The experimental evaluation of permeability parameters encounters different difficulties, depending on the features of the materials. Traditional steady-state techniques for refractory castables and other highly dense ceramics demand arduous work and require reliable measurements at high pressure and for extremely now flow rates. In this paper, a recent and promising technique known as vacuum-decay permeametry is modified to consider the effect of gas compressibility and fluid inertia. Darcy's law is replaced by Forchheimer's equation, which has been proved to better represent the interactions between fluid and a castable porous medium. The modified method is modeled and applied experimentally to refractory samples with different permeability levels. Results that have been compared to a well-established steady-state permeametry method confirm the validity of the model.