I think I’m right in saying that Bavaria don’t use a cored layup as such. Some makers use a sandwich of outer grp, mid layer of balsa, inner layer of grp laminated together with the balsa providing an element of strength to the structure: hence problems when it gets wet and rots.
Bavaria use the mid layer as thermal insulation, not as a structural element: you can remove the headlining panels to expose the foam below. Thus, getting water into the foam is not the issue it would be if the foam were in any way structural. Also makes replacing wet foam easier as you can remove the headlining panel to repair/replace the wetted foam without the need to start digging up the deck.