Another help is to get moisture absorbing desiccant packs and put them in the container with the ammo. Keep the container off of concrete floors. Moisture seeps through them. A high shelf is to where heat rises. Keep the container between the floor and ceiling.
Desiccant packs are available from Midway.
They sell three sizes of desiccant packs, depending on the number of cubic feet you need to dehumidify. Properly stored ammunition will last for many decades.