Reliable values of apparent thermal conductivity have been obtained under steady-state conditions and at low temperatures (between liquid nitrogen and room temperatures) together with air flow permeability (defined according to the Darcy law) of glass fiber materials. Both sets of data were used simultaneously to calculate actual conditions of heat transfer in porous media from recent theoretical analysis. It is shown by computation of filtration Rayleigh Number from these results that it is possible to design insulating structures to minimize convective heat transfer.