That is a really good point, I live in the mountains so I don't have that humidity issues as bad as some parts of the country. We definitely don't want that bad food..just attracts feed mites and the like.
I really like the initial framing. This seems like you could easily add sub coops/runs as needed. Could theoretically just keep adding boxes onto the sides and still maintain the ease of access. Also with that setup, ever thought about adding in a mini grain silo/chute like they use for dairy cows?