How do i keep the humidity in my iguanas cage?

im planning on getting an iguana. my plans for the cage is 6'hx6'lx3'w. im goin to make the frame with 2"x4" and make 3 of the walls with ply wood and also make the ceiling and floor ply wood. the other wall i was thinking of making out of some wire mesh that is plastic coated. how can i keep the humdity in the iguanas cage without making the whole room as humid(idc if its a litle bit humid in the room)