Integrating multi-objective optimisation and life-cycle assessment in early-stage building design

Abstract

The built environment is responsible for roughly 40% of global carbon emissions, compelling the need for targeted strategies that address impacts across the building life cycle. Early-stage building design still lacks integrated optimisation frameworks that quantify how material choices and envelope configurations influence life-cycle emissions and thermal comfort. This gap is critical in tropical climates, where the interaction between thermal mass, envelope performance, and operational loads is complex. Each stage of a building’s life cycle generates different carbon profiles. Designers often rely on single-metric assessments, leading to design decisions that appear sustainable but perform poorly across the full life cycle. This study develops a combined multi-objective optimisation (MOO) and life-cycle assessment (LCA) method to evaluate and balance these competing objectives across alternative design scenarios. A DesignBuilder (V6) MOO algorithm was applied to an existing tropical case building constructed with sandcrete blocks. Two optimised substitutes were developed: one combining burnt brick external walls with a clay slate roof, and another with burnt brick external walls with a green roof. Preliminary simulations show that, although material substitution reduced embodied carbon, it increased operational carbon and worsened thermal discomfort. The optimisation processes generated 626 iterations and yielded 57 non-dominated solutions, with Pareto front analyses revealing a complex interplay between design variables and performance outcomes. Subsequent LCA of the buildings using OneClick LCA revealed that horizontal structural elements accounted for up to 89% of embodied carbon in the A1-A3 (materials) stages. While embodied carbon represented 19% of total life-cycle emissions in the case building, operational carbon dominated overall impacts, reaching 87% in the second optimised building. The study concludes that Pareto-based multi-objective optimisation presents a robust framework for navigating conflicting design goals. However, it must be integrated with life-cycle evaluation to identify design interventions that can deliver the greatest environmental and thermal performance gains.