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Unread 04-02-2022, 09:52 PM   #1
Fractal20
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Tub surrounds questions: liquid membrane VOCs and backing material

There seems like endless information out there and I have been looking a bit on the John Bridge forums, but I figured it might be worth actually posting at this point. I am going to redo the bathroom in my house, it currently just has large plastic panels glued to drywall. I have a few questions... I'm pretty unsure at the moment what approach to take.

1) I'm leaning towards doing hardi backer or another similar backing board. This is largely motivated by price and the availability locally for things like Kerdi boards. The existing drywall is 1/4 inch and I have read that 1/2 hardi backer is suggested for shower walls. Are there suggestions for how to deal with the gap?

2) The bathroom is the only one in the house, and it is in the center of the house, without the best ventilation. I'm a little worried about the fumes, and have heard the Red Guard can be pretty smelly. I've tried doing some searched about comparing liquid membranes in terms of off gassing, and it seemed like there were some claims that Hydro Ban is a little better. Does anybody have ideas about this?

3) I know Kerdi would avoid the fumes. It does look like the Kerdi membrane directly on dry wall might be cheaper option that would fumes. If I went this route, could it be applied to the existing 1/4 dry wall? The dry wall also has a bit of glue on it that the plastic panels had been applied to. I'm not sure if that would make it impossible to apply the membrane to the existing dry wall?

4) I've read a lot of different opinions about how worried to even be about water proofing on vertical shower walls. It would be simple to just do a plastic vapor barrier behind the hardi backer and not do any liquid membrane on the surface. But again, I've heard varying opinions about this. Any additional varying opinions now?

Any advice would be great. Thanks!
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Unread 04-02-2022, 11:27 PM   #2
jadnashua
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1. One of the advantages of Kerdi and associated options like KerdiFix is its 0% VOCs.
2. I've played with both, but it's been a long time ago, so I don't really remember one way or the other.
3. You really need 1/2" drywall to have a stiff enough surface in case you lean or slip and apply too much pressure against the wall and crack the tile or wallboard. Now, HardieBacker does allow 1/4" on the wall. Note, HardieBacker is a bit tougher to work with, but it can be done.
4. While neither tile, grout, nor cement board products are waterproof, there are millions of showers built without waterproofing on the walls. My preference is to make the shower waterPROOF versus just water resistant. The pan is always waterproof, and I also prefer to have that waterproofing directly beneath the tile rather than having a damp setting bed in a conventional shower pan where water in is expected, as long as it can weep out if built properly.

Putting up the Kerdi membrane isn't that tough when you prep things properly. Don't try to spread too much thinset at once, and wipe the wall well with a wet sponge first, especially if it's HardieBacker, which will suck huge amounts of water out of the thinset making the job really tough if not possible to embed the membrane properly.
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Unread 04-03-2022, 08:29 AM   #3
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Okay, so if I was to do the Kerdi membrane I'd definitely do replace the current dry wall with 1/2 dry wall. If I stuck with hardi backer (whether doing a liquid membrane or kerdi membrane), should I do that as 1/2 inch as well.

In either case above, assuming I do a 1/2 inch thick backing board of some type, would you have suggestions of how to deal with the transition to the 1/4 dry wall? I am only going to tile a portion of the existing walls, so there will be a transition near the ceiling and on the edge from whatever backing I put in to the 1/4 inch thick existing dry wall. I was planning on doing Schluter style trim, but that would still start a 1/4 of an inch up higher off the old dry wall.
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Unread 04-03-2022, 09:15 AM   #4
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If your bathroom has poor ventilation, I'd correct that situation with a proper exhaust vent as step #1 (regardless of going with redguard/ kerdi options).

The VOC from redguard and aquadefense did not strike me as particularly strong, but I did have my vent turned on while applying to large areas.

If it were my house, I'd just remove all the 1/4" drywall and start with new 1/2".
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Unread 04-03-2022, 09:22 AM   #5
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Welcome, Bob.

A geographic location in your User Profile would likely be helpful.

I agree with Phil on both counts.

Having the bath in the center of the house would certainly make adding an exhaust fan more difficult, but I'd try to find a way.

RedGard odor has never bothered me much, but I've never done an actual shower with it.

My opinion; worth price charged.
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Unread 04-03-2022, 09:36 AM   #6
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I've updated the profile, I am in Colorado. So short of replacing all the drywall, there aren't alternatives for the transition to the 1/4 inch drywall? Thanks for your time everybody.
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Unread 04-03-2022, 09:50 AM   #7
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You can certainly make such a transition, Bob. I wouldn't find 1/4" gypsum drywall over (presumed) 16" stud centers to be an adequate wall in any application, but if that's what you've got, and if you're satisfied with it, Just transition from your CBU to what you've got. Any number of ways to do that, depending upon your circumstances, which we do not know.

And, as has been pointed out, you could use the 1/4" Hardiebacker on your shower walls to make the transition easier. I wouldn't do that, but the manufacturer says it's acceptable, and Hardiebacker is a good bit more rigid than most CBUs.
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Unread 04-03-2022, 02:06 PM   #8
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There are two classes of cbu, and HardieBacker is classified as a fiber-cement board. THe fibers are cellulose (wood fibers), and they act like reinforcement in the board, making it stiffer than a regular cbu which is why the manufacturer says it can work on a wall instead of the more typical 1/2" stuff (note, 1/2" Hardibacker isn't really 1/2" thick like most other boards, it's thinner!).
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Unread 04-04-2022, 09:50 AM   #9
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First off, thanks for your patience with my questions. I can get the Hardie hydro defense boards locally. The installation instructions specify that a liquid membrane should still be used on joints if complete waterproofing is necessary. Does anybody know if something like Kerdi band can be used instead of painting on a liquid membrane? Again, my motivation is to avoid the fumes.

I've tried to do a little searching about this. The installation guide does explicitly use the word liquid membrane. This website mentions kerdi band or an equivalent for the seams though: https://www.tileshowerdiy.com/hardie...-hydrodefense/. Any thoughts?

One more thing, the installation instructions mention a flexible sealant for fixture penetrations. Could you just do a similar sealant on the joints?
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Unread 04-04-2022, 11:05 AM   #10
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Bob, if the avoidance of fumes is a decision point you can simply add a moisture barrier behind the Hardie backer wall boards, then tape and mortar the seams, omitting the liquid water proofing all together. The little bit of moisture that may penetrate the seams is of no concern. Zillions of showers have been built that way, and the method is code compliant.
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Unread 04-04-2022, 11:10 AM   #11
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That sounds great to me Dan. I have read that searching around, and come across some conflicting opinions. I think that is the main thing that has made making a decision hard is that there are so many approaches that different people use.

One follow up question on that Dan, I would be tempted to do the hydro defense water proof boards and the vapor barrier and foregoing a liquid membrane on the seams. But would you advise against that for potentially trapping the moisture on both sides in the hydro defense board? I've also read about avoiding "mold sandwiches" where water can be stuck between two waterproof layers, so I'm not sure if this would be one of those cases?
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Unread 04-04-2022, 11:20 AM   #12
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A valid concern. Avoid the potential issue by using regular Hardie panels. Available in 1/4" and 1/2" thicknesses.
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Unread 04-04-2022, 10:08 PM   #13
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Yet more questions, thanks for bearing with me. Also, I'm an idiot and all the dry wall is 1/2 inch, so I don't need to worry about doing a 1/4 inch backer to match at transitions.

1) I like the simplicity of doing a vapor barrier behind and not worrying about a liquid membrane. However, I also had been considering putting in a shower niche (probably a pre-built Kerdi one). I imagine that doesn't go well with using a vapor barrier though, unless it's okay to have a rectangular hole in the vapor barrier for the niche to go into or something? So would a vapor barrier and a niche be incompatible?

2a) The tub flange sticks up 1 inch and is a 1/4 inch thick. The studs have not been previously furred out to be flush. If I did furr out the studs, that will exacerbate making it match the existing drywall at transitions. A couple of places, https://www.diytileguy.com/tile-tub-flange-gap/, https://diy.stackexchange.com/questi...the-tub-flange, seem to suggest that having the backer board end on top of the flange and applying silicone to the gap is okay. Do you think that approach would be valid?

2b) I have a feeling that how I deal with the flange might depend on if I'm using a vapor barrier or not. Would you have a different opinion on question 2a if I was using a plastic vapor barrier behind that came down in front of the flange below the backer board? Or alternatively, if I was doing the hardi backer hydro defense board that is water proof (and no vapor barrier behind), then it seems like to me that any water would nicely run along the surface of the board and then across the silicon and into the tub. e.g. it would be hard for the water to get behind the flange.

2c) I'm assuming this is a no go since I haven't come across anything suggesting it, but could the bottom inch of the cement board have a 1/4 shaved off (so the bottom inch was only 1/4 thick) so it would overlap the flange without having to furr out the studs? I haven't worked with cement board before, but I'm guess "shaving" it isn't reasonable, but I figured it's worth asking.
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Unread 04-04-2022, 10:40 PM   #14
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From what I hear, KerdiBoard is sometimes hard to find with the supply situation, but if you were to use it on the walls instead of the drywall, it's pretty easy to put a rabbit joint on the bottom to go over the tub's tiling flange. That would also allow you to seal it to the flange and make the walls waterproof, making your niche install easier, too.

But, depending on the size of the tile, as long as at least 1/2 of it is supported on the wall above the tub, you can stop the wallboard slightly above the flange so it doesn't potentially squeak if it were touching it.
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Unread 04-05-2022, 07:24 AM   #15
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That was another question that occurred to me. The tile is 4 by 12 subway tile. So if the backer doesn't come down over the flange there is a 1 inch gap. I assume the mortar won't really stick to the flange? Does it seem like the tile could be secured just by the top 3 inches? Or does this mean I definitely should figure out a way for the backer to come over the flange?
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