Old and unique house with condensation problems

This house that my girlfriend owns was built as a cottage in 1969. It is simply 12 large arches cladded with a metal roof sort of like a half cylinder. The arches are quite big so there is plenty of height for two stories. Other than the ends of the "cylinder", the walls, which are just the roof, follow the curvature of the arch. The unknown builder put pink fiberglass insulation right on the metal and then covered it with paneling. We noticed water puckering some of the paneling several months ago but didn't get around to investigating until today when we ripped the carpet out in preparation for wood flooring. The plywood subfloor near the puckering shows a large spot of water. We wondered if there was a leak in the roof. We ripped off a segment of paneling between two arches on floor 1 and noticed the metal was uniformly wet! We went upstairs and ripped the paneling off between the same two arches. The metal there was wet too. The insulation was a bit wet on the side where it contacted the metal. It appeared to wick water and drip it onto a poor 2x4 which is now rotting. After reading all these posts, I see that there should be a vapor barrier and good airflow. However, that makes sense if we had a traditional house with an attic. I was thinking that maybe we could accomplish a vapor barrier if we ripped down all the paneling and old insulation and then sprayed onto the inner-side of the roof a foam that sticks to it and also insulates. Then we cover it with paneling again. (Paneling because it is easy to bend along the curve defined by the arches. Is that a good idea? To coat the metal with a spray-on foam? Thanks, John