This work investigates the impact of space planning, interior porosity and variable occupancy on the energy use in offices that is attributable to ventilation/infiltration and air movement. TAS, Lightscape, and Excel software packages were used to simulate and analyse airflow and thermal loads in different office layouts. These layouts were created by varying the internal configurations of a base case shell. Constructions of the roof, floor, external walling, windows and vents of the base case shell were based on good practice recommendations, as were the specifications for ventilation, thermal and lighting conditions in the simulations. The results show significant variations of: natural ventilation/infiltration with plan regime; internal air movement with internal porosity; and consequently thermal loads with layouts and occupancy levels. The choices of layouts for given occupancy patterns and the design and control of interior apertures should be carefully made since their impact on ventilation in offices can significantly influence a building’s energy consumption.