The Bubnov-Galerkin method was applied to the problem of onset of convection in a horizontally infinite liquid layer set between two blocks with different thermal conductivities and thicknesses. The critical Rayleigh number (Ra_c) and wavenumber (k_c) were determined for a range of conductivities and thicknesses. Computations, based on the finite element method, for the steady conductive regime (motionless liquid) suggest that lateral walls, with different thermal admittances appropriate to a finite vessel, have no appreciable effect on Ra_c and k_c when the aspect ratio is larger than two. Values calculated from the theoretical model are in reasonable agreement with experimental results.