Conference PaperPDF Available

Carbon footprint of pre-fabricated wood buildings

Authors:
a
Corresponding author: Alejandro-de-jesus.padilla-rivera.1@ulaval.ca
SIMPÓSIO BRASILEIRO DE DESIGN SUSTENTÁVEL +
INTERNATIONAL SYMPOSIUM ON SUSTAINABLE DESIGN
Belo Horizonte, 1 – 4 August 2017
Carbon footprint of pre-fabricated wood buildings
Alejandro Padilla-Rivera
,1,a
, Pierre Balnchet
1
1
Université Laval-CIRCERB, Faculté des sciences de bois. Pavillon Gene-H.–Kruger Université Laval 2348, rue de la Terrasse Québec
(Québec) G1V 0A6, Québec, Canada. +1 418 656-2131 ext. 16292.
Abstract.
This study reports on a study examining the potential of reducing greenhouse gas (GHG) emissions from the building sector by
substituting steel and concrete building structures with timber structures, as well as traditional construction for prefabricated
methods. A multi-storey timber residential building in Quebec City (Canada) was chosen as a baseline scenario. This building has
been constructed according to the concept of green environmental protection and sustainable development. Life Cycle Assessment
(LCA) approach is applied to compare the climate change impact (CC) of timber structures. In this scenario of this research,
material production and construction (assembly, waste management and transportation) were assessed. Additionally, a LCA that
comprises eight actions divided in four low carbon strategies, including low carbon materials, material minimization, reuse and
recycle materials and adoption of local sources and use of biofuels were evaluated. The results of this study confirm the positive
effect using prefabricated approach in buildings as an alternative construction method based on timber-frame-materials in Quebec.
By using the CO2 emissions as global indicator, the CC saving per m2 floor area in baseline scenario produces up to 25% fewer
emissions than traditional buildings. If the benefits of low carbon strategies are included, the timber structures can cause up 38%
lower CC than the original baseline scenario. The analysis suggests that CO2 emissions reduction in the construction of buildings as
climate change mitigation is perfectly feasible by following different working lines. We concluded that the four strategies
implemented have an environmental benefit in reducing greenhouse gases emissions. The reuse wood waste into production of
particleboard has the greatest environmental benefit when considering temporary carbon storage.
Keywords. Timber structures, carbon reduction strategies, life cycle assessment, climate change, carbon
footprint.
SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
1 Introduction
The Intergovernmental Panel on Climate Change (IPCC)
documents the science, impact and mitigation options of
climate change. In 2010, the IPCC (Graham et al, 2014)
reported that buildings accounted for 32% of total global
energy use, 19% of total Green House Gas (GHG)
emissions. GHG mainly include six gases with proven
global warming effects, including carbon dioxide (CO
2
),
methane (CH
4
), nitrous oxide (N
2
O), hydrofluorocarbons
(HFCs), perfluorocarbons (PFCs), and sulphur
hexafluoride (SF
6
).
In the other hand, the United Nations Environment
Programme (UNEP, 2016) has declared that due to rapid
urbanization and the inefficiency of existing building
stock, GHG emissions will be duplicated by 2030 unless
mitigation strategies are implemented, therefore, GHG
emissions reductions in the building sector is an
important focus of research. Consequently, reducing the
energy use and climate change impacts of buildings is
seen as a critical climate change mitigation measure by
the IPCC (Skullestad, Bohne, and Lohne 2016).
The last decades have seen extensive efforts to increase
the efficiency of buildings operation, to reduce the
related energy use and GHG emissions. With the
adoption low energy buildings, renewable energies and
the reduction of energy consumption and GHG
emissions during the use phase, the relative contribution
from building materials increased. In new energy-
efficient buildings, the embodied energy use related to
construction, transport and production of building
materials and demolition can constitute, according to
different authors, from 10-50 % of the total life cycle
energy consumption Cellura et al. 2014, Suzuki & Oka
1998, Dixit et al. 2013).
It is important to take into consideration a wider range of
impacts. In this sense, a strong analysis of the degree of
energy or GHG emissions reductions is important for
understanding the contributions buildings to achieve
efficiency measures that can make to climate mitigation.
The LCA technique provides better decisions support
when optimising environmentally favourable design
solutions that consider the impact cause during the entire
lifetime of building (Malmqvist et al. 2011).
The LCA of buildings is rapidly developing field. While
the use of LCA is not yet a standard building practice
(Malmqvist et al, 2011), the method is rapidly adopted in
the development of green buildings, especially those
which have low GHG emissions. According to Mao et
al. (2013), several studies evaluating GHG emissions
with LCA approach, have been concentrated on
quantifying emissions during operation and end of life
stages of buildings (operating carbon), where as only a
few are focus on material manufacturing, construction
methods and assembly (embodied carbon). In this sense,
a study performed by Dahlstrøm et al. (2012) indicated
that the production phase impacts are, in most cases,
lower than use phase impacts, but for some specific
measures in buildings with very high energy efficiency,
the trade-off can be negative. The next step in building
sector is thus to minimise impacts related to the
production, transport and fabrication methods of
buildings.
The combination of population growth and GHG
emission reduction targets stimulates construction of
more densely concentrated urban areas with “nearly zero
buildings” that comes with stronger structures and have
fewer use of materials per floor area. Thus, choosing
environmental friendly construction materials is
especially important for buildings. Structural systems
have traditionally consisted of steel and concrete.
Production of these materials is energy emissions
intensive, and accounts for a great portion of total GHG
emissions from materials production in the building
sector. Timber building material prove to cause
considerably lower climate change impact than materials
of steel and concrete.
Oka et al. (1993) quantified the energy consumption and
carbon emissions produced by construction in Japan
while Buchanan & Honey (1994) has performed a
detailed study on the embodied carbon of buildings and
resulting CO2 emissions from wood, concrete, and steel
structures for office and residential purposes in New
Zealand and concluded that wood structures have less
embodied carbon than concrete and steel structures. In
Europe the quantity of greenhouse gases avoided by
replacing steel with wood in buildings in Norway and
Sweden is 0.06-0.88 kg CO
2
-eq per kg input of timber;
while replacing concrete with wood reaches 0.16-1.77 kg
CO2-eq/kg (Petersen and Solberg 2005).
Furthermore, there is potential to reduce embodied
carbon requirements using building strategies that
produce less GHG emissions during manufacturing
(Shadram et al, 2016). These strategies can include the
use of low-carbon materials, material reuse, recycling
and minimization, selection of optimal structural system
and structural optimization and optimization of
construction operations, such as prefabrication methods
(Roh et al. 2017,Shadram et al. 2016,Yeo & Gabbai
2011,Thomark 2000). These approaches are translated
into reductions in cost, time, defects, health and safety
risks, and a consequent increase in quality, predictability,
whole-life performance and profitability (Jaillon & Poon
2014, Tam et al. 2015, Cao et al. 2015).
Another aspect to take into consideration is the uptake of
GHG emissions from the atmosphere during the
SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
photosynthesis, a unique process feature of plant
biomass. The transformation of biomass (and its
embodied “biogenic” carbon) into products represents in
effect a removal of CO
2
, via its continued storage in the
product over a period of time. Bio-based products can
thus contribute to reduce the CO
2
level in the atmosphere
and address climate change. In this sense, for carbon
footprint accounting purposes, biogenic carbon
embodied in a product should be considered as a CO
2
reduction or avoided emissions. Since this study is
limited to embodied GHG emissions, it is not calculated
environmental impact resulted from the biogenic CO
2
circulation, the CO
2
storage in the buildings for years of
use.
The goal of this study is to estimate CO
2
reduction
strategies as climate change mitigation-measures from
the building sector by considering a baseline building.
Life cycle assessment is applied to assess the potential of
reducing GHG emissions (embodied carbon) from heavy
structures versus timber structures. In addition, a
comparison of the impacts due to implementation of
carbon reduction strategies, e.g. construction
optimization strategies, waste reduction practices, low
carbon materials substitution and local sourcing and
transport minimization was performed.
To facilitate comparison and reporting, an aggregate
measure, known as carbon equivalent, is usually used to
quantify and report the overall global warming impact
cause by various greenhouse gas emitted during a
process. Throughout this paper, the term embodied
carbon emissions is used to refer to carbon equivalent
emissions. This cradle-to-gate study considers only
embodied carbon emissions (building materials during
all processes of production, on-site construction and
disposal). The obtained results should enable to target
the specific sources of GHG emissions in the production
phase and construction phase and establish knowledge
for building professionals such as engineers, architects
and interior designers for developing guidelines for
application in other countries and to efficient the results
to final users and clients.
2 Methods
The key steps of the methodology are summarized
below. Each of the steps outlined are described in more
detail in the following sections.
2.1 Overview
This section describes the case study building that was
analysed and the methods used to determine the GHG
emissions associated with both timber and concrete
construction approaches for this building
.
2.2 Life Cycle Assessment (LCA)
This section describes the case study building that was
analysed and the methods used to determine the GHG
emissions associated with both timber and concrete
construction approaches for this building.
LCA is a cradle to grave approach for
product/service/system that evaluates the environmental
effects associated with any given activity from the initial
gathering of raw materials from the earth (petroleum,
crops, ore, etc.) to the point at which all the materials are
returned to the earth (Figure 1). This evaluation includes
all side stream releases to the air, water, and soil. LCA is
an attempt to comprehensively describe all these
activities and the resulting environmental release and
impacts. In order to carry out this research and to
quantify GHG emissions, the recommendations of the
ISO 14040 series (ISO, 2006) were followed. This LCA
study has been modelled using Open LCA 1.5.0
software. All data are collected from the Société
d'habitation du Québec (SHQ), the Province of Quebec
Social Housing Agency.
Since the focus on this research is the construction of
buildings, the process of manufacture, delivery and
assembly of the components are included in the system.
Meaning that, use and demolition and final disposal are
not considered. The system boundary of this study is
shown in Figure 2.
For this study, a Global Warming Potential (GWP)
indicator has been evaluated based on TRACI 2.1 (EPA,
2012) that utilizes 2013 IPCC characterisation factors
considering a time horizon of 100 years (IPCC, 2001).
Figure 1. LCA Methodology (ISO, 2006).
2.2.1 Functional unit
The functional unit defined as a measure of the
performance of the functional outputs of the product
system. In LCA, the functional unit provides a reference
to which the inputs and outputs are related. According to
SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
Gustavsson & Sathre, (2011) different functional units
could be use in the energy and carbon studies of
buildings. These units include 1 m
2
of a building’s gross
or usable floor area, total gross or usable floor area and
the complete building. Thus, to establish a baseline to
evaluate the prefabricated building and to make
comparison, the functional unit for this study is defined
as 1m
2
of floor area.
2.2.2 Case study building (baseline)
An existing multi-storey building is used as reference to
model the wood-frame building system explored in this
study. The building selected is part of a 93,000-m
2
area
located on the lands of the Cité Verte, in the Saint-
Sacrement district of Quebec City. This residential
complex, consisting of condominiums, townhouses and
offices and businesses, aims to integrate sustainable
development elements from the design and exploitation
of energy management, waste management,
transportation and construction techniques.
The reference building is a mass timber structure. It is a
four-story building and contains a total of 20 apartments
with a total living area of 1512.3 m
2
(75 m
2
per unit).
The multi-residential building currently biomass district
heating system for its space heating and domestic hot
water needs, storm water management to reduce energy
use associated with water, and innovative planning to
encourage alternative modes of transportation.
Figure 2. Systems boundaries
2.2.3 Life Cycle Inventory
The LCI component identifies and quantifies the
material and energy resource inputs as well as the
emissions and product outputs from the unit processes in
accordance with the functional unit and boundaries. The
LCI is an iterative process that requires time and amount
of research, fortunately the use of databases help in the
process. The U.S Life Cycle Inventory Database,
developed by National Renewable Energy Laboratory,
and Ecoinvent database, which has been developed by a
Swiss initiative haven been selected as a reference in this
study. All the unit processes selected in this database
were cradle-to grave processes. A report from the SHQ
on sustainable building has been used for the modelling
this research. For the current study, Table 1 shows the
main construction materials with the respective
quantities and processes used for the prefabricated
building construction.
Table 1. Materials and quantities used in each stage under
evaluation.
Life cycle stage Material/process Quantity
(per 1m
2
)
Cross Laminated timber
(CLT) 0.27 m
3
Glue laminated 0.0019 m
3
Steel 3.98 kg
Gypsum fibreboard 4.92 m
2
Concrete 0.15 m
3
Vinyl boards 3.15 kg
Granite 3.88 kg
Ceramic tiles 1.83 kg
Primer 0.499 kg
Paint 1.35 kg
Bricks 50.23 kg
Windows 0.03 m
2
Metal doors 0.06 m
2
Wooden doors 0.30 m
2
Fiberglas 1.72 kg
Spray polyurethane 0.06 kg
Elastomeric membrane 0.28 kg
Coverage bitumen
membrane 0.08 kg
Material acquisition
and material
production
Transportation to site
assemblya
188.81 t-
km
Electricity 85.65 kwh
Excavation 0.0264 m
3
Diesel 0.9919 kwh
Assembly
Transportation to site 27.9 t-km
SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
construction (100km)
Waste Managementb
(WM) (wood) 9.91 kg
WM (gypsum) 5.26 kg
WM (Iron metals) 6.11 kg
WM (plastic) 0.1656 kg
WM (carton/paper) 3.83 kg
WM (insulation wool) 0.4228 kg
WM (Polythene) 3.46 kg
WM Transportation (25
km)
0.6298 t-
km
2.3 Description of carbon reduction strategies
In order to investigate the sensitivity of embodied
carbon emissions from this building, design
configuration was changed through a hypothetical way.
Four hotspots approaches were defined in line with the
commonly carbon reduction strategies. They were
conducted to single out those having the greatest
potential for reducing environmental impact in baseline
scenario. These strategies are divided and described into
the following categories: 1) low carbon materials, 2)
material minimization, 3) reuse and recycling strategies
and 4) local sourcing and transport minimization.
3
Results
3.1. Global warming potential of baseline
scenario
The total CO2-e. emissions are presented as the Global
Warming Potential (GWP) per 1 m
2
of floor area. The
estimated carbon footprint of the building material
manufacturing phase and construction process was 275
kg CO
2
-e. /m
2
. Compared to authors’ previous study of
different frames/construction methods, whose system
boundary is almost the same as in this study. It shows
that embodied CO2-e emissions of building per
functional unit of reference (1m
2
) of base case is fewer
(35 to 672 kg CO
2
-e) than 66% of literature analysed,
but 33% higher (183 and 205 kg CO
2
-e) than references
in Table 2. This gap is because almost all buildings
materials in building #2 and #3 use lower carbon
materials, such as wood products, and the use of
concrete and steel, principal CO
2
-e emitters are limited.
The transportation energy during
fabrication/construction also contributes to reduce CO
2
-
e emissions.
The embodied carbon difference by case base and
reference buildings 3 to 9 is attributed principally to
modular construction technic, light frame materials and
low energy during assembly process. As it can be seen
the base case showed an average decrease about 150%
CO2-e emissions, this was achieved for the use of wood
as a low-carbon construction material than conventional
concrete and steel in the case building considered,
highlighting the importance of material selection in
reducing the overall carbon footprint in Canada. These
results agrees with (Nässén et al. 2012) who indicate
that, in New Zeeland, increasing the use of wood around
15% may result in a 20% reduction in carbon emissions
due to the manufacturing construction materials, thus,
about a 1.5% reduction in the country’s total carbon
emissions.
It should be noted that the results in Table 2 make an
interesting reading. It turns out that bigger constructed
areas produce more carbon dioxide per m2 than small
ones. That may come as a surprise, given that there are
so many economies of scale at work in bigger
constructions, however Fragkias et al. (2013) emphasizes
the importance of size as a major factor in determining
the intensity of CO
2
-e emissions, although large urban
areas are more innovative than smaller ones, they may
lack capacity in steering eco-innovations towards their
local markets for fossil fuels. These important
hypotheses remain untested and need to be addressed in
future research.
Furthermore, the selection of modular/semi-
prefabrication method over conventional construction
suggests that the emissions of prefabricated wood-frames
technology may result in lowest life cycle carbon
footprint, attributed to the higher level of prefabrication
of the base case under study, that according to (Pons,
2014) a higher degree of prefabrication could contribute
to greater benefits on environmental impact ,such as
GHG emissions.
Table 2 Comparison of embodied carbon among different
literatures
Author Name/Reference Kg CO
2
emissions
Current project Base case
(Quebec, Canada) 275
Paya-Marin et al.
(2013)
70 Building #1 (UK)
Kim 2008 92 Building #2 (USA)
(González and
García Navarro
2006)
196 Building #3
(Spain)
(Oriol Pons and
Wadel 2011) 310 Building #4
(Spain)
(Mao et al. 2013) 336 Building #5
(China)
(Roh et al. 2017) 477 Building #6
(Korea)
(Aye, et al. 2012) 578 Building #7
(Australia)
(Suzuki and Oka
1998) 790 Building #8
(Japan)
(Su and Zhang
2016) 947 Building #9
(China)
SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
3.2 Baseline scenario vs different buildings
frame materials
The raw material extraction, material production,
including transportation account for 243 kg CO
2
eq.
(88% of the total). The assembly, that includes the
modular fabrication, and construction waste management
is 32 kg de CO2 (12%). If we see the contribution from
the all the sub-process in the two life cycle stages, the
fabrication of materials is responsible for 75% of total
GWP, transportation to site construction 13%, waste
management 11% while the excavation, electricity and
diesel accounts only for1%.
These results are in line with other similar studies in
Canada. Canadian Wood Council (CWC) (2010) and
Société d’habitation du Québec (SHQ) (2016) that
conducted life cycle analysis of environmental impacts
of wood frame, steel and concrete buildings in Quebec.
The embodied carbon footprint, materials production and
assembly phases have of different buildings are shown in
Figure 3.
Findings suggest that the emissions of fabrication
materials is the principal contributor to total GHG
emissions in the five buildings, following by assembly
phase. According to the results, the wood frames causes
lower carbon emissions than concrete and steel frames.
The baseline building presents significantly lower
embodied CO2-e emissions (275 kg/m
2
) compared to the
wood frame 1 (302 kg/m
2
) and wood frame 2 (287
kg/m
2
), while concrete frame (442 kg/m
2
) and steel
frame (353 kg/m
2
) are the greatest carbon contributors.
Figure 3 . Total GHG emissions of baseline scenario vs
different buildings (kg CO
2
eq/m
2
)
3.3 Implementation of carbon reduction
strategies
One of the most important skills required for the
implementation of sustainable strategies in practice is the
ability to evaluate the effectiveness of such strategies in
the context of a real building. Quantifying the embodied
carbon reductions achievable by the adoption of different
low carbon reduction strategies provide keys insight into
the design of low-climate-change buildings. This section
combines the results of this study (baseline scenario) and
the implementation analysis of factors influencing
carbon emissions by mainly considering four strategies
(Section 2.3).
The results of carbon reduction strategies show that
overall reductions of life cycle GHG emissions are 106.5
kg CO
2
-e /m
2
(38.8%). Figure 7 shows the breakdown in
terms of life cycle stages and carbon strategies wherein
the implementation of recycled of wood waste (waste
management) that accounted for the largest share -11 kg
CO2-e /m2 (14.6%) of the total GHG emissions have
been considered to be the main hotspot. The local
sourcing of materials and components (transportation)
also reduce significant portion of GHG emissions 32 kg
CO
2
-e /m
2
(11.7%). In the production of building
materials (178.2 kg CO
2
-e /m
2
), the use of clinker
materials to produce cement (3.8%) and light clay bricks
(3.6%) have been found to be the low carbon materials
actions. All other actions such as cork flooring system,
reduction of waste ratios, cellulose insulation and use of
biofuels together contributed to the remaining reduction
of GHG emissions 13.6 kg CO
2
-e /m
2
(5%).
Figure 4 GHG emissions of baseline scenario, carbon
reduction strategies and new carbon emissions (kg CO
2
-
e)
4 Conclusions
The results of this study confirm the positive effect using
prefabricated approach in buildings as an alternative
construction method based on wood-frame-materials in
Quebec. According to the obtained environmental
impacts, these conclusions can be drawn as follows:
By using the CO
2
emissions as global indicator, the
CC saving per m
2
floor area in baseline scenario
produces up to 25% fewer emissions than
traditional buildings built with steel or concrete.
SBDS+ISSD 2017, 1 - 4 August 2017, Belo Horizonte. Universidade Federal de Minas Gerais.
During the life-cycle of baseline scenario, total
embodied carbon emissions of 275 kg CO
2
-e
was calculated. The fabrication of building
material phase contributed the most (75%) to
the carbon emissions, while transportation
(13%), construction (1%) and waste
management (11%) contribute to 25%.
The four strategies implemented have an
environmental benefit in reducing greenhouse
gases emissions. The analysis of low carbon
strategies showed an overall carbon reduction
approximately 104 kg CO
2
-e (38%) in
comparison to baseline scenario.
The CO
2
emissions reduction in the construction of
buildings as climate change mitigation is perfectly
feasible by following different working lines. The four
strategies implemented have an environmental benefit in
reducing greenhouse gases emissions.
It was demonstrated that use of wood-based building
materials can contribute to a sustainable built
environment based on resource-efficient systems with
low environmental impact. Wood building products from
sustainably managed forests are a renewable resource
that can provide multiple benefits during their life cycle.
Reuse of construction materials can lead to significant
resource savings together with other environmental
benefits from a reduction in waste disposed of in landfill
and the energy required for the production of virgin
materials.
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... The results of Padilla and Balnchet [44] confirm the positive effect of using the prefabricated approach in buildings as an alternative construction method based on wooden-frame materials in Quebec. By using the CO2 emissions as a global indicator, the climate change impact (CC) saving per m2 floor area in the baseline scenario produces up to 25% fewer emissions than traditional buildings. ...
... The results of Padilla and Balnchet [44] confirm the positive effect of using the prefabricated approach in buildings as an alternative construction method based on wooden-frame materials in Quebec. By using the CO2 emissions as a global indicator, the climate change impact (CC) saving per m2 floor area in the baseline scenario produces up to 25% fewer emissions than traditional buildings. ...
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