Flooring Tile - Insulation behind tub (shower)

We recently removed the tile and walls from the tub surround. We are now faced with framing studs and the exterior cement block wall. The studs are approximately 1" thick. Before we have the walls replaced with cement board, I am wondering if we should put insulation between the exterior wall and the cement board? If so, since the space is so thin, what should we use? Also, do we need to worry about water/rot in this area?

Id put as much of the polystyrene board as I could on the block wall for sure there. Also down below the tub line. Water wont hurt the poly board.

ED ;)

What sort of moisture issues are involved with insulating a tub/shower when it's directly next to a brick/block wall?

I've got a 1st floor (above ground level) bathroom in a 50's era brick-on-block construction house. That is, it's block with a bit of an air-gap and then brick facing. Quite the bomb shelter for a DC area house...

The original construction had simply a single sheet of tar paper, some 1" furring strips and tile set in concrete mud. No insulation whatsoever. It's now firred out with about a 2" gap. Can I just use the foam board in this space? What sort of barriers should (or shouldn't) I have next to the durock and the block wall? Should I tear down the old tar paper?

And can I use regular fiberglas insulation around the tub itself? It's a 42" x 60" kohler acrylic unit and I'd like to be able to retain as much heat as possible.

Is there a particular brand that I should use for the insulation? Would some brand be better in the bathroom than another brand?

Home depot an lowes both have the polystyrene board for there.

ED ;)

Thanks for your help!