Taping Hardi with mesh tape
Here’s the plan and the questions:
Hardibacker 500 inside showers will be taped with Alkali Resistant Mesh tape and thinset.
Hardi outside of showers will be taped with ARM tape and joint compound (and textured with all purpose joint compound).
–Hardi says not to use paper tape—alkali environment will destroy it. You must use ARM mesh tape.
–Some subs and “experts” say that taping with mesh tape and all purpose joint compound will crack, although newer “cross-fiber” weave mesh tape is stronger/better than the older Leno weave.
–Some say that the solution is to use ARM tape and setting compound—supposed to be as strong as paper taping.
–So, I’m all set, right? (ARM tape and setting compound)……. Well no. The bag of setting compound says it must be applied with 1/8″ minimum thickness to avoid drying out prior to the chemical reaction taking place.
—So, here’s the questions:
-1/8″ seems too thick for a first pass on a mesh tape joint. Am I wrong?
-Would it help if I primed the Hardi with something like Kilz2 to keep the Hardi from immediately sucking out all the moisture from the Joint Compound. One sub claims that texturing Hardi with all purpose joint compound will eventually lead (years?) to the texture falling off the wall, but had no explanation as to why. Maybe the primer would help that situation also?
–So, what do I do? I would appreciate anyone’s comments and experiences on these issues.
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Replies
John,
My advice is to follow the manufacturer's instructions. When HardieBacker 500 is used on walls, joints are filled with mortar, not drywall compound. The instructions require:
"1. Fill all joints with mortar (see “Materials Required”).
2. Embed 2" wide high-strength alkali-resistant glass fiber tape in the mortar and level."
In the "Materials required" section, the mortar requirement is described:
"Mortar for walls: Latex or acrylic modified thinset (complying with ANSI A118.4) or Type 1 mastic (complying with ANSI A136.1)."
Here is the link to the document (the installation instructions):
http://www.jameshardie.com/d2w/installation/hardiebacker-us-en.pdf
Thanks, Martin, for the response.
Has it been your experience that Hardi joints taped with ARM tape and modified thinset (in exposed/visible areas outside the shower that will be painted) don't typically crack?
Why are you installing cement board in areas that are not subject to bulk water? I could understand that approach if you planning to tile, but that does not appear to be part of your plan. (Or am I misunderstanding? Would not be the first time.) A mold-resistant drywall would be easier to finish.
There are areas where the shower wall continues into the room. So, rather than transition to gyp rock where the tile stops, some folks just continue the Hardi into the room--i.e. just hang one continuous piece.
It's easier to hang, there is no transition to worry about, and it provides protection from any errant water spray out of the shower (or shower/tub, or tub). There are also no worries about the thickness difference between Hardi and gyp rock at that intersection. And in general, it could be considered an upgrade from greenboard.
So, once that decision is made, the question is how to best tape and finish it.
Thanks for clarifying, John.
I read on another site that the mastic on the seams needs to be smooth out with a standing block before the product completely sets up. That wait-time help explains why some contractors prefer drywall compound (even if it's violates the manufacturers instructions).
John,
Maybe I'm not understanding what you are proposing, but unlike when you transition to green board or regular gypsum, don't you have to completely skim coat the Hardi backer where it isn't covered by tile to get a smooth wall? That seems like a lot of work, and the mud won't be water resistant.
Malcom, Hardi backer has a smooth side and a rough side. The smooth side can be painted (actually either side can be painted). If you plan to texture with joint compound, then it really doesn't matter that much which side you use. If your expectations are for a super smooth painted wall, then the "smooth" side would have to be evaluated based on your definition of smooth.
In any case, a skim coat is not required unless you want a super smooth wall. But this is also true for gyp board (the skim coat and super smooth part).
No, the joint compound is not water proof, but it will be coated with paint. And the underlying substrate (cement board) is water and mold resistant. No, it's not perfect, but if you wanted perfect, I suppose you could texture with thinset.
What it can do for you is to eliminate the gyp board immediately outside of a tub/shower that can turn to mush if your occupants are not careful about water spraying on the wall (or dripping down the side of the tub) every time they take a shower.
John,
Q. "Has it been your experience that Hardi joints taped with ARM tape and modified thinset (in exposed/visible areas outside the shower that will be painted) don't typically crack?"
A. Sorry -- I don't have enough experience in this area.
When I tiled my own bathroom, I used cementitious backerboard. This was in the pre-internet era, when research was more difficult. I ended up taping the joints between the sheets with fiberglass mesh tape and my own homemade mortar (sand, cement, and water). Thirty-six years later, everything is fine.
Martin, thanks for the empirical results.
Are any of your successful joints exposed or are they all behind tile?
Extrapolating from your success, I would think that modified mortar would be a step up.
Obviously this isn't the only thing that can cause a crack (building movement/framing movement, etc), but it's encouraging.