Enclosures made of highly permeable and highly conductive materials are used as passive shields to reduce the static magnetic field and magnetic disturbances for fundamental physics experiments and biomedical diagnostics. The two most important specifications of a magnetic shield are the shielding factor and the residual static magnetic field. We present an alternative strategy to improve the prediction of shielding factors taking into account the influence of gaps and overlaps in the real complex structure, which reduce the shielding properties of the Magnetically Shielded Rooms (MSRs) in comparison to virgin shielding material. The airgap $\delta$ between the sheets turns out to be the most critical parameter with respect to the shielding factor. By using a reasonable value $\delta$, it is possible to theoretically estimate a nonlinear equivalent permeability of the shielding wall structure which leads to a realistic prediction of the frequency dependent shielding factor up to about 20 Hz. The residual static field distribution inside the shield is hard to predict because it depends on the magnetic history of the shielding material. A reproducible residual field stable in time can only be obtained by a demagnetization process. The presented simulation of the complete demagnetization process considering the complex structure of an MSR with material gaps, overlaps, feed-through openings and different arrangements of the degaussing coils is able to predict the residual field distribution inside MSRs for the environmental field around the chamber. The prediction of the limit obtainable by degaussing can explain the surprisingly low magnitude of the residual field observed and reproduced inside a three-layer MSR.