Independent Study, Spring 2019
• MASS TIMBER CONSTRUCTION •
The built environment is responsible for at least 31% of anthropogenic carbon emissions in the United states (EPA). Some of that carbon is emitted in energy production that goes towards interior climate control, and can be somewhat mitigated by intelligent building designs and increased insulation. However, a large portion (approximately 30%) of the emissions associated with a building are embodied in materials or incurred during construction (Fenner et al). This means the emissions that occurred during onsite construction and the production of glazing, steel, and concrete for a given building make up almost a third of the lifetime energy consumption of the building. Given this more complete accounting system, emissions-conscious building design must aim to find materials and construction processes with lower carbon emissions profiles.
There are many different accounting systems with different scopes. Some only account for the operating emissions over the lifetime of a building, some include both embodied emissions and operating emissions, or only embodied material and construction emissions (cradle to gate), while some very complete studies look at embodied emissions, operating emissions, and demolition emissions including disposal of materials. These complete audits are known as ‘cradle to grave’ lifecycle analyses or LCAs.
There are many sustainable building certification programs like LEED (Leadership in Energy and Environmental Design), Living Building Challenge, and Passivhaus, which aim to tackle environmental impacts of building through complex accounting procedures. Factors like lifetime energy efficiency, toxicity of materials, transport emissions, renewability of building materials, building site and even locality of sources are weighed and balanced in complex audits that determine whether a project receives a certain level of certification. LEED is the most commonly used and attainable system, offering certification, silver, gold, and platinum project levels. The LEED system offers credits for completing a cradle to grave lifecycle analysis of a project that shows improvement over a comparable building, and in 2013 LEED announced credits for mass timber designs because the embodied emissions are ostensibly low (LEED).
Mass timber is wood harvested from forests, processed into engineered wood products, and sequestered during the lifetime of the building. In global carbon cycle terms, this means that carbon moved from the atmosphere to trees where it was used to grow woody tissues. It was then harvested and moved through processing and construction into the structural framework of a new building, where it will likely stay for up to 100 years. Re-growth of the harvested forest will continue to capture carbon from the atmosphere, possibly resulting in a net sequestration of carbon. So if the harvesting, transportation, and processing emissions are less than the carbon captured in the wood tissues, then this product could result in a net reduction of building emissions in a cradle to gate audit of a building. However, even if it is not a subtraction of carbon emissions, it seems to be a significant reduction in embodied emissions over concrete and steel construction of comparable buildings (Lippke et al).
Mass timber construction is a relatively new frontier in building design, following from European technological advances in engineered wood products like cross-laminated timber or CLT. Constructed of many layers of wood glued together with adhesive, engineered wood is lightweight, strong, and flexible (Brandner et al). It has be used as the structural frame for buildings with up to 18 stories so far, replacing traditional concrete and steel construction. With a greater strength to weight ratio than steel, it also reduces the size of concrete building foundations, resulting in further emissions reductions.
There are some barriers to implementation of a new construction method like this. Building codes have been slow to react to the new material, so rules against wooden structures in cities with a traumatic fire histories are some hindrance in application. Mass timber is actually quite fire safe, developing a layer of char on the surface of beams that insulates the inner wood from flames and retains structural integrity in a fire. Controlled burn tests show that it can perform as well as steel does (Franji et al). There are some questions about the environmental harm associated with the adhesive used, but it still seems to be an improvement on cement and steel. North American supply chains are new still developing there are only 15 production facilities on the continent, resulting in large transport emissions from Europe or limited availability (see maps linked below). There have also been setbacks, like the adhesive failures during construction on the University of Oregon campus last year from use of production facilities without climate control (Post).
Carbon accounting systems for wooden products show that products kept in use over 15 years can result in sequestration, but also make some assumptions about soil carbon stability (Kilpelainen et al). Short-term use wood products like toothpicks and paper fail as carbon capture strategies. A disposable product requires so much processing, and has such a short lifetime, that it practically goes straight to the landfill. In the landfill it’s assumed that the wood anaerobically decomposes and forms methane, an even more potent greenhouse gas (Ingerson). The embodied emissions of wood products also depend on the management of the forests used for harvest and their emissions profile. Effective LCAs ought to include forestry practices in their scope (Kilpelainen et al).
It is important to consider the timeline of emissions as well: if there is a net sequestration for the next 100 years, or the life of the building, what about when the structure is torn down and landfilled or combusted? In an LCA, how are the disposal emissions accounted for? Based on neutrality assumptions that underly biomass electricity generation, some systems of accounting assign neutral emissions to combustion of wood at the end of the building’s lifetime (Takano et al). As discussed in the plants section of this independent study, those assumptions are dubious because of potential for harvest practices to result in a net reduction of soil carbon. While 100 years of use is likely long enough for plant life to recapture carbon dioxide sufficient to replace the harvested wood, avoiding the time-lag issue with biomass electricity generation, there is no guarantee that land-use decisions will have resulted in that recapture (Wiloso).
Replacing concrete and steel in construction is a reduction in embodied emissions, even if the net emissions of mass timber are not negative. However, it’s important to consider how LCA audits account for end of use disposal. Assumption that combustion emissions of wood after lifetime use are neutral is problematic, and forestry practices should be rigorously researched and included in the scope of the audit.
Mass Timber North American Production Maps: https://archpaper.com/2019/01/timber-industry-bloom-mapped/ (linked My 31 2019)
Brandner, R., Flatscher, G., Ringhofer, A., Schickhofer, G., Thiel, A. (2016) Cross laminated timber (CLT): overview and development. European Journal of Wood Products. 74: 331. https://doi.org/10.1007/s00107-015-0999-5
EPA. Inventory of U.S. greenhouse gas emissions and sinks: 1990–2016. EPA 430-P-18-001; 2018. https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions#electricity
Fenner, A. E., Kibert, C. J., Woo, J., Morque, S., Razkenari, M., Hakim, H., Lu, X. (2018) The carbon footprint of buildings: A review of methodologies and applications. Renewable and Sustainable Energy Reviews. 94: 1142-1152. https://doi.org/10.1016/j.rser.2018.07.012.
Franji A., Fontana M., Hugi E., Jöbstl R. (2009) Experimental analysis of cross-laminated timber panels in fire. Fire Safety Journal. 44:1078-1087. doi:10.1016/j.firesaf.2009.07.007.
Ingerson, Ann. (2011) Carbon storage potential of harvested wood: summary and policy implications. Mitigation and Adaptation Strategies for Global Change. 16: 307-323. DOI 10.1007/s11027-010-9267-5
Kilpelainen A., Strandman H., Kellomäki S., Seppälä J. (2014) Assessing the net atmospheric impacts of wood production and utilization. Mitigation and Adaptation Strategies for Global Change. 19: 955-968. DOI 10.1007/s11027-013-9454-2
LEED green building certification. Retrieved 5/31/2019 from https://new.usgbc.org/leed
Lippke B., Gustafson R., Venditti R., Steele P., Volk T., Oneil E., Johnson L., Puettmann M., Skog K. (2012) Comparing Life-Cycle Carbon and Energy Impacts for Biofuel, Wood Product, and Forest Management Alternatives. Forest Products Journal. 62(4):247–257.
Meneghelli, A. (2018) Whole-building embodied carbon of a North American LEED-certified library: Sensitivity analysis of the environmental impact of buildings materials. Building and Environment. 134:230-241. https://doi.org/10.1016/j.buildenv.2018.02.044.
Pajchrowski G, Noskowiak A, Lewandowska A, Strykowski W (2014) Wood as a building material in the light of environmental assessment of full life cycle of four buildings. Construction and Building Materials. 52:428–436
Post, N. (2018) ‘Regluing' Oregon State's Showcase for Mass Timber. Engineering News Record. September 5, 2018. https://www.enr.com/articles/45125-regluing-oregon-states-showcase-for-mass-timber
Rajagopalan N. & Kelly S. S. (2017) Evaluating Sustainability of Buildings Using Multi-Attribute Decision Tools. Forest Products Journal. 67(3/4):179–189.DOI:10.13073/FPJ-D-16-00028.
Takano, A., Hafner, A., Linkosalmi, L., Ott, S., Hughes, M. (2015) Life cycle assessment of wood construction according to the normative standards. European Journal of Wood and Wood Products. 73: 299. https://doi.org/10.1007/s00107-015-0890-4
Wiloso E.I., Heijungs R., Huppes G., Fang K., (2016) Effect of biogenic carbon inventory on the life cycle assessment of bioenergy: challenges to the neutrality assumption, Journal of Cleaner Production, 125:78-85. https://doi.org/10.1016/j.jclepro.2016.03.096.
LEED homepage: https://new.usgbc.org/leed
Living Building Homepage: https://living-future.org/lbc/
Passivhaus USA: https://www.phius.org/home-page
World’s Tallest Mass Timber Building (March 19, 2019) https://www.dezeen.com/2019/03/19/mjostarne-worlds-tallest-timber-tower-voll-arkitekter-norway/
Mass Timber North American Production Maps: https://archpaper.com/2019/01/timber-industry-bloom-mapped/