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Comparative life-cycle assessment of a mass timber building and concrete alternative

Authors:
COMPARATIVE LIFE-CYCLE ASSESSMENT OF A MASS TIMBER
BUILDING AND CONCRETE ALTERNATIVE
1
Shaobo Liang
Postdoctoral Research Fellow
E-mail: sshliang@gmail.com
Hongmei Gu*
Research Forest Products Technologist
E-mail: hongmei.gu@usda.gov
Richard Bergman
Project Leader and Research Wood Scientist
USDA Forest Products Laboratory
Madison, WI 53726
E-mail: richard.d.bergman@usda.gov
Stephen S. Kelley
Professor
Department of Biomaterials
North Carolina State University
Raleigh, NC 27695
E-mail: sskelley@ncsu.edu
(Received January 2020)
Abstract. The US housing construction market consumes vast amounts of resources, with most structural el-
ements derived from wood, a renewable and sustainable resource. The same cannot be said for all nonresidential
or high-rise buildings, which are primarily made of concrete and steel. As part of continuous environmental im-
provement processes, building life-cycle assessment (LCA) is a useful tool to compare the environmental footprint of
building structures. This study is a comparative LCA of an 8360-m
2
, 12-story mixed-use apartment/ofce building
designed for Portland, OR, and constructed from mainly mass timber. The designed mass timber building had a
relatively lightweight structural frame that used 1782 m
3
of cross-laminated timber (CLT) and 557 m
3
of glue-
laminated timber (glulam) and associated materials, which replaced approximately 58% of concrete and 72% of rebar
that would have been used in a conventional building. Compared with a similar concrete building, the mass timber
building had 18%, 1%, and 47% reduction in the impact categories of global warming, ozone depletion, and
eutrophication, respectively, for the A1-A5 building LCA. The use of CLT and glulam materials substantially
decreased the carbon footprint of the building, although it consumed more primary energy compared with a similar
concrete building. The impacts for the mass timber building were affected by large amounts of gypsum board, which
accounted for 16% of total building mass. Both lowering the amount of gypsum and keeping the mass timber
production close to the construction site could lower the overall environmental footprint of the mass timber building.
Keywords: Cross-laminated timber, environmental assessment, life-cycle analysis, tall wood building.
INTRODUCTION
The building industry is a heavy user of resources
and is responsible for more than 30% of total global
energy consumption and for about 40% of global
* Corresponding author
SWST member
1
This article was written and prepared by US Government
employees on ofcial time, and it is therefore in the public
domain and not subject to copyright.
Wood and Fiber Science, 52(2), 2020, pp. 217-229
https://doi.org/10.22382/wfs-2020-019
© 2020 by the Society of Wood Science and Technology
carbon dioxide emissions (Jones et al 2016; Berardi
2017). Developing energy-efcient and low-impact
buildings has become increasingly important. The
use of wood as a building material can provide
substantial economic and environmental benets
(Ritter et al 2011). Mass timber products, including
cross-laminated timber (CLT), glue-laminated tim-
ber (glulam), and nail-laminated timber, have been
demonstrated to be green building materials with a
lower carbon footprint than their concrete and steel
alternatives (Perez-Garcia et al 2005; Karacabeyli
and Douglas 2013; Bowers at al. 2017; Gu and
Bergman 2018). Also, CLT and other mass timber
products offer additional advantages such as faster
erection times, easier material handling, a high level
of prefabrication at the material manufacturing site,
and less waste generation and noise pollution during
the construction stage (Kremer and Symmons 2015;
Connolly et al 2018; Smith et al 2018). In particular,
the construction phase of a mass timber building can
result in substantial savings with quicker erection
times, more than 50% faster than other alternative
materials (APA 2019).
CLT is a massive structural composite panel fab-
ricated with kiln-dried dimensional lumber stacked
in three to nine layers arranged perpendicular to
each other (APA 2012). The production of similar
perpendicular engineered wood products dates back
to the early 20th century in the United States
(Walch and Watts 1923). The use of CLT in mid- to
high-rise buildings began to appear several decades
ago in European countries (FII 2016; Espinoza
and Buehlmann 2018). More recently, CLT and
other mass timber technologies have captured the
interest of designers, developers, property owners,
industry, and governments in North America
(Podesto and Berneman 2016; Williamson and
Ross 2016). The revised 2021 International
Building Code includes provisions for new con-
struction Type IV-A/B/C for up to 18 stories for
business and residential buildings using mass
timber (Breneman and Richardson 2019). Using
mass timber in building systems could be a boost to
the wood industry sector, but to gain the support of
green building advocates, rigorous scienticanal-
ysis on the environmental impacts is required. The
whole-building life-cycle assessment (LCA) is a
method to analyze building environmental impacts
based on ASTM E2921 (ASTM 2016) and EN
15978 (EN 2011) standards. However, only few
LCA studies on CLT and other mass timber
buildings are publicly available (Cadorel and
Crawford 2018). These studies all agree that mass
timber buildings have better environmental perfor-
mance such as lower greenhouse gas (GHG)
emissions compared with alternative concrete
buildings, although different study periods or sys-
tem boundaries were applied (Robertson et al 2012;
Durlinger et al 2013; Grann 2013; Bowick 2015,
2018). LCA case studies for mass timber buildings
in the United States are very limited (Gu and
Bergman 2018; Pierobon et al 2019) because few
buildings have gone beyond the concept stage.
More importantly, there are very few studies that
directly compare the LCA implications for mass
timber and concrete steel buildings, with similar
size, function, and operational energy perfor-
mance. It is critical to conduct more studies to
assess the environmental impacts of mass timber
buildings and to further analyze the impacts on
local communities, forest health, and the regional
economy.
This study conducted a building LCA for a 12-story
mixed-use tall wood building in Portland, OR, that
comprises CLT and glulam as the main structural
building materials. The LCA of this mass timber
building was compared with a functionally equiva-
lent concrete building system, with no wood struc-
tural elements. The environmental impacts of the two
buildings were categorized using the Tool for the
Reduction and Assessment of Chemical and other
environmental Impacts (TRACI) 2.1 impact method
(Bare et al 2012), and a detailed contribution analysis
and carbon accounting metrics were performed. This
research is part of a more comprehensive project
investigating the CLT supply chain along with po-
tential economic contributions and environmental
implications of increased CLT and other mass timber
building construction (Kelley and Bergman 2017).
Results generated from this study will provide solid,
transparent evidence on the environmental perfor-
mance metrics for CLT and other mass timber
buildings that can inform the public, building de-
velopers and owners, and policy makers.
WOOD AND FIBER SCIENCE, APRIL 2020, V. 52(2)218
MATERIALS AND METHODS
Goal and Scope
The goal of this study was to quantify the environ-
mental impacts of a tall wood building built primarily
with CLT and glulam structural elements and compare
those impacts with a functionally equivalent building
with traditional concrete materials. The target building
for this cradle-to-site LCA study is an 8360-m
2
, 12-
story, mixed-use ofce and apartment complex. The
building was designed to be built in Portland, OR, and
to have the same re-proong performance, insulation,
and energy consumption outcomes as a functionally
equivalent concrete building design. Both building
designs were completed by LEVER Architecture
(Portland, OR) with additional structural design and
analysis from their partner, KPFF Engineering
(Seattle, WA). Both buildings comply with Type 1B
re-resistant construction code with noncombustible
capacities of 2-h exterior walls, 2-h structural frame,
2-h ceiling/oor separation, and 1-h ceiling/roof
assembly (Heppner 2019). The building structure
components to compare include ceilingroof,
oors, foundation, postbeams, and walls.
Functional Unit and System Boundary
The functional unit for this study is dened as 1 m
2
of
oor area of the whole building. The system
boundary of this building LCA is dened as cradle-
to-site, as illustrated in Fig 1, and includes the
modules A1-A3 Product Stage and modules A4 and
A5 Construction Process Stage. More specically, in
this study, module A1 Raw materials supply covers
raw material acquisition (eg from tree seeding to log
harvest, or cement and aggregate mining and pro-
duction); module A2 Transport covers the trans-
portation of raw materials to the manufacture plant
(eg truck loading logs and transporting to primary and
secondary wood products manufacturers, or trans-
portation of cement and aggregate and rebar); module
A3 Manufacturing covers the gate-to-gate production
of secondary products (eg mass timber products
processing and packaging at the plant, or formulated
concrete); module A4 Transport covers the trans-
portation of materials and products from the factory
gate to the building site (eg trucking all construction
materials to the building site), whereas the trans-
portation of construction equipment to and from the
site was excluded; and module A5 Construction
installation process only covers the energy
consumption to install building materials into the
building (eg diesel usage by crane to lift CLT and
pour/pump concrete), whereas ground works,
labor assembly, land use, and other things were
excluded.
Figure 1. Cradle-to-site life-cycle assessment system boundary for the mass timber and concrete buildings.
Liang et alLCA OF A MASS TIMBER BUILDING AND CONCRETE ALTERNATIVE 219
Life-Cycle Inventory (LCI) and Impact
Assessment
The LCI phase in this study measures the materials
use (modules A1-A3), transportation (module A4),
and direct energy (module A5) inputs for the
construction process of the proposed CLT and
concrete buildings. Table 1 summarizes the
quantities of building materials used for the two
buildings, which were designed with the same
U-value. These quantities were provided by the
building designer (LEVER Architecture). The
building materials transport distances to the
Table 1. Quantities of materials and life-cycle inventory data sources for mass timber and concrete buildings.
Building section/material Unit Mass timber building Concrete building Database/source
Ceiling and roof
Hollow structural steel kg 11,449 7415 AIE
CLT m
3
0.95 Chen et al 2019
1-inch mineral wool m
2
285 285 DataSmart
Acrylic latex paint L 3096 1548 DataSmart
1-inch polystyrene board m
2
144 144 DataSmart
Steel sheet kg 5693 5693 DataSmart
5/8-inch gypsum board, re-resistant m
2
14,907 5945 AIE
1/2-inch gypsum board, regular m
2
4154 3337 AIE
Floors
Acrylic adhesive L 117 117
CLT m
3
1279 Chen et al 2019
Coated steel deck kg 110 110 AIE
Hollow structural steel kg 400 400 AIE
Concrete m
3
932 1878 DataSmart
Mortar kg 4737 4737 DataSmart
3/8-inch plywood m
2
661 661 DataSmart/USLCI
1-inch polystyrene board m
2
4067 4067 DataSmart
Rebar kg 53,177 170,348 DataSmart
Steel sheet kg 4193 875 DataSmart
Steel welded wire mesh kg 110 110 AIE
60-mil TPO membrane m
2
351 351 AIE
Foundation
Concrete m
3
125 149 DataSmart
Rebar kg 38,590 57,884 DataSmart
Post and beam
Hollow structural steel kg 43,527 39,230 AIE
Composite wood I-joist kg 60 60 DataSmart/USLCI
Concrete m
3
162 DataSmart
Glulam m
3
557 DataSmart/USLCI
Rebar kg 22,089 DataSmart
Steel sheet kg 830 823 DataSmart
Walls
Aluminum extrusion kg 31,039 31,051 DataSmart
CLT m
3
502 Chen et al 2019
Hollow structural steel kg 31,947 30,026 AIE
Concrete m
3
48 438 DataSmart
Concrete masonry unit kg 71,031 70,908 DataSmart
3/8-inch plywood m
2
3230 3230 DataSmart/USLCI
Mortar kg 90,113 89,824 DataSmart
Acrylic latex paint L 9100 5143 DataSmart
1-inch polystyrene board m
2
7643 7644 DataSmart
Silicone sealant L 503 503 DataSmart
Rebar kg 12,078 125,951 DataSmart
5/8-inch gypsum board, re-resistant m
2
57,330 47,097 AIE
WOOD AND FIBER SCIENCE, APRIL 2020, V. 52(2)220
construction site were provided by LEVER Ar-
chitecture (Heppner 2019). It is noteworthy that
concrete used a transportation distance of 24 km,
and the CLT and glulam were sourced with a
transportation distance of 320 km. Direct energy
inputs for the construction installation were
estimated as diesel usage using an empirical
equation provided by Athena Sustainable Ma-
terials Institute (Finlayson 2019). The LCI of the
CLT production process model was provided by
the University of Washington (Chen et al 2019).
Other building materials, transportation, and
energy LCI data were sourced from the USLCI
and US Ecoinvent 2.2 (DATASMART 2019)
and Athena Impact Estimator (AIE) databases
and from the Pacic Northwest forest resources,
as listed in Table 1.
The life-cycle impact assessment (LCIA) inte-
grates the LCI data of each building stage
(modules A1-A5) to quantify the total life-cycle
environmental impacts, following the ISO 14040
(ISO 2006a) and ISO 14044 (ISO 2006b) envi-
ronmental management standards. The total en-
vironmental impacts were modeled using data
sources in SimaPro 8.5 software (PR´
e Sustain-
ability, Amersfoort, the Netherlands) and AIE for
Building 5.6 software (Athena Sustainable Ma-
terials Institute, Ottawa, Canada), and the TRACI
2.1 impact method (Bare et al 2012) was used in
SimaPro and AIE. The primary energy consump-
tion, categorized as nonrenewable (fossil and nu-
clear) and renewable (biomass, solar, wind, and
hydropower), was calculated using the embedded
cumulative energy demand (CED) method v1.10 in
SimaPro and AIE. The impact indicators calculated
for each building material from the two software
were extracted into Microsoft Excel spreadsheets
and then integrated for further analysis.
Sensitivity Analysis
Although the United States has signicant lumber
manufacturing capacity, there is limited CLT
manufacturing capacity. However, with increasing
interest in this emerging product, new capacity is
developing across the United States. Thus, CLT
transportation distances to the construction site
were varied in determining the portion of this
specic variable on the whole-building environ-
mental impact. The current distance was 320 km,
which assumes transport from a nearby local fa-
cility. Sensitivity analyses included middle dis-
tance by truck (768 km) from Inland Northwest
(INW), long distance by truck (465 km) and rail
(4189 km) from eastern United States; and oversea
distance by sea (21,333 km) from Europe, rail
(1103 km), and truck (335 km).
RESULTS AND DISCUSSION
Comparison of Building Materials
The quantities of building materials of each
building section (ceilings-roof, oor, foundation,
postbeam, and wall) for the mass timber and
concrete buildings are shown in Table 1. Gen-
erally, the mass timber building uses a total of
2376 m
3
of wood products with 98% CLT and
glulam. In this study, mass timber usage is about
0.28 m
3
per m
2
of oor area. Other CLT and mass
timber building designs have used between 0.1
and 0.45 m
3
per m
2
of oor area (Gustavsson et al
2010; Oregon BEST 2017; Gu and Bergman
2018). Specically, the walls use 502 m
3
of 7-
and 9-ply CLT, the oors use 1279 m
3
of 5-ply
CLT, and the postbeams use 557 m
3
of glulam for
columns and beams. In addition, the mass timber
building also uses a signicant amount of con-
crete and steel, eg 1104 m
3
concrete and
103,845 kg rebar. The concrete and steel are used
on the foundation and also to stiffen the CLT oor
elements. The concrete building uses no mass
timber but uses 2627 m
3
of concrete and 376,272 kg
of rebar. In addition, to comply with Type 1B re-
resistant construction code, approximately 36%
more gypsum board is applied to the mass timber
building than the concrete building.
As shown in Fig 2, the total mass of the mass
timber building is about 68% of the functionally
equivalent concrete building. It is important to
keep in mind that the density of concrete
(2400 kg/m
3
) is much higher than the density of
the two mass timber products and wood building
products in general (550 kg/m
3
). The total mass
of the concrete building is 7.5 million kg for all
Liang et alLCA OF A MASS TIMBER BUILDING AND CONCRETE ALTERNATIVE 221
ve structural sections, whereas the total mass of
the mass timber building is only 5.1 million kg. In
the mass timber building, except for the ceilings-
roof which uses more gypsum board and attached
materials for re resistance purposes, the mass of
other building sections is about 64%, 81%, 76%,
and 66% of the concrete building for oors,
foundation, postbeams, and walls, respectively.
Light-weight mass timber buildings tend to have
lower carbon footprints and lower costs than
heavier concrete buildings (Connolly et al 2018).
Environmental Impact Analysis
The comparative cradle-to-site whole-building
LCIA results for 1-m
2
oor area of the mass
timber and concrete buildings are shown in
Table 2. Global warming contribution of the mass
timber building was found to be 18% lower
(193 kg CO
2
-eq/m
2
) than that of the concrete
building (237 kg CO
2
-eq/m
2
). The mass timber
building performs better in eutrophication than
the concrete building (47% lower), whereas the
concrete building has better performance in the
impact categories of smog and acidication (3%
and 16% lower than the mass timber building,
respectively). In addition, the two buildings are
essentially the same for ozone depletion (1%
difference), which is dominated by the use of
polystyrene insulation boards (XPS). The two
buildings used about the same amount of XPS.
Primary energy consumption, also called CED,
which describes the direct and indirect energy use
throughout the life cycle of products, is an
Figure 2. Total mass for the mass timber and concrete buildings.
Table 2. Life-cycle impact assessment results for 1-m
2
oor area by building types.
Impact category Unit Mass timber building Concrete building Percentage difference (%)
Global warming kg CO
2
eq 193 237 18
Ozone depletion kg CFC-11 eq 1.91E-04 1.93E-04 1
Smog kg O
3
eq 15.74 15.22 3
Acidication kg SO
2
eq 1.03 0.89 16
Eutrophication kg N eq 0.19 0.36 47
Total primary energy MJ 2868 2673 7
Nonrenewable, fossil MJ 2344 2371 1
Nonrenewable, nuclear MJ 198 242 18
Renewable MJ 326 61 439
WOOD AND FIBER SCIENCE, APRIL 2020, V. 52(2)222
important driver of environmental impacts and is
indicative for many environmental problems
(Huijbregts et al 2006). As shown in Table 2, the
mass timber building has 7% higher CED than the
concrete building, which is mainly caused by the
relatively higher unit CED of 2629 MJ/m
3
for
CLT than 1540 MJ/m
3
for concrete. In the mass
timber building, the large mass of CLT results in
CLT having the highest CED (23% of building
CED), followed by 22% for glulam. In the al-
ternative concrete building, rebar has the greatest
CED, accounting for 47% of building CED,
followed by 21% for concrete. In addition, fossil
fuel accounts for 82-90% of building CED for
both buildings, and renewable energy accounts
for 12% and 2% of building CED for the mass
timber and concrete buildings, respectively. As
expected, the mass timber building also uses
substantially higher renewable energy than its
concrete alternative building. This was mainly
because of the wood products manufacturer using
mill residue as an alternative heating source to dry
the lumber before CLT and glulam production
(Bowers et al 2017; Chen et al 2019).
The normalized environmental impacts and en-
ergy demand for the mass timber and concrete
buildings at different life-cycle phases are illus-
trated in Fig 3. The product phase (modules A1-
A3) is the dominant contributor, accounting for
88-98% in all impact categories for the two
buildings. The transport phase (module A4)
contributes 3-8% to the impact categories of
global warming, smog, acidication, eutrophi-
cation, total primary energy, and fossil fuel
consumption for the mass timber building. Be-
cause of the longer transportation distance of
wood building materials from the manufacturer to
construction site, eg 320 vs 24 km for materials
for the concrete building, the transportation im-
pacts are greater in the mass timber case than in
the all-concrete case.
The construction installation phase (module A5)
contributes a small fraction (1-5%) in all impact
categories except for ozone depletion, although
the impacts are consistently lower for the less
dense mass timber materials than for concrete.
The assessment for this phase, module A5, is based
on the estimation of diesel consumption used for
lifting all the building materials (Finlayson 2019).
When more empirical data on diesel consumption
by construction equipment and time for mass
timber building construction are collected, more
precise LCIA results can be reported for this phase.
Figure 4 compares the product phase (modules
A1-A3) environmental impacts from each
building section for the mass timber and concrete
buildings. Floor is the largest contributor for both
Figure 3. Normalized impacts for mass timber (left column) and concrete (right column) buildings at different life-cycle
phases.
Liang et alLCA OF A MASS TIMBER BUILDING AND CONCRETE ALTERNATIVE 223
buildings and accounts for 40-50% in the impact
categories of global warming, smog, and eutro-
phication, followed by the wall sections at 31-
39%. Specically, the building materials of CLT
and concrete/rebar in the mass timber building, as
well as concrete/rebar in the concrete building,
dominate the impacts of the oor component (93-
99%). For acidication, wall is the largest con-
tributor, accounting for 39-48% of impacts for the
two buildings. This is caused by the use of
gypsum board. Wall and oor together contribute
98-99% of the ozone depletion for both buildings,
which is because of the insulation material, eg
polystyrene boards (XPS). Although XPS ac-
counts for only 0.4% of the total mass quantities
in wall components in the mass timber building, it
contributed more than 99% of total ozone de-
pletion impacts. We strongly suggest that an
alternative product be considered. The ceilings-
roof, foundation, and postbeams together con-
tribute less than 27% to all impact categories for
the two buildings, although the mass timber
building contributes a higher fraction than the
concrete building in these building sections.
These differences are caused by the greater use of
gypsum board in the ceilings-roof, rebar in the
foundation, and glulam in the postbeams.
MasterFormat is a standard for organizing
specications and other written information for
commercial and institutional building projects in
the United States and Canada. It provides a
structured hierarchy for building construction
requirements and associated activities (Tecchio
et al 2018). To further analyze the environmental
impact contribution, the building materialsLCIs
were grouped into individual construction divi-
sions based on MasterFormat (CSI 2016), eg
Division 03: Concrete;Division 04: Masonry;
Division 05: Metals;Division 06: Wood, Plastics,
Composites;Division 07: Thermal and Moisture
Protection; and Division 09: Finishes. Figure 5
shows the comparison in the product phase
(modules A1-A3) of the environmental impacts
from individual construction divisions for the
mass timber and concrete buildings. The two
buildings have signicant differences in all im-
pact categories except for ozone depletion, which
was dominated by Division 07 for both buildings,
eg XPS. Because the insulation packages are
essentially the same, materials such as XPS,
aluminum panels, mineral wool, sealants, adhe-
sives, and TPO membranes dominate the impacts.
The total environmental impact of the concrete
building is dominated by Division 03, consisting
of concrete and rebar, which accounts for 72%,
72%, 61%, and 86% in the impact categories of
global warming, smog, acidication, and eutro-
phication, respectively. These numbers range
Figure 4. Normalized impacts (A1-A3) from different building sections for mass timber (left column) and concrete (right
column) buildings.
WOOD AND FIBER SCIENCE, APRIL 2020, V. 52(2)224
from 17% to 50% for the mass timber building.
Approximately 58% concrete and 72% rebar
in Division 03 of the concrete building are
substituted by 2339 m
3
CLT and glulam in Di-
vision 06 for the mass timber building, which
resulted in Division 06 in the mass timber
building contributing 28%, 49%, 45%, and 22%
to the impact categories of global warming, smog,
acidication, and eutrophication, respectively.
The two buildings have no signicant differences
in Division 07. In addition, in the mass timber
building, Division 09 contributes relatively more
to all impact categories than it does in the con-
crete building, which is largely caused by the use
of 36% more gypsum board in the mass timber
building compared with the concrete building.
Carbon Analysis
Biogenic carbon refers to CO
2
emissions that
originate from biological sources such as plants,
trees, and soil (Harris et al 2018). In LCA studies
of durable wood products, it is assumed that
harvested timber products will be replaced sus-
tainably by new growth in managed forest land,
and therefore, the biogenic CO
2
emissions are
considered to be carbon neutral from the climate
change prospective. The Intergovernmental Panel
on Climate Change (IPCC 2006) also supports
the carbon neutral hypothesis, which considers
the CO
2
emissions from biomass as part of the
natural carbon cycle. The carbon in wood is
accounted for as stored CO
2
during the lifetime of
the product or building. Such stored carbon is
estimated by average carbon content of 50% of
dry mass of wood products. This analysis as-
sumed a service life of 100 yr for the mass timber
building, which equates to the same time frame
used for accounting for GHG emissions in this
study. As shown by this LCI analysis and Fig 6,
biogenic CO
2
emissions from the mass timber
and concrete buildings were 81 and 3.4 kg/m
2
of
oor area, respectively. These results were
closely aligned with the consumption of renew-
able energy in the two building types. The
Figure 6. Comparison of CO
2
emissions for mass timber
and concrete buildings under different carbon accountings.
Figure 5. Normalized impacts (A1-A3) of different construction divisions for mass timber (left column) and concrete (right
column) buildings.
Liang et alLCA OF A MASS TIMBER BUILDING AND CONCRETE ALTERNATIVE 225
sequestered CO
2
values in wood products were
about 276 and 4.3 kg/m
2
of oor area for the mass
timber and concrete buildings, respectively. As
previously calculated, the GHG emissions for the
CLT and concrete buildings were 193 and 237 kg
CO
2
eq/m
2
of oor area, respectively, which is
about 18% different. More signicant differences
were observed when biogenic carbon and se-
questered carbon were combined, resulting in
values of CO
2
emissions for the mass timber and
concrete buildings of 2.7 and 236 kg/m
2
of oor
area, respectively.
Sensitivity Analysis
Transportation accounts for about 5% of the total
global warming impact emissions for the mass
timber building. But given that there is a limited
production infrastructure for CLT, it is worth
considering the effects of different production/
transportation alternatives on the overall global
warming impacts of the building. CLT trans-
portation has a global warming impact of 3.4 kg
CO
2
eq/m
2
of oor area when assuming local
production and transportation (320 km). Thus,
with limited CLT production in the United States,
it is important to understand the implication of
longer transportation distances. Figure 7 shows
the global warming impacts of different CLT
transport distances and, specically, the effects of
sourcing CLT from INW, the eastern United
States, or Europe. This analysis shows that the
global warming impact of CLT transportation
increased from 3.4 to 8.2, to 16, to 47 kg CO
2
eq/m
2
of oor area with the increasing levels of
distance, respectively (Fig 7), which also ac-
counts for 59%, 73%, and 89% of total impacts in
the transport phase (module A4) for the three
projected regions. Meanwhile, as shown in Fig 7,
the transport phase (module A4) contributes less
than 5% to the global warming impact compared
with the module A1-A5 impacts under the local
(within 320 km) assumption, and this ratio would
increase to 7%, 10%, and 22%, respectively, if
sourcing the CLT material from the three other
locations.
The CLT transport distance revealed substantial
changes to the differences between the mass
timber and concrete buildings in environmental
impacts. The mass timber building outperformed
the concrete building inglobal warming with 18%,
16%, and 13% lower impacts when sourcing from
local and projected INW and eastern US regions.
However, the mass timber building had 0.2%
higher global warming impact than the concrete
Figure 7. Global warming impacts from transportation of cross-laminated timber (CLT) materials to the building site
(assuming CLT manufacturer in-state with a distance of 375 miles; INW: Inland Northwest region in the United States,
Eastern: eastern United States, and Oversea: Europe) and the fraction of A4 impact in total A1-A5 for the mass timber building.
WOOD AND FIBER SCIENCE, APRIL 2020, V. 52(2)226
building if CLT was sourced from Europe because
of the long transportation distance.
Study Strengths and Limitations
A strength of this study was the detailed building
design information for two equivalent buildings
by our highly qualied architectural and struc-
tural engineering partners. The study evaluated
the structural elements, modules A1-A5, with the
expectations that the bill of the materials for the
two buildings incorporated the same U-value.
Although mineral wool was incorporated into the
analysis at the same quantities, its impacts were
less than 0.3%. A limitation of this study was the
lack of detail on the CLT manufacturing process.
This lack of detail includes energy consumption
in the manufacturing, proprietary details such as
resin use, and the yield of CLT from dimensional
lumber. In this study, the building components
were limited to elements that were most different,
eg the ceilings-roof, oors, foundation, post-
beams, and walls. Other equivalent components
such as windows, doors, plumbing, and elec-
tricity were excluded in the scope of this study.
Construction site data including equipment use,
electricity use, and labor count were not available
for this study. Therefore, an empirical equation
provided by the Athena Sustainable Materials
Institute and based on building height was used.
As with any LCA, additional data on specic
processes, eg CLT manufacturing to concrete
transportation, will improve the value of the
analysis.
CONCLUSION
In this study, a comparative cradle-to-site LCA of
a mass timber, tall wood building, and a func-
tionally equivalent concrete building was con-
ducted. This study shows that the mass timber
building outperformed the concrete building on a
number of environmental impact categories, eg
global warming, ozone depletion, and eutrophi-
cation, whereas the concrete building showed
better performance on smog and acidication, as
well as total primary energy demand. The product
phase (modules A1-A3) contributed more than
66% of total cradle-to-site (modules A1-A5)
impacts in all impact categories for both build-
ings. The mass timber building was much lighter,
68% of the total weight of the concrete building.
Even with the use of concrete and steel in the
foundation and CLT oor systems, the mass
timber building used 58% of the concrete and
72% of the rebar of the concrete building. Floors
and walls were major environmental contributors
in building sections. CLT and concrete were the
hotspot for the tall wood building, and concrete
and rebar were the hotspot for the concrete
building. The required use of more gypsum board
for mid- to high-rise mass timber buildings to
comply with building codes increased global
warming, smog, acidication, and eutrophica-
tion. The CLT building had lower CO
2
emissions
than the concrete building when biogenic carbon
and sequestered carbon were included. Sensi-
tivity analysis showed that the environmental
impacts for CLT transport distance, including
sourcing from European countries, could reverse
the advantages of all impact categories for the
mass timber building in this study. Further work
will focus on environmental impacts at the use
and end-of-life phases.
ACKNOWLEDGMENTS
This project was nancially supported by a joint
venture agreement between the USDA Forest
Service, Forest Products Laboratory, and the U.S.
Endowment for Forestry & Communities, Inc.,
Endowment Green Building PartnershipPhase
1, no. 16-JV-11111137-094. External reviews
were performed by Jonathan Heppner, Lever
Architecture; Cindy Chen and Francesca Pier-
obon, University of Washington; and Kuma
Sumathipala, American Wood Council. The au-
thors also acknowledge the anonymous reviewers
from the Wood and Fiber Science journal.
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Liang et alLCA OF A MASS TIMBER BUILDING AND CONCRETE ALTERNATIVE 229
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