Convection inside the insulation materials representing porous media may be responsible for important increase of the heat flow density through the building element, when the climatic conditions are very cold. A numerical study, based on CFD simulations, is performed on a geometry very close to that of a real attic. The most significant heat transfer phenomena - conduction in solid elements, convection in porous medium, and convection in air spaces are taken into account. This is done exclusively by using the mass, momentum and energy conservation equations. In parallel, a testing campaign was conducted in the Large-Scale Climate Simulator testing facility at Oak Ridge National Laboratory. The results are in agreement with the CFD simulations. The results are expressed in terms of critical Ra* numbers and Nu* variations against Ra*. The critical Ra* is about 27, according to calculations and in the range 26 to 28, according to testing.