it does help but does not block. foil can make it so the distance someone has to be to read your cards is less then a inch away from you. but that's better then a few feet.

I do scanner checks for people coming into federal building and we can tell foil in wallets by the color it comes up as. its transparent and we can tell there is no wires or batteries so you wont have to worry about questions or a search if your have foil in your wallet