I had a problem with a Rover 75 where the air extract vent was loose and allowing water to stream into the boot. The previous owner had drilled holes in the spare wheel well to allow water to drain off, but it had also soaked the carpets under the rear seats and the foam in the rear seat cushions had soaked it up like a sponge. I took out the seats so they could dry out, and dried out the carpets with an electric fan heater, placing blocks of wood under the carpets so the warm air could dry them out from underneath. The air vent was concealed under the rear bumper, I had to take it off and clip the vent back into place, problem solved.
On the i30 I think the vents are in the rear panel, concealed under the bumper. I would feel around for any damp carpets or hidden pools of water that are providing a constant source of moisture. The Rover also suffered from leaves blocking the drain hole near the cabin filter, so that would get wet and make the incoming air moist.