This work deals with the evaluation of the performances of phase change materials (PCMs) coupledwith a ventilated ceiling system in order to reduce summer cooling loads and improve the internalcomfort in buildings, thus enhancing the positive effect of night ventilation.Object of the study is the numerical simulation of the thermal behaviour of a typical open space officelocated in three Italian cities during the summer. Indoor climate is only controlled by means of passiveventilation; a usual ventilation system configuration (i.e. the outdoor air is supplied directly inside theambient) is compared with another characterised by the PCM activation by means of a ceiling air-PCMexchanger. Both systems provide daily and night ventilation and are aimed to maintain, as much aspossible, thermal comfort conditions during the office working hours. The numerical simulations areperformed using an explicit dynamical model, with a finite difference discretization, developed on thesoftware Simulink. The PCM is modelled with the equivalent heat capacity method. The results showthe positive effect of PCMs, if activated by means of intensive night ventilation. This measure allows toappreciably reduce the peak cooling loads and to achieve a good indoor thermal quality.