison of Cost Between the PPB and TPB during their Respective Life Cycles
It was necessary to conduct an economic analysis from the perspective of the whole life cycle.
Energy saving in each stage of building is of great significance to the reduction of the environmental
load. The promotion, application, and economy of energy saving technology should also be
considered. Pure energy saving without considering the cost will limit the market application

Sustainability 2020, 12, 2609 11 of 18
potential of the technology. It can be seen from the calculation results (Figures 6 and 7) that the cost
of the two types of building in the operation stage accounts for approximately 60% of the total
throughout the life cycle, while the construction phase accounts for nearly 20% of the total. However,
the material manufacture and construction should be considered comprehensively, as they are
closely related to the energy consumption of the building operation stage. The PPB was found to cost
less than the TPB at all stages of their life cycle, reducing the price per square meter by 10.62%. The
construction phase cost was found to be reduced by 17.08% compared with that of the TPB. The use
stage cost was shown to be reduced by 5.97%, and the demolition stage was found to be reduced by
16%.
Figure 6. Comparison of cost between the PPB and TPB during their respective life cycles.
Figure 7. Percentage of cost at various stages of the PPB and TPB in their respective life cycles.
The reasons for this are detailed in the following evaluation. First, Japan has a complete
industrial chain of prefabricated components for material production and construction. This
effectively reduces the production cost of fabricated components. At the same time, the prefabricated
construction method shortens the construction period and saves labor costs. In addition, the rejection
rate of cast-in-situ components in field construction is higher than that of prefabricated components,
which increases the input of raw materials. Furthermore, construction machinery is used more
frequently than prefabricated construction, which also increases the construction cost of traditional
buildings.

Sustainability 2020, 12, 2609 12 of 18
Generally, the quality of prefabricated components is higher than that of cast-in-situ
components. Some special construction methods improve the service life of components as well,
which can reduce the renewal frequency of building components in the operation and replacement
stage. Moreover, the prefabricated insulation partition effectively improves the insulation
performance of the building, which reduces the energy consumption in the operation process.
Essentially, this indicates that the money is saved.
Finally, in the demolition stage of the building, industrial components adopt a modular design,
which can be reused easily. These products are used extensively during the construction process,
which means that plenty of products can be reused. In other words, it can effectively improve the
bulk recycling utilization of building components. Some long-life parts—such as metal doors and
windows, steel stairs, and light shields—can be reused after a simple repair. As a consequence, the
recovery rate of components of the PPB is higher than those from the TPB, reducing the cost of the
demolition stage. Moreover, the production of construction waste is reduced in the PPB, which means
the waste treatment cost can be reduced as well.
4.2.4. Comparison of Ecosystem Damage Between the PPB and TPB during their Respective Life
Cycles
The performance of two kinds of building in terms of ecosystem damage is indicated in Figure
8. The bars under the x-axis describe the percentage of energy consumed during the material
production and site construction stage, while the bars above the x-axis indicate the proportion of
energy consumption during the use and demolition phase. The PPB was found to perform better at
reducing global warming, acid rain, and health damage in every stage by more than 15%. This can
be explained by the fact that PPB construction and operation consumes fewer materials and less
energy, leading to eutrophication and global warming, for instance, materials such as steel, concrete,
and wood and energy sources such as electricity and natural gas. Additionally, the emissions of
harmful gases, such as CH
4
, SO
2
, CO
2
, and NO
x
, in the production of relevant materials and the use
of fossil fuel is further reduced, thereby achieving the goal of reducing the environmental impact.
Figure 8. Comparison of ecosystem damage between the PPB and TPB during their respective life
cycles.

Sustainability 2020, 12, 2609 13 of 18
4.2.5. Comparison of Different Assembly Rates and Prefabricated Components
The case studies conclude that the carbon emissions and cost of prefabricated buildings are
superior to those of cast-in-situ buildings. Accordingly, the impact of the assembly rate on the carbon
emissions of prefabricated buildings was analyzed from the perspectives of carbon emissions and
economy. Most previous research on prefabricated buildings has focused on building components
on the ground but has rarely involved prefabricated pile foundations. Thus, further analyses of the
impact of prefabricated pile foundations on carbon emissions were conducted. The influences of
structures with different assembly rates (Cases 1–4) and prefabricated pile foundations (Case 5) on
the carbon emissions of PPB during the life cycle are indicated in Figures 9 and 10. As we can see
from the bar charts, the carbon emissions of prefabricated buildings decrease when the assembly rate
rises, bottoming out when the assembly rate is 60%. Then, the emissions increase generally when the
assembly rate is added. This can be explained by the following three aspects. The first point with
respect to this is that the main body of the building structure is basically formed when the
prefabrication rate of the structure exceeds 60%. After this, increasing the assembly rate cannot
effectively reduce the use of wood formwork.
Figure 9. Carbon emissions of different assembly rates.
Figure 10. Comparison of carbon emission between the PPB and PPB with prefabricated foundations.

Sustainability 2020, 12, 2609 14 of 18
Second, as the rate of assembly goes up, some special shapes and structures with fewer
applications need to be prefabricated in factories, which will increase the carbon emissions as well.
This is because, during the prefabrication process of these components, the reuse ratio of the steel
template is not obvious, and the production processing duration of these components is longer.
Besides, the particularity of these components also causes a reduction in production efficiency,
resulting in the waste of materials and excessive energy consumption in the production process.
Finally, in the site construction stage, when some special-shaped components (such as special-
shaped beams and t-shaped floor slabs) are assembled on site, the construction difficulty will increase
the working hours and mechanical energy consumption required, leading to an increase in carbon
emissions as well.
The relationship between the cost of prefabricated buildings and the assembly rate shows a
similar trend to that shown in Figure 11. More specifically, the cost of prefabricated buildings drops
firstly, reaching the lowest value when the assembly rate is 60%. After that, an upward trend is shown
as the assembly rate increases. It is evident that, with less usage of some building components, the
production cost of the components in the prefabrication production process rises significantly.
Moreover, the construction cost is greater.
Figure 11. Cost of different assembly rates.
The comparative study (Figure 12) of the PPB and case 5 outlines that prefabricated pile
foundations increase the carbon emissions of component manufacturing and construction
dramatically. The reason for this is that there is a small number of building foundations with special
shapes and large volume employed that make the material utilization rate of the prefabricated
component production process lower, and the production cycle longer. For example, the steel
formwork of prefabricated pile foundation has poor versatility, thereby increasing the consumption
of steel. Steel is considered to have a major environmental impact factor, of which the impact occurs
during the production and processing. In addition, it has a considerable impact on resource depletion
and harmful gas emissions. Consequently, the use of prefabricated piles will increase the carbon
emissions of buildings obviously. In the site construction stage, compared with cast-in-situ
foundations, the use of prefabricated foundations requires more hoisting equipment to be employed.
Furthermore, the precast foundation is not convenient for construction due to the high accuracy

Sustainability 2020, 12, 2609 15 of 18
requirement of foundation positioning in construction, which increases the construction time and
leads to an increase in carbon emissions in the construction stage.
Figure 12. Comparison of cost between the PPB and PPB with prefabricated foundations.
5. Conclusions
This study analyzed the impact of each stage of the life cycle of prefabricated buildings on the
environment based on a hybrid model. The application of the model was based on existing data to
guarantee the integrity of the system boundary and the accuracy of the calculation results. In the case
study, the influences of prefabricated buildings and traditional cast-in-situ buildings on the
environment during the life cycle were compared. Moreover, the carbon emissions of prefabricated
buildings with prefabricated pile foundations and different assembly rates were studied.
Compared with the TPB, the PPB has a reduced EI at all stages. The most significant energy
consumption reduction was found to occur in the operation stage, 66.62 MJ/year∙m
2
, due to factory-
prefabricated insulation, which improves the thermal performance and durability of the walls. The
energy saving effect during the construction phase was also shown to be obvious: 17.94 MJ/year∙m
2
.
Although the energy saving in the demolition phase was found to be the least with 1.823 MJ/year∙m
2
,
the energy saving ratio was as high as 11.29%. During the construction phase, the use of wood
formwork was significantly reduced by using prefabricated components. The consumption of
materials at the construction site was also reduced. The PPB was shown to have reduced carbon
emissions and energy usage by 7.17% and 7.54%, respectively. Prefabricated buildings also showed
higher recycling rates than traditional buildings. The performance of ecosystem damage of the PPB
was found to be better than that of the TPB, which can reduce global warming, acid rain, and health
damage by 15%.
The analysis of buildings with different assembly rates indicated that the carbon emissions of
PPB will increase and then decrease as the assembly rate increases. The assembly rate has the best
improvement effect on carbon emissions during the construction process and has little impact on the
operation and design stage. With the assembly rate rising gradually, the carbon emissions and cost
of prefabricated buildings drops, bottoming out when the assembly rate is 60%. After that, there is
an upward trend as the assembly rate increases. The prefabricated pile foundation is not suitable for
fabricated components, which will significantly increase the carbon emissions and cost during the

Sustainability 2020, 12, 2609 16 of 18
construction phase. Therefore, it is suggested that the cast-in-situ construction method should be
adopted for the building foundations.
The use of prefabricated buildings in Japan effectively reduces the EI and energy consumption.
The results are based on Japan's construction industry structure and social production level, which
can help recognized the current prefabricated buildings development in Japan. The successful
construction experience could provide useful information and guidance for other countries. This
paper concluded various environment performance improvement potential in different building
construction phase, which can help to make targeted measures to implement and promote
prefabrication technologies for specific phase. The comparison of prefabricated buildings with
different assembly rates points out that an excessively high assembly rate will not decrease the carbon
emissions and energy consumption of the building, which remind some countries where
prefabricated buildings development are just in its infancy that do not blindly seek for excessively
high assembly rates.
In the initial stage of prefabricated building development, due to the incomplete supporting
industry, the energy consumption of units for the prefabricated component production process may
be increased. Prefabricated building development also faces many challenges. It requires high
precision in the manufacture of components, which requires excellent ability of workers and strict
management of prefabricated factories. The construction duration will be delayed if the prefabs are
damaged during lifting or transport. Generally, prefabricated factories have to be close to the
construction site to provide convenience for transportation. The durability and safety of prefabricated
buildings depends on the assembly of prefabricated components, which also requires strict
management of the construction sites and professions of workers. Therefore, the prefabricated
building and traditional cast-in-situ building methods still coexist. Multiple factors are considered
when deciding which type of building is appropriate. It is suggested that experienced and mature
design companies and prefabricated parts from manufacturers are considered in the early stage of
the industry’s development, which will help to improve their application. Considering the
characteristics of carbon emissions during the life cycle of Japanese prefabricated buildings, it is
necessary to give priority to prefabricated components of walls to improve the thermal insulation
performance of buildings, which can significantly reduce the carbon emissions during the life cycle
of buildings.
Author Contributions: Conceptualization, H.W.; methodology, H.W.; software, H.W.; validation, H.W., Y.Z.
and W.G.; formal analysis, H.W.; investigation, H.W.; resources, H.W.; data curation, H.W.; writing—original
draft preparation, H.W.; writing—review and editing, Y.Z., W.G.,S.K; visualization, H.W.; supervision, W.G.;
project administration, H.W.; All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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