Help! Should contractor use my leftover 1/4" backerboard plus the Kerdi board?

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Julie M.

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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?
 
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jadnashua

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CBU is not waterproof, but is not (generally) damaged by being wetted. If you're using Kerdiboard, or Kerdi membrane, your intent is to make the enclosure waterPROOF, so while you could have cbu beneath it, it should be covered with either Kerdi membrane or the Kerdiboard, and all of the seams properly sealed to create the waterproof shield.

At the tub flange, you generally do not want the cbu to be tight against the horizontal surface, and to be held back some. To keep things from bowing out over the tiling flange, depending on the tile size, you can often end the support just above the flange's edge, if it won't lap over it easily. WIth Kerdiboard, it's not hard to use a sharp knife or a dado blade on a saw to make a rabbit joint to lap over the tiling flange so everything stays nice and flat. You want to use some KerdiFix to attach things to the tiling flange.
 
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