Is Mass Timber the Future of Sustainable Construction?

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.May 14 2024

A recent study published in Buildings quantified and compared the embodied carbon (EC) of a mass timber structure (Adohi Hall, a residence building at the University of Arkansas, Fayetteville) with a steel equivalent. All materials in the building structure were assessed for their global warming potential (GWP) impact through the life cycle assessment (LCA) method.

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Background

The construction industry is responsible for almost 40 % of total carbon emissions, contributing significantly to global greenhouse gas (GHG) emissions. In 2021, roughly 10 % of global CO₂ emissions were associated with steel manufacturing. Additionally, cement, the main element in concrete, accounts for about 8 % of the world’s GHG emissions.

Traditional efforts to reduce carbon emissions from the building sector focus on optimizing energy consumption by improving insulation, using energy-efficient lighting and HVAC (heating, ventilation, and air conditioning) systems, and optimizing building designs. These measures have effectively reduced operational carbon (OC). However, this has scaled up EC-related GHG emissions during building manufacturing, transportation, construction, and demolition.

Thus, recent research has concentrated on using low-carbon footprint materials in construction to mitigate EC emissions in buildings. In this context, this study explored the EC reduction potentials of a mass timber structure compared to an equivalent steel structure, focusing on the institutional use of these building materials in the United States.

Methods

The Tally^® building LCA tool was used for the comparative analysis in this study. A cradle-to-construction site system boundary was applied in the LCA comprising modules A1 to A4 according to standard EN 15804 definitions. Modules A1-A3 cover raw material supply, their transport to factories, and manufacturing processes, respectively. Meanwhile, A4 assesses the environmental impacts of materials and equipment transported across the construction site. The GWP and carbon emission were considered as the variables in this study.

Adohi Hall's mass timber was replaced with a steel structure comprising a corrugated steel deck and concrete topping. To fully comprehend the GWP impact, the inventory of equipment and materials quantifications were also included in the analysis. These material quantities were extracted from the Revit model obtained from the Hall’s design team and contractor. The overall substitution benefit from this construction case was studied through the displacement factor (DF) quantification following the standard process.

The research team omitted the construction phase from the current analysis due to the absence of empirical on-site consumption data about the hypothetical steel equivalent building. Additionally, other environmental factors were excluded in the GWP assessment.

Study Results

The mass timber building weighed approximately 35 % less than its steel structure equivalent, highlighting the significant material resource efficiency of mass timber building designs. Additionally, it exhibited superior environmental attributes considering the carbon dioxide equivalent (CO₂ eq). Emissions per square meter of gross floor area for mass timber was 198 kg, in stark contrast to the 243 kg CO₂ eq recorded for steel structures.

In the product stage (modules A1-A3) LCA, GHG emissions of the Adohi Hall were approximately 2853 tons of CO₂ eq. Alternatively, the steel equivalent generated much higher GHG emissions of around 4478 tons of CO₂ eq. Thus, the mass timber structure demonstrated about 36 % lower GHG emissions than the steel model during the product stage.

This substantial reduction in carbon emissions can be attributed to the intrinsic sustainability of mass timber as a building material. Alternatively, steel production is an energy-intensive process involving mining and smelting, contributing to higher fossil-based carbon emissions. Additionally, concrete production is responsible for about 70 % of each structure's total GWP impact.

Module A4 analysis revealed that the transportation phase of Adohi Hall generated a higher carbon footprint because of the overseas material sourcing, amounting to 825 tons of CO₂ eq emissions. This is due to the fewer mass timber manufacturers in the US, consequently amplifying the environmental impact of mass timber constructions. Nevertheless, there was still a significant reduction in the carbon footprint by replacing steel structures and concrete floors with mass timber in the building, as evidenced by the obtained 0.28 DF. 

Conclusion

Overall, the mass timber building achieved a 19 % reduction in carbon emissions compared to the functional equivalent steel structure within the building modules A1 to A4 studied. Regarding carbon storage, about 2757 tons of CO₂ eq were stored in the mass timber building, presenting further benefits of carbon emission delays for the structure's life span. Thus, Adohi Hall has achieved a remarkable weight reduction and significantly minimized its environmental impact using a mass timber structure instead of steel. 

This comparative study provides insights into making more environmentally efficient decisions in buildings and helps move forward to reduce GHG emissions and address GWP mitigation. Further, it underscores the significance of incorporating LCAs into architectural design decisions. Using this information, architects and engineers can make informed choices that prioritize sustainability and minimize the environmental impact of the construction sector. The researchers aim to include the operational stage (modules B6 and B7) of Adohi Hall in the future GWP analysis.

Journal Reference

Hemmati, M., Messadi, T., Gu, H., Seddelmeyer, J., & Hemmati, M. (2024). Comparison of Embodied Carbon Footprint of a Mass Timber Building Structure with a Steel Equivalent. Buildings, 14(5), 1276. https://doi.org/10.3390/buildings14051276, https://www.mdpi.com/2075-5309/14/5/1276

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