A numerical study of the thermoconvective instabilities at high time in a habitat filled with Newtonian fluid is conducted. The gable roof of the habitat is subjected to a heat flux of constant density, and its side walls and floor are, respectively, adiabatic and isothermal. Based on the Boussinesq assumptions, the summer thermal and dynamic conditions are numerically studied using unsteady natural convection equations formulated with voracity and stream-function variables. The finite volume method is used to generate the set of equations, which are solved by the iterative under-relaxation line-by-line method of Gauss-Seidel. Sudden changes in the average Nusselt number and in the extreme values of the stream functions at Ra = 1 x 10(8) show that the initially unicellular flow in a pseudo-conductive regime becomes a multicellular flow with the emergence and disappearance of cells. In time and space, the change of the flow behaviour is observed more rapidly if the active walls are close to the cold horizontal wall. (C) 2016 Elsevier B.V. All rights reserved.