No. You need either a vapor barrier behind the cement board

OR

a waterproofing membrane on top of it.

Yes, both would create a moisture sandwich.

Kerdi is used in steam shower applications, so there should be no vapor transmitting across it, if all the overlaps are correctly sealed.

Mongo had a beautiful drawing on how to use it over a tub, and Weddyacres saved it from internet purgatory, see link below.

It's a really nice product. Really great folks for both creating and saving this awesome tutorial.

The tub application is at the very bottom of the long page.

Here is a link that might be useful: Weedy's copy of Mongo's Kerdi School page

NO - DO NOT us a vapor barrier behind Kerdi.

It is a vapor barrier. If you put another behind it you'll end up with a moisture sandwich btw. the two.

I noticed most of the pics on Mongo's original post don't show up anymore, neither do on Weedy's copy.

I wonder if Mongo has put it some sites where the pics hosting has an expiration date.

Mongo...if you see this, can you check into? These pics are so valuable they should be up 24x7 :-)

Thanks

I know what happened.

My very original kerdi thread was posted on another forum. When I reposted it here. I linked the photos from that forum over to here. Same thing when weedy "saved" the thread.

That forum just changed their hosting software a couple of day ago, changing from Prospero to Drupal.

ALL photos on that site, and there were thousands, were lost. The forum folk are trying to see if they can bring them back from the dead, but right now everything is showing up as broken links.

The photos that do show up are ones I added later as replies, those links are referencing a personal hosting account which is why they still show.

If they don't get is squared away, I can repost, as I still have my original photos either on my computer of or one of my photo hosting sites.

Yup, a bummer for sure, because the ability to refer people to that thread alone has has saved me from having to reply to several hundred emails.

That explains it Mongo. Thanks!

If they don't resolve it for another week, can you report the with the pic? My tile installer won't begin til next Jan. I would love to have the thread back with pics so I can share it with him.

Question - I am ordering tiles from fastfloor. On the shower, do you have to use cove base and corner base or they are nice to have but not by necessity?

Will 7x7 be difficult to work with on the shower floor due to the preslope?

Also, on the Kerdi side, what about KERDI SEAL-MV (ones that have rubber gaskets and are used to seal pipe protrusions), do you normally use it?

Thanks again.

If they don't resolve it for another week, can you report the with the pic? My tile installer won't begin til next Jan. I would love to have the thread back with pics so I can share it with him.

Well, I'm not going to make any promises about WHEN I'll get the thread back up. I'll certainly do it, but no promises on the timeline. I be berry berry busy and it's going to take a bit to reconcile the photos on my computer with the text in the thread. Regardless, the drawings you want are in weedy's link, scroll to the very end.

Question - I am ordering tiles from fastfloor. On the shower, do you have to use cove base and corner base or they are nice to have but not by necessity?

I never use cove base in a shower. Sometimes in a laundry room, or in a bathroom where the homeowner is a neat freak and wants to swab the floors down with a mop every day. But very seldom do I use a cove or sanitary base.

Will 7x7 be difficult to work with on the shower floor due to the preslope? I'll say it's right over the limit of what I like to work with. 6x6 is usually the largest I recommend. The larger the tile the more you have to honor the drain-to-corner floor "creases" and put a cut line there. If you don't you can run into lippage issues, etc. Plus the larger the tile the fewer grout lines. That can be good if you;re anti-grout, but the grout lines do provide traction underfoot.

Also, on the Kerdi side, what about KERDI SEAL-MV (ones that have rubber gaskets and are used to seal pipe protrusions), do you normally use it?

No I don't use them.

Thanks again.

You're welcome!

Got a message from the administrators at the other forum, they're trying to re-establish the photos. If they do I'm not sure if the photos on my Kerdi Shower thread here at gardenweb will point to the "new" addresses over there.

So this will be a bit of a waiting game for me to see what they come up with.

You know, I am sure the photos showed up just before I posted it. But now they are gone. How bizarre.

I thought I had "downloaded" the page... how sad!

Alas. At least the tub application pic is still there for the OP.

Best of the holiday spirit and wishes to all!

Mongo:
If you if you end up saving them to a more permanent and controllable site, like photobucket, let me know and I'll work on getting them updated on my saved copy. Yeah, that reconciling, especially without a hard copy of what was in there, will be a pain.

How does kerdi-durock-6 mil create a "moisture sandwich"? If the kerdi is waterproof, the 6 mil is waterproof, how could moisture get into the durock? I ask this because it seems like USING the 6 mil would be cheap insurance just in case the kerdi got punctured, etc.

Everything always has a bit of natural "drying" ability. Spill a little water on your floor and it'll evaporate. Set up a slow drip and if the rate of accumulation outpaces the rate of evaporation you'll end up with water damage.

Same with moisture in house walls. Moisture vapor can be driven into wall assemblies, mostly via air movement. Some insulations are better at restricting air movement than others. Foam and dense-packed cellulose are great air infiltration barriers, fiberglass is not so good. When moisture vapor does get into walls, some wall assemblies can "hold" the moisture better than others. Cellulose can hold moisture fairly well, fiberglass insulation not so well.

Same with liquid moisture. Liquid moisture can get into walls via a leak in the plumbing, cracked grout, a roof leak, etc.

When moisture gets into an assembly, it needs a way to escape. It usually escapes via the same path it entered. Moisture is accumulated during a wetting cycle, and given up during a drying cycle. If the wetting exceeds the drying, then over time you get water damage. Rot, mold, etc.

Building science generally recognizes the idea of wetting and drying cycles. It recognizes that some materials hold moisture better than others. It recognizes that moisture may get into assemblies. Because of that, we also want to provide a back door through which it can escape in case it can't exit through the way which it entered.

That's why building code prohibits two vapor barriers in a wall assembly. Moisture that gets in could be trapped between the two barriers, creating what's referred to as a "moisture sandwich." There's a way in but not enough of a way out.

I certainly understand there are all sorts of "but what if..." and "but how could it..." on this subject. But that's the short long answer.

The long long answer is really long...and evolving. Just like building science.

This is right. Little molecules (like H20) can get into anything, and they do. Water is a weird substance because it also sticks onto things, a bit like a lubricant or liquid glue. So H20 in particular can pile up between two large areas of water-repellant surfaces that have little air to blow the molecules out. It does happen a lot more frequently than anyone would think... There has to be a path to remove H20 or else it piles up. It can arrive by any means, and then kinda lock itself in. This was learned the hard way, when people began getting sick and houses began rotting fast once they had been "properly" sealed by good people back in the 60's and 70's. I know someone who survived sick building syndrome in their home in southern Sweden from this time. It all had to be stripped and rebuilt.

Ambient humidity.

It gets sealed up in the wall cavity, and you're screwed.