# Bathroom Vapor Barrier



## milkmoney (Apr 3, 2008)

Hello
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!!

Thanks!!!
Mark (Western New York):thumbsup:


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## AtlanticWBConst. (May 12, 2006)

You want it to be seamless. If you do have seams, seal them with Tyvek Housewrap tape.


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## milkmoney (Apr 3, 2008)

Thanks for reply. 
If you look at the attached illustrations, can I install the vapor barrier with two sheets on the exterior wall that runs along the tub? The first sheet would extend from the ceiling to the floor (seamless), and the second sheet would be installed from the ceiling to the bathtub lip, extending over the lip so any moisture will shed into the tub. Would two sheets of plastic installed in this manner - negligible air space between them - invite mold/mildew?
Or should I just live with a seam between the upper sheet that extends over the tub lip and the lower sheet that runs from the floor to a few inches above the tub lip (Option 1 on drawing)
Thanks!


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## mdalli (Jun 29, 2006)

That's really weird. I was just about to post an almost identical post asking basically the same thing.

And my name's Mark too.

Another question I had was this: on the exterior wall, there is already faced insulation. Should I remove the faced insulation and replace with unfaced? Any brands of vapor barrier you'd recommend?


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## Maintenance 6 (Feb 26, 2008)

Ideally, you don't want any seams if you can help it. Condensation should not be an issue. If it is, then you have a serious problem. The plastic vapor retarder should always be at a temperature above due point, so that moisture cannot condense on it's surface. With a tile face on the wall, practically no moisture should ever reach the plastic. If you can't help but have a seam, then seal it like Atlantic says. And be sure your renovation includes an exhaust fan for the shower. Most moisture problems from condensation in baths and kitchens could easily be avoided with proper exhausting.


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## Maintenance 6 (Feb 26, 2008)

mdalli said:


> That's really weird. I was just about to post an almost identical post asking basically the same thing.
> 
> And my name's Mark too.
> 
> Another question I had was this: on the exterior wall, there is already faced insulation. Should I remove the faced insulation and replace with unfaced? Any brands of vapor barrier you'd recommend?


If you plan to install another vapor retarder over top of your existing insulation, take a utility knife and slash the paper barrier about every 3 inches or so. You never want two vapor retarders. 6 mil polyethylene plastic is pretty much standard for a residential vapor retarder. Foil is about the only product I know of that has a permiability rating of close to zero. So I guess it comes close to a "barrier". Everything else is a retarder.
The idea of a vapor retarder is to prevent vapor laden air from penetrating the insulation where it could eventually reach due point and condensation could form inside the wall. Vapor retarders need to be installed on the "warm in winter" side of the insulation.


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## John316 (Mar 10, 2009)

*Vapor Barrier before hardibacker board?*

I am remodeling a bathroom to install wall tile. Here is what we have done so far.
1. Removed all old wall plaster to studs (this was plaster over wallboard so did we need to tear out or could we have tiled over this)
2. Installed new plumbing fixtures and cast iron tub-then leveled
3. Installed Hardibacker board and taped joints
4. Started to fill in Taped joints with thinset 

Okay, here is our questions after first listening to the home center department manager of his tile department, then tonight watching an installation video of hardibacker board.
1. Was told no vapor barrier needed due to popular use of this board and it being moisture resistant
a. if we need a vapor barrier-can I use something else as the thought 
of tearing the board off is like thinking I going to get a big stimulus
2. The gap between the tub and backer board is a little more than 1/4"
a. Can I fill this gap with scrap board, then thinset, then caulk? (yes, we should have dropped the board down another 1/8" but for some reason didn't see this. I know that is where water will set so I want to fill with thinset, then caulk if this sounds agreeable. :thumbup:

We are ready to tile but the vapor barrier question is bugging me before I tile. Is there something to go over this before I tile that thinset will adhere too?

We are using 12x12 with 3/8 x 1/4 trowel

 Thank you for your help.


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## John316 (Mar 10, 2009)

Your PDF's show exactly what I should have done. I am waiting for a response for someone to tell me there is a solution to cover the outside of the board with an agent that will prevent water going into the board before we decide if we want to unscrew the boards and do it all again. I have not put the the tile on yet.


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## Maintenance 6 (Feb 26, 2008)

The board you used is moisture resistant and will serve as a vapor retarder. It is NOT water proof, so if it gets wet, it will absorb some moisture that will destroy it.


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## Rivethead (Dec 26, 2008)

http://www.custombuildingproducts.c...terproofingAntiFractureMembranes/RedGard.aspx


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