Using Computational Fluid Dynamics (CFD), four different cooling systems used in contemporary office environments are modeled to compare energy consumption and thermal comfort levels. Incorporating convection and radiation technologies, full-scale models of an office room compare arrangements for (a) an all-air overhead system (mixing ventilation), (b) a combined air and hydronic radiant system (overhead system with a chilled ceiling), (c) an all-air raised floor system (displacement ventilation), and (d) a combined air and hydronic radiant system (displacement ventilation with a chilled ceiling). The computational domain for each model consists of one temperature varying wall (simulating the temperature of the exterior wall of the building during a 24-hour period) and adiabatic conditions for the remaining walls, floor, and ceiling (simulating interior walls of the room). Two sets of computations are conducted. The first set considers a glass window and plastic shade configuration for the exterior wall to compare the four cooling systems. The second set of computations consider a glass window, a phase change material layer and the plastic shade configuration for the exterior wall to examine the effect of the phase change material (PCM) layer on the cooling energy requirements. Both sets of simulations assumed an external wall that changed temperature as a function of time simulating the temperature changes on the exterior wall of the room during a 24 hour period. Results show superior thermal comfort levels as well as substantial energy savings can be accrued using the displacement ventilation and especially the displacement ventilation with a chilled ceiling over the conventional overhead mixing ventilation system. The results also show that the addition of a PCM layer to the exterior wall can significantly decrease the cooling energy requirements.