Is a vapor barrier under CBB necessary?

[papazian]

I am installing a tiled shower in my basement. The bathroom was roughed in when we built the house and the CBB was put up as part of the rough in. They left room to intall the waterproof liner on the bottom but there is no plastic sheet under the CBB as recommended by some tiling books. One wall is also an outside wall so it has insulation between the CBB and the CBB's. Should I take off the CBB's and install a plastic vapor barrier befor I tile? It's a bit of work because I have to remove some moldings and retape but if neccessary I am willing to do it.

[ddmoit]

Welcome to the forum. Please introduce yourself; we like to be on a first-name basis here.

Cement backer board is not harmed by water, and so it is sometimes called waterproof or impervious. But, it is not a moisture or vapor barrier. That means that water can and will pass through it. Anything behind it that can be harmed by water will be harmed by water in the absence of a moisture barrier.

Since your backer board is already installed, you might decide that a roll-on membrane like RedGard is your best solution at this point.

[papazian]

Hi Dan: My name is Peter and I live in Golden, CO.

Thanks for your suggestion on a surface moisture barrier. I will have to check with my local Home Despot and see if they have any. It sounds easier than removing my CBB's. One issue I still have is that the 1/2" CBB's were also screwed directly to the studs without the recommended 1/4" spacer to allow room for the rubber shower pan liner folds. I bought some 1/4" CBB's and though I could use them on the bottom to allow extra room and then add extra thin set on top to make it level. Is this the way to go here?

PS: I hope I am posting correctly. I did try to post a picture but couldn't because it was too large even though I have downloaded IrfanView.

Peter

[Tool Guy - Kg]

Welcome to the forum, Peter.

1/4" CBU is not the way to go because it's not stiff enough to span from stud to stud and support the fragile tile and grout.

But answer a question, if you would. Do you have a moisture barrier over that insulation that you mentioned? That would change the answers we give you.

[papazian]

Hi Again: I guess the devil is in the details.

No I don't have a moisture barrier over the insulation. I think this is not recommended as I am in the basement and moisture barriers over insulation adjacent to foundation concrete walls are a no no. They don't want moisture from the concrete to be trapped in the insulation. So this does seem to cause a problem with moisture barriers on the outside wall

PS:I have managed to attach a file showing the existing interior walls and the shower base I just poured.

[Tool Guy - Kg]

We need a better picture then that. When you are saving the photo in IrfanView:

Click on "Image" on the menu bar on top of the screen and a drop down menu will appear. Then click on "resize" to a reduce the picture size itself to no more than 600 x 800 pixels (you have this part down).

Then click on "File" and a another drop down menu will appear. Then click on "save as" and a 2 boxes will appear. Look in the big box near the bottom......see the "Save as type:"? Use the drop down menu to select "JPG - JPEG Files" there. Then go in the upper right hand menu box and click on the "Save as progressive Jpeg", then change the quality percentage down to about 80% or so. Then save it. Your picture quality will skyrocket.

[papazian]

Hi Tonto:

Thanks for the advice on th picture. Hope this one is better. If 1/4" CBU's are too thin it sounds like I should take off the CBU's and install a vapor barrier behind them. What about the outside wall with insulation in it against concrete?

Any more suggestions on how to proceed?

Peter

[Dan in Texas]

Just a quick comment. Are those 2X10 blocks between your studs or 2x4. They look like 2x4 but it might just be the picture. I would suggest you block up to a BARE min of 8 inches so you have at least that far to tack your liner. Even if the CBU is not shimmed out you can still notch the blocking and stud for the liner. I used a 4 inch angle head grinder and a 50 grit pad. The dust is a MESS. Dan

[papazian]

Hi Dan:

I have a mixture of blocking sizes because my walls are floating 4" above the basement floor due to possible floor movements. If you look closely you will see ther is a gap on the rear wall between the green sill 2x4's and the blocking. However the floors seem stable so hopefully they won't move.

If you look at the seat it has taller blocking. All my blocking is at least 8" above the sloped mortar base so it should be fine. I was thinking of putting some compressible foam blocking blocks in the gap to help support the the final shower base while I am casting it

I guess I could set the majority of the blocking back 1/4" aqnd then grind or chisel down the studs although because of the framing some studs are laid flat instead of on their edges.

Thanks
Peter

[Andrew21]

Here's a pic of the redgard. They do sell it at home depot. Just gotta look for it. Its by the thinset and stuff..

[papazian]

Hi Andy:

Do you tape and grout the corners where sheets of CBU join with thinset before using the redgard or only use redgard for sealing between backer boards? Thanks for the picture.

Peter

[muskymike]

Hi Peter, yes tape all the corners and flats then thinset over them getting them smooth as possible leaving no voids. Let it dry then Redgard it.

[Tool Guy - Kg]

Hi Peter, good job on the picture.

Since the bottom 10"-12" (however tall your liner is) of cement board isn't fastened with mechanical fasteners (to avoid puncturing the liner), and you don't have much more height than that below the existing cement board, you have a couple things going against you. I don't like the idea of fitting in short pieces of cement board that will barely be fastened....even if the studs were already shimmed to accommodate the thickness of the liner. What I would do to kill 2 birds with one stone is to remove the cement board and shim the studs (to accommodate the liner). Install the liner. Then re-install that cement board, but lower those pieces down to 1/2" from the pre-slope. Then cut some smaller pieces of cement board to fill-in on top. I'd encourage you to go with the RedGard for a couple of reasons. First, the waterproofing is directly under the tile and that keeps for a drier shower. Second, you have a bench to deal with and that's taken care of quite easily with the roll-on waterproofing.

BUT BEFORE YOU GO ANOTHER STEP.... I forgot about how you folks in Colorado hang your stud walls in the basement from the first floor's floor joists and levitate them off the floor to account for slab movement. The last shower I had anything to do with in Colorado was intentionally put in the middle of the house and all the walls were anchored to the slab with a floating tie-in to the joists above to keep the movement joint away from the crackable tile below. This was done to get away from the very problem you will have if you install a bunch of stiff tile and don't allow for movement between the base and the walls.

I don't know what y'all do to account for this problem. Let's see if we can get another pro familiar with this to shed some light on this challenge.

[jadnashua]

Have you looked at Kerdi from www.schluter.com? You'd have to change the drain, but it would make the whole thing waterproof, not just the floor with the liner. It dries out much quicker, therefore it has little chance to grow nasties...It would allow you to keep the bench intact easier because the whole surface would be waterproof.

[John Bridge]

Hi Peter,

I think you will have to re-engineer your shower, as Kurt mentions. I think your shower walls will have to have the movement gap up above and be anchored to the floor instead of the ceiling. There is no way the shower is going to stay together not anchored to the floor.