Am not getting the vapor barrier thing, at all?

saxmaan1February 28, 2007

OK, redoing my shower and using an acrylic shower pan. And I read a million posts about the vapor barrier. So now I have the following questions:

1) I just do not understand this water flowing down behind the cement board and back into the tub? How does it get throught the tile, grout, thinset, backerboard in the first place. And when it "flows" down the vapor barrier, how does it get into the shower base? I thought the gap between the tile and shower base is caulked? Wouldn't water fill up behind the wall? And how much water flows down this system during a shower...gallons, teaspoon full, ???

2) What is the correct vapor barrier process. After it is installed to the walls, do you drape it over the tile flange? And if so, how can this be done because I thought the gap between the backerboard and the top of the shower pan flange should be can it be caulked with a piece of plastic sticking out or over the top of it? From literature, after this gap is caulked, you tile over the backerboard and let the tile hang over the flange until it is about 1/8" above the shower base. And then this gap gets caulked. Now we have 2 (!!!) caulked joints preventing water from getting back into the shower pan.

3) What is better...a poly vapor barrier, or redguard over the backerboard?

4) Isn't a vapor barrier really for condensation of a hot tiled wall and a cooler internal wall? Versus a mechanism for water flower? I am relating this to the term vapor barrier of house insulation.

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well...., I have torn out a lot of bathrooms when I worked with a general contractor. Let me assure you, moisture often finds it's way to the wood, and destroys and loosens the nails. (when a vapor barrier was absent) When you gut a bathroom, you will notice the moisture damage in the form of blackened wood around nails, thin rusted out nails, and veiny growths of mold. Mold is a mushroom, mushrooms are basically the earths digestive system as regards things like celulose (wood) This is bad.. mold merely needs humidity, and that is all. After that, once the roots grow, mold collects it's own moisture from the air to condense on itself into a muculingous enduring slime. Termites also like this kind of wood. This damage you see when a vapor barrier was not present. You will notice the damage stops abruptly outside of the shower area, the nails will not be rusted at all outside the shower. Remember that heat from hot water splashing on walls can equal indirect moisture; condensation and even if that is only on nails maybe tha's all it takes - all the way behind the cement board even if water does not actively get through, remember the fact that grouts breathing is often considered a good thing to dry things out. opinions vary. Even grout sealers breathe vapors. I prefer thick roof paper, it acts as sound and thermal insulation anyway even if you apply it beyond the shower area so why not? I also use a cement board sealer gook that stops moisture before the actual board but I do'nt really want to print that in a public forum but yeah my thinking is stop the moisture before the cement,and 2 put the paper on anyway for it's many benefits. Home Depot sells redguard maybe you will want to try that I use different stuff but same process. As far as moisture not getting behind cement, try pouring some water on cement board, see if you get a sponge effect, cement is some absorbent stuff, now wether or not humidity in the air will dampen it..?... why take a chance. just use common sense in installation. Actually, the cement board being so absorbent means it may well be drying out the humid air to quench mold which could mean it does not need a vapor barrier as much compared at least say to greenboard hmmm.. (it is not always what you think will work but what actually does)

To drive the point home regarding cement board, my girlfriend who does not have a drier dries even sleeping bags blistering fast by laying them on her back lawn - lay a wet towel in the hot sun on cement, after a while lift it up, the cement will be shiny wet acting like a sponge, and the towel will take all day to dry. Lay the towel on grass, and the air will flow under the towel and it will dry in one hour or 2. the drying ratio is around 4 to 1. The Redguard process therefore between the towel and cement would surely help that towel dry faster. try laying a wet towel on plastic versus absorbent cement. if the result is the same either way,......

    Bookmark   March 1, 2007 at 12:53AM
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I am curious too after reading this...what about the poly sheeting and the caulking question...??

    Bookmark   March 1, 2007 at 2:48PM
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Any water that condenses on the vapor barrier will roll down to the bottom of it, and yes, the caulking at the bottom will act as somewhat of a dam. But the moisture will evaporate thru the grout joints, back into the air, rather than into the wall cavity where it can do some real damage. This is the biggest reason for having a good strong vent fan. Contrary to popular belief, it's not for people with a good strong constitution when they use the john! :-) It's to pull moisture out of the air as quickly as possible after bathing, showering, or any way that you introduce moisture into the air, so that all surfaces will have a chance to dry out as quickly as possible.

    Bookmark   March 1, 2007 at 3:34PM
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Kerdi waterproofing membrane is a preferred premium method for doing a shower.

Using it over cement board with sealed joints and 6 mil vapour barrier on the studs should cover all bases.

    Bookmark   March 1, 2007 at 9:30PM
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That's what I want now if I could find someone who is familiar with it in Atlanta.....

    Bookmark   March 2, 2007 at 7:15AM
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What you might do is go to Schluter's site and find out who in the Atlanta area sells their products. Once you have that list, call a couple of the tile warehouses and find out who buys alot of kerdi from them.

    Bookmark   March 2, 2007 at 8:39AM
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Or you can buy Kerdi from They will give you great customer service.

    Bookmark   March 2, 2007 at 2:41PM
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They're looking for an installer-- not the product. :-)

    Bookmark   March 2, 2007 at 4:43PM
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I told my contractor about Kerdi. He said that if it costs more both in product and labor that most of his customers would not want to use it. How much more does it cost to install Kerdi?

    Bookmark   March 2, 2007 at 6:33PM
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Installed, it's about 2.50- 3.00 a foot for the membrane, plus the drain.

    Bookmark   March 2, 2007 at 6:53PM
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if you're using Kerdi, it is a vapor barrier, so you won't need poly behind the cement board.

Putting poly behind the CB and Kerdi over it results in two vapor barriers, something you want to avoid.


    Bookmark   March 2, 2007 at 6:55PM
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Thanks for clarifying that, Mongo. My eyes popped out after reading that comment as our Kerdi is being installed right now and we didn't install a separate vapor barrier membrane. I can breathe normally again.

    Bookmark   March 2, 2007 at 7:15PM
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Like saxmaan, I have been wondering about this myself. On this forum and the john bridge forum, without exception, a vapor barrier is mandated behind your cement board. However, both contractors working in my bath (plumbing and general) do not recommend it. In fact, my GC refuses to do it. He says that any water wicking through will have no chance to dry out if it becomes trapped by the vapor barrier, and you increase the chances of mold growing between the vapor barrier and cement board. This makes sense to me! Like others state in this thread, where is the water going to go when it gets to the bottom? It can't drain into the tub due to caulking, and now you've trapped it in the substrate where it has to wick back through the tile/grout into the bathroom, which seems to be the path of most resistance. Since my contractors are the ones guaranteeing their work, I am going with their judgment and not installing the vapor barrier behind the cement board. Additionally, I can't find anything on the USG (manufacturer of durock) or James Hardie sites (see link) that mandates the vapor barrier. Closest thing I can find is "use a vapor barrier if required by local building codes."

Here is a link that might be useful: hardibacker installation

    Bookmark   March 3, 2007 at 12:08AM
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Like others state in this thread, where is the water going to go when it gets to the bottom?

Like I answered others-- it'll evaporate thru the grout joints--the same way it came in-- rather than into the wall cavity.

It's mandated by TCNA and ANSI spec.

Here's the thing. If you've been to John's site, you've been told by countless pros that it's necessary. None of us "have a horse in this race" so to speak-- it doesn't matter to us one way or the other. We don't make any money in here. We're here strictly to help homeowners out so they don't run into some of the nightmares you read about. If you go without the vapor barrier, your shower won't have HALF the life it would have WITH it. it's up to you what you do with this information. The tile police are not going to show up at your door, if you decide to follow your GC's advice and go with out. :-)

    Bookmark   March 3, 2007 at 7:56AM
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Can someone clarify the caulk issue. When the poly vapor barrier gets install over the studs, and then over the tile flange, you need to cut it clear and caulk that this correct. In reality, when you cut the poly sheet, it no longer hangs over the flange...won't this make the water than drain into the wall cavity at this point?

    Bookmark   March 3, 2007 at 10:10AM
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You don't cut it back that far. What I'll usually do is cut it right at the point where the flange turns and becomes the deck. Then the water will hold there until it evaporates thru the grout.

    Bookmark   March 3, 2007 at 11:05AM
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Great info here. I am installing a Kohler air bath tub. Old tub is out, ripped out the old tile glued right to the drywall. I did get confused when I read to use a vapor barrier or waterproofing, not both. It makes sense not to use both, I thought redguard waterproofing might be better, but I will go with the roofing paper. I have one or 2 questions myself. I read somewhere, the backerboard should stop about 1/2 inch above the tub lip to prevent water from wicking upwards? with a tub, where should I stop vapor barrier? I was thinking go to the floor and have one piece go over the tub lip. Would it help to waterproof the floor under the tub. I am going to make a mud base for support with poly liner, so tub does not stick (per kohler instructions. I will hopefully tile, or use a 3 piece wall kit if I run out of time.

    Bookmark   June 17, 2007 at 5:20PM
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"He (GC) says that any water wicking through will have no chance to dry out if it becomes trapped by the vapor barrier, and you increase the chances of mold growing between the vapor barrier and cement board. This makes sense to me!'

How can mold and mildew form and thrive between poly sheeting and cement board? Mold and mildew need food to survive. That food it cellulose. Cellulose is wood or wood-derived products like paper. Paper facing on kraft-faced insulation, paper facing on drywall, etc.

Moisture trapped between a 6-mil poly vapor barrier/drainage plane and the cement backer board has no cellulose in sight, so the green stuff can't thrive. The moisture will eventually dry out back into the bathroom, just as Bill described.

No poly vapor barrier? Then when the moisture gets behind the cement board and wets the the kraft paper facing of the insualtion, or the framing, then you get the funky green stuff. Moisture, spores, and food equals the dreaded green funk.

It's tough to get your head wrapped around these concepts, but trust me, the way Bill and others have said is correct.

Your GC is wrong. He may have good intentions, but he's just uneducated.

Building Science is a continually evolving process.

Changes to the building code normally lag bulding science by as much as a decade. Sometimes even longer.

Too many builders don't learn about or apply changes made to the code until several years after the code has been changed.

Many "rotted wall" stories are from having vapor barriers behind greenboard. Trapped water and the paper facing food source on greenboard leads to the dreaded green funk, soon followed by the wetting and rotting of the gypsum core of the greenboard.

But who here has been tiling over greenboard? Only the guys who like greenboard because it's cheaper than cement board and easier to cut and hang than cement board.

Greenboard was allowed by code as a tile backer in wet areas until January of '06. Yet I've NEVER used it as a tile backer.

Doing what's allowed by code isn't always the right way to build. Remember, code is the MINIMUM building standard. In some areas of construction the minimum is good enough. In other areas it's far below. That's why the code evolves.


    Bookmark   June 17, 2007 at 6:08PM
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Cement board is not waterproof! It wicks moisture through to the vapor barrier. The tub and board manufacturer's tell you to leave a 1/4 inch gap above the tub tile flange and then seal the gap with silicone, but the vapor barrier should overlap the tile flange. Doesn't that mean that you are applying the silicone to the vapor barrier which will cause the moisture to build at the base of the wall? Brick layer's leave weep holes at the base of brick walls to solve this.

    Bookmark   March 6, 2011 at 11:13AM
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