Insulating Roof

[damnyankee36]

In my quest to get my cab as quiet as possible, I've been thinking about filling the area with some sort of insulating material between the ceiling and roof panels on my 66.

Originally, I thought about strategically drilling holes and squirt in some expanding foam to fill the space. Someone thought that might cause a future rust problem if the foam trapped condensation. I can see the possibility of that. However, here in New Mexico, that might not be much of an issue.

Then, I thought about Vermiculite. I just need to figure a good way to seal the A and C pillar areas to keep it it from spilling out. I can flip the cab upside down to fill the space.

Has anybody else tried to insulate this area?