Drywall / ceiling choice for stacked bathrooms with plumbing in between
We've got two bathrooms stacked - one small one (5x5) on the first floor adjacent to a laundry room, and a second one on the 2nd floor with a small shower. There's space between them for the plumbing (waste and water lines), but the area is generally closed off by joists.
I was going to buy some drywall as a replacement ceiling, and I like the mold-resistance and fire rating of XP.
QUESTION: Is the moisture resistance a plus or a minus for a ceiling between bathrooms like this?
The bathrooms have small windows, so there are no fan(s). I'm wondering if the 2nd floor plumbing will generate some condensation that will get "trapped" due to the moisture resistance of the XP below. Should I put a small vent (no fan) in the 1st floor ceiling to get air into the space between the bathrooms?