Whole life embodied emissions and net-zero emissions potential for a mid-rise office building constructed with mass timber

Abstract

Increasing global interest in sustainable buildings has generated significant investment in the mass timber industry in recent years. Timber products including Cross-Laminated Timber (CLT) and glue-laminated timber beams (glulam) are a structurally sound, aesthetically pleasing, and potentially carbon reducing alternative to traditional steel or concrete framing. This study uses Life Cycle Analysis methodology to determine the total embodied carbon emissions for a four-story (10033 m2) office building using two design scenarios: a traditional design using structural steel framing and a mass timber alternative utilizing CLT and glulam. The work is founded on detailed material lists for the two building designs that were generated by an architectural contractor with both designs subject to strict evaluation regarding building and construction codes. In addition to a comparative embodied carbon emissions analysis, this study utilized building energy consumption models and existing utility data to estimate operational emissions throughout the lifetime of the building. Energy production with rooftop solar arrays was estimated to evaluate grid emissions avoidance and determine the potential to offset whole life embodied carbon emissions when evaluated over a 30-year time horizon with temporal grid emissions accounting. Three end-of-life scenarios were evaluated for the mass timber products to determine the fate of biogenic carbon stored in the wood biomass. For the three scenarios, different mass fractions were sent to the landfill, directly re-used, or processed into particleboard. Results for the cradle-to-grave embodied carbon emissions analysis show the steel design embodies 3999 t CO2-eq. (398.5 kg CO2-eq m−2) whereas the mass timber design offers an 80–99% reduction in embodied carbon totaling 50–795 t CO2-eq. (5–79 kg CO2-eq m−2) dependent on the end-of-life treatment of mass timber products. Of all the evaluated cradle-to-grave scenarios, the smallest required solar array to fully offset embodied carbon emissions in 30 years was 665 kW with the mass timber building and the end-of-life scenario where 75% of CLT and 95% of glulam were directly re-used with the remaining fractions manufactured into particleboard. Discussion focuses on the challenges and uncertainty in using solar arrays to offset emissions from building construction, operation, and end-of-life treatment, requirements for achieving true net-zero carbon buildings, and methodological limitations.