Selection of the optimum values for net zero energy buildings (NZEB) is a complex task, due to the existence of multiple objectives. In the current study, cold and semi-arid climate was considered for investigation, and DesignBuilder was used for simulating a residential NZEB in Shiraz, Iran, as a case study. First, optimum solutions for minimising the cooling and heating demands were obtained by obtaining a Pareto front including 21 points. Then, phase changing materials (PCMs) were integrated to the envelope to further optimise energy consumption. The obtained results illustrated that using PCMs on both sides of the envelope can significantly reduce the energy consumption. Findings of this study are beneficial for the future plans of Iran, particularly with regards to improving sustainability within the building sector.