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Velkro615
06-06-2018, 01:38 PM
I've looked hard for a clear answer on this but I've been unable to find.

I am remodeling my bathroom, fiberglass tub is coming out, and a walk in shower is going in.

I am putting up CBU, probably Wonderboard since its at a local store.

I am going to Redgard the CBU and I understand that if I use Redgard then I do not need a moisture barrier on the other side (inside the wall) since double moisture barrier is a big no no.

My question is can the CBU touch the wall studs directly? I always heard cement on wood was also a no no since the wood could pull moisture from the cement.

Do I wrap just the stud with a moisture barrier plastic? Like maybe wrap just the face and half the sides, stapling at the sides?


Thanks,
Velkro

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Lou_MA
06-06-2018, 02:55 PM
No. Fasten directly to studs.

Velkro615
06-06-2018, 03:12 PM
Thanks Lou!

Can you tell me the "why" behind why you can attach CBU directly to the wood studs?

I'm likely just paranoid, its just hard for me to get past cement against wood. Since I didn't find much of anything on wrapping studs, this shouldn't be a problem. But then I thought the lack of findings was due to most folks using a full moisture barrier inside the wall and no Redgard.

ZZZK
06-06-2018, 04:35 PM
Velkro you are probably thinking about wood over a slab on grade or wood in a basement over concrete floor/walls. Concrete and cement, like wood, are hydrophilic meaning they absorb water like a sponge or a paper towel (which is actually made of wood). They will readily absorb moisture from their environment.

In the case of a slab on grade or a basement the concrete is in direct contact with the soil (which is also hydrophilic) and the soil is what gets raine on and possibly in direct contact with the water table. The soil generally acts as an aquifer. A storage medium for water which is why plants grow their roots in it. So given all the above you can see why concrete in slabs and basements can become quite moist materials especially during the wet season.

Also, because the ground remains relatively cool throughout the year so does your concrete slab on grade and basement. This means when the air is hot and humid and the dew point is right the moisture in the air will readily condense over the cooler slab/basement providing an additional source of moisture. All of this is fine until you put untreated wood over said slab/basement which will be in direct contact with the moist concrete. Because the wood is also highly hydrophilic it will pull moisture right through said moist concrete. However, unlike concrete, wood is organic and thus fungus can grown on it and eat it if it has enough moisture to survive. That is where we have to worry about placing untreated wood in direct contact with potentially moist concrete.

Now when it comes to the cement board hanging on your studs it is no different then the paper and gypsum drywall that is hanging on the studs in the rest of your house (which is also highly hydrophilic). All these materials will readily absorb moisture but only if there is a source of excessive moisture. These materials will all absorb some moisture from the air but not enough to cause a problem in your typical climate controlled house unless there are outside factors such as consistent big temperature differentials between the ambient air and the wall while there is a high relative humidity in the air. That is exactly why we use insulated A/C ducting because in humid environments that can become a source of moisture and mold/rot problems.

Velkro615
06-08-2018, 11:15 AM
Thanks Z, that makes sense.

So when I am making my shower pan preslope base, what do I use to keep the
"deck mud" off of the wood studs? Felt paper? I attached a picture of the area I'm working with.

I've looked at a lot of videos, most are for wood subfloors and not on a slab and are never clear on what keeps the deck mud off of the studs.

cx
06-08-2018, 01:15 PM
You should have blocking between all of the studs at the bottom so you'll have something to which to attach your pan liner. Don't worry about edges of the deck mud touching the studs/plates/blocking. If you wanna be real paranoid, put some roofing felt or polyethylene sheeting over the wood, but it's really not at all necessary.

I would strongly recommend you center your shower drain before you go any further.

My opinion; worth price charged.

Velkro615
06-08-2018, 01:24 PM
Thanks CX.

I was thinking about chopping out some of the slab to get centered. Looks like I'd need to chop out about 15 inches or so and make a trench to get to the center. I guess that would make the preslope task easier?

Lazarus
06-08-2018, 02:30 PM
Oh yeah... :yeah::yeah:

Velkro615
06-08-2018, 03:41 PM
Since the plumbing is in front of the studs, I guess they got away with that due to the fiberglass tub, will I need to reroute these pipes? Seems like the cement board wonít be able to hang flush with these pipes in the way. Picture attached, I think, Iím on my phone now instead of the PC, ha!

cx
06-08-2018, 05:25 PM
Or fur out the studs to accommodate the current location.

My opinion; worth price charged.

Velkro615
06-08-2018, 05:34 PM
Got it. What would you do with the gap between the shimmed out cement board and the existing drywall for the rest of the room? Use a rounded trim piece to cover the difference? I assume Iíd need to cut the drywall out to the ceiling in that wall now

Velkro615
06-12-2018, 05:25 PM
I have all my new plumbing in. I kept everything in front of the studs as it was and will go with furring out the wall. The finished wall needs to sit flush with this construction protection cap that sits on the shower valve. Looks like Iíll need to shim out roughly 2 1/4Ē for the finished tile to sit even with that valve cap. Is that too much of a distance to shim?

Dealing with the same pic as my last post, the cement board is going to stick out 2 inches from that drywall in the rest of the room. Whatís the best way to fill that gap? Strip of cement board turned sideways?

PS, I learned to sweat copper and my joints look amazing Iím very proud lol