Got a problem: Daughter bought a house in Southern MS. Right after she bought it, had a sewer backup that caused a floor drain in bath to overflow & spread about 1 inch of water all over floor. Amazingly, it ran out under the sill plates into the garage & some got into an adjacent bathroom. House id 30+ yrs old. Why on earth there is a floor drain is beyond me.

Had no idea the magnitude of the damage till about a month later when daughter decided to replace filter in return air duct that ran behind shower in bath that had overflow. Opened louver & found black mold along back of wallboard forming one wall of return air duct. Also discovered that her shower head chrome nipple had corroded & been leaking down into same space, probably compounding the mold problem. This meant that return air was picking up mold spores & spreading around entire house. Simultaneously, she decided to repaint second bath on other side of return air duct and discovered that wallboard at base was crumpling & paper peeled off face. Black mold had a foothold on that wall, also. While cleaning this mess out, I wore an MSA mask w/ a P100 filter. Called the MSA folks first & they said that was adequate protection against black mold.

I have since (Like this past week) installed a real galvanized return air duct to isolate any black mold in wall space. There was about 11 feet of duct that was nothing more than 2X4’s & wallboard. Have removed all the moldy wallboard from several areas, having to go about 2 feet up wall to get to sound wallboard, so am going up 4 feet to replace – figured that it is no more work to go 4 feet than to go 2 feet.

Now to the question – I can see the back side of the bathroom wallboard at the sillplate – which is on a slab. The bath has 4 inch tile along the base of the wallboard that has the black mold growing on it inside the wall cvity. As soon as I get the current areas closed in, I will remove the tile & its walllboard. Is there any reason why, when replacing this mess, I cannot put a 3 1/2 inch band of 1/2 inch Hardiebacker along the sillplates to adhere the replacement tile to then continue up with the new Densearmor anti-mold wallboard to a horizontwal joint w/ the current wallboard? I figure that the HB would not wick water up into the new WB & the band of Dense3armor would protect against moisture & mold in the lower spaces of the wall.

Don