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Old 08-13-2015, 02:23 PM   #1
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cement board or denshield over greenboard?


Remodeling a bathroom with new shower with one exterior shower wall. That wall is existing typical w/faced insulation and 1/2" greenboard drywall. Want to avoid tear out of existing drywall. Considering two approaches; 4mil poly behind hardieboard (concern is could be a double vapor barrier) or denshield directly on the greenboard.
Also the lower half of that wall needs to be bumped out about 5" as shower base doesn't fill the space. in this 5" bumpout we'd create another wall cavity behind the shower wall and the exterior wall.
We're in chicago area so winters do get cold.
Looking for advise/council on this project.
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Old 08-13-2015, 03:06 PM   #2
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From what I've read, you can use cement backer board over drywall, if you don't want to remove the drywall. However, you should treat the installation of the backerboard the same way you would if it was not over drywall-meaning that you should install some sort of vapor barrier behind the backerboard prior to installation (in your case, over the drywall), or some sort of waterproofing membrane (redgard, etc) over the backerboard after installation. BUT NOT BOTH.
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Old 08-13-2015, 04:00 PM   #3
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I would NOT put a vapor retarder/barrier between concrete backer & wallboard especially when there's faced insulation involved. That's called a moisture sandwich.

The waterproofing/moisture barrier should go on the surface of the backer, just under the tiles.

So, if you use concrete backer, use Redgard, Hydroban or a sheet membrane such as Kerdi.

If you decide on DenShield, don't apply any additional membrane cuz the surface has it built in already.

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Old 08-13-2015, 09:24 PM   #4
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Tear out the greenboard as it does not meet code with the asphalt-paper (Class 2-- vapor retarder); http://publicecodes.cyberregs.com/ic...9_7_par016.htm

Most "sandwiches" have a vapor barrier (the bread) on both sides of the insulation, not a barrier and retarder on the same side. Green board (23 perms) will grow mold from the impervious tile/poly stopping moisture, unlike asphalt-paper rated at 1 perm (dry) but when wet, jumps to -10 perms to "throttle" the moisture through the wall- hence "retarder (Class 2 vapor barrier); http://buildingscience.com/documents...uth-assemblies

Ginger vs. Mary Ann; http://buildingscience.com/documents...024-vocabulary

Fill the air space with insulation and use Denshield with the impermeable facing. No poly; http://www.ctioa.org/reports/fr46.html

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Old 08-13-2015, 09:40 PM   #5
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Quote:
don't apply any additional membrane cuz the surface has it built in already.
although this is true, according to denshield website the product is rated for a steam room IF the seams are filled with fiber tape & mud then red guarded. so that's what i did (also covered the screws too) even though its not a steam shower. i had a huge thread on this website about project but i think all the pictures are somehow gone. i liked the denshield, super easy to work with, waterproof surface where cement board is porous. tiles stuck easy to it.
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Old 08-14-2015, 12:06 PM   #6
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Thanks for the council. Just to be clear I need to remove the greenboard and the faced insulation. Insert unfaced insulation and install denshield direct to the studs.
Or, if i decide on hardiebacker, remove greenboard and faced insulation, install unfaced insulation and a vapor barrier before the hardiebacker goes on.
Do I have this correct?
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Old 08-14-2015, 12:59 PM   #7
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We counsel as a hobby to try to help people with their projects.

You can keep the faced insulation if it's still in good condition and hasn't drooped, just slashed the facing. Or if it's the old 3.5" R11, you can switch to the newer R13, easy job.

I highly recommend you use a surface membrane instead of tarpaper or plastic on the studs behind your favorite CBU. Why allow any moisture to get the CBU damp/wet in the first place? You'll get a better sealed job and the tilework will look better and dry out faster after each use.

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Old 08-14-2015, 02:53 PM   #8
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It wouldn't be my first choice, but can't Kerdi be installed directly on drywall?

If so, frame the bump out, cover with cement board, the cover the whole shower with Kerdi. (After reading A LOT, I did my new shower with Kerdi. I'd do my next one that way, too.)
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Old 08-14-2015, 05:03 PM   #9
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You're right ratherbefishing. For some reason I didn't suggest a third option when told to choose from the 2 options listed. I think I was answering another thread with a tub enclosure instead of a full shower. I would use Kerdi especially if the floor was also tiled.

OK so, is this a full shower? Sounds like the floor of the shower is a molded shower tray, not a tiled floor.

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Old 08-18-2015, 09:54 PM   #10
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I would think especially in this situation where the shower base doesn't fit the space, it's begging for a custom mud base/kerdi installation. I did one in a new house a few years back, and have had zero problems and when I finish the basement, I'll use it again.
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