The original bulkhead, at least on my TCS was made from particle board., which is probably as absorbent as cardboard. As I understand it, its main function is to stiffen the body perpendicular to the fore/aft axis.
Since mine was decomposed, I replaced it with 1/2”? The right thickness of marine grade plywood. But others have used honeycomb aluminum and other extremely stiff, and fireproof, panels. Others still have added soundproofing to the cabin side. Ill probably add that when I do the interior in the next few months