Tiling conundrum - Question about construction of niche
We're in the thick of a bathroom remodel. We were hoping to create a custom niche in our tiled tub/shower surround, and we're running into some issues resulting from our unique situation.
Our house is a split level. The back wall of the tub area is partly facing the lower level of the home and partly facing attic.
For the tub surround we're planning to screw Hardibacker to thin strips of wood (to bring it out past the tub flange) which will be attached to the studs. We were planning to use modified thinset, and then Laticrete Hydro Ban.
Based upon our proposed niche location, the bottom half of the niche location is currently backed by drywall-- the lower level of the house. The top half is backed by fiberglass sheathing--the attic.
My question relates to the piece of Hardibacker that would become the back surface of the niche. Is it even OK to do a niche since it's so close to the drywall on the other side?
Should we use plywood to cover the fiberglass and drywall before attaching Hardibacker? How should it be attached? Is this even feasible or should we give up on the niche idea?
Here's a photo of the tub area.
Here's where we were planning to put the niche. What do you think? Can this be done?