Which mud for drywall & hardie joint

[Do It Myslelf]

I’ve come to a point where I’m not sure what to do in my bathroom remodel. The hardie board and drywall joint is boggling me for some reason. So my question is which joint mud should I use in the pictured joint. Would I use drywall mud with fiberglass tape or mortar with tape made for cement board application?
As you can guess, I am a DIY girl, I’m new at this and still learning but I want my project done correctly because I’m also a bit OCD.
Can you tell that from all the mudding spots? Lol!
I need guidance, please!

 

[Jadnashua]

At this stage, you have a couple of choices.

One thing that appears to be missing is a moisture barrier behind the HardieBacker. When building a shower surround, you have two ways to manage moisture, you either put a moisture barrier behind the boards on the studs and lap it over the tiling flange, or you put waterproofing on top of the wall board. If you don't want to take all of the Hardie off to install a moisture barrier (a sheet of plastic or roofing felt is usually used), then, you need to deal with surface waterproofing. There are a couple of different methods:
- a sheet membrane, sort of like wallpaper
- a painted on waterproofing material

If you choose a sheet membrane, the membrane itself will take care of your CBU (cement backer board) joints. If you want to do a painted on waterproofing, you must use a special alkaline resistant mesh tape on the seams and use THINSET, not drywall mud...let that cure, then add the painted on membrane. That tape may look the same as drywall tape, but it is not...it must be alkaline resistant, or it will disintegrate over time in the mortar. Hardiebacker is VERY thirsty, and if you don't wet it down first, it will suck all of the moisture out of both any thinset or the waterproofing membrane and make further installation difficult. It's not too wet unless there is actual liquid water on the surface...saturated is fine, dripping is not. This will make a HUGE difference in how easy it is to deal with.

Pay particular attention on the tub to wall sealing of your moisture control.

Should you want to go with a sheet membrane, one company that has good products and excellent videos to see what I'm talking about, is www.schluter.com . Should you decide to go with a painted on waterproofing, RedGard, Aquadefense, and several others all work. While it might seem easier to use a painted on membrane, I think it's a little harder to ensure even coats without runs or pinholes; plus, you have to prep the area first by taping and thinsetting the seams, and waiting at least a day before you can paint on the waterproofing (you have to wait in between coats of that as well).

Pay particular attention to the niche you created. The cement board is NOT waterproof, it's just not damaged by being wet, and it is a compatible surface and stable for putting tile on top of.

 

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[Do It Myslelf]

There is a heavy mil plastic sheeting behind the hardie board. And I will be using the thinset for the hardie joints. But my question is should I use that same tape and thinset on the hardie/drywall joint to the left of the window?

 

[Jadnashua]

The plastic should come in over the tiling flange and be sealed to the tub.

But, if the tile will be covering the cbu/drywall joint, use the same thinset and tape as on the other seams of the cbu. Thinset sticks to drywall just fine. If it's painted, scuff it up to take any shine off, rinse any dust off, then do it.

 

[Do It Myslelf]

Ok that’s what I’ll do then. The pic isn’t that great, but the plastic is hanging down over the tub flange and just touching to top of the tub. When you say “sealed to the tub” you mean with a bead of silicone between the plastic and flange? That’s what I’m planning on, unless there is a better way. Sorry for all the questions! And thank you for your help on this.

 

[Jadnashua]

Yes on the silicon...bond it to the tub so if there's ever any moisture behind, it can flow into the tub and not behind it into the wall.