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Convective flow patterns result from the interaction of flow and transport processes, where fluid density is the crucial coupling variable. The formation of circular flow patterns, so called convective cells, appears in natural and artificial systems of various scales: in geology, meteorology, hydraulics, etc.! These flow patterns are of special interest for heat transfer, i.e. for insulation or cooling of technical devices of many kinds. We deal with 2D convection rolls, as they appear near the onset of convection, which is describing by the Rayleigh number (Ra) surpassing a critical threshold. The heat transfer, described by the Nusselt number (Nu) depends on the role pattern, i.e. number of rolls per unit length or the aspect ratio. The knowledge of the quantitative dependency of heat transfer on the convection cell size can be utilized in the design of insulating or cooling devices. Minor technical modifications on the horizontal walls can favour the emergence of a certain cell size, i.e. the cell pattern with lowest heat transfer for thermal insulation and the pattern with highest heat transfer for cooling purposes. A model set up by COMSOL Multiphysics using coupled flow and heat transport in porous media simulates the convective motions. The heat transfer through the system depends on the Rayleigh number, in which material properties are gathered, and the size of the convection cells. In the model the latter is shaped by the initial conditions. The resulting figure gives the heat transfer, described by the dimensionless Nusselt number in dependence of the Rayleigh number and the flow pattern