Abstract The ability of mat slabs (foundations) to shield in-door air space from contaminants vapor or/and gaseous pollutants migration from the subsurface has been explored. An expert system that employs the Dusty Gas Model (DGM) has been developed to assess the steady state, isothermal, and isobaric gas transport through concrete mat foundations. The DGM combines different gas transport flux mechanisms, such as the molecular, Knudsen, non-equimolar and viscous fluxes. The gaseous system has been modeled as a ternary system composed of the pollutant gas mixed with natural air components of Oxygen (O 2 ) and Nitrogen (N 2 ). Results indicate that the contaminant vapor flux is strongly affected by the water content's cement ratio and the radius of pores. For water cement ratio less than 0.6, capillary pores are not connected and diffusion through the gel pores is dominant. For water cement ratio greater than 0.6, capillary pores are connected and occupy a major part of cement paste and control diffusion. The effect of increasing radius of pores on total fluxes has been found negligible when Knudsen diffusion becomes insignificant compared to molecular diffusion. Also, Fick's law of diffusion has been found inadequate to study gaseous flux for small pores in which the Knudsen diffusion is significant.