I have enough powder poured into cans to satisfy my needs for shooting, and the bulk is stored in my outbuilding, unheated or air conditioned. The humidity here is low except for some oppressive days during the summer, and the bulk powder, in plastic bags and cardboard boxes, is unaffected.
A friend who supplies our powder stores his in a freezer. He says that the house can burn to the ground around the freezer, yet his meat and the powder will be unharmed. Another friend of mine who lives in a mobile home (trailer) just recently lost everything in a fire. His bulk powder burned, and most of his cans went too, but many survived. But the thing that lifted the roof off was two 20# propane bottles stored in his bathroom. His toilet must have burned up on re-entry, because it has not been found.