Planning on insulating the roof of my 91 245 sometime too and was interested in seeing what others have done.
I've had what I consider great results on a TB budget using Frost King duct insulation. I got as far as the floors, butt cheeks and rear doors in my 85 245 before I sold it.
To my ears it made a huge difference in the sound damping.
In my 91 245 I've gotten as far as the floor from the firewall back to under the rear seats, butt cheeks and inside the rear panels. Also under the hood. What's interesting there is that on the occasionally cool mornings here in Florida where the car is covered in condensation, the insulated hood area remains covered in condensation and you can see the pattern of the under hood bracing (not insulated) after driving for about fifteen minutes.
I really put it there more as a test and it turned out to really help the engine noise and heat insulation and has stayed put without any loss of adhesion.
So I am going to do inside the doors next and unless someone comes up with something else better here, one of these days the roof with the same stuff.
I'm actually surprised it hasn't been mentioned yet. There's quite some videos on the You Tube.
Frost King Duct Insulation