ᐅ Intermediate ceiling with composite panels – Is a vapor barrier necessary?

garnickel
Our 40-year-old weekend bungalow, a solid construction, received a new flat roof this year. The roof includes integrated insulation: OSB boards, polystyrene insulation boards, OSB boards, and then roofing felt. It has side cladding as well. Of course, this does not create an airtight seal. About half of the space under this roof contains embedded roof beams that were left in place for structural reasons. These are now intended to serve as the base for a ceiling in between. In the areas of the bathroom, kitchen, and children’s room, a layer of moisture-resistant boards has already been installed underneath the roof beams. In the living room, the beams still look good and will remain visible, with laminated panels installed between the beams. For the bathroom, kitchen, and children’s room, I plan to add a row of laminated panels as a second layer. Now I wonder if a vapor barrier is necessary here? The main insulation is in the roof. The laminated panels are just meant to add a slight improvement. Could there be issues with moisture in wet areas when using, basically, drywall, polystyrene, drywall?
Thanks to everyone who responds!