An alternative approach based on the use of reverberation time measurements to characterize the boundary conditions on Finite Element room acoustic simulations is proposed. The methodology relies on the estimation of the mean absorption of the room using Sabine/Eyring reverberation equations. From this information, diffuse absorption coefficients are calibrated and the specific acoustic impedance of the absorbing surfaces is calculated. The implications of assuming that the specific acoustic impedance is entirely resistive are also considered. An analysis of the accuracy of the proposed approach is presented for a reference case. The results indicate that the suggested methodology leads to a more accurate prediction of the modal behaviour of the enclosures compared to geometrical acoustic simulations. However, the assumption of a purely resistive specific acoustic impedance leads to shift in frequency which can be compensated for by synthesizing complex impedance phases that provides the same diffuse absorption coefficients.