# Cement Board Questions



## Bud Cline (Mar 12, 2006)

> IF I bring it down to the top of the tub flange, do I need horizontal support (or a 2x4 horizontally) between my vertical 16 on center 2 x 4 studs?


Nope! Not if you use 1/2" board and the stud spacing doesn't exceed 16" o.c.


----------



## SuperHans (Feb 11, 2011)

If I'm understanding the manual right, they show the wall material brought down to the top of the flange and then the tile hangs off the wall material by the height of the flange. 

I suppose that would work but a viable alternative would be to fur all the walls out 1/4 inch after the tub goes in. That way you can bring your cement board all the way down and you don't have any partially supported tile. If you had the cement board flush with the studs and bend it over the flange, the finished surface may end up looking funny. Good Luck.


----------



## Gary in WA (Mar 11, 2009)

I may be seeing it incorrectly but I believe the poly wants to be inside the tub flange to channel any water/moisture through the b.b away from the framing.......

Gary


----------



## ccarlisle (Jul 2, 2008)

Walleye:

Gary is right in commenting that the poly vb should come down the wall and end _inside_ the tub...the problem becomes a minor one about how to finish off that gap created between the cement board, the tiles and the poly. Some caulk it, some grout it - but there isn't one definitive answer that I know of, if you choose to use that system...add to that the preferred use of total vapour barriers (right down to the floor) in cold climates - and you have a whole variety of options.

That's why many have opted for the Kerdi membrane - or a liquid waterproofing membrane _on top of (not behind)_ the cement board. Just one less thing to worry about.

You may already be passed the stage where Kerdi etc membranes come into the picture, however. So, Gary's observation is the next best thing IMO: over the lip.


----------



## trophywalleye (Dec 4, 2010)

*Reply*

Thanks for the comments.

I will not be furring the wall, nor will I overlap the flange with the cb.

If I do not require the horizontal supports, meaning the vertical supports are sufficient for rigidity, I will run the cement board down to the top of the tub flange. 

My best plan then, unless specified otherwise, is to run the kerdi all the way down the cement board to the tub, using kerdi-fix sealant to adhere the kerdi to the plastic tub, and non-modified thinset to adhere the kerdi to the cb.

Finally thinset in between the tub flange and the tile.

Any comments?



As for the vapor barrier, I believe I have it right. I'm in a cold climate, so it runs the whole wall (meaning behind the tub). I will not be draping a second piece to hang over the tub flange as dual layers may cause more problems.



p.s.
http://ths.gardenweb.com/forums/load/bath/msg111135518894.html


RE: Kerdi Shower

clip this post email this post what is this?
see most clipped and recent clippings
Posted by bill_vincent ([email protected]) on Fri, Nov 23, 07 at 16:31
Thanks, Sally. 
Do you "caulk" the Kerdi to the tub flange?

You basically caulk it to the tub's edge (the flat part between the flange and tub itself).

Does the backer board still go over the flange?

You stop it right at the top of the flange, otherwise it'll kick out the bottom of your wall.

Do you still leave a gap at the bottom of the backer board?

Asked and answered

If so, do you attach the kerdi to the tub or just the backerboard and then caulk the gap?

Caulk the gap with Kerdifix, and make sure it's bonded to the bottom of the Kerdi.


----------



## ccarlisle (Jul 2, 2008)

Yeah; there should be two beads of caulk at the tub/tile area: one bead joining the cement board to the top of the flange and one bead sealing the tile to the tub...


----------



## Bud Cline (Mar 12, 2006)

No vapor barrier is to be used behind the wall boards when using KERDI on the surface.


----------



## ccarlisle (Jul 2, 2008)

Ahhhh, 1947 was such a good year...:laughing:


----------



## trophywalleye (Dec 4, 2010)

Hi Bud,

Ok now I'm worried... I have my cement board up over the vapor barrier. The one wall with the vapor barrier is an exterior wall, which is per the building code in Ontario. Kerdi is not technically considered a vapor barrier from what I have read. 

Should I not use the Kerdi now? I really don't want to take the cb down to remove the vapor barrier... however I really want to use the Kerdi as per the price I paid, not to mention the credibility of water protection. Will it destroy my outer wall by installing it. Will it really trap moisture in between? Currently we do not have air conditioning, but possibly in the future.


----------



## trophywalleye (Dec 4, 2010)

And to note, if there is a bit of moisture the cb is 100% mold proof. Not sure how much moisture will form.


----------



## Bud Cline (Mar 12, 2006)

I think you are now over-thinking the process.

KERDI is not sold as a vapor barrier per se, but of course it is, it has to be, simply due to its composition.

I think if it were me I would keep going rather than change anything at this point. But that is up to you.


----------



## ccarlisle (Jul 2, 2008)

Just slash the poly vb you already have wtih a sharp knife...no need to take it down.

Schluter's Kerdi membrane is a vapour retarder not a vapour barrier; there's a big difference...Kerdi therfore doesn't let physical water molecules through it but it does let water vapour through it. The whole point is to keep the water molecules from entering the wall cavity - but at the same time, we want the wall+tile assembly to dry to the inside, in northern climates.


----------



## trophywalleye (Dec 4, 2010)

*Solution?*

Hi Guys,

One last note...

I talked to a Schluter supplier... and since I already have the Kerdi, and feel more comfortable with it... I was told to drill a bunch of 1/4" holes through the cb and vapor barrier to break the seal so it can breath.... I will then thinset the holes and kerdi the wall...

Does this sound like it may work?

Please let me know.

Thanks!


----------



## ccarlisle (Jul 2, 2008)

Why cut holes and then thinset them?

Cut the holes - or slash it - like he says, then put up the cement board using appropriate screws, then thinset the Kerdi to that...


----------



## trophywalleye (Dec 4, 2010)

*Schluter*

Just for anyone who is seaching for similar answers. 

I, as well another fellow that I found online both independently contacted Schluter, and Schluter did not recommend to slash or cut the vapor barrier open. Apparently it should be used on all exterior walls, regardless that Kerdi is applied. So I will be keeping mine on the 5 x 8 exterior wall. Also, two of the Schluter suppliers also noted here in Ontario that vapor barrier is kept behind the cb prior to applying the Kerdi.

Thanks for all the posts though.


----------



## ccarlisle (Jul 2, 2008)

Well, all I can say is that if you ask 10 Schluter guys about Schluter products and they'll all say the same thing; if you ask 10 tile guys about Schluter products in a tiled shower, you'll probably get a 50;50 split. If you ask about Kerdi products in a tiled Ontario shower system - if you can find 10 of those - you'll also get a mixture of answers.

The fact is debatable and it is far too early in the existence of "Kerdi showers in Ontario" to be able to say what the right way is; but one thing Kerdi won't tell you about is how we here in Canada like to have our showers dry to the interior and we use Kerdi because it is (a) waterproof and (b) a vapour retarder. If you are worried that the small amount of vapour getting passed the Kerdi, the cbu and the vapour barrier, then by all means keep the vb intact. IMO there is greater evaporation 'draw' of vapour to the inside of a 20deg C house in winter than there is tendency for that same vapour to exit via the wall cavity.

But that's just me....


----------



## Gary in WA (Mar 11, 2009)

When you make a hole or slit in an *air-barrier* on a house, it will compromise it greatly. In a *vapor barrier *it has almost no effect except the width/length/size of the hole/slit, the rest of it is still working, (not like a balloon when a pin-hole ruins it- more like a brick wall with a few bricks missing)

Gary


----------

