Can I waterproof old drywall before battening it and then fixing cement boards in shower area for tiling?

Question

I intend to install a shower in an area that has existing drywall in situ.

My idea is as follows:

1. Paint out the drywall area with a waterproof paint.
2. Then screw 2x1" battens on to the waterproofed drywall and apply waterproof paint to the battens.
3. And then screw the cement boards on to the battens ready for tiling.

My theory is to damp-proof the entire area internally in the knowledge that there would be a gap separating the cement board and the old waterproofed drywall created by the battens.

Any advice or comments would be greatly appreciated.

Answer 1

Why do all this extra work? Just tear the drywall off and install cement board. Waterproof that and you're in better shape.

I see no advantage to keeping the drywall and then add battens and all the other stuff. Then you have the issue of the wall being 1 1/4 inches thicker than everything else.

Just keep it simple and don't do it.

Answer 2

This appears to violate good practice - you want only one waterproof layer so that the rest of the wall can dry. If you have two waterproof layers, water can be trapped in the wall. And if your idea is to only waterproof the old drywall to have one layer, there's a LOT of stuff to get wet between the tile surface and the old drywall - and the "old drywall" waterproof layer would have to be connected to the drain pan for the shower, which is going to be a bit of a problem with the additional construction on the face of it.

Also, you're wasting space on this that you would not need to waste if you simply remove the old drywall and replace with cementboard as suggested in the comments. I've been in showers that were definitely too small - I've yet to meet one that was "too large."

Answer 3

I'd do KERDI membrane since it gets applied to the drywall with thin set.

As long as the drywall is in good condition that should work. Of course if the paint is pealing or the drywall is crumbling, then applying that KERDI membrane over it isn't going last.

I've used the KERDI membrane for my shower and its working pretty well after a year, but that was over fresh unpainted drywall. Your mileage may vary.