Some follow-up questions re bathrooms I am currently redoing. I have gutted 2 bathrooms – 1 above the other and on the upstairs bathroom will be putting on the floor Schuter Ditra and tiles (which is waterproof membrane.). The floor will also have an electric radiant floor mat. For the downstairs bathroom, I want to isolate the noise from the upstairs bathroom and provide some insulation for the radiant floor heating. I also want to to put some sound isolation into the wall with the shower plumbing to somewhat lessen the noise. The shower will be treated with the Schuter Kerdi waterproof membrane. So, questions are:

1) I was going to use unfaced fibergalss batt in ceiling, without a vapor barrier. My thinking is that with the Ditra waterproof membrane above, by putting another vapor barrier on the ceiling below, moisture could be trapped in between. Any thoughts? The one thing that worries me is that by not having a vapor barrier, I am allowing more moisture to enter into the cavity to begin with. I have a large exhaust fan in both batrooms, so that should help alot.

2. The wall where the plumbing is going in the downstairs bathroom is a 2×4 interior wall. I don’t want to blow foam (as I am doing on the exterior walls) because of the plumbing, but I want to reduce the noise of the water running. Owens Corning makes stuff called Qietzone, which is supposed to reduce noise and is supposedly ok for use in bathrooms. It is fiberglass, which I know is not great, but the only other solution I could think of is mineral wool. This would be behind the Scluter Kerdi on cement board, so I figure not using another vapor barrier. Any thoughts on Quietzone v. Mineral wool?