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Vapor Barrier Confusion


Thank you for this forum!!

I am remodeling my bathroom and I have a few questions regarding installing a vapor barrier on the exterior walls that run along a three-wall alcove bathtub. The wall spaces (2x6) are insulated with unfaced, fiberglass batt insulation. My plan is to install a 6mil plastic sheet vapor barrier on the exterior walls, over the unfaced insulation. Then, I plan to install 1/2" hardibacker cement board inside the bathtub wall areas, directly onto the wall studs, and then install ceramic tile. My question is in regards to how the plastic vapor barrier should be installed on the exterior walls that run along the bathtub. I have attached a file with hand drawings that illustrate three options for installing the plastic vapor barrier.

Option 1 has two sheets of poly installed on exterior wall; one sheet above tub where the bottom of the sheet extends over the bathtub lip, and the second sheet runs from the floor up a few inches above the bathtub lip. This would leave a seam between the two overlapping sheets.

Option 2 has just one sheet of plastic that runs from the ceiling to the floor - no seams. Concern: any moisture that condenses on the sheet will run behind the tub, onto the floor.

Option 3 has a seamless sheet that runs from the floor to the ceiling. And then another sheet is installed on the wall from the ceiling to the bathtub lip, where the bottom of the sheet overlaps the bathtub lip.

I'm at a loss with what option would be most appropriate.
My concerns are of course moisture penetration into the walls spaces and behind the bathtub.

Any thoughts or suggestions would be greatly appreciated!!

Mark (Western New York)
Note: I posted this same thread in Interior - General Section. I posted it there and then found this section to be more appropriate for this question.

Re: Vapor Barrier Confusion

There is plenty of confusion as to the function of the vapor barrier.

The main role of this barrier is more in preventing warm moist air from passing through into the exterior wall cavity and mixing with the colder air.... which would condense and result in moisture. In other words think of this as more of an air barrier..... which also aids in stopping drafts.

The plastic is of low air & moisture permeability ...pretty much zero .... meaning it doesn't allow moisture laden air to penetrate ... and typically 6 mil is usually recommended because it's thicker and won't puncture as easily than 4 mil ... which in some areas is the minimum code.

Now having said that .... it's important to install this barrier with as few seams as possible and those that are present should be well sealed .... forming a continuous monolithic envelope. This can be done using a house wrap tape.
If this step isn't properly done then the idea of this barrier is greatly reduced , negating any advantage it offers.

One additional step that can be done to ensure a premium seal is to use a caulking like acoustical sealant along the top and bottom sills and each stud with a continuous bead. This will seal any punctures from the fasteners and maintain that monolithic envelope.

Hope this helps.:)

Re: Vapor Barrier Confusion

Thanks for the reply canuk
Im not certain if you took a look at the three illustrations I drew in the attached file. Which option would be most appropriate for installing the vapor barrier on the exterior wall. Could I install two sheets of plastic? The first sheet would extend from the ceiling to the floor (no seams), and the second sheet would extend from the ceiling to the bathtub lip, extending over the lip so as any moisture that collects on the plastic will shed into the tub and not behind it.


Re: Vapor Barrier Confusion

In my opinion the second sheet isn't nessessary..... and probably more bother than it's worth.:)

Re: Vapor Barrier Confusion

I agree that the second sheet isn't necessary. The idea of the vapor barrier is to prevent condensation, not provide a place for it to happen.

Re: Vapor Barrier Confusion

Years ago I was taught by an old tile person that a vapor barrier was necessary because tile and backer board are both porous. Because of this will convey moisture through them (upwards of 7% on glazed and up to 25% unglazed tile). Because of this you need a water barrier that is the last line of defense. He instructed me that you should be able to shower with out tile and backer board in place and have no leaks. I would not feel comfortable in installing a tiled shower without a moisture barrier in place. I know that many home owners do not reseal their tile and keep up with the grout repairs. This leads to leakage and larger problems.

Re: Vapor Barrier Confusion

Thank for replies.
I only suggested two sheets because somewhere I have read that a plastic vapor barrier around a tub should extend over the bathtub lip so if any mositure was to penetrate the tile grout it would run/shed into the tub, not behind it. I have also read that it is prefered to install a plastic vapor barrier without any seams if all possible. So I saw the two sheets as a means to achieve both of these requiremnets. The first sheet would extend from the ceiling to the floor (behind the bathtub) giving no place for any vapor to infiltrate the wall space and insulation batting. The second sheet would extend from the ceiling to the bathtub lip, where the bottom of the sheet would overlap the bathtub lip. That way if any moisture were to infiltrate the tile grout, it would shed into the tub and not behind the tub.

I have a funny feeling I am thinking myself into a corner with all of this information.


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