Transparent building envelope components have been considered traditionally as the highest source of energy loss in buildings, in terms of heat transfer through the envelope and ventilation. Double Skin Facades (DSFs) are one of the most effective methods of managing outdoor-indoor heat exchange and ventilation within a glazed component, along with providing occupant comfort, high architectural quality and energy flexibility. In the context of an EU project, a novel concept of thin cavity double skin façade was conceived to improve the performance and building integration of DSFs, by integrating an Air to PCM Latent Heat Exchanger (a-PCM LHEX) in series with DSF cavity. The aim of this work to evaluate and optimise by design the overall building energy performance of this novel fenestration component. This was achieved by evaluating the energy performance of different combinations of design variables (i.e. coatings adopted on the different glazing layers, PCM melting temperature etc.) by means of building performance simulation of a South oriented reference room in a temperate climate, and of comparison with benchmark glazing and DSF technologies. To this aim a co-simulation framework was conceived between EnergyPlus and a purposely built lumped parameter model of the a-PCM LHEX. The use of a-PCM LHEX shows consistent improvement in DSF performance, reducing heating and cooling needs and potentially enhancing thermal comfort. Glazing configurations with lower g-value and U-value, in combination of a-PCM LHEX provide the minimum energy uses.