Unsteady natural convection phenomena in a rectangular room heated from below is studied numerically for a Rayleigh number of 1.5×10⁵ and a Prandtl number of 0.71. Two-dimensionl and Boussinesq approximations are applied to the equations of motion describing buoyant circulation of air in the room with or without a window. The vorticity and stream functions are used for numerical computations. During the simulation the flow behaviors such as appearance of multicellular flows, formation and destruction of symmetric and asymmetric streamlines, vorticity changes, and temperature profiles are observed. The finite-difference computational methods used in the numerical experiments are stable and rapidly converging iteration schemes, and can be utilized for further studies in this field.