In order to control moisture in old buildings, a new technology consists in incorporating a ventilation system into the air space located between the thermal insulation and the wet wall. This technology is considered as one of the effective solutions to achieve the comfort of the occupants, the durability of the insulation. A mathematical approach based on the darcien model was developed, in order to analyse the influence of effective coupled parameters which are heat, moisture, and air on the evaporation performance of a porous layer. A hybrid coupling combining both the Boltzmann method and the finite volume method is proposed. The results show that blowing air in winter conditions affect markedly the heat and mass exchanges at the interface between the porous layer and the channel. However, blowing air in summer conditions provide drying the wall and decrease the risk of development of mould.