Waterproofing behind CBU for tub surround [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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10-09-2006, 11:11 AM
I'm about to install my first tub/shower walls/surround and have Qs about the waterproof barrier. I'm using 1/2" Hardibacker CBU w/greenboard above the tile at the very top. Porcelain tile to be installed.

1. 4mil poly or 15# building paper/tar paper? I'd prefer poly, but I'm concerned about the staples to the studs compromising the integrity of the barrier. In the Michael Byrne book he really shows the tar paper in detail and says to seal all joints and staples w/roofing cement.

2. After I trim poly or tar paper, should I caulk it to the tub rim w/silicone (behind the tile)?

3. I know I need a 1/4" gap between the CBU and the tub rim. Should I caulk that gap w/silicone? Seems like no but I saw it mentioned as optional at the "askthebuilder" website.

4. I'm almost certain this is a bad idea but at "askthebuilder" website they recommend caulking between the CBU sheets instead of, or in addition to, fiberglass tape w/mortar. Thoughts? I'm really not a fan of that site I just ran across it while googling.


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10-09-2006, 11:24 AM
Hey there nash -- got a first name for us? We won't bite :)

You can do the poly for your vapor barrier. Put it in in strips, stapled to the studs. Overlap the strips "shingle-style" bottom to top, with the staples from one row covered by the overlap from the row above. Bring the whole barrier down so it hangs into the tub, then you can trim it off so it drapes just over the tub flange. Not sure about the caulking it to the rim, though I think so -- hang tight for more answers.


10-09-2006, 11:29 AM
Welcome, what should we call you?

I don't have a problem with covering the staple holes in the vapor barrier, but it's really not needed as it's a vertical surface. If you really want waterproof there are numerous surface membranes made to go over the CBU. Kerdi by Schluter is an example of a sheet membrane, Redguard by customs is a trowel applied membrane . Many others in this catagory that work equally well. This would eliminate the need for a vapor barrier behind the cbu and essentially put a waterproof barrier just under the tile.

At the junction where you meet a plastic/fiberglass pan/tub flange. If you use a traditonal vapor barrier behind the cbu it overlaps the tub/shower flange...at this point you sould put a bead of sealant behind the tar paper/poly. I wouldn't put a sealant in the gap under the backerboard because in my opinion it stops and holds the water from exiting easily from behind the tile (running out off the vapor barrier and into the pan) thus creating a spot for mold/mildew to build up.

Just my opinion. :D

Mort 801
10-09-2006, 11:30 AM
I've got a similiar question for this thread. Do I even need the vapor barrier if all my bathroom walls are interior walls? There is lots of discussion on vapor barriers, but they all discuss use of vapor barriers on exterior walls. Should they be used on bathroom walls (mine is a tub/shower combo) regardless of interior or exterior? Thanks. Paul

10-09-2006, 11:37 AM
Paul, there is a dreat debate about the pro's and cons regarding vapor barriers and condensation...In some parts of the country where a high tempature swing between the house and the outside is present it seems a more relavant concern. But in a shower, my feeling is you need some waterproof or vapor barrier or you'll have much greater mold and moisture concerns to contend with . As I stated above there is a number of suface membrane options that exist that eliminate the need for a vapor barrier over the studs/insulation in the wall. I am a big fan of these..Particularly Schluters Kerdi system. It's almost all I install in showers anymore for a number of reasons. Primarily because after using so many different types it's the one I trust in my mothers home and mine. It's easier for me to install with thinset (Kerdi)as I never bash my thumb with the hammer stapler :yeah:

10-09-2006, 11:38 AM
Hi Paul,

You definitely want a vapor barrier behind any walls that will be exposed to water -- shower/bath for sure. Water will get through tile/grout and your CBU and get into the wall cavity otherwise.

No need for a vapor barrier behind other walls though...

Mort 801
10-09-2006, 12:16 PM
Ok, thanks. I figured I needed a vapor barrier even though they are interior walls. (Note, when I tore down the old walls, there was no vapor barrier). I plan on using Hardibacker on the floor and walls under the tile, and GP Densarmor Plus on the painted portion of the walls and ceiling. Looks like I can use the Schluter Ditra on the floors on top of the Hardibacker and under the tile, and the Schluter Kerdi on the walls, correct? One last thing, one wall is common with my neighbor (row townhouse), and when I tore it down there were two layers (one drywall, one cement board). Is this a fire code thing or just noise isolation? Can I use the 1/2" Densarmor Plus as the first layer, and the 1/2" Hardibacker as the second (for the tiled areas)? My tub is a three wall alcove install, but the tub is about 2 inches shorter than the width of the framing. Is it better to build out the wall with the shower/tub controls (the common wall), or the other wall?

10-09-2006, 12:28 PM
Hi Mort,

Alla these questions, I think it'd be worth starting your own thread so's not to leave Nash :jacked:.

That said, you've got a lot going on there and not all of it's necessary. If Kerdi's in the plan, you don't need or want an additional vapor barrier on the studs. You also don't need CBU or DensArmor -- plain sheetrock will do just fine. Start up a thread yourownself and we can hash out the details...

10-09-2006, 12:31 PM
I just retiled my master bathroom shower because of a leaking valve in the wall. I live in Florida. I purchased my home brand new and it is now 14 years old. I had no vapor barrier or moisture barrier of any kind when I ripped the old walls out. Except for the area around the leaking valve, the walls were bone dry. I installed HARDIEBACKER board (1 inch total) and tiled right over the top of it.


Mort 801
10-09-2006, 12:47 PM
Thanks. Didn't intend to "jack" Nash's thread; thought the topics were similar. Concur that i don't need the extra vapor barrier with Kerdi. Paul

10-09-2006, 10:03 PM
OK, I'm just learning etiquette (names) on this board -- a little different from Saabnet. Anyway, the name is Derek.

Thanks much for the tips! I know I will be back on this board in a few days a little later in the job. We are ordering tile from the store tomorrow (a real tile store not big box) and are excited.

I'll install poly horizontally w/overlaps between the sheets. I will drape it down into the tub, cutting off excess carefully when done, but I will caulk the poly to the tub rim. Will not seal the joint between tub and CBU/hardi.

I think I'll stick with poly since I have lots on hand and I won't forget where the joists are. Plus the bathroom won't smell like tar paper, making my wife happy.

I just found a good picture of the waterproofing installation (w/tar paper) at the CBP website under bathtub walls.

Cheers, Derek

10-10-2006, 10:12 AM

sounds good...don't be a stranger..keep us posted on the progress :tup1: