Over recent decades, phase-change materials (PCMs) have gained prominence as latent-heat thermal energy storage systems in building envelopes because of their high energy density. However, only PCMs that complete a full daily charge–discharge cycle can deliver meaningful energy and carbon-emission savings. This simulation study introduces a methodology that simultaneously optimizes PCM integration for storage efficiency, indoor thermal comfort, and energy savings. Two new indicators are proposed: overall storage efficiency (EC_n), which consolidates heating and cooling-efficiency ratios into a single value, and the performance factor (PF), which quantifies the PCM’s effectiveness in maintaining thermal comfort. Using EnergyPlus v8.9 coupled with DesignBuilder, a residential ASHRAE 90.1 mid-rise apartment was modeled in six warm-temperate (Cfb) European cities for the summer period from June 1 to August 31. Four paraffin PCMs (RT-22/25/28/31 HC, 20 mm thickness) were tested under natural and controlled ventilation strategies, with windows opening 50% when outdoor air was at least 2 °C cooler than indoors. Simulation outputs were validated against experimental cubicle data, yielding a mean absolute indoor temperature error ≤ 4.5%, well within the ±5% tolerance commonly accepted for building thermal simulations. The optimum configuration—RT-25 HC with temperature-controlled ventilation—achieved PF = 1.0 (100% comfort compliance) in all six cities and delivered summer cooling-energy savings of up to 3376 kWh in Paris, the highest among the locations studied. Carbon-emission reductions reached 2254 kg CO₂-e year⁻¹, and static payback periods remained below the assumed 50-year building life at a per kg PCM cost of USD 1. The EC_n–PF framework, therefore, provides a transparent basis for selecting cost-effective, energy-efficient, and low-carbon PCM solutions in warm-temperate buildings.

Building construction and operation consume one-third of the world's total energy, and thermal loads account for 34 percent of the total building energy consumption. Phase change materials (PCMs) emerged as a solution to reduce energy consumption in buildings due to their high energy storage density and the ease with which PCMs can be incorporated into buildings. This study presents a systematic literature review of 32 peer-reviewed journal papers based on relevancy to the topic, published between 2008 and 2024, to identify critical parameters that enhance the energy efficiency of buildings incorporating PCMs in their building envelopes. The findings indicate that PCMs' effectiveness heavily depends on their melting temperatures, layer thicknesses, climatic conditions, and proper ventilation at nighttime to solidify the PCM for the next cycle. A relatively thick PCM layer with a high melting temperature in moderate climate zones, coupled with appropriate night ventilation, offers the most efficient thermal energy savings. Future research should focus on the storage efficiency of PCMs, optimizing PCM use with ventilation, and conducting life cycle cost assessments. The study concludes that a single PCM cannot provide consistent energy efficiency globally, making efficiency analysis essential before integration into building structures.