Quantifying construction-stage (A5) energy consumption in mass timber buildings: A dual-track assessment framework

The building sector accounts for approximately 37% of global energy-related carbon emissions, and as operational efficiency improves, the share attributable to embodied carbon continues to grow. Mass timber has emerged as a promising structural alternative because of its lower embodied carbon, biogenic carbon storage, and potential to accelerate construction schedules. However, construction-stage energy use (A5) is often oversimplified in building life-cycle assessment, with previous studies frequently approximating it as a fixed percentage ( ∼ 3%) of total embodied energy rather than explicitly inventorying on-site activities. To address this gap, this study develops a dual-track framework for quantifying A5 energy use, combining a theoretical lower bound with a schedule-based upper bound and Monte Carlo uncertainty analysis. Crucially, this study evaluates A5 as a standalone module and does not assess its relative contribution within the full life-cycle profile of the buildings. The framework is applied to two completed large-scale mass timber projects in the U.S. Midwest (ASHRAE Climate Zone 5A) and extended to functionally equivalent reinforced concrete (RC) and structural steel options. Results show that in cold-climate projects, temporary heating dominates A5 energy use (52%–60%), while mechanical fastener installation contributes less than 0.2% regardless of climate. Compared with functionally equivalent alternatives in the same climate zone, mass timber achieves approximately 30% lower A5 energy use than RC and approximately 13% lower than structural steel. A complementary climate-sensitivity analysis extends the framework across ASHRAE Climate Zones 1A through 5A and shows that the mass timber advantage holds across this range, although the absolute A5 totals decline substantially as temporary-heating demand diminishes. In addition, a 44% learning-curve productivity improvement in cross-laminated timber (CLT) installation suggests further potential for reducing construction-stage energy consumption. External validation using Bakers Place, another completed mass timber project, supports the transferability of the methodology, with observed values for all three structural systems falling within their respective 90% credible intervals. These findings indicate that A5 energy use in cold-climate mass timber projects has been substantially under-quantified in previous studies and that the proposed framework provides a systematic method for evaluating construction-stage energy impacts.