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Unread 08-11-2021, 02:02 PM   #1
MinnMax
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Bathroom Remodel Questions

Hi Everyone. I’ve been reading this forum, watching videos, and scouring the internet to get prepared for my small bathroom remodel project. I think I have most of it figure out, but I have one last question (for now).

I am redoing an alcove shower with one exterior wall, which is opposite the plumbing wall. I have removed the tile and drywall on all three walls and have decided that I want to use Johns Manville GoBoard for the walls and then tile, but I'm wondering how to handle the exterior wall.

According to the Minnesota Residential Code (which is almost identical to IRC 2018) , "A class I or II vapor retarder is required on the interior side of frame walls in Climate Zones 6 and 7. Class II vapor retarders are permitted only when specified on the construction documents." (I'm in climate zone 6.)

I have removed the R11 Kraft-faced fiberglass batts that were on the interior side of the exterior wall and I'm wondering what is acceptable to put in their place in MN. Can I put non-faced batts and then GoBoard on the wall to act as the vapor retarder?

In this thread from 20218 it is mentioned that ESR-3767 "specifically requires that a vapor barrier be used behind the GoBoard in shower applications", but it appears that the ESR was updated in 2021 and no longer has that language. The current document says, "GoBoard composite backer boards are for use as a water resistive barrier when installed in accordance with Section 4 of this report." I'm guessing that's different than a vapor retarder, but I'm not sure.

Any help would be appreciated.
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Unread 08-11-2021, 08:20 PM   #2
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Max, here's my problem with your dilemma: I don't know what GoBoard is. I've never seen nor touched a piece. From discussions here on the forums I got the idea that it was another foam board, but when I read their ICC-ER, ESR-3767, I see this description, "GoBoard® composite backer boards....used as a tile backer board provide equivalent performance to ANSI A118.9 cementitious backer units (CBU)." I don't know what their "composite" is. I see further that it says the GoBoard "provides equivalent performance to ANSI A118.10." That is the standard for bonded waterproofing membranes in the tile industry. So, it acts similar to a CBU, and it also acts similar to a bonded waterproofing membrane, but is actually neither? And they previously required a vapor barrier behind the board in a wet area application, but now they don't? What's a fella to do? I dunno.

If you wanna use the product, I recommend you have a talk with your local code compliance official and see what you can work out. If he sees the product and reads the specs and the ICC ER and is content to have you use it and is content not to have an additional vapor barrier material behind it as required in your local code, fine. If he still requires an additional vapor barrier material over your stud walls in addition to the GoBoard, go with that.
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Originally Posted by Max
I have removed the R11 Kraft-faced fiberglass batts that were on the interior side of the exterior wall...
I find it difficult to believe that anyone would build in your climate and use only R11 insulation in the walls, but that's a whole 'nother discussion, eh?

My opinion; worth price charged.
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Unread 08-12-2021, 07:16 AM   #3
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I appreciate the feedback. Maybe someone who has used GoBoard will chime in, but I think your right that I should ask our local building inspector.

I agree about the insulation. The house was built in 1970, so I don't know what the requirements were then.
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Unread 08-20-2021, 03:25 PM   #4
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In case anyone is curious.

I called the local building inspector. He read the ESR and called me back. He still wasn't 100% sure, but his reading of the use of the term "water resistive barrier" in the ESR vs. vapor retarder in the code suggested to him that I should have an additional vapor retarder behind the GoBoard used on the interior side of the exterior wall. He suggested that I reach out to Johns Manville to get a definitive answer.

I spoke with someone in JM support and he said that while GoBoard is waterproof, he is not an expert our building code. I asked if using an additional moisture barrier (Kraft-faced insulation or plastic sheet) behind the GoBoard would be a problem, and he said it would not. He also made the point that unless the entire wall is GoBoard, it would not be a continuous barrier, in which case I should definitely have something behind it that is a continuous vapor retarder. I hadn't planned to tile all the way to the ceiling, so there will be a foot+ of drywall.

I think I'll use Kraft-faced insulation in that wall, which is what was there.
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Unread 08-21-2021, 02:09 PM   #5
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GoBoard is a polyisocyanurate foam as opposed to an extruded or expanded polystyrene, plus it has a facing on front and back like other tile-able foam boards.

Max, your question isn't so much of a tile question as it is an insulation/building science question. I've asked the same question but I don't know what the "official" answer is. I think you're probably handling it the best way, though.
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Unread 09-03-2021, 11:41 AM   #6
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I have more questions about this project, this time about the floor. We're hoping to use 12" x 24" porcelain tile for the bathroom floor, but I have a number of questions before proceeding.

1. When using the deflecto tool, does a stud wall in a finished basement running perpendicular to the joists have any bearing on the joist length I would enter? (Or does the tool only care about load bearing supports).

The joists are 2x10, unknown wood in good condition. I was unable to find any markings on the wood, but the house was built in 1970 in the Minneapolis/St. Paul suburbs if that's enough of a clue for anyone to figure out what type of wood it might be. They run 14' from the foundation wall to an I-beam in the center of the basement. There is wall built in the basement 10' from (and parallel to) the foundation wall. Using the 14' calculation for unknown wood I get a a deflection of L / 290. If I run the tool using 10' for the length or change the type of wood to SYP or Douglas Fir, then I am above L / 360.

2. There is an adjacent bathroom that has 12" x 12" tiles for the floor that shows no sign of any issues. That tile has been in place for at least 11 years so I'm inclined to believe that tile in the other bathroom (5'x5', in case that makes a difference) should also be ok. Does that make sense, or would adding additional weight potentially cause problems?

Assuming I go with tile, my next questions are about the subfloor. The subfloor is 1/2" plywood with 3/4" plywood on top of that in the area to be tiled. There is currently vinyl sheeting on the floor that's basically flush with the hardwood floor in the next room. The tile we had in mind is 3/8" thick, so with those tiles and 1/4" CBU, we'd have a 5/8" rise coming into the bathroom. We could live with that but would prefer a smoother transition.

3. Would it make sense to remove the 3/4" plywood and put thinner plywood in it's place?

4. If so, how thin could it be?

5. Does using Ditra vs. CBU change the need for that additional layer of plywood or how thick it needs to be?

FWIW, the tile floor in the other bathroom is level with hardwood in the hallway, so whoever did that tile figured out a way with seemingly no ill effects. It was done by prior owners and I can't see what's under the tile.

I understand that I can do whatever I'm comfortable with, but I'd rather live with a 5/8" transition into the bathroom and/or use some other kind of flooring than end up with cracks in my floor.


And can a moderator change the title of this thread? How about Bathroom Remodel Questions.
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Unread 09-03-2021, 01:22 PM   #7
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1. Joist deflection is based upon the unsupported span of the joists. If the wall below is load bearing, it divides the span; if not, it doesn't.

2. You need to evaluate the joist structure and the subflooring independently in each area to be tiled.

3. No. It would make sense to remove both layers and put nominal 3/4" plywood as the first layer. Could technically be the only layer if your joist spacing is 16" on center or less.

4. See #3. You could theoretically remove the 3/4" plywood and glue-laminate a second layer of nominal 1/2" plywood if the first layer is in pristine condition and you use a full spread of wood glue and lots of screws to do the laminating. Far easier to type than to do.

5. No, but it would make the finished floor slightly lower.

My opinion; worth price charged.
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Unread 09-03-2021, 03:04 PM   #8
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Thanks, CX. That all makes sense. I forgot to mention that the joists are 16" on center.

Now it's up to me to decide what to do, since the joist structure as it is is not rated for tile.
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Unread 09-03-2021, 08:41 PM   #9
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Wait…didn’t you say you’ve got a wall 10’ from the foundation that reduces the span so that you’re up over L/360?

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Unread 09-06-2021, 10:51 PM   #10
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Max, you still there?

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Unread 09-07-2021, 08:44 AM   #11
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Yep, I'm still here. Just took the weekend off

There is a wall 10' from the foundation, but as far as I know it's not load-bearing. It was built to separate the laundry/mechanical room from the finished part of the basement (long before I moved in).
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Unread 09-07-2021, 08:54 AM   #12
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Could it be made to be load bearing?
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Unread 09-07-2021, 11:09 AM   #13
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I'm not sure if the wall can be made load-bearing. Below is a picture of where the wall meets the joists under the span that the bathroom floor would be on. There is some space to add cross-bracing 3 to 6 feet from the foundation wall, if that would help with deflection.
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Unread 09-07-2021, 04:00 PM   #14
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You'd need to at least be able to put a header across the top of that door, Max. That's a standard 6'8" door frame?

The rest of the wall, presuming it's sitting on a sufficient surface, could be made load bearing by "stacking" studs directly under the joists that are well offset from the existing studs. You could just build another wall against the existing, but you'd still have the problem of the door.

My opinion; worth price charged.
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Unread 09-08-2021, 08:54 AM   #15
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Yes, it's a standard 6'8" door frame. I don't see a way to put a header in, since I don't plan to rebuild that wall/door.

I'm wondering if installing metal bridging between the joists would have any effect on the deflection. I have space where I could do this about 4' from the foundation.

Baring that or somehow determining that the joists are Douglas Fir or SYP, I'm thinking vinyl plank flooring might be in order.

I feel like it's worth mentioning that the other bathroom with a tiled floor is above the same set of joists, just further from the outside wall. I just wanted to throw that out there before giving up on tile. I guess I'm hoping someone will reassure me that since there have been no problems with that floor, there likely won't be issues with tiles over a different part of the same span.
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