My contractor stopped by today to take a look before his guys and the plumber come out. Husband has started demolition of the greenboard - and we discovered an inch of foam board insulation behind it, which unable to preserve, except on the studs. The foam created extra space between the studs and the tub flange, so my husband left the foam/greenboard on the studs for the installers to work off of, and contractor said he needed another quarter inch of material for atop the tub flange. Since we have several sheets of 1/4" backerboard in the basement from a a former renovation, I volunteered its use. So - the final configuration should be - space between studs - with loose insulation, then backerboard as a space holder on the lip of the flange, with kerdi board in front of that, then tile. (I am not sure where or if the Kerdi membrane needed). Does this sound feasible in terms of 1) Moisture and mold prevention, 2) Substrate stability - resting on the old insulated foam board on each of the studs and ultimately 3) Tile stability?