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Mold on the OUTSIDE of the vapor barrier in bathroom

Posted by kredding (My Page) on
Tue, Jan 3, 12 at 18:23

The wallboard above our shower enclosure was starting to delaminate due (probably) to high humidity in the bathroom. Whoever lived here before us didn't caulk/seal between the wallboard and the shower enclosure in this bathroom. As I cut out the wallboard along an exterior wall, I could see dark black splotches in the pink rolled insulation through the vapor barrier. Further inspection revealed more dark splotches ON the vapor barrier on the side in contact with the insulation. There was no mold on the wallboard itself or on the side of the vapor barrier in contact with the wallboard. Has anyone seen anything like this before? My guess is that the humid air from the shower worked its way into the wall space through a small gap between the wallboard and the shower enclosure. Does that sound plausible? If so, then is it reasonable that I could eliminate the source of moisture by replacing the insulation, installing a new vapor barrier, and making sure the vapor barrier is sealed?

And yes, I will be installing a bathroom vent to get rid of the humidity.


Follow-Up Postings:

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RE: Mold on the OUTSIDE of the vapor barrier in bathroom

Including a link to a Flickr photo set that shows the black spots in the insulation and the splotches on the vapor barrier.

Here is a link that might be useful: Photos of the issue


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RE: Mold on the OUTSIDE of the vapor barrier in bathroom

What is the "vapor barrier" you see the mold through?

It sounds like the humidity is being trapped by a double vapor barrier. I presume the exterior of your house has a vapor barrier, i.e. asphaltic felt paper, Tyvek, or something. The other vapor barrier (the one you see the spots through when you cut out the wallboard), is it plastic? That is the problem. Plastic should not be used as a vapor barrier around a shower. (or ever) Plastic ends up trapping and condensing water vapor, and does not allow the condensation to evaporate.

The steam from the shower enters the wall cavity. (Not having a fan ensures this will occur) The steam condenses on the tar paper or house wrap, because it is cold from contact with the cold exterior wall. This condensation wets the insulation. The side of the insulation in contact with plastic can't dry out, because plactic can't breathe. The side in contact with the breatheable vapor barrier dries out just fine. The heating and cooling of the exterior wall also enables that side to dry out. The plastic just stays wet and grows mold. No fan can circulate air through an airtight membrane like plastic.
The solution (in addition to installing a fan) is to use a breatheable vapor barrier, NOT plastic. Tar paper is a better choice. Schluter Kerdi is a great choice. Just stay away from plastic!

Hope this helps.


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I didn't even look at the photos first!

Ummm, yup. Just as I expected. I really didn't see the photos... As I responded, the OP was posting the pic's

Get rid of the plastic man!


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RE: Mold on the OUTSIDE of the vapor barrier in bathroom

First of all you do not even say where you live, so I can properly place the vapor retarder, so I will try to help the best I can. The correct location for a vapor barrier, or more correctly, a vapor retarder, since it is very hard to actually have a real vapor barrier is on the warm side of the assembly. Vapor pressure always moves from warm to cold, because warmer air has more pressure than does cold air. Plastic on studs over unfaced fiberglass followed by MR wallboard is a good system, and provides a pretty good vapor retarder, from my experience. Much better than faced insulation, if you live in a northern climate where the vapor drive is from inside to outside. From your pictures it looks to me like you might have an actual water leak behind the plastic. If the fixture is in the wall you are showing, remove the shower head, cap it off and turn on the pressure to see if you have leaks in the connections. I would not ever use wallboard in a bath or shower area. You want to use a cementitious board like wonderboard.It looks to me like someone may have reused the insulation that you have in the wall, then installed the plastic and new wallboard without actually solving the real problem. Look at it this way, If cold air is getting into the cavity, the warm bathroom air will condense on the inside of the platic, not the outside, because the warm air contains the moisture. If the mold is not widespread, and consistent throughout the wall, but is occurring in one place it it probably a leak that has not been corrected.


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