a contract friend recommends I cover all my joists and beams of my deck (which is PTW) with a Bituthene type flashing. he says this will prevent any chance over moisture getting in cracks between joints and causing rot. Im imagining its another $100 of material which is nothing compared to the total cost and labor I put into it.

does anybody see any issues with doing this, one concern is it could trap harmless moisture that my PTW would absorb and allow to slowly dry and instead my decking would be subjected to this, is that a ridiculous thought?