Multi-objective optimization of building envelopes for energy efficiency and carbon emission: evaluating PCM configurations across four climate zones in Iran

• NSGA-II optimizes the PCM application to enhance building efficiency. • Optimizing PCM melting temperature, placement and thickness is considered. • Energy load reductions of cities are 41.8% for Yazd and 41.8% for Bandar Abbas. • CO 2 emissions decrease the most in Yazd and the least in Bandar Abbas. • Thermal discomfort hours in Bandar Abbas decreases, while increases in Tehran. Phase change material (PCM)-based technologies present a promising strategy for enhancing the efficiency of buildings. Their effectiveness is primarily governed by the thermo-physical properties of the materials and the climatic conditions specific to the building’s location. This study presents a multi-objective optimization framework for building design, incorporating PCMs into the envelope based on the distinct climate zones of Iran. Four scenarios were developed based on melting temperature (MT), thickness, and PCM placement and the interaction of different MTs to minimize energy consumption and CO 2 emissions, while considering thermal discomfort hours (TDH). The NSGA-II algorithm was used to determine the optimal trade-off between these objectives. The results showed that climate significantly influence the type of PCM, with the optimal solution consisting of three layers placed internally. Consequently, the highest reductions in total annual energy loads were observed in Bandar Abbas (9.6%), Yazd (41.8%), Tehran (40.15%), and Tabriz (30.53%). Regarding CO 2 emissions from the LCA, Yazd exhibited the highest emission reduction at 41.6%, while Bandar Abbas recorded the lowest at 9.7%. However, TDH in Bandar Abbas increased by 4.5%, whereas in Tehran, it decreased by 8.6%.