Contractor doesn't use vapor barrier behind tiles in tub

Anna VegiaAugust 11, 2008


After following this forum for awhile, I've noticed that it is strongly recommended to install some sort of plastic sheeting behind the cement backerboard behind tile installations that will be getting wet.

I spoke with my contractor today who said they put up Hardiebacker or Durock cement board, and that it waterproof enough, so they don't put up any other vapor barrier. It sounds like they've been doing it that way for years. But I could sense some hesitation, almost like he was thinking in his head "wow, I wonder if I should have been putting that stuff up?". He said that leaks would usually occur around the shower valve, and plastic sheeting isn't going to help there anyway.

Should I insist on it? What should I do?



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Hi Jenna -

Your contractor has been doing it wrong for years -- many have...

Hardiebacker and Durock are more water-resistant than ordinary sheetrock, but they are certainly not waterproof. If you ever see a scrap of Durock that has been left outside in a wet environment for a few weeks, you'll see what I mean.

In a finished shower, moisture will go through the grout lines very easily, and even through the tile faces themselves to some extent. That moisture will seep into the Durock where it sits until it evaporates (slowly) or continues on to the wooden studs behind, causing them to rot. A moisture membrane behind the Durock helps slow this process down A LOT, preserving the life of the wooden framing studs behind.

What's even better is a relatively new product called 'Kerdi' that's applied on top of the Durock. Do some reading on this board, and have your contractor do some research on Kerdi, and the benefits will become apparent very quickly. The over-simplified version is that Kerdi is a moisture membrane that goes between the tile and the Durock. The Kerdi layer prevents the moisture from ever reaching the Durock, dramatically lengthening the life of your installation.

Of course, for leaks that occur at the valve, none of these precautions will help much... There are moisture alarms you can buy that will notify you if there's a leak, and leaving an access panel behind the valve will be invaluable then.

    Bookmark   August 11, 2008 at 11:07AM
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I've noticed that it is strongly recommended to install some sort of plastic sheeting behind the cement backerboard behind tile installations that will be getting wet.

Actually, it's not. It's REQUIRED.

    Bookmark   August 11, 2008 at 11:41AM
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Anna Vegia


Based on some of your comments in earlier threads, I had a feeling it was required. The problem is, I can't figure out which county/state/federal agency to contact. I want to be able to back up what I say with cold, hard facts before I call the contractor back. Do you have a section of the IRC can I can refer him to?

This makes me nervous. I want everything built to code, and I wonder what other important precautions he's not aware of, or chooses not to follow!


    Bookmark   August 11, 2008 at 1:02PM
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Not IRC-- TCNA (Tile Council of North America). If he checks Method B412 (method for constructing bath walls where a showerhead is located), it specifically states:

membrane behind backer unit[backerboard]to lap over face of flange of tub unit or prefabricated shower receptor

These specs are all ANSI Based (American National Standards and Specifications Institute-- which is where ALL building specs are found here in the U.S.).

Matter of fact, the definition of "membrane" can be found in ANSI 108.2, section 3.8:

3.8 Membrane or cleavage membrane
Membrane material shall be one of the following:

3.8.1 Roofing felt: ASTM D226 (15 lb. asphalt saturated) or ASTM D227 (13 lb. coal-tar-saturated [which is next to impossible to find any more]).

3.8.2 Reinforced asphalt paper, duplex type: ASTM C171

3.8.3 Polyethlene Sheeting, atleast a nominal thickness of 4-mil (100 microns): ASTM C171

    Bookmark   August 11, 2008 at 6:33PM
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FYI, one reason you can't find coal-tar saturated roofing felt is because coal tar is a known carcinogen linked to neuroblastoma (a nervous system cancer most common in children and teens). Shouldn't be around people in large quantities. Another is that sources have dried up. It was a common by-product of coal gassification (the process of making natural gas from coal) commonly used in the first half of the last century, but now mostly abandoned.

    Bookmark   August 12, 2008 at 9:11AM
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Anna Vegia

Thanks Bill. I spoke with the designer yesterday, and asked her if we provide the poly sheeting will the crew put it in for us, and she basically said no. She said that having any kind of plastic in the walls makes her nervous because of mold. Then she said it's like a Ziploc bag, where mold would just sit and not be able to evaporate. I'm wary of having these guys put it in if they've never done it before, they could potentially make a bigger mess than we're trying to avoid. I don't know what to do.

ButI'll definitely steer clear of the coal tar :)

    Bookmark   August 12, 2008 at 9:33AM
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Roofing felt will work to and will "breath" somewhat. Should be a good compromise to all. Kerdi is a better solution but is more $$$ and more difficult to the thick headed and ignorant. Tell you designer / contractor whoever that their tile job is incorrect like Bill V. says. TCA is a wonderful resource. Also tell her it's not really up to her anyway - "It's my home" and mold NEVER evaporates, once started you've got it till remediated. The water that help cause it can dry up but the mold is still left behind. Better to keep moisture from getting to the mold's food source (wood studs). That's what the sheeting is doing! Besides her insurer is most likely not going to cover her/you for mold related claims anyway.

    Bookmark   August 12, 2008 at 11:48AM
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Want to hear the kicker? Mold is one of the things that the vapor barrier will help ELIMINATE. Without it, moisture can get into the voids of the wall. Moisture hits the vapor barrier and stops there, where it evaporates back out through the grout joints.


Jennalarc, You can tell your designer for me that she's a shoemaker, and if she's got any questions, you show her where to find me!!

    Bookmark   August 12, 2008 at 4:53PM
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Text this to your designer:

"OMG, lol, Mongo sez u r a dadwdks!"

Can't figure out what I wrote? Use your imagination. Then use it some more. And you're probably still coming up short in my description of her knowledge of best common building practices.


    Bookmark   August 12, 2008 at 5:22PM
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Not to hijack a post, but is this sheeting required behind solid surface shower panels? I'm planning on Swanstone. Thank you.

    Bookmark   August 24, 2008 at 5:52PM
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