Hardibacker Over Drywall for Shower, Exterior Wall

[DIY Michigan]

I need some advice about double vapor barriers. I want too install hardibacker over drywall. I read the previous posts and methods. But my situation is not that straight forward.

I have a condo that had a one piece fiberglass tub-shower. For fire protection all of my drywall is 5/8". Studs are 24" oc. One side of my bathtub alcove is an exterior wall.

I removed the previous fiberglass unit. Drywall existed behind the fiberglass unit. Including the drywall the alcove is about 60" wide. So I left the drywall in place. I considered removing all the drywall. But that is not so simple. It would require some major rebuilding of the bath / shower alcove structure. Also I would need 5/8 backer board to replace the 5/8" drywall. I can not find a local store that sells 5/8' backer board.

So my plan is this. Cover the drywalled alcove with tar paper or plastic film. Slide a cast iron tub into the alcove. Above the tub attach hardibacker to the drywall. Then tile the hardibacker. I assume that the tar paper or plastic film behind the hardibacker will prevent moisture from transfering to the drywall.

Here is my delimma... One of my walls is an exterior wall. Therefore that drywall already has a plastic vapor barrier on the exterior side. I plan to attach a vapor barrier on the inner side of the drywall. That will sandwich the drywall between two vapor barriers. I assume that arrangement will prevent the drywall from breathing. Is that acceptable? Will dual vapor barriers create a modly mushy mess of the drywall?

[cx]

Welcome, Anthony.

I, for one, do not recommend that procedure for the very reason you suggest.

Sheetrock is cheap, and 5/8th" sheetrock is common as dirt. If I intended to install a tub/shower in that alcove and wanted to have sheetrock for my walls, I'd consider no other waterproofing method other than Schluter's Ditra, which has been approved for that application.

There may well be some argument from your code compliance inspector, but you should be able to get it approved.

Otherwise, I'd recommend installing a CBU instead of your sheetrock. Should be no problem getting the code people to accept that as a replacement for the Type X sheetrock. You can install a moisture barrier behind that or a waterproofing membrane on the inside.

In any case, I'd recommend you add framing to the wall to reduce the spacing to something 16" on center or less, even if you intend to stay with the 5/8th" sheetrock.

My opinion; worth price charged.

[Higher Standard Tile]

CX suggestions are good and Durock does make 5/8" panels but you won't find them at HD maybe from a building supply company that specializes in commercial products.

Another approach. Others can correct me if it would not work. Skip the moisture barrier directly over the drywall and attach the cement board directly over the drywall. But then apply a 2 coats of liquid waterproofing membrane like Laticrete Hydroban to the surface of the cement board. No moisture will get to the drywall and you won't create that moisture sandwich.