How thick are your sheets of R13 insulation ?
and R13 is fiberglass isn't it ?Funny how insulation changes from different places in the world. Here in denmark we use Rockwool which is fibres made from molten rock. Glasswool is available though.
I have som Rockfon, room acoustic damping material (dense rockwool) around 1 inch thick. I plan to use this in my pro four type Pi's maybe two layers layers.
Take care
Andreas