Environmental impact assessment of mass timber, structural steel, and reinforced concrete buildings based on the 2021 international building code provisions

Building construction and operation accounts for over a third of global anthropogenic greenhouse gas (GHG) emissions and energy consumption. To determine whether more prevalent use of mass timber (MT) construction could significantly reduce environmental impacts of construction, we performed whole building life cycle assessment (WBLCA), using Athena Impact Estimator for Buildings (IE4B) software, on a series of nine multistory residential buildings of three varying materials and heights. Three MT buildings of 18-, 12-, and 8 -story heights, respectively represent IV -A, -B, and -C occupancies of the 2021 and 2024 editions of the International Building Code (IBC), which facilitate regulatory approval of tall wood buildings. When benchmarked with six functionally equivalent structural steel (SS) and reinforced concrete (RC) buildings and accounting for cradleto-grave (A -C) system boundaries pertinent to embodied energy, MT buildings lowered global warming potential (GWP) by an estimated range of 39-51% and 28-34% compared to functionally equivalent RC and SSbuildings, respectively. Extending system boundaries to consider module D to account for biogenic carbon of mass timber and recyclability of steel and concrete, the GWP reduction of MT buildings further reduced to ranges of 81-94% and 76-91%, respective to RC and SS buildings. Reducing the reference study period (RSP) to 60 years generally lessened GWP 11-21% relative to the standard RSP of 75 years. Finally, greater exposure of MT components permitted in the 2024 IBC further reduced GWP up to 1.05% by using less structural fire encapsulation material (e.g., gypsum wallboard) than previously prescribed.