Spatial Performance Optimization of High-Altitude Residential Buildings Based on the Thermal Buffer Effect: A Case Study of New-Type Vernacular Housing in Lhasa

High-altitude cold regions suffer from severe diurnal temperature fluctuations and limited energy supply, posing persistent challenges for maintaining indoor thermal comfort. This study investigates how the spatial configuration and thermal buffer effect can be optimized to improve the energy and comfort performance of new-type vernacular housing in Lhasa, China. Based on field-measured data, two representative housing prototypes—a self-built U-shaped dwelling and a government-designed resettlement house—were modeled and validated using EnergyPlus through the Rhino/Grasshopper platform. Parametric simulations and multi-objective optimization employing the NSGA-II algorithm were conducted to optimize both annual heating load and heating-season comfort percentage. Results show that optimized configurations combining south-facing sunspaces, north-facing enclosed corridors, and attic buffer cavities can reduce heating load by up to 80% compared with the baseline model without buffer spaces, and increase comfort duration by more than 50% under identical envelope and climatic conditions. The findings quantitatively reveal how spatial hierarchy and boundary buffering synergistically enhance passive solar utilization and thermal stability. This research establishes an integrated form–space–boundary optimization framework for energy-efficient housing design in extreme climates and provides a transferable reference for sustainable building strategies in other high-altitude regions.