1. No category

A Comparative study of housing life-cycle carbon emissions
for the characteristics of structural materials
Ji-yeon Park1, Chang-u Chae 2, Sung-hee Kim3
1
Korea Institute of Civil Engineering and Building Technology, Republic of Korea
Korea Institute of Civil Engineering and Building Technology, Republic of Korea
3
Hanyang University, Republic of Korea
2
Abstract: Buildings composed of various construction materials and components
manufactured through various processes, induce enormous environmental pollution, such as
global warming throughout the entire life cycle from production to disposal stages. In
particular, cement, concrete and steel materials which are major construction materials
constituting a building, are regarded as typical CO₂ generating materials and it is necessary
to develop an alternative to those materials to reduce greenhouse gas. In the assessment, the
LCCO₂ technique as an evaluation tool for life-cycle CO₂ emissions is adopted for the
comparative analysis of the environmental impacts. Through the comparative analysis of the
houses based on the evaluation results, this study attempts to identify potential carbon
emission reductions from the reuse and reduction of housing materials and propose ecofriendly development potentials in the housing market.
Low Cabon Technology, Life Cycle Assessment, Green Building, Building materials
1. Introduction
1.1. Background of the study
While multi-faceted efforts for greenhouse gas reduction are being carried out worldwide,
environmental measures have been advanced from various sectors in Korea to reduce
greenhouse gas emissions.The greenhouse gas emissions in domestic construction sector
accounts for about 40% of total emissions for material production and building management
and various efforts for developing technologies are also being accelerated to reduce the
emissions. In particular for building materials, concrete, cement, and steel materials are
regarded as typical CO₂ generating materials and it is necessary to develop low-carbon
materials or an alternative to those materials to reduce greenhouse gas. Also, for the
sustainable and eco-friendly development of the construction sector, efforts should be made
for eco-friendly material and technology development to reduce its environmental impacts.
1.2. Necessity and purpose of the study
This study intends to analyze modular houses that tend to increase in a compact housing
market and conventional concrete houses from the perspective of LCA, and compare their
environmental impacts generated throughout whole life cycle. A modular house is an
industrialized house that can be mass-produced in the form of a unit in a factory and
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prefabricated on site for being produced in large quantities for a short period. The modular
house is designed with a prefabricated structure, allowing recycling and reuse of building
materials through the assembly and dismantling of its structural members to reduce waste
material. Besides, it has a number of advantages over conventional construction types
including cost savings and site personnel reduction and a wide variety of configuration and
styles in the building layout as a result of the lightweight construction and large supply of
modular houses. Also, the modular house is expected to be effective in reducing materials and
energy consumption compared to conventional construction types such as reinforced concrete
structures, eventually reducing greenhouse gas emissions and the environmental impact.
Through the comparative analysis of the houses based on the evaluation results, this study
aims to identify potential carbon emission reductions from the reuse and reduction of housing
materials and to propose eco-friendly development potentials in the housing market.
2. Life cycle assessment (LCA) methodology for building
2.1. Overview of LCA methodology for buildings
In the assessment, environmental impacts are evaluated by comparing reinforced concrete
structure houses to modular houses from the perspective of LCA. As an evaluation tool for
environmental impacts of environmental burden materials that occur in the life cycle of
products and systems, LCA is intended to evaluate quantitatively potential impacts associated
with inputs and outputs generated from the life cycle of a product on the environment in the
production, construction, use and disposal stages.Among the tools available to evaluate
environmental performance, LCA provides a basic structureand principle to evaluate
environmental performance and impacts on sustainable development by considering the
potential environmental impacts from all stages of manufacture, product use and end-of-life
stages, and as a result, to establish criteria for evaluating the environmental impacts in the
building life cycle.
2.2. LCA methodology
A building LCA includes setting a functional unit for a product system, providing a
reference flow to which the inputs and outputs can be related, and system boundaries to define
the scope of data collection for the product. An LCA starts with an explicit statement of the
goal and scope of the study, and then sets the functional unit and reference flow.
The functional unit quantifies the function and scope delivered by the product to be
assessed, setting the amount of product required for conducting the assessment. Further, the
functional unitis set in consideration of the life time and performance of a target and the end
user's perspective. Reference flow is also taken into account to set the functional unit for the
comparative evaluation. The reference flow is determined by the final outcome necessary for
the functional unit by identifying the process flow of a building. To develop a LCA, system
boundaries for the life cycle of a product is to be defined. A clear definition is required for the
system boundaries to be assessed and a clear explanation is required for the portion not to be
assessed. The system boundary for the LCA of a building comprises all unit processes from
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the entire life cycle to be evaluated, including raw material extraction, transportation and
product manufacturing stages.
The unit process included in a system boundary is referenced based on the inputs and
outputs in a system boundary. Because raw materials, products and energy that are inputted
vary with the unit process of the life cycle, data categories are chosen to collect input and
output data. If recycled·reusable materials are to be reintroduced to the process within a
system boundary, reuse·recycling processes are included in the system boundary for
evaluation. Using the corresponding LCI DB module on the basis of the calculation formula
of the data, collected data are multiplied by a greenhouse gas emission factor to calculate the
greenhouse gas emissions for each data set. Carbon dioxide emissions per data to be
calculated include the amount of carbon dioxide emissions in each stage of the life cycle and
the one emitted during the life cycle of the building.
3. Overview of assessment targets and assessment methods
3.1. Overview of assessment targets
The buildings to be compared include a reinforced concrete apartment house and a modular
house. The reinforced concrete apartment house uses a electric boiler heating system in 98
square meters with a 4-person household. The modular house with for a 1-person household
uses an electric boiler heating system in a unit type prefabricated structure.
Type
Building type
Area
Number of occupants
Heating system
Heating energy usage
R.C. Apartment Housing
Apartment Building
Modular Housing
Modular Unit
98
4 persons
electric boiler
22
1 person
electric boiler
54 kwh/ ·year
49kwh/ ·year
㎡
㎡
㎡
㎡
Table 1 Overview of target buildings
3.2. Assessment methods
For the houses to be compared that differ in housing area and number of occupants, the
basis of comparison for analysis results is required.
For the LCA analysis in this study, greenhouse gas emissions during the life cycle are
analyzed from one household house. In the case of a modular house, because the life-span of a
house varies with the materials used, it is difficult to define the life cycle of the house. Thus,
in consideration of the service life of a concrete building, the life cycle was set to 30 years.
The reference flow was set for the analysis on the amount of energy and materials that are
inputted in the production, construction, use, and disposal stages of a one household
residential building to be used for 30 years.
The system boundaries for the reinforced concrete house and modular house are set
differently in the production and construction stages of LCA, For the reinforced concrete
house, the production stage includes material production and all the processes before
materials are delivered to the construction site after processed, and the construction stage
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comprises the processes in which materials are transported to the construction site and a house
is built. The processes of house repair·maintenance and energy usage by residents are
included in the use stage. The disposal stage is associated with the amount of energy of
equipments caused by house dismantling and waste material transportation, and all the
processes for the reuse of waste materials, recycling, incineration and landfill.
To conduct a LCA, the data of the raw materials and transportation of the life cycle is
calculated by setting a scenario. Data is calculated within the range that does not affect the
evaluation results. The scope of data collection is typically determined by 95% or 99% of the
cumulative mass contribution. For this assessment, the materials corresponding to 99% of the
cumulative mass contribution are used for data calculation.
For this assessment, the top-down approach is chosen to determine the data values by
reducing the unit of comparison targets. For the criteria that can compare by quantifying
targets, data is compared by setting a comparison unit for area and number of occupants,
eventually, for emissions per household, unit area, and person.
4. Analysis of life cycle carbon emissions
4.1. Life cycle CO emissions
The analysis results of the life cycle total emissions showed the modular house generated
emissions of 39,375kgCO eq when using general materials and 37,976kgCO eq when using
reusable·recycled materials. For the reinforced concrete house, the emissions were
113,321kgCO eq from general materials and 90,798kgCO eq from reusable·recycled
materials.
Emissions
Type
Production stage
Modular house
using general
materials
Modular house
using reusable
·recycled materials
Reinforced concrete
house using general
materials
Reinforced concrete
house using recycled
materials
Total emissions
kgCO eq/household· kgCO eq/㎡· kgCO eq/person·
30years
30years
30years
6,413
292
6,413
%
kgCO eq/person·
30years
16.3
Construction stage
110
5
110
0.3
Use stage
32,820
1,492
32,820
83.4
Disposal stage
32
1
32
0.1
Production stage
5,014
228
5,014
13.2
Construction stage
110
5
110
0.3
Use stage
32,820
1,492
32,820
86.4
Disposal stage
32
1
32
0.1
Production stage
233,475
2,382
58,369
51.5
Construction stage
6,086
62
1,522
1.3
Use stage
197,505
2,015
49,376
43.6
Disposal stage
16,217
165
4,054
3.6
Production stage
143,383
1,463
35,846
39.5
Construction stage
6,086
62
1,522
1.7
Use stage
197,505
2,015
49,376
54.4
Disposal stage
16,217
165
4,054
4.5
39,375
37,976
113,321
90,798
Table 2 Total carbon emissions by stage during life cycle
4.2 Comparative Analysis
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From the perspective of LCA, the carbon emissions of a reinforced concrete house and a
modular house were compared and analyzed for each stage, based on the life cycle of 30 years
and one household. Life cycle greenhouse gas emissions were analyzed and eventually a
comparative analysis was applied by dividing into emissions per unit area and per person.
Total emissions
kgCO eq/
household
·30years
Reductions against
RC house using
general materials
kgCO eq /
㎡·30years
Reductions against
RC house using
general materials
kgCO eq/
person·
30years
Reductions against
RC house using
general materials
Modular house using
general materials
39,375
413,908
1,790
2,835
39,375
73,945
Modular house using
reusable·recycled
materials
37,976
415,307
1,726
2,899
37,976
75,344
Reinforced concrete house
using general materials
453,283
0
4,625
0
113,321
0
Reinforced concrete house
using recycled materials
363,191
90,092
3,706
919
90,798
22,522
Type
Table 3 Comparison of carbon emissions during life cycle
(1) Comparative analysis of emissions per household
The comparative analysis of life cycle carbon emissions showed that the total carbon
emissions of the modular house was 39,375kgCO eq per household when using general
materials and 37,976kgCO eq when using reusable·recycled materials. The reinforced
concrete house using general materials showed the total emissions of 453,283kgCO eq,
indicating the modular house was more effective by more than 80% in reducing the emissions
than the reinforced concrete house. Further, the emissions from the reinforced concrete using
reusable·recycled materials were reduced by more than 20%, compared to the reinforced
concrete house using general materials.
(2) Comparative analysis of emissions per unit area
The analysis results of the life cycle total emissions per unit area for 30 years showed the
reinforced concrete house using general materials generated emissions of 4,625kgCO eq.
Also, the emissions from the modular house were 1,790kgCO eq when using general
materials and 1,726kgCO eq when using reusable·recycled materials. It is also shown that
the total emissions of the modular house were reduced by approximately 60% compared to
the reinforced concrete house.
(3) Comparative analysis of emissions per person
The comparative analysis of life cycle carbon emissions per person showed that the total
carbon emissions of the reinforced concrete house was 113,321kgCO eq per person when
using general materials and 90,798kgCO eq per person when using reusable·recycled
materials. The modular house showed the total emissions of 39,375kgCO eq per person for
using general materials, and 37,976kgCO eq for using reusable·recycled materials. This
indicates the modular house with reusable·recycled materials was more effective by
approximately 66% in reducing the emissions than the reinforced concrete house. Further, the
emissions from the reinforced concrete with reusable·recycled materials were reduced by
approximately 20%, compared to the reinforced concrete house with general materials.
5. Analysis of assessment results
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From the perspective of LCA, the carbon emissions of a reinforced concrete house and a
modular house were compared and analyzed in each stage. Depending on the assessment
method, the life cycles of 30 years for two houses to be compared were divided into
production, construction, use and disposal stages to collect data, and the emissions were
calculated by using an emission factor.
From the results regarding major environmental impact factors in the life cycle of each
evaluated house, it was analyzed that the outputs of concrete in the production and
construction stages of the reinforced concrete house, gas usage by heating in the use stage and
the amount of recycled waste concrete in the disposal stage were major environmental impact
factors. For the modular house, the amount of steel frame and plasterboard was regarded as a
major environmental impact factor in the production stage, but the emissions were
significantly reduced when reusable and recycled steel frames were used. In the use stage,
electricity usage by using an electric furnace, and the carbon emissions due to the landfill of
materials in the disposal stage were regarded as major environmental impact factors.
However, it was analyzed the reduction of outputs due to the reuse of materials in the
disposal stage exerted the greatest effect on the reduction of emissions. It is shown that the
modular house using general materials can reduce the emissions by 413 tons over 30 years,
compared to the reinforced concrete house with general materials, and the modular house with
reusable·recycled materials can reduce the emissions by 415 tons. The modular house with
reusable·recycled materials is shown to be more effective in reducing 415 tons of greenhouse
gas emissions over 30 years than the reinforced concrete house with general materials, which
is equivalent to the amount of carbon dioxide that 28,000 30-years old pine trees absorb for
one year.
When using these modular houses for 90 years, it is expected that 1,200 tons of greenhouse
gas emissions can be reduced, as compared to reinforced concrete houses with general
materials. Therefore, by the improvement of the performance of modular houses resulting
from future performance development of modular houses and the introduction of energysaving technologies, potential reduction is expected to be larger.
6. Conclusions
In this study, to reduce greenhouse gas in the life cycle of a building, we analyzed the
environmental performance due to building materials by comparing and assessing the life
cycle of a reinforced concrete house and a modular house to provide the direction of ecofriendly development of building materials.
A LCA was applied to the reinforced concrete house using general concrete materials and a
recycled material of slag concrete, and the modular house using general materials and
reusable·recycled materials, to compare their environmental impacts arising from the use of
reusable·recycled materials. The analysis results of life cycle carbon emissions per person
over 30 years showed that the total carbon emissions of the reinforced concrete house was
113,321kgCO eq when using general materials and 90,798kgCO eq per person when using
₂
₂
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reusable·recycled materials. Further, the emissions from the use of a recycled material of slag
concrete were shown to decrease by approximately 20%, compared to the use of general
materials. The modular house showed the total emissions of 39,375kgCO eq per person over
₂
₂
30 years for using general materials, and 37,976kgCO eq for using reusable·recycled
materials, resulting in a reduction of carbon emissions by 1.4 tons. This indicates the modular
house with general materials was more effective by approximately 65% in reducing the
emissions than the reinforced concrete house. Factors affecting the emissions of greenhouse
gases appeared slightly different in the two types of house. The use of concrete and rebar in
the production of the reinforced concrete house and gas usage by heating in the use stage were
shown to be major environmental impact factors. For the modular house, steel frame and
plasterboard in the production stage and electricity usage in the use stage were regarded as
major environmental impact factors. Difference in the heating system in the use stage was
also found to affect the difference in greenhouse gas emissions. It is also found that the reuse
or recycling of building concrete and steel materials is capable of reducing greenhouse gas
emissions in the production stage.
Further, modular houses are expected to contribute more actively to environmental
protection and resource saving compared to existing reinforced concrete houses, by reducing
the emissions and relatively reducing energy consumption in the production stage from the
environmental aspect related to the response to climate change.
However, there is a weakness of modular house, such as fire safety performance and
durability as compared to conventional reinforced concrete buildings, and it is necessary to
solve the technical challenges including noise and indoor environment to address the need for
technology development associated with performance improvement. As a result, it is needed
to consider the development of a building that can reduce the environmental impact by
applying the modular housing method to slag concrete and complementing its performance
and safety.
In future, ongoing technology development and research of building materials
complemented with environmental advantages are essential to cope with climate change and
reduce an environmental impact.
References
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1~2 person household, Seoul
Kim Gyuntae et al, (2011), Scenario development of Korean modular house construction,
Architectural Institute of Korea
Heo SungHoon et al, (2011), A study on the small sized housing supply according to the
changes in family composition, Architectural Institute of Korea
Heo Tak et al, (1995), Principle of life cycle assesment, Korea employer’s federation
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