Comparative LCAs of Conventional and Mass Timber Buildings in Regions with Potential for Mass Timber Penetration

: Manufacturing of building materials and construction of buildings make up 11% of the global greenhouse gas emission by sector. Mass timber construction has the potential to reduce greenhouse gas emissions by moving wood into buildings with designs that have traditionally been dominated by steel and concrete. The environmental impacts of mass timber buildings were compared against those of functionally equivalent conventional buildings. Three pairs of buildings were designed for the Pacifc Northwest, Northeast and Southeast regions in the United States to conform to mass timber building types with 8, 12, or 18 stories. Conventional buildings constructed with concrete and steel were designed for comparisons with the mass timber buildings. Over all regions and building heights, the mass timber buildings exhibited a reduction in the embodied carbon varying between 22% and 50% compared to the concrete buildings. Embodied carbon per unit of area increased with building height as the quantity of concrete, metals, and other nonrenewable materials increased. Total embodied energy to produce, transport, and construct A1–A5 materials was higher in all mass timber buildings compared to equivalent concrete. Further research is needed to predict the long-term carbon emissions and carbon mitigation potential of mass timber buildings to conventional building materials.


Introduction
Atmospheric carbon dioxide is the major contributor to global warming, making carbon emissions one of the world's most urgent environmental challenges. Recent research has indicated that afforestation can offer the single greatest opportunity for carbon mitigation [1][2][3]. However, delivering on this potential implies the afforestation of hundreds of millions of hectares in the next decade [4]. Forests have the ability to take up carbon dioxide and release oxygen back into the atmosphere through photosynthesis while storing carbon for decades or centuries in trees. For each metric ton (ton) of carbon stored in trees, 3.67 tons of carbon dioxide emission is removed. As forests age, their initially high carbon sequestration rates decrease, and eventually, carbon fux (i.e., sequestration and release) reaches a balance [3]. After disturbances, much of the stored carbon may be released back into the atmosphere relatively quickly via increased mortality, fre, or decomposition. Recent controversies about how forests best can offset carbon emissions have focused on the question of whether or not forests can increase their positive contribution to the carbon cycle if trees are harvested and the sequestered carbon is stored in long-term products, such as buildings [3,[5][6][7].
Over the next decades, economies will grow because of an increase in population, resulting in a construction surge of buildings, bridges, and other structures. Particular attention has been given to the potential impact of mass timber (MT) penetration into these markets. Awareness of this comes from the expected increase in demand for wood products and the ability of forests to sustainably support the demand using carbon mitigation strategies, such as MT storing carbon in structures for decades. Mass timber is a defned category of engineered wood products (e.g., cross-laminated timber (CLT), glued laminated timber (glulam), mass plywood, and others) that enables the construction of tall buildings with wood [8][9][10]. Mass timber construction can have a greater carbon displacement beneft because it moves wood into building designs that traditionally have been dominated by steel and concrete materials.
Cross-laminated timber is at the forefront of the MT movement, enabling designers, engineers, and other stakeholders to build taller wood buildings. CLT panels are made by laminating dimension lumber orthogonally in alternating layers. Cross-laminated timber and other MT products store carbon and generate virtually no waste at a building site, as panels and beams are generally prefabricated before delivery.
Life-cycle assessment (LCA) has evolved as an internationally accepted method to objectively evaluate a product by identifying and quantifying energy and materials used and wastes released to the environment. Life-cycle assessment studies can evaluate full product life cycles, often referred to as "cradle-to-grave" or incorporate only a portion of the product's life cycle, referred to as "cradle-to-gate" or "gate-to-gate".
Life-cycle assessment studies of engineered timber products such as glued laminated timber (glulam) and CLT in construction have highlighted their environmental advantages over conventional materials such as concrete and steel [11][12][13][14]. However, no studies have yet compared the environmental impacts of MT buildings and conventional buildings for different building heights and across different United States (U.S.) regions. Cradle-to-gate product LCAs indicated net negative carbon emissions for MT products, which positions them with a high environmental advantage over nonwood materials [15][16][17][18].
The impacts that increased wood product utilization might have on forests and climate are complex. The current increase in wood demand from the MT movement is minimal. The maximum annual production capacity of North American MT manufacturers is 1.67 million m 3 , consuming about 2.2% of the total softwood lumber production in North America [8]. However, because of the COVID pandemic, which put a lot of MT projects on hold, and the high cost of lumber in the U.S. relative to that in Europe and resulting increased imports of CLT, the softwood lumber usage in North American MT products was only 20% of the maximum capacity in 2020 [19].
The production of concrete and steel currently represents approximately 11% of annual global building greenhouse gas (GHG) emissions [20]. The global building stock, which primarily uses concrete and steel, is projected to double over the next 40 years, with most of that growth expected to occur in the southern hemisphere. To reduce the impact of this building expansion, MT buildings may offer a potentially appealing alternative to concrete and steel [11,[21][22][23][24][25][26][27][28][29].
Whole-building LCA (WBLCA) studies have quantifed and compared the environmental impact of MT buildings with that of traditional concrete and steel structures [11][12][13]21,22,[25][26][27][28]. In one case study of midrise buildings [11], total carbon emissions for a fve-story MT building were dominated by the manufacturing stage (77%), while the construction stage represented only 3% of the total carbon emissions. Total carbon emissions for the CLT building showed emissions of +1153 tons carbon dioxide equivalents (CO 2 e) and storage of a total of −5315 tons of CO 2 e, resulting in a net negative emission of −3847 tons CO 2 e. Carbon emissions for an equivalent steel and concrete designs were +1372 tons CO 2 e and +1718 tons CO 2 e, respectively [11]. In summary, the CLT building produced 33% less carbon emission than the equivalent steel building and 16% less carbon emissions than the concrete.
In another case study on a midrise northwest building, the environmental benefts of using CLT in hybrid midrise structures compared to using concrete resulted in a 26.5% reduction in carbon emissions and an 8% reduction in nonrenewable fuels [25]. The hybrid CLT building stored −1556 tons of CO 2 , offsetting the emissions from product manufacturing and construction and resulting in a net negative emission of −1222 tons of CO 2 e.
Cross-laminated timber is a relatively new product, and research is ongoing to track how production changes and building designs result in lower embodied carbon than conventional materials and designs [12][13][14]25]. Clearly, CLT buildings have potentially greater benefts if effcient reprocessing at the end of building service life is implemented for reuse and recycle [3]. Increased benefts are also manifested when the timing of emissions is considered [12,27].
Building with wood provides an important climate change mitigation opportunity by storing carbon for decades and displacing emissions from nonrenewable materials together with reducing dependence on nonrenewable resources. Taking advantage of this opportunity requires sustainable forest management, which ensures that carbon sequestration is optimized in the forest while increasing carbon pools in harvested wood products for long-term storage [3,5,[30][31][32][33] (Gu Johnston perez). This study is the frst step in flling the knowledge gap on comparing functionally equivalent conventional buildings to those constructed using MT. The goal of this study was to determine the embodied carbon and energy contribution for three building heights, in three U.S. regions, using both conventional materials and MT products in the buildings' assemblies (structure, envelope, and interior walls).

Architectural Building Designs and Assumptions
The whole-building life-cycle assessment (WBLCA) was designed to compare MT buildings with functionally equivalent conventional concrete structures for their cradleto-gate environmental impacts. A total of eighteen different modeling conditions were selected for the comparative building LCAs in the U.S., composed of three geographic locations: (1) the Pacifc Northwest (PNW), (2) the Northeast (NE), and (3) the Southeast (SE). The building designs covered three building heights under the ICC TallWood Building Code, Type IV-A for 18-story buildings, Type IV-B for 12-story buildings, and Type IV-C for 8-9-story buildings (Supplementary Materials S1) ( Table 1), and two building materials (MT and conventional concrete-and-steel) ( Figure 1). It should be noted that all of these mid-and high-rise buildings were in fact hybrid buildings. The concrete buildings utilized both steel and concrete, just as the MT buildings utilized both concrete and steel for certain building elements as well as CLT and glulam. Other key assumptions (Supplementary Materials S1) included the different structural and constructability requirements for the PNW's seismic region (Supplementary Materials S1). All buildings were designed with mixed usage in mind. Floor-to-foor dimensions were 4.11 m for the commercial foors and 2.95 m for all residential foors. The basic building type was a simple rectangular shaped building with a central elevator and stair core with a foorplate of 25.91 m × 45.72 m (1185 m 2 ). For complete descriptions of the architectural plans and materials takeoffs for all building designs and regions, see Supplementary Materials S1.  The buildings were not designed with any particular site in mind, except for their broad geographic regional differences. However, for some of the structural analysis, certain site assumptions had to be made given the need for appropriate soil analysis for soil pressure. Based on potential markets and production of MT the following three building sites were chosen for the WBLCA: (1) Seattle, Washington, (2) Boston, Massachusetts, and (3) Atlanta, Georgia.
Life-cycle inventory (LCI) datasets for the building materials used a combination of primary data [34] (CORRIM) and public databases [35][36][37] for the WBLCA modeling in this study. The study followed international standards for LCA methods and WBLCA analysis (ISO 14044, EN 15978, and ISO 21930) [38][39][40] as well as the building designs and assumptions in Supplementary Materials S1. Datasets and methodology were described further by Gu et al. [41]. The declared unit was 1 m 2 of the total foor area of the building. The system boundary for this assessment was cradle-to-gate and included modules A1-resource extraction, A2-transportation of materials to product manufacturing, A3-product manufacturing, A4-transportation of materials to construction site, and A5-construction energy use ( Figure 2). Excluded from the study were modules B, C, and D [41].
Each region represents different energy mixes and timber species, and in the case of the PNW, additional seismic considerations drove differences in the building designs from those in the other two regions [41]. The species and MT production sites (actual and assumed) are listed in Table 2. For timber buildings, the density of the wood species infuences the weight contribution of MT [12,41] (Table 3).  Table 2. Geographical regions, species, and mass timber production sites.
Fire protection of the MT structural elements was a critical factor in determining the allowable heights and uses for MT in mid-and high-rise buildings (Supplementary Materials S1). All MT building designs followed the new approved codes set by the International Code Council [42], which were adopted in the 2021 International Building Code. All MT elements in Type IV require some level of fre protection (Table 1). Type IV-A (up to 18 stories) requires noncombustible protection over all MT elements. Types IV-B and -C allow some exposure to MT [42]. The noncombustible material used in this study was either 1 2 " gypsum or 5/8" Type X gypsum sheathing (Supplementary Materials S2) [41]. Reporting of embodied carbon was based on the Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI) evaluation method [46], and embodied energy was determined using the cumulative energy demand (CED). The building embodied carbon represents the total GHG emissions from cradle-to-gate of all the manufacturing of materials, transportation, and installation. Embodied energy is the sum of all energy consumed (renewable and nonrenewable) by all of the processes (including electricity use) associated with each building, from the mining and processing of natural resources to manufacturing, transport, product delivery, and construction. The WBLCA was modeled using the SimaPro LCA software [41,47] equipped with the USLCI [35], EcoInvent [36], and DATASMART 2019 [37] databases.
All materials used in buildings were assumed to be produced and sold domestically; therefore, only road and rail transportation modes were used. For distances shorter than 805 km (500 miles), the materials were assumed to be transported by truck, while for distances longer than 805 km, they were assumed to use a combination of truck and rail transport [41].

Results
The WBLCA results demonstrated the embodied carbon and embodied energy of using MT in mid-to high-rise buildings when compared to those of using conventional concrete. Highlights of the building differences in embodied carbon and energy demands by life cycle stage, regions, building height, materials, and assembly are presented in this paper. Since carbon emissions were the main focus of the study, global warming potential (GWP) expressed in carbon dioxide equivalents (CO 2 e) is the main metric reported for embodied carbon and megajoules (MJ) for CED. Additional environmental impact indicators (smog, acidifcation, eutrophication, ozone waste, and fossil fuel depletion) are reported in Supplementary Materials S3 for all building heights and regions.

Building Mass
For both MT and concrete building designs, concrete represented the largest contribution by mass ( Figure 3). In the MT building designs, concrete representation of total building mass was as low as 35% in the SE 8-story building and as high as 46% in the 18story building in the PNW (Figure 3a). In the concrete building designs, concrete accounted for over 90% of the total mass of all the buildings (Figure 3b). The 8-story buildings had the largest contributions of CLT at 23%, 315, and 35% for the PNW, NE, and SE, respectively. Glulam contributed to under 10% in the 8-story buildings. Cross-laminated timber represented 22-31% of the mass for the 12-story buildings and 16-24% of the mass in the 18-story buildings. Glulam was below 10% of the mass in the 12-story and 6-10% in the 18-story buildings. Glulam had the highest representation in the PNW buildings, representing 10% of the mass in all building heights.
The mass contribution of MT was highest in the 8-story buildings and lowest in the 18-story buildings in all regions because the building code for taller buildings requires greater use of gypsum as an interior fre protectant. As a result of the strict fre codes, the 18-story building required over 11 times more gypsum than the 8-story MT design and over 2 times more gypsum than the 12-story MT design. Floors represented the largest mass contribution in the 8-story MT buildings, representing about half the total mass of these buildings, while the foundations had the highest mass contributions for the 12-story buildings, ranging from 38 to 42% depending on the region (Figure 4). In the 18-story buildings, the largest mass contribution was the interior wall assembly, which represented 42-52% of the MT whole-building mass. Gypsum wallboard represented about 29% of interior wall mass (Table 4), while for the whole building system, the gypsum wall contributed 14% to the total building mass, including the gypsum used in the exterior wall.

Embodied Carbon
Over all regions and building heights, the MT buildings held lower embodied carbon than their functionally equivalent concrete buildings ( Figure 5). In general, embodied carbon per unit of area increased with building height in MT buildings as the quantity of concrete, metals, and other nonrenewable materials increased. The NE MT buildings had the largest reduction in embodied carbon compared to the corresponding concrete buildings, with the SE MT buildings showing the smallest reduction, because of the regional electricity grid, wood species, and transportation differences. Across all the building heights and regions, MT buildings exhibited reductions in embodied carbon varying between 22% and 50% compared to the corresponding concrete buildings ( Figure 5). The results of the whole-building embodied carbon analysis are shown in Figure 6. The PNW concrete 12-story buildings had the highest embodied carbon per unit area of all building designs and regions (Figure 6b). This was attributed to the components needed to meet the PNW building code requirements for seismic protection, as well as the mat footing foundation design used only for the 12-story buildings (Supplementary Materials S1). The equivalent 12-story MT building in the PNW had a 44% reduction in embodied carbon. The largest reductions in embodied carbon were in all 8-story MT buildings, for which the results showed reductions of 40-50% compared to the equivalent concrete buildings (Figures 5 and 6).
Embodied carbon of the MT buildings was greatest in the A1-A3 life cycle stages, which represented 85-91% of the carbon emissions. Transportation (A4) accounted for 5-11% and construction (A5) for 3-4% (Table 5). For concrete designs, the A1-A3 life cycle stage represented 94-96% of the carbon emissions. The biggest difference was in the A4 modules. For concrete buildings, the A4-transportation accounted for about 2%, 1%, and 0.5% of carbon emissions in the PNW, NE, and SE, respectively.  In the 18-story MT buildings, the interior wall represented 50-59% of the total embodied carbon impact of the whole building depending on the region (Figure 7). Gypsum wall board was used in both interior and exterior wall systems. Within the interior wall assembly of the 18-story MT buildings, gypsum wall board represented about 29-30% of the interior wall mass and 30-33% of the embodied carbon depending on the region (Table 4). For the 18-story MT building systems in all regions, gypsum wall board contributed 13-15% of the mass (Figure 3a) and 16-21% of the total embodied carbon (Figure 8), while the two MT structure components (CLT and glulam) contributed 28-39% and concrete (including gypsum-concrete) 30-35% of the whole-building embodied carbon. This included the gypsum in the exterior wall.

Energy Use
Both renewable and nonrenewable energy were consumed during extraction, production, transport, and manufacture of the materials used in all building designs. In all building designs, total embodied energy was higher for the MT buildings compared to the equivalent concrete buildings ( Table 6), independently of the region. Total (A1-A5) nonrenewable energy (fossil and nuclear) was lower in the MT than in the concrete designs for the 8-and 12-story buildings (Table 6), while the 18-story MT buildings (NE and SE regions) consumed more nonrenewable fuels than the equivalent concrete designs. This higher nonrenewable fuel consumption in these two regions and not the PNW was primarily from electricity use for regional production of building components (e.g., CLT and glulam). The PNW regional grids use a higher percentage of renewable fuels.
The transportation distances of the CLT and glulam from the manufacturers to the building site over all regions ranged from 332 to 490 km [41]. The transportation of MT components was the driver in the A4 stage. Transportation (A4) from production to construction accounted for 5-8% of nonrenewable energy use for MT buildings and 1-3% for concrete buildings. Construction (A5) energy used only diesel fuel and accounted for 3-5% on the nonrenewable fuel use for MT buildings and 5-6% for concrete buildings.
Renewable energy use originated mainly from the production of the lumber that was the feedstock for both CLT and glulam. For MT, most of the renewable energy was generated by combustion of biomass such as bark, sawdust, chips, and other waste generated during the milling processes [43][44][45]. The total renewable energy used, from A1-A5, in the MT buildings represented 25-40% of the total energy, depending on the region and building height ( Table 6). The percentage of renewable energy decreased with height; it represented 33-40% in the 8-story buildings and 25-33% in the 18-story buildings, wherein there was greater use of on nonrenewable materials such as gypsum.

Embodied Carbon (A1-A5)
Mass timber buildings had lower overall embodied carbon than equivalent concrete buildings within the cradle-to-construction gate system boundary. There were also differences in regional buildings' embodied carbon discovered in this study, which could be attributed to differences in electricity grids, the distance of transporting materials, and wood species. In addition, the upstream impacts of producing the softwood lumber used to make CLT and glulam were transferred downstream with the lumber inputs for the production of MT [43][44][45]. Most of the regional differences came from softwood lumber, which was primarily a result of species density, green moisture content, and type of energy used for drying the lumber with different kiln-drying schedules. These upstream impacts were seen in the overall results in MT buildings over the three regions.
While MT buildings produce carbon emissions during their production and installation, MT buildings also offset their carbon emissions by storing carbon for the time of building is in use (Figure 9). In all three MT designs in all regions, more carbon was stored in the building than was released during production and installation (Figure 9), with results similar to those of earlier published studies [11,13,14,25] in which net carbon (storage minus emission) ranged from −1222 to −5315 tons CO 2 e [11,25] for the whole buildings. By life-cycle stage, 88-90% of the embodied carbon was generated during A1-A3 (extraction through manufacturing), 6-11% during product transportation (A4) and 3-4% during construction (A5). Salazar and Puettmann [11] reported 87%, 8%, and 5% for A1-A3, A4, and A5, respectively, results comparable to this study. Current standards on the reporting of embodied carbon (global warming potential) do not include biogenic carbon emissions released from the combustion of renewable fuels as emissions under sustainable forestry practices.

Embodied Carbon-Assemblies
In MT and concrete 12-story buildings in all three regions, the foundation had the highest embodied carbon contribution. This was due to the mat footing design for the 12-story MT buildings. This required more cement and rebars than the spread footing design for the 8-story buildings and the pile foundation design for the 18-story buildings (Supplementary Materials S2) [41].
Following the requisite code performances as required under the new building codes for mass timber buildings (Supplementary Materials S1), there was additional consideration given to the fre and life safety code requirements. Interior walls represented the largest contribution to embodied carbon for the 18-story buildings because of strict fre codes requiring nearly 11 times more gypsum than for the 8-story buildings and 2 times more gypsum than for the 12-story buildings. Gypsum wall board was assumed as the requisite noncombustible protection and was required only for the MT assemblies and not for the equivalent noncombustible concrete assemblies [41] (Supplementary Materials S1 and S2).

Embodied Energy
All MT buildings used more energy to produce than the equivalent concrete buildings. As mentioned earlier, the energy requirement to produce lumber was transferred to MT production and again to the whole-building cradle-to-gate energy use. Energy consumption was not directly in line with embodied carbon, and energy content of the fuels used was not equal. Wood fuels have a lower heating value than fossil fuels. Recently produced life-cycle assessment reports [43,44] on the production of softwood lumber in the United States showed that nearly 100% of the energy was from renewable biomass, mostly generated at the facilities. When these burdens were transferred with the quantity of MT used in the whole buildings along with all the materials used in the buildings, the use of renewable energy ranged from 33-40% in the 8-story buildings, to 25-33% in the 12-story buildings, to 27-35% in the 18-story buildings. Over all regions, 88-91% of nonrenewable fuels used in the MT designs were from modules A1-A3. In the concrete buildings, the maximum amount of renewable energy use was only 3%.
Transportation of MT to construction sites (A4) had minimal impact on the total wholebuilding energy use (5-8%). On the other hand, concrete transportation to the construction site was limited to only 1-3% of the buildings' total nonrenewable energy. This was due to the short local transport of concrete and the fact that CLT is a customized product and is more diffcult to be sourced locally. Current regional production facilities for MT are limited to either one or none in certain regions, making transportation distances longer. Our assumptions were based on having only one CLT and glulam facility within each region. Distances ranged from 354 to 473 km for CLT and 332 to 490 km for glulam [41]. Concrete transport distances were short, limited to under 52 km to the construction site.
As an example of the potential impact of A4, when the transportation distance for CLT and glulam was doubled for the 8-story MT building, the A4 energy use contribution increased to 15% for the whole building. We mention this because some of the current whole-building design embodied carbon models available use environmental product declarations that might not include the A4 module in the total embodied carbon of the product. Therefore, preferred purchasing based solely on A1-A3 embodied carbon could have unintended consequences on the overall embodied carbon of MT buildings.

Conclusions
Manufacturing of all building materials and construction of buildings consume energy and emit carbon. Sustainable use of wood products gives the opportunity for reducing global greenhouse gas emissions by: (1) growing more trees; (2) managing forests sustainably for yield; (3) using local wood sources and products to reduce transportation impacts; (4) producing wood products used in long-term service; (5) building for deconstruction with reuse and recycling potential of all wood elements; (6) replacing fossil-based, energy-intensive materials with wood products in low-, mid-, and high-rise buildings; and (7) using wood residues for energy generation during wood product manufacturing which displaces fossil carbon emissions.
This study demonstrated embodied carbon (global warming potential) reductions when replacing concrete and steel with MT in all three levels of building, 8, 12, and 18 stories, in all three U.S. regions studied. Reductions of 22% to 50% in carbon emissions were achieved compared to the functionally equivalent concrete buildings based on cradleto-construction gate assessment. Regional differences in the embodied carbon of buildings were due to the regional building code requirements for MT building designs, MT feedstock production differences, and regional electricity grid differences. Mass timber products, if sourced from local forest resources and produced locally, can keep the whole-building embodied carbon impacts lower and avoid unintended consequences as a result of long transportations.
This study clearly showed the potential of carbon emission reductions that could be achieved in MT construction compared to the construction of traditional concrete mid-to high-rise buildings. However, it also indicated the need for updates and improvements in research and testing so that building codes and materials use can refect actual risk, as we showed with the impact of gypsum wall board on the 18-story buildings.
A plethora of data exist on the favorable environmental performance of wood as a building material and its role in carbon mitigation. The opportunities for improvement in the use of wood as a building material are endless, including improving material and building designs, innovative products, building codes that allow the use of MT for high-rise buildings and displace fossil-intensive alternatives, and better communication and education on how to improve the effciency of wood use and avoid unintended consequences.