Phase change materials are increasingly used in low-energy buildings to moderate indoor thermal fluctuations, shift peak loads, and reduce heating and cooling demand through latent heat storage. This review provides a critically comparative synthesis of PCM-integrated buildings across walls, roofs, ceilings, floors, glazing, hybrid envelopes, cementitious composites, and emerging 3D-printed systems. It links material design, envelope placement, climatic suitability, thermal comfort, and building-scale energy performance within a structured comparative framework. It presents that PCM performance is governed by the interaction among melting temperature, latent heat, thermal conductivity, PCM thickness, layer position, solar exposure, and operating conditions. Climate-resolved and mass-specific comparisons derived from the meta-analysis in this review further show that the highest total energy savings do not always correspond to the most efficient PCM utilization. The review also examines optimization pathways, including model predictive control and simulation-coupled workflows, together with key barriers such as leakage, low conductivity, incomplete daily cycling, durability concerns, fire safety, toxicity, and cost. Future priorities include standardized comparative metrics, full-scale validation, cycle-aware performance assessment, climate-responsive optimization, and integration with hybrid low-carbon building systems.