Do I have a double moisture barrier problem?

18 years ago

I am ready to start installing the Kerdi. While rereading my notes, instructions, etc. I suddenly realized that I may have a double moisture barrier problem.

The tub-surround shower alcove has 1 exterior wall, shares 1 wall with another room, and the third wall is a partition in the bathroom.

The tub is in and all the plumbing is completed. The Durock is installed.

These are the layers on the exterior wall of this 1911 house. 1. Cedar shingles. 2. Old tar sheeting of some kind. The places where I have removed a couple of exterior wall shingles this black stuff has been crumbly, however, until bothered it is probably doing the job. 3. Next is Owens Corning Pink insulation with the paper covering. Paper looks like a sandwich with a thin tar covering. Paper covering is toward the inside of the house. 4. Durock. 5. Kerdi after I install it.

Layers on wall shared with another room. 1. Wallpaper. 2. Lathe and plaster. 3. Same pink insulation with paper next to other room - insulation side toward bathroom. 4. Durock. 5. Kerdi after I install it.

There are no braces in the space between the studs. I put up a few supports but they don't totally obstruct the stud space. There are two niches, however, there is about 1" between the back of the niche and the adjoining wall because of the way the plaster was keyed. Generally, the insulation runs the full length of the chase, down to the floor behind the tub.

There is a low plumbing access door on the bathroom partition. I haven't built this door yet, so I could put a vent in the bottom part of it. Might allow more moisture into the joist spaces than it would let out. I have built and will tile the tub skirt so I could put a vent here also. But same question, would this let moisture out from the stud spaces, or allow more in from the steam in the bathroom? We have put a fan in this bathroom and there is also a window.

Thanks in advance for the help. I am going to hold off on the Kerdi until this issue is solved.

Edwena

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Comments (5)

bill_vincent
18 years ago
The only thing you might do is take down the Durock on the exterior wall, cut some slits in the paper on the insulation, and then reinstall the Durock. Other than that, you should be okay.
edwena
Original Author
18 years ago

Thanks for the answer Bill.

The exterior wall Durock was the first piece I put up. It is about 1 1/2" inches under the right window trim, 3" under the sill, and also the trim under the sill. The Durock is screwed under the trims. Trust me, I put that trim up to stay. I could take it down but I am afraid that I will rip it apart - particularly the sill.

Do the slits in the insulation have to be distributed uniformily? There is really only one significant tall chase. Maybe I could put a raiser blade on something and work my way partway up. I can definitely get to the chase under the sil without removing the Durock.

I also thought of drilling a couple of small holes in the bathroom floor close to the chase, from underneath, and putting a vent in the ceiling of the basement room below. Is it correct to assume that if there is moisture in the chase, that it will not be a problem if there is air exchange? Will air move up and down the chase?
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edwena
Original Author
18 years ago
OK After sleeping on the issue, I can probably get the two trim pieces out and leave the sil in. The Durock may have to come out in pieces and the replacement Durock could be two pieces instead of one.

Think I should also take down the Durock on the interior wall because the adjacent room is a bedroom that is quite cold in the winter? Maybe 45. Is the issue is change of temp between sides of the wall so there is condensation?

There is one part of this whole thing that has me puzzled. If this were new construction and Tyvek was wrapped around the house, would I be restricted from having a shower on an exterior wall?

Appreciate the help.
bill_vincent
18 years ago
No, The problem comes when you've got one vapor barrier right up against the cement board on one side, and another right up against it on the other. Then there's no way for ambient moisture in the cement board to move at all.
edwena
Original Author
18 years ago
That makes sense Bill. Truth is Bill, I was hoping that you would tell me I didn't have to take off the molding, etc. Oh, well, so I will get at it. Thanks.