tin foil
tin foil
Tin foil will shield for everything that is an "electric field" and for much low energy particle radiation, alpha and beta, the charged helium nucleus, stray electrons, low energy solar radiation, and cell towers. For very high energy EM above the high ultraviolet, xrays and gamma are very penetrating, you could be talking about lead plus concrete, if you have a source you need to shield from. Xrays, gamma, and neutron radiation are ionizing, very different animal from non ionizing EM.
Magnetic fields are penetrating, permeability. Similarly, first you need a source, high current carrying equipment, before you have a shielding problem. Field strength drops rapidly with distance, unless you are in the magnetic field path, then intensity does not drop with distance. Tin foil will have some induced eddy current shielding for stray magnetic field. It provides a path other than through the air and through the customer, the eddy current makes a counter emf. Iron is preferred by magnetic fields for path, magnetic field density is ~ 1000 x air in iron. Magnetic fields will penetrate but they will be countered in conductive materials by induced counter emf and in the presence of iron will take the path of least impedance, through the iron. Magnetic flux is trying to close the loop from north to south pole.
In a residence, I would expect no source of strong magnetic fields like from high current power distribution. Similarly very high energy cosmic radiation, ionizing radiation, nuclear radiation do happen but are low probability of occurance. An EMF survey is a likely start but could be likely inconclusive, you could find nothing to shield from, does not mean shielding is ineffective.
Liability should be a piece of cake on this one. Offer mitigating recommendations and exclude health performance guarantees, exclude "full satisfaction" guarantees, exclude "personal satisfaction" guarantees. Offer simply to warranty the work as customary in the industry and have the customer preapprove the work plan. Foil faced insulation is standard commercial grade material, nonstock but easily available, and inexpensive as an adder. I have the foil faced foam board in my ceilings and it's very difficult to tune in FM and TV signals from inside the house.. The antenna is inside the attic above the foil and gets everything.
No shielding will cover everything, the sources will be inside the house, appliances and devices. These will have to be eliminated or mitigated also.
Ask the power company for a single dedicated transformer for the service if available. As already noted if the service transformer and the water line are shared, there will be multiple neutral to GEC connections and shared neutral current on the water line and ground paths. If that is the case install an isolating dielectric fitting on the incoming water line and bond on the customer side. Consider not connecting to the utility water line as a GEC, if it is acting as, arranged as, a shared neutral conductor. Use a rebar in the footing GEC and ground rods. RGS for the service and EMT for anything over 25 amps is a good bet. Most important thing is to find and eliminate any unbalanced current flows. All circuits must balance exactly in current to result in zero net external magnetic field.
