Comparative Life Cycle Assessment of Vertical Greenery Systems in High-Density Residential Retrofits: Carbon, Thermal, and Water Trade-Offs Under Climate Variability

Vertical greenery systems are increasingly adopted in high-density residential retrofits, yet existing carbon performance assessments often overlook the embodied carbon of system components, the methodological ambiguity of carbon sequestration boundaries, and the water consumption required for maintenance. This study develops a cradle-to-grave comparative life cycle assessment model to quantify the carbon–thermal–water trade-offs between modular living walls and green facades, applied to two climate regions, hot summer and warm winter and cold, with water-saving irrigation as a moderating variable, testing three research hypotheses. The results show that the embodied carbon at the material stage of the modular living wall reaches 144.42 kgCO2eq/m2, 10.5 times that of the green facade at 13.74 kgCO2eq/m2, dominating the carbon payback gap between the two systems. The modular living wall achieves a carbon payback period of 22.7 years in the hot summer and warm winter region and 54.7 years in the cold region, both exceeding the 20-year reference service life, with exceedance probabilities of 58% and 97%, respectively; the green facade achieves 3.0 and 6.6 years, with 0% exceedance probability in both regions. The carbon sequestration accounting boundary determines whether the modular living wall achieves a net carbon benefit but does not alter the ranking between the two systems. Water-saving irrigation reduces water consumption by 45% and 35% while extending the carbon payback period by only 2.6% and 3.3%, with thermal benefit retention above 93%. The green facade is identified as the robust choice across all scenarios, while the modular living wall requires carbon payback failure risk to be explicitly accepted or mitigated through substrate replacement cycle extension. A staged decision framework and climate-differentiated policy recommendations are proposed.