My research into EMP and Faraday cages indicates that if a Faraday cage can be totally isolated, then not grounding it is fine. However, if there is any type of conductivity between the cage and any earth connected ground, then the Faraday Cage MUST be PROPERLY grounded. That includes a chance of arcing from a conductive item to the Faraday cage.
A room with a concrete floor. A room with any penetrations in it. A metal box sitting on a concrete floor, or even on a rubber pad (since there can be some bleed over the pad if there is any dampness). A metal cabinet or safe placed close to an electrical outlet where a high intensity pulse could arc from the outlet to the metal. All of these situations call for a very good grounding system. Far beyond a normal house grounding. The entire outer surface must be conductive and every part of it must be electrically bonded to every adjoining part. And then a high capacity ground system is connected to the resultant Faraday cage.
A Faraday cage can have penetrations. Even conductive wiring running from outside to inside. But each and every one of those penetrations has to be both shielded and have protective devices connected to them. That includes plumbing pipes, electrical supply lines, communications lines, antenna lines, and HVAC vents. It is not that difficult, but it is something that can be very tedious to do, as well as expensive if there are a lot of penetrations or the space is very big and requires a lot of ventilation and a bathroom.
One other thing. A Faraday cage CAN be made out of metallic mesh (20opi or finer), preferably copper. It does not have to have solid walls. But the coverage does have to be complete, with no large holes, or parts not completely bonded to the rest of the cage, including the entire perimeter of any openings such as doors or hatches. The current has to be able to flow smoothly over the surface of the entire cage.
There has been some discussion of using an all metal building such as a garage or pole building as a huge Faraday cage. This can be done as well. But it must be remembered that the FLOOR has to be part of the cage. It cannot just be a concrete floor, even if it is reinforced. The floor needs to have the same type of protection as the rest of the Cage, and it has to be bonded to the walls along every edge. And pipes and electrical lines coming up through the floor have to be protected, just like any other Faraday cage, as a strong EMP can penetrate the ground far enough to induce currents in underground wiring and piping. Not super deep, but deep enough to induce currents in many residential and light commercial and industrial installations.
As to the whole house Faraday cage, this can be done, too, using Denny 245 double sided aluminum foil on kraft paper that is wrapped around the entire structure, with every seam bonded, and every edge bonded to the next. Then the window and door are made of or encased in metal, and copper mesh used to cover the windows and doors, which is bonded to the metal frames. Remember, this has to go on all six sides: Floor, roof, and all four walls, without any holes or gaps. Not easy, but doable.
As is protecting solar PV panels. The same 20opi copper mesh can be used to protect PV panels, using two layers, and enclosing all six sides. You need to increase the size of the array to compensate for the loss of sunlight through the mesh, but it does not take all that much extra.
Anyone that wishes to see my documentation to support these statements can contact me and I will be glad to show it to them, and explain it to the best of my ability. I can be contacted through e-mail at jerrydyoung@outlook.com and we can set up a time and place to meet and go over the various reports, pamphlets, and books.
Just my opinion.