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My bathroom was gutted to the studs. Now we are to the point of mudding drywall. I'm wondering if I need to use red guard on the sub floor and green board (around bathtub shower) before I install cement board. Or do I just wait and paint red guard on the all the cement board once it is installed.

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    So you're adding the cement board over the drywall?
    – JACK
    Jul 17 at 22:17
  • A picture of the walls that will be tiled would help. CBU over drywall is unusual. Is tub shower installed now ? Is a gap left between tub shower and drywall? My shower had cbu placed over drywall then tiled. A polyethylene vapor barrier was placed between drywall and cbu I now use the [schlutter system ](schluter.com/schluter-us/en_US/schluter-us/en_US/Shower-System/… waterproofing before tile
    – Kris
    Jul 18 at 2:23
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You can do either. Just don't do both.

Behind the cement board is more similar to older membrane systems. On the cement board surface means the waterproofing is what your tile thinset is directly adhered to, which is supposed to be fine, if the backer is correctly primed with the waterproofing compound, but some folks prefer having that attachment be directly to the backerboard with the membrane back one layer.

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  • Just one data point. I used Red Guard on the outer surface of the concrete backer board - the surface the tiles are attached to. I have not had any issues with tiles coming loose or water penetration (that I can tell, anyway). This was done 6 years ago.
    – SteveSh
    Aug 17 at 14:48

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