The design and realisation of natural ventilation systems is an important research topic into the ability of buildings to respond to climatic conditions, using parts of the buildings themselves as indoor microclimate control systems. This research aims to evaluate how the stairwell can be an essential element of natural ventilation in low-rise buildings. In this study, the main innovation is the different architectural and functional conception of traditional building components such as the stairwell. The stairwell is used as a chimney in order to increase the air exchange rate in cold and hot seasons. While previous works of the author were focused on the ventilation in winter, when the heating of dwellings enhances the stack effect in the stairwell, in this paper the summer behaviour of similar systems has been be investigated. The hypothesised driving forces causing air movement are the stack effect and the solar chimney effect. In this paper the application of the described systems is analysed on common building types, such as blocks of "in-line" housing. These consist of three to five storeys with a single stairwell and two apartments on each floor. The natural ventilation system studied is characterized by easy implementation in energy retrofitting of buildings as well as inexpensive installation and management. In order to increase the solar chimney behaviour, two different technical envelope solutions have been evaluated, namely painting external surfaces black and external insulation. The effects produced by the implementation of different faade systems (equipped with PV systems, transparent insulating materials, air collectors and advanced envelope systems) will be compared in future works. Finite elements and Computational Fluid Dynamics codes were used in order to design and verify the intervention efficiency and the behaviour of the system as a whole . The first results of CFD simulations here presented highlight that the fluid patterns can play a fundamental role. The complex geometry and the dynamic thermal boundary conditions of a typical stairwell substantially modify the hypothesised solar chimney behaviour of the system at night.