I’m planning to renovate an existing bathroom. I’m located in central MA (zone 5).

It’s a 5′ x 8′ room that will have a tub/shower along the 5′ wall, which is the exterior wall. All plumbing will be contained within the 8′ interior partition wall which separates the bathroom from the kitchen. Exterior walls are 2×4, fiberglass batts, plywood sheathing, tyvek and wood shakes.

Since the walls will be tile, I’d like to use Kerdi or some equivalent waterproofing membrane under the tiles. My initial thought was to use cement board with kerdi for the 3 walls of the shower and moisture-resistant drywall for the remainder of the room. Apart from the shower, the walls will have wainscotting on the lower portion and paint above. I’ve also recently seen ads for Kerdi-board, which is the waterproof membrane fixed to XPS. At most, it’d be 3/4″ thick for my stud-spacing. Is that worthwhile for increased R value/thermal bridging? Would I be better off taking out the fiberglass and blowing in cellulose?

I haven’t priced spray foam, but given that it’s such a small section of wall I can’t imagine it’d be cost-effective for a company to come out and spray.

It’s currently the only bathroom in the house, so when I do finally pull the trigger on this project, I want to make sure I have all the angles covered so I can limit the down-time.

Thanks.