Vaper barrier for kerdi lined shower walls




Last Updated February 19, 2016 01:09 AM

I'm using kerdi membrane for my shower area with cement backerboard. Should I still add a layer of plastic vapor barrier behind the backerboard. Normally that's required to prevent the studs and insulations from wicking up water from the cement board, but I've seen discussion about water being trapped in the backerboard if the board is lined with Kerdi on one side and water vapor barrier on the other side. What about the rest of the bathroom, vapor barrier behind greenboards? Any definitive answers?

Answers 1

Kerdi is your barrier if properly installed it will stop water from migrating. I don’t recommend a barrier behind green board it is better than standard sheetrock for moisture but sealing it may cause more problems than can be gained. A good quality exhaust fan with the lowest noise or Sones will do be the best way to go.

Ed Beal
February 18, 2016 23:08...

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If you need to decide which method is best for you I have a free shower waterproofing manual that you can download here. Shower waterproofing manual. Go get it – it’s free! And I’m not gonna use one of those damn annoying pop-ups! I hate those things…

There are several ways to prepare the wall of a shower for tile. Depending upon what was originally there, what stage the shower rebuild is currently in, and what type of tile you plan to install plays a minor part in choosing which method to use.

The most critical aspects of which product to choose are: how much work you’re willing to put in and how much money you’re willing to spend. The end result should be the same – a waterproof box. The methods used to accomplish that vary in effectiveness and cost. So we’ll start with what I consider the most bullet-proof method.

Kerdi Shower System

A company called Schluter makes a shower system called Kerdi. The entire system, which can include everything from the...

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Update: The 5 posts in this series have been updated and combined into one post which you can find in this link. Enjoy!

Part 5: Conclusion

The final post of this series will review the three different methods for waterproofing a residential bathroom shower that have been previously discussed and the whys, or why nots, for each method.

Traditional showers

The good:

lowest cost

tried and true


water is allowed to soak into substrate which may lead to mold growth and minerals coming to the surface (efflorescence) through the grout

Liquid surface waterproofing

The good:

blocks water at the surface, rather than letting it absorb

easy to install


lack of vapor protection

more prone to user error by not getting spots thick enough

Waterproof sheet membranes (and foam board)

The good:

blocks water at the surface, rather than letting it absorb

excellent vapor...

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I am building a bathroom in my basement, and was wondering whether I should use kerdi for the shower along the outside wall. Since it is an outside wall, normally (with no shower) I would have from the inside out: drywall, vapour barrier, insulation, concrete basement wall. For the shower I am using cement board, so if I use the Kerdi, it would be from the inside out: tile, kerdi, cement board, poly vapour barrier, insulation, cement basement wall.

I was wondering, since kerdi is a vapour barrier already, would that trap moisture between the kerdi and the poly, keeping the cement board wet and cause mold? should I not use the poly vapour barrier? Or should I skip the kerdi since I am using cement...

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With all of the plumbing in and the rough approval complete I set about installing a vapor barrier for the bathroom shower surround. I actually did this back in September before my crazy period of travel this fall, but I didn't have time to organize the photos and write about it until now.

Although the porcelain subway tiles that I will tiling the shower surround with are waterproof the grout and the cement backer board is not so it is important to protect the wood studs with a vapor barrier. I choose to use clear 6 mil polyethylene behind my cement backer board for my vapor barrier.

There are other ways to waterproof the shower, like using RedGard or Kerdi waterproofing membrane, but the important thing is that you should only install one vapor barrier (i.e. do not install sheeting behind the cement backer board and then RedGard over the cement backer board) so that moisture won't get trapped between the barriers.

Because this is a barrier against moisture...

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The installation instructions below represent the use of Schluter(r)-KERDI-LINE in floor applications. Installation instructions for the use of KERDI-LINE in shower applications can be found in the Schluter®-Shower System Installation Handbook.


1. Any leveling of the floor must be done prior to the installation of the channel body. For installation adjacent to the wall, the channel body must be aligned in accordance with the thickness of the wall covering. For intermediate installation, use the supplied filling strip with peel-and-stick adhesive layer to make the channel support symmetrical.

2. KERDI-LINE can be installed in conjunction with the provided channel support as described below or set in loose mortar. KERDI-LINE is connected to the waste line with the appropriate mechanical no-hub coupling in accordance with the coupling manufacturer’s instructions.

Channel body installation

Access to plumbing from below When there is access...

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Sadly code allows this type of installation.

Your technique is better in my opinion but I would double check you are getting the right coverage with two coats of liquid.

We use liquid all the time and getting a proper 22mil thickeness can be tough with 2 coats only.

When prepping for liquid membrane we wash all bond breakers and dust away. The first coat is burned into the backer by brush and we work it in all directions. It almost dries straight away so we then go back and hit it with the "Johnny Roller". That is coat one.

Second coat is a roll on coat.

When your done measure your container and determine if you have the proper amount of product left. If not a third and final coat will be needed. Expierment with different brushes and rollers until you find a pair that does the job.

If you wish - here in Vancouver you can just bang up Hardie, tape the seams and your good to go. This gets done all the time. There is no shortage of work in...

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The bottom layer (pre-slope) since it is on concrete, can taper from near zero to the walls at a slope of 1/4" per foot, so that thickness will depend on the size. Then the liner is installed, then a second layer parallel to the first (therefore the same thickness all over since the first layer is sloped) normally needs to be about 1" thick to have enough strength to hold together.

An advantage to using Kerdi, is that you only need one layer, then you install the Kerdi, then the tile.

The drain you need differs between the two systems and the Kerdi drain is more expensive. You need a clamping drain to use a liner, and you need the Kerdi drain that will allow the membrane to stick to it (no clamp) when using that system.

If the size of your shower is close to that of one of Schluter's premade pans, you could use that. They're a high density foam, and can be cut to size, but work best if you can do that evenly all around (or use it whole). This will keep the outer edge...

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Can I use a product like Schluter-KERDI as a vapor barrier in the situation described below?

- Application on top of concrete board in a three-wall shower/tub stall (to be followed later by application of tiles on top of the Schluter-KERDI)

- One wall of this shower/tub stall is an insulated exterior wall on the north-facing side of the house in a cold, dry climate (Colorado)

- There is currently no other vapor barrier installed behind the concrete board, which has been installed directly onto the studs

... or will this cause problems down the road?

The reason I am asking: the standard out here in Colorado seems to be clear plastic vapor barriers BEHIND the concrete board.

I guess the question could also be asked like this: if tile could be applied directly on top of a clear plastic vapor barrier, would these vapor barriers all be installed on top of the concrete board instead of behind it?

Thank you SO MUCH for your assistance....

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+ Fast & Easy: These systems have the fewest steps and the fewest components for a quick, simple installation. You can build an entire shower stall in under an hour and tile same day!

+ Completely Customizable: You can easily build a completely custom shower, purchasing only the components you need for your specific installation.

+ Lightweight Materials: Without any heavy cement board or mud pan sand mix to haul around, preparing for your installation is a much less strenuous process.

+ Instructional Videos: The complete official series of instructional videos on installing the Wedi system and HydroBlok are on YouTube..

+ Numerous Prefabricated Options: You can purchase a preformed ADA-compliant wheelchair ramp, recessed niche, shower curb, bench, curved wall, nautilus-shaped shower pan, steam rooms, and...

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I just finished up my second Kerdi bathroom. The first was a full shower (have posted pics in the past on some thread in here....) complete with the foam ramp, foam bench, foam curb, and really spendy linear Kerdi-line drain/cover. I also used Ditra-Heat on the floor. I can't post links to that forum (John Bridge) but I have a whole thread of my bathroom build. Search on: Chris' Orange Tile Project.

Kerdi is more or less like hanging wallpaper. IF the underlying stud framing is sound, square, flat, and plumb, tiling will be smooth and clean. So take care to put some effort into getting the studs in shape. However, an experienced tiler can get things flat even with a bumpy wall floating thinset and although that isn't really advisable, it is done all the time. Someone skilled in floating mud won't care how your wall is...he makes his own flat, coplanar, plumb, square walls himself.

The second bathroom was a 5x10 hall bathroom with alcove tub. Only the walls above the tub had...

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These barriers block water from reaching the framing of your home while providing an outlet for water vapor. Wall vapor barriers can also be bought with a built-in flame retardant. Follow these steps to install wall vapor barriers for a more energy efficient home.

Remove any visible water from the surface to which you are applying the wall vapor barrier. Walls should be free of any debris and completely dry. Caulk any areas such as around windows or gaps within your wall surface itself.

Roll vapor barrier onto the wall making sure to it falls at least 2 inches below footings. This will stop water from seeping in as it rolls down. All joints should have double layers of wall vapor barrier by at least 4 inches.

Staple or nail wall vapor barrier, spacing about 2 feet apart to secure it in place.

Seal seams created by the vapor barrier to walls using special seam tape. Alternative taping may compromise the seams and allow water...

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Here's the answer from my Fine Homebuilding article, Tile Backerboard Options:

"Manufacturers of cement backerboard generally recommend that a moisture barrier of some sort (WonderBoard calls for #15 felt or 4-mil polyethylene sheeting) be installed behind the backerboard when used in a wet location. According to some tile contractors, however, this is bad advice. “Plastic is a bad idea because you are nailing it on and putting holes in it,” says Tom Meehan. “When there is plastic, I’ve found mold behind the plastic. It locks any moisture behind it, and the moisture can’t dry.” If you want to waterproof the wall, Meehan recommends the use of a liquid-applied membrane such as
Laticrete Hydro Ban or Mapei Mapelastic AquaDefense on top of the backerboard. For a steam shower, he prefers a sheet membrane from Kerdi or Noble."

Answered by Martin Holladay

Posted Nov 7, 2013 4:24 PM ET

Edited Nov 7, 2013 4:25 PM...

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