You can set up a simulation with plane wave excitation in HFSS for different materials and evaluate the fields in different regions.

You can find the shielding effectiveness of materials by simulating it in a virtual waveguide environment. It is a rather easy process. Set up a waveguide based on your frequency cutoff and place the material of your choice inside the waveguide. Then assign the electromagnetic properties of your material. Give proper excitations and analysis set up. You will get the S-parameters as your output once your analysis is completed, from which you can find the shielding effectiveness of your material.

Waveguide model is one of the best choice in HFSS software. For the material model you should take care, because the HFSS software cannot meshing too complecated geometrical model., for example like one piece of fabric in yarn level. So you should use the HFSS integrated software to build your own model. You can check my published paper for more detailed information.