I never see water inside the kiln unless the roof is leaking. But then I always air-dry first; maybe I would see water if I started with green wood.  It gets so hot in the kiln though, and with the fans blowing during the day, the moisture must either evaporate or escape through the vents.  My inner walls are coated with a thick black sealant that can be found in the roofing section at any building supply store.  I'm hoping it protects the inside of the walls; I didn't use plastic under the wall plywood, relying instead on the coating and the kraft paper barrier on the fiberglass insulation.  Plastic sheeting does protect my floor though, which is 3/4" treated plywood.  You are welcome to stop by  -  send me a message and we'll arrange a time.