This work examines the permeability behavior of sandstone when exposed to hydrogen and nitrogen under both dry and irreducible water saturation conditions, with the aim of deepening the understanding of gas transport processes relevant to underground hydrogen storage (UHS). Controlled laboratory experiments were conducted using two steady-state permeameters under constant temperature. The results show that once backpressure is applied, the measured permeabilities to hydrogen and nitrogen under dry conditions become nearly identical, as the slippage effect is effectively minimized. In the presence of irreducible water, both gases exhibit a marked reduction in effective permeability, underscoring the influence of residual water and its distribution at the pore scale.