The demand for buildings with high quality indoor environments is growing, especially in developing countries, where more and more energy will be consumed in the near future. Air flow pattern, air temperature and humidity are among the main parameters that contribute to indoor thermal comfort. Care must be taken to design the most energy-efficient air distribution system that provides comfort for the occupants. To achieve this it is very helpful to know the air flow patterns and the temperature and humidity field in a building at the design stage. This paper presents results derived from the numerical modelling of air flow and heat and mass transfer within an apartment building in a densely populated urban neighbourhood. Typically, such apartment blocks are in close proximity to each other and cross contamination between apartments in the same and adjacent buildings can occur. As the Reynolds number is large, the air flow is fully turbulent. After validation of the numerical procedure and the turbulence model against benchmark cases, the k- turbulence model was employed to evaluate the air flow pattern, temperature, and contaminant concentration in the ground floor and top floor apartments. Based on the numerical results, suggestions are given for the design of hybrid ventilation systems for use in high-rise buildings in densely populated residential regions. For the configuration of shared ventilation shafts, considered in this paper, it is concluded that a built-in fan should be used to control the direction of the air flow and to avoid cross contamination. It is also concluded that this, fan assisted, ventilation system approach provides thermal comfort in a temperature band somewhere above 20C and below 30C. This approach it is much cheaper to operate than full air-conditioning.