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I don't think that is how EMP shielding works. Also would your electronic need to be shielded before hand, so having materials to shield them later is irrelevant?
It is. Nuclear EMP is a (very powerful) radio burst; you shield against that by putting the item in a Faraday cage - an enclosure of conductive material (which, when exposed to EM fields, will generate transient currents that cancel out the field inside it; this is why phones don't work very well inside metal vehicles). Aluminium foil is conductive (aluminium is, in fact, frequently used for power transmission, as it's the most conductive material per mass short of superconductors), so it works, although because you can't exactly seal it into a solid enclosure without significant equipment, you need multiple layers at different angles and you ideally want the items to be relatively small.
(The use of metal foil to block EM radiation is the reason that literal "tinfoil hats" were invented; the physical principle's sound, although of course the schizophrenics' worry about people mind-controlling them with radio waves is nonsense, and Faraday cages need to completely or almost-completely surround the shielded item so a "hat" isn't really that effective.)
Correct, but a) some items, like the emergency radio, aren't useful outside a crisis, so you can just keep them wrapped up permanently; b) nuclear war usually comes out of an existing military conflict, which means that upon hearing about said conflict (WWII was not secret; the Cuban Missile Crisis was not secret; Able Archer 83 and the Soviet concern about it was AFAIK not secret) you can wrap things up then against the possibility of EMP attack.
I get that encasing something in aluminum with no gaps would shield it. Aluminum is the fourth most conducive metal and a good shielding design made out of aluminum would work.
But merely wrapping something in aluminum foil would leave small gaps that I think would defeat the shield. I get that you'd overlap the aluminum layers and not have large gaping holes, but I don't think that's good shielding. I sometimes deal with EMI issues at work and it is much harder than you'd think to shield parts. "Wrap it in foil" is not a clear path to success. I have literally seen parts wrapped in foil in misguided attempts to shield them. They were really quite leaky from an EMI point of view. "Put it in a metal enclosure whose lids or openings are sealed with metal EMI gaskets" is what actually works.
I think if the typical person tried shielding a radio with aluminum foil, they would screw it up.
I wouldn't worry about small gaps.
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