You know, I don't know if you really need to worry about moisture with mics. I would be more worried with your preamp/ad/recorder, or anything that has a circuit board where moisture can easily get. Mics are a little bit more resilient to moisture. Though, it isn't a bad idea to take some precautions. I would worry about throwing silica packs around your other electronics first--just make sure they aren't going to rip open and get all over the place.
Bean, that foam will not soak up the moisture like a silica pack.