The residential sector is a major contributor to Japan's emissions and plays a pivotal role in achieving net-zero targets. As operational emissions decline with power-sector decarbonization, embodied emissions from building materials are becoming an increasingly critical component of mitigation efforts. However, the predominance of short-lived buildings and the evolving housing demands associated with an aging population present significant, and yet insufficiently examined, challenges for building-sector decarbonization. This study assesses the mitigation potential of low energy and material demand (LEMD) strategies in Japan's residential sector, explicitly incorporating demographic aging within the MESSAGEix-Buildings framework. Results show that population aging may increase building material demand through higher per-capita floor area consumption, a factor often overlooked in decarbonization modeling. The moderate sufficiency measure, reducing per capita floor space among elderly households through a shift toward compact multi-family housing, achieves a 10% reduction in embodied carbon emissions relative to the Reference scenario in 2050. When combined with other LEMD measures, total avoided embodied emissions reach 25–40%. Although extending building lifetimes may increase operational energy use by retaining less efficient structures, the net mitigation effect remains positive, as the reduction in embodied emissions outweighs the additional operational emissions. Overall, deep decarbonization of Japan's residential sector requires aligning LEMD strategies with aging-sensitive demand trajectories and accelerated electrification powered by cleaner electricity.