Second Thinking Shower Waterproofing

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Cleveland, OH
I finished my rough in and pan install for my basement shower. The plan was to use HardiBacker and some form of membrane to seal it. I read that Hardi is a pain in the ass to apply anything to because of how absorbent it is, so I am thinking of taking those down and going a different route. I have a few options and I would really like the opinions of some professionals on this one. Cost is not restrictive, I want to do this right the first time.

Confusing things:
Hardi calls for modified thinset
Kerdi calls for unmodified thinset
AquaDefense calls for a mortar with polymer

Will the unmodified stick well to the Hardi, what about porcelain tiles and glass mosaics? Old school thought is to use modified for those.

My options:
Kerdi over wet Hardi
AquaDefense / Redguard over Hardi (with a primer coat)
Replace Hardi with Kerdi board

My concerns are that I am a long shower taker and wall leaner. I want the wall to be strong and not flex and crack. The studs are on 12in centers. It is very possible that I am being paranoid and all of those methods will last a long time, but I don't do this for a living and I want to do something that will last a long time with minimal maintenance.

My current thought is to tape and mud the joints on the Hardi with good modified thinset. Then put on a prime coat of the roll-on (primer G or diluted redguard), then a full coat and then tile with UltraFlex 2 or equivalent.

I'm gonna go have a beer, because this is confusing as hell!!!!

Thank You


Retired Defense Industry Engineer xxx
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New England
Hardiebacker is classified as a fiber-cement board. The TCNA handbook (the industry bible) lists two different conventional shower build methods for cement boards, based on whether they are fiber-cement or not. With Hardiebacker, you must maintain a gap above the pan to the board. The fiber in Hardiebacker is listed as cellulose, and while that makes the Hardie stiffer, it also means that getting it soaked (like from the end if it's buried in the mud pan), that it can degrade.

Hardie is very thirsty, and that makes attaching anything to it more of a pain. There's a guideline on prepping any surface for best adhesion called SSD, saturated, surface dry. IOW, on ANY surface, you'll get better performance if you first achieve SSD prior to applying your thinset (or, with RedGard, applying a primer coat of diluted RedGard). What you don't want is to have all of the moisture sucked out of the material before it has a chance to cure or set up. Keep in mind that with a cement based thinset, the water does two things:
- makes it spreadable
- gets chemically combined with the cement to actually allow it to cure (excess will eventually evaporate)

If you're planning on Kerdi, you CAN apply it over HardieBacker, but it would have been much less expensive and easier to just do the walls in drywall. If you're using Kerdi over it, you might find that a garden sprayer works well to mist the wall first. You should still wipe it first with a wet sponge to remove any dust, but it can often take more than a few passes to achieve SSD with that stuff. It's not too wet unless the surface has liquid water on it. Because Hardie is so absorbent, if you get a bit too much on, you generally don't have to wait too long.

With the stud spacing you have, the 1/2" Kerdiboard would work fine. It is designed to work at up to 16" OC, so 12" would be even stronger. If you're doing a complete Kerdi Shower, if your Hardiebacker is in the pan, it's not an issue, as the whole thing is then considered in a dry area.

CBU (aka cement boards) are NOT waterPROOF, but nominally, not affected by being wetted as long as they are used as specified in their installation instructions. The same is true for a shower, the liner is the waterproofing and the pan must be waterproof. The walls, just need to be water resistant, but making them waterproof has some advantages, but it must be done per manufacturer's instructions and according to industry guidelines for reliability.

For building a shower, my go-to place is where you'll find lots of professional tile setters and lots of help to build a reliable, long-term shower (or anything tiled).
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