Coupling Mechanisms Between Spatial Adaptability and Embodied Carbon Emissions in Residential Buildings: Identifying an Upper Fixed-Wall-Ratio Threshold Through Dynamic Life-Cycle Scenarios

The aim of this study is to identify, through layout-coverage testing, the upper fixed-wall ratio at which the reconfigurable layout set can still fully satisfy mainstream spatial demand, and to quantify, under consistent structural, temporal, and accounting conditions, how 90-year cumulative embodied carbon emissions change across δ scenarios and how crossing this threshold alters the residential update pathway. A design-sample database of 954 residential floor plans was used to develop a common “basic unit,” and a wall-based index, δ, was defined as the ratio of fixed-wall length to total interior-wall length. A multiple-response survey of 168 households was then used to establish a mainstream layout set; layouts were ranked by selection frequency, and the smallest ranked set whose cumulative share exceeded 50% of all selections was retained. Eleven δ scenarios were assessed over a potential 90-year service-life scenario that incorporated interior reconfiguration, I-wall renewal, and possible demolition–reconstruction. The results identify δ ≈ 0.72 as the upper fixed-wall-ratio threshold compatible with full coverage of the mainstream layout set. When δ ≤ 0.72, internal reconfiguration maintains mainstream-layout coverage, and cumulative embodied carbon ranges from 224.88 to 313.46 tCO₂e, equivalent to 1772.10–2470.13 kgCO₂e/m². When δ > 0.72, at least one mainstream layout becomes infeasible and the modeled update pathway shifts to demolition–reconstruction, increasing emissions to 402.62–503.95 tCO₂e, or 3172.73–3971.24 kgCO₂e/m². Dynamic trajectories for the representative δ = 0.72 and δ = 0.81 scenarios show that the former accumulates emissions gradually, with limited increases associated with I-wall renewal at Years 30 and 60, whereas the latter undergoes sharp jumps at the same nodes because insufficient layout coverage triggers demolition–reconstruction. These findings provide an operational basis for using the fixed-wall ratio as an early-stage carbon-screening variable while retaining the need for project-specific multi-performance verification.