When I removed my Malibu's headliner, I noticed that there is this material that was glued to the inner metal roof skin. It looks kind of like roofing felt, or a heavy tar-paper. It was peeling away in several areas, and I could see the rust underneath it,. I cut and scrapped it all off, so I could clean up the rust and repaint. Now I am ready to put the headliner back in, but am wondering what I should do about replacing this tar paper stuff.
Originally, this material was actually run between the roof skin and the center support rib, and also ran under the front. rear and side roof frames.
I could just adhere some roofing felt to the skin, right up to the edge's of the rib, and frames, but don't see how it would be possible to tuck it under the framing. Not sure if that would be an issue or not?

I am not really sure what this stuff was even used for, but I suspect it may have acted as an insulation perhaps? I know that since I removed the headliner, during certain weather conditions, (cold nights sunny days), I get lots of water condensing, on the inside roof skin.

Another idea I had was to spray the roof skin with a thin layer of expanding foam. It would have to be very thin so I can make sure the styrene headliner base, will fit properly. I felt that would act as an insulator.

Anybody have any idea, exactly what GM put this stuff on the roof for, and what might be the best plan of attack to replace it?

Thanks!