Attic Bathroom

I am renovating an existing attic bathroom with a tub. Part of the bathroom ceiling is the roof line. Should any insulation be placed between the roof rafters; and if, so what kind? Also, the manuals I've read said to staple 4 ml plastic to the beams before installing the cement board. Should this include the ceiling? Over the roof rafters?

Thank you.

Depends on how the attic and roof are ventilated. If you have a ridge vent, then the channel must be maintained between the eaves and the ridge. They make foam panels for that, then you can put insulation over it.

Tiling the ceiling is possible, but often not necessary. If you wish to do it, you need a vapor barrier behind the cbu. you don't want multiple ones, so you'd probably want to either use unfaced fiberglass, or faced that you slit to compromise it, then put plastic over it.

Not to answer your question, just bringing up a new one - is the bath original, or was it added? A tub full of water weighs a lot, and your ceiling joists might not be strong enough to take the load.

Frenchie beat me with the question about the ceiling joists. I would urge you to have a serious heart to heart with the building inspector, an architect, or similar person to get sound advice with this. Water weighs something like 8 pounds per gallon, so a tub of water really puts a load on the floor/ceiling below it.