Electromagnetic shielding and EMP
This question relates to a special area of electrical engineering. Protection from intentional EMI and EMP was/is a big subject in defense industry, and tons of studies were conducted in 1960s -1980s on the subject, with some unclassified parts published and groups. There are published magazines as "Interference Technology", there are lots of IEEE publications,  and books were written like this one or this.

Propagation of EM waves across complex media may be not an intuitive thing. To find answers on effects of complex layered materials, the right approach is to use 3D modeling software tools, like the one offered by Keysight. It will be difficult to answer your particular question about field strength without solving your case with details of particular materials and exact geometry.