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Unread 10-11-2019, 07:04 PM   #1
Marydoesntknow
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Will this create a “mold sandwich”

Hi, I apologize for my ignorance but I’m trying to understand the correct way to build a shower. We are hiring a company in Los Angeles to add a bathroom to our home. I’ve seen some posts about double moisture barriers later causing issues, but because of my unfamiliarity with the industry terms I am unsure if this would create a problem. Our contract states they would:

1. Frame new stand up shower
2. Install new “moisture barrier” double black paper and chicken wire for shower walls
3. Apply new cement float at tile working areas only
4. Apply RedGard waterproofing and crack preventing membrane on top of new cemented areas

This is a licensed firm with a good reputation and I have been happy with my interaction with them so far. I just want to be sure because it is a lot of money for us. I really appreciate any insight anyone may have. Thank you!
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Unread 10-11-2019, 07:37 PM   #2
jadnashua
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Yes...none of the waterproofing systems is totally vapor proof, so any moisture that gets into the wall, with a vapor retarder on both sides, will potentially build up...it needs to be able to dry out at least one side.

A mudded wall with a vapor barrier behind it should be all you need on the walls. Being CA, are they doing a hot mopped pan?
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Unread 10-11-2019, 07:49 PM   #3
cx
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Welcome, Mary.

While I think I understand generally what he intends, I don't fully understand just what he might mean by "at tile working areas only" I suppose that could mean in the entire wet area of the shower, which would be the customary way of dong things.

As for the "double black paper" and chicken wire, that would depend somewhat on just what that paper consists of. If he's using a pre-papered "stucco mesh," that would be less vapor permeable than a couple layers of #15 roofing felt. If he plans to use a direct bonded waterproofing membrane (RedGard) on the interior face of the shower, he should actually forego the moisture barrier behind the wall mud. Unfortunately, I've never tried that over "chicken wire" and don't know how well it would work. Works OK when using expanded metal lath instead of the chicken wire, and that's the industry recommended material to use in that application. But a whole lot of wall mud has been installed over chicken wire, 'specially in California, over the years.

Bottom line is that you will not have an actual vapor barrier behind your wall mud, only a vapor retarder and your wall cavities may allow for adequate ventilation, depending upon whether they are interior or exterior and how they are built. And there is also some consideration for how the shower receptor is to be constructed. This will be a typical Left Coast "hot-mop"construction?

Vague enough to suit you?

My opinion; worth price charged.
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Unread 10-11-2019, 08:21 PM   #4
Marydoesntknow
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Thank you all! Yes, it will be a hot mopped shower pan.

Is the mud you guys are referring to the cement float he mentioned in the contract?

If I’m understanding correctly, I should ask him not to do the “moisture barrier double black paper”? It is on exterior walls. Thank you again!
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Unread 10-11-2019, 09:22 PM   #5
cx
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The "mud" we're referring to is what the ANSI standards refer to as a Portland cement mortar. Unfortunately, mortar is a term used in many other applications in the ceramic tile industry.

In various regions of the country the Portland cement mortar used for walls will be called wall mud or fat mud and in other countries it may be called render. I don't think I've heard it called "cement float," but I clearly haven't heard them all. It's almost universally a mixture of sand, Portland cement, and slaked lime.

Portland cement mortar is also used for floors, but then it is only sand and Portland cement and is generally called floor mud or deck mud or dry-pack or.......other.

Yeah, it can be very confusing if you're not used to it. For your purpose, I think it's safe to assume that your contractor is talking about fat mud. Simple enough to just ask him, though, and then you'll be sure. He'll tell you it's 4 parts sand, 1 part Portland cement, and 3/4 or 1 part lime. Or something similar to that. Then you'll know, right?

Lots of guys in California still do showers the old way with wall mud. It's a dying art, but it makes a very good shower when done correctly. I just wish those guys would get into at least the 20th century and get rid of those hot-mopped pans.

My opinion; worth price charged.
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Unread 10-12-2019, 06:35 AM   #6
Davy
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Nearly all the old mud showers I've tore out had lath stapled right to the studs and I've seen a few with a rotten stud here and there. Most had no moisture problems at all in the walls and no rotten wood, and that's with no tar paper at all. Tar paper does breathe well and in my opinion allows any moisture to dissipate.

I've never mudded over chicken wire. It's not actual chicken wire but it looks similar to it. It's a heavier version made for plaster work. Maybe it's possible to apply a scratch coat to chicken wire nailed right to the studs but I would think the tar paper needs to be behind it to keep the mud from falling thru.

Even with the tar paper on the studs and lath over it, I add Aqua Defense over my mud in niches overlapping onto the wall. I feel the tar paper breathes enough to avoid a moisture sandwich. I tile right to the mud on my walls and have for many years without any issues.

In my opinion, I'd let them use the tar paper and leave the Redgard off except in the niche (if you have one).
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