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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 on which 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)

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.:)

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