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Unread 04-16-2021, 04:41 PM   #1
rickurukus
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Aquadefense over Hardiebacker

Hey all,

Just getting finishing touches before I slap up all these walls and make this beast waterproof! Currently have mold resistant sheetrock up and next going to be placing hardiebacker up. I have Mapei Aquadefense for a liquid membrane that I will be using and was curious if I should have rough or smooth side of hardiebacker facing out? Whole shower will be tiled and I plan on using the recommended thinset from Mapei for the Aquadefense over the membrane to attach the tile. Thanks all!

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Unread 04-16-2021, 04:51 PM   #2
jadnashua
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It doesn't really matter which side is out.

HardieBacker is VERY thirsty...I know with RedGard, when used over Hardie, they want you to thin with water some Redgard and then use it as a primer. Read the Aquadefense instructions carefully to check if they want the same thing...otherwise, what tends to happen is that the brush or roller sticks, as it almost immediately sucks most of the moisture out of the material. So, you might be applying three coats. Buy a wet film thickness gauge and learn how to use it properly, as you really DO want to get the coats between the min/max thicknesses (other than say the primer, if required). Most any paint store should have one, or buy it online...they're cheap and it's the only way to know for sure that you've got your technique down...pressing too hard, or going over it too many times, and you'll end up with it too thin...too thick, and you'll risk runs. Just like in Goldilocks, you want it 'just right'!
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Unread 04-16-2021, 06:08 PM   #3
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Welcome, Rick.

Where and for what reason are you using the MR Board? This is part of your shower construction?
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Unread 04-16-2021, 09:52 PM   #4
rickurukus
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Thanks for the responses and glad to be here! Seems to be a treasure trove of information. My contractor and I are using the MR board as the first layer against the studs and the hardiebacker over the top of it. Once the hardiebacker is up I plan on taping all seams and using the aquadefense to create the liquid membrane over the top of the hardiebacker.

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Unread 04-16-2021, 10:12 PM   #5
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Well, that prompts me to ask almost the same question again, what is the reason for the MR Board behind the Hardiebacker? James Hardie doesn't want it there, why do you and your contractor want it there?
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Unread 04-17-2021, 09:32 AM   #6
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Ahhhh I really don't have a clue as I don't do this for a living and that is the reason I am here asking for any advice on the layers behind the shower tile. Contractor is the one who says he does it in all his showers so I am here looking for verification to make sure he is doing it properly. I also don't know who James Hardie is and why his opinion matters.
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Unread 04-17-2021, 10:14 AM   #7
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James Hardie is the manufacturer of Hardiebacker and that's why his opinion is very important. You and your contractor will want to read and follow his instructions, available online, for installation of his product in your application.

The only reason I can conjure up for installing the Hardiebacker over gypsum drywall would be to create a faux mortar wall to give the appearance of an old-style installation if the appropriate tile trim (mud cap) is available for the tile you propose to use.

If you're doing something other than that, I'm without a clue as to why you'd wanna install the CBU over gypsum drywall.

My opinion; worth price charged.
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Unread 04-17-2021, 10:23 AM   #8
rickurukus
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Very well then, thanks for the advice will have them remove the MR board and just use hardiebacker->aquadefense->modified thinset->tile

What got me wondering this was having another board off the wall sticking out past the dry wall plus us having tile over it, thought it would look funny
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Unread 04-17-2021, 10:34 AM   #9
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I'd be curious on how he planned to finish the edge of the tile, Rick. Two edges actually, the Hardie and the tile, as both would be proud of the surrounding drywall.
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Unread 04-17-2021, 11:05 AM   #10
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I would also suggest you consider using a different CBU. While Hardiebacker is perfectly suitable to the application, the Hardiebacker 500 is not 1/2" thick. It's more like 7/16" thick and folks frequently have difficulty transitioning from the Hardiebacker to their gypsum drywall. Other real CBUs (ASTM C1325) are actually a full 1/2" thick and transition nicely. Just a suggestion.

My opinion; worth price charged.
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Unread 04-17-2021, 03:30 PM   #11
jadnashua
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There are two classes of CBU, and the installation details differ some between them...IOW, they're not universally interchangeable. HardieBacker is stiffer mostly because it has fibers mixed into it...those fibers are listed as cellulose (i.e., wood fibers) which is the main reason why the installation instructions differ. FWIW, wood fibers = mold food...now, they're encased in cement, but cement isn't waterproof (none of the cbu boards are). SO, how they're installed does make a difference!
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