We measured gas and water permeability in sets of nano-composite silica gels, (silica gel network with aerosil particles), which are highly porous but low permeability materials. Experimental results showed that gas and liquid permeability increased with aerosil content, allowing control of transport properties. Our results also showed that gas permeability was more than one order of magnitude higher than water permeability. This difference was analysed in view of the gas slippage effect. To this end, we tested and compared different correction factor models (Knudsen and Klinkenberg correction factors) proposed in the literature. We showed that the Klinkenberg factor increases with porosity, and discuss the effect of tortuosity. Models that account for porosity and tortuosity improve the accuracy of the prediction. The Knudsen correction increased the accuracy of the experimental data and made it possible to extrapolate the intrinsic permeability from the gas permeability with quite reasonable accuracy.