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Old 12-19-2017, 09:32 AM   #1
pruff
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How to insulate behind cement board for tiled shower

We are in the process of installing a corner tiled shower with an acrylic shower pan. Two of the walls are exterior walls with faced kraft insulation currently installed. We've purchased Hardibacker and the shower pan, but after reading quite a bit on-line, I'm not sure what we need to do next. Do we just put the cement board on the studs or do we have to put up poly sheeting, then the cement board. IF we do the poly sheeting, do we then need to slash the paper faced insulation ? We do not plan on using Redgard, if that helps.

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Old 12-19-2017, 10:55 AM   #2
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Welcome to the forum, Kraul.

If you're not planning to have a surface-applied membrane, then use either four or six-mill plastic on the studs directly behind the cement board. If you have paper-faced insulation you can slit the paper to defeat that barrier.
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Old 12-19-2017, 12:13 PM   #3
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Thanks Kevin.
I have another question regarding the shower pan. If I bring the poly sheeting inside the flange of the shower pan, do I still caulk where the cement board meets the top of the shower pan flange or no since I want any moisture to flow into the pan ??
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Old 12-20-2017, 07:59 AM   #4
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Correct, you want any moisture between the poly and the backer to be able to drain into the pan so don't caulk that backer to the pan flange joint.

Additionally, if your pan doesn't have built in drainage channels, you will want to leave some small weep holes when you caulk the tile to the pan.
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Old 12-20-2017, 10:35 PM   #5
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Welcome, Kraul.

I'm gonna disagree with Dan on that. I would recommend you not caulk between the backerboard and the polyethylene at the bottom edge.

I am in agreement with leaving some spaces in the flexible sealant you use between the bottom of your tiles and the shower receptor, though.

My opinion; worth price charged.
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Old 12-21-2017, 06:54 AM   #6
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I was going with the "or no" portion of Kraul's sentence CX.
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Old 12-21-2017, 10:38 AM   #7
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Understand.
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Old 01-08-2018, 12:24 PM   #8
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is mortar under acrylic shower pan necessary

We purchased an acrylic shower pan and are going to tile the walls. It's new construction and the pan sits level where we want to install it. My questions is , since it's level, do we still need to put mortar underneath or will just securing the shower pan flange to the studs be good enough ?

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Old 01-08-2018, 01:05 PM   #9
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Kraul, let's keep all questions related to this project on this thread so that questions and answers aren't duplicated, and the history is in one place.

You should go by what the manufacturer of the pan says to do. Mortar is really to provide support for the floor of the pan. It can prevent the floor from cracking under the weight of someone standing on it. The floor can sometimes get a little soft with hot water running on it.
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Old 01-09-2018, 08:42 AM   #10
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Mortar under the pan will also make it quieter, deadening the sound of the water falling on it.
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Old 01-09-2018, 01:19 PM   #11
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mortar under shower pan

The manufacturer does say to put mortar under the shower pan for leveling, but since the pan sits level on the floor, my husband doesn't think we need the mortar underneath. Also, he is concerned, if we had to replace the pan for some reason down the road, it would be much easier to slide out if it's not mortared down. I'm just trying to find out what the risk is if we don't do the mortar before we start the project ! Thanks for all advise
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Old 01-09-2018, 02:11 PM   #12
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Kraul, it's entirely up to you whether you follow the manufacturer's recommendations for the installation of that shower receptor, but the mortar under the receptor usually provides stabilization and prevents flexing of the receptor when in use as well as aiding in leveling. I'd recommend you use it.

My opinion; worth price charged.
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Old 01-09-2018, 04:25 PM   #13
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Usually, the mortar doesn't make a great bond, as said, it's more for support to prevent flexing, and level the thing, if required. You wouldn't use thinset for this, as thinset, when applied thick enough to fill any voids beneath the pan, will tend to shrink and crack...not what you want. Thinset is designed to be installed 'thin'! In general, you'll find 'sliding the pan out' isn't an option, so the fact there's mortar underneath or not isn't in the equation.
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Old 01-10-2018, 07:09 AM   #14
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I think it's been discussed here before; if sliding the pan out sometime in the future is a concern can't one place a sheet of poly under the mortar so it doesn't bond to the floor?
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Old 01-10-2018, 04:49 PM   #15
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Busting up whatever you use under the pan is easy. A poly sheet makes it easier. Like Jim said, the mortar doesn't really stick to the pan. It's just there to support it. I just took out a Jacuzzi from the 90's set in concrete. A few whacks with a baby sledge and it was in manageable pieces and it didn't stick to the floor.
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