[en] Natural convection is numerically and experimentally studied in vertical side-by-side porous and fluid layers. Two typical configurations are presented: at first, the ''Three-Layers'' system where a central fibrous layer is placed between two fluid spaces, and the ''Double-Layer'', with just one fluid partition. In each configuration, the case of a non confined fibrous material, i.e. with permeable boundaries is considered: for the ''Three-Layers'' one, a fluid flow through the porous layer is observed, while on the contrary no penetration (or a low one) occurs for the ''Double-Layer''. A modelization for the natural convection in the ''Three-Layers'' configuration, using an original treatment for the fluid-porous interface problem is presented