Hi, I am remodeling a 12x6' guest bathroom. Removed old 4" floor tile and am preparing to install 13x13" porcelin tile in its place. The original tile was installed over a
2" mortar bed which held up extremely well. Reading some of the forums, I decided to
remove the mortar bed and install additional subfloor and 1/2" hardibacker in its
place to get closer to the origianl height.. So the base is 1/2" plywood on 2x10 joists
16"OC. To my dismay, I discovered that I had inadvertently purchased and
installed 19/32" indoor plywood as my second layer rather than an exterior
plywood. It's screwed down to the joists using deck screws so *could* be
removed if absolutely necessary. So I'm trying to determine whether there is an
adequate method for providing a vapor barrier to avoid having to tear out
the incorrect subfloor I just installed... and of course cost is a factor here too.
If the correction costs $200 then out it comes. I plan to use thinset/epoxy coated screws to install the hardibacker when the time comes. Thanks

If you are putting dowm 1/4" Hardiebacker, screwed and glued, you don't need a vapor barrier.

Hi, thanks for your response. I thought the reason for exterior plywood was due to risk of moisture getting through grout/hardibacker and that interior plywood could come apart if exposed to moisture for any significant amount of time? Also, my plan is to put down 1/2" hardibacker using thinset and screws. My understanding is that thinset does not have any adhesive or moisture blocking characteristics. Please correct any misunderstandings I might have here. Thanks

Yes you are off just a hair. You can use either 1/4" or 1/2" Hardiebacker. 1/4" is all thats required but if you need more height then 1/2" is fine. If moisture starts getting through both grout and Hardiebacker hundreds of us will be sued out of existence, it just doesn't happen. I used modified thinset to set both the backer and the tile, I also tape and butter the backer joints with same. You would only use thinset if you were setting backer on cement.

Thanks for setting me straight on a few things.. why the requirement then for exterior plywood as the subfloor then? I see it specified constantly.. Is that more preference than a requirement?

As long as the plywood you purchased was APA underlayment rated (bc sanded plywood or better) and not listed as sheathing or CD rated you are on your way to being fine. Don't confuse "exterior" with "pressure treated" as you never want pressure treated anything going under tile.

You do have at a minimum, 1 problem...

The underlayment plywood never gets fastened to the joists. It gets fastened to the subfloor only, not the joists, with fasteners spaced every 6" around the perimeter of each panel and every 8" within the field. That was a definite error.

Also, the underlayment gets installed with the long edge (face grain) crossing the joists. It does not get rotated 90 degrees to the subfloor. Long panel edges get offset from long edges below and the short edge should end 2" past the joist, no land on them. No glue either unless it's full spread glue. Bead of adhesive from a caulk tub leave unsupported voids between the underlayment and the subfloor.

Set your cement board in a bed of thinset. Hardi and durock allow either unmodified or modified underneath. Set your tile and tape the joints with modifed. Hardi is the thirstiest of all backers. Make sure you wet it down good prior to combing your thinset across it. Taping the joints as you tile eliminates bumps to ride across with your tile that result from taping joints ahead of time.

No sort of vapor barrier gets used between any of the layers. Actually, it wouldn't be a vapor barrier by the time you are done installing all those fasteners through it. If you want the floor to be waterproofed, you can do that with a roll on waterproofing membrane such as Hydroban or Redgard.

Thinset, cement boards, and grout (even when sealed) all allow moisture to pass through. This is not an issue though, with the intermittent amount of water that will get on the floor, dripping from someone gettingout of the shower or from mopping. If you have small children that like to play in the tub and fill containers with water that will sometimes end up getting knocked onto the floor, then a roll on waterproofer is a good idea. It would go on after the Hardi was installed.