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Moisture at top of stud cavities

robinasu | Posted in General Questions on

Hi all, I’ve found some moisture on the bottom of 10 stud cavity top plates, and probably the rest of the 75′ wall is similar. I drilled a hole 10′ away from the last open cavity and it seemed dry, but I saw evidence of previous moisture and a musty smell. Luckily, there is a horizontal drywall joint 10″ from the ceiling. The non-paper faced loose fiberglass batts are damp at the top 6″- two are moist as far as my arm can reach. Here is some background info:

Building located in SF
The wall assembly is 5/8″ cement board, tyvek wrap, probably 1/2″ or 5/8″ plywood, 5/8″ gypsum fire board, 2×6 studs, then 5/8′ drywall.
The building is two stories, each story has 16’8″ ceilings. I’m on the 2nd floor and the full 16’8″ exterior of my condo unit has clear SW exposure. The story below me is blocked by a next door building.
Above the wet cavities the wall extends upward creating a parapet wall 4′ above the roof.
The moisture seems to be condensation forming on the underside of the bottom top plate. There is another plate on top.
The previous exterior wood siding on the wall was replaced about 6 years ago. The wood siding was replaced with the cement board and tyvek.
The torch down roof was covered with a Dura-Last roof about 5 years ago. It seems solid.
I opened a hole in one ceiling cavity to check for moisture above the top plates and there was none.
Before I opened the wall, the worst cavity was warm to the touch on the interior drywall, while the adjacent cavities were cool (but were also moist when opened).
I suspect condensation. Attached is a picture of the worst cavity right after I opened it. I’ve since dried it out and covered the opening with sealed saran wrap to see if moisture returns. The saran wrap bulges out a bit, so there is some noticeable upward air pressure.

The previous wood siding was in bad shape before it was replaced. A lot of drywall screws I’ve inspected show evidence of rust all up and down the wall. The drywall 12″ from the ceiling and down seems fine- the rear paper is still in good shape- but it seems there was moisture all around. The top 10″ of drywall has to be replaced- the rear paper is mush. Luckily, there is a very small amount of mold. It’s unclear how long the moisture has been in the wall. I found this issue as I was removing blistered and cracking drywall tape all over the 16’8″ x 75′ wall. The tape generally pulled off easily.

Is it possible moisture was locked in the wall after the cement board and tyvek was installed and just now most of the wall is drying out? I guess this would be the best case, considering.
How can I check for air infiltration?
Should a dense packed insulation be installed? I’ll most likely activate insurance.

Any help is greatly appreciated! Sorry for the long post- let me know if I missed anything.

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  1. GBA Editor
    Martin Holladay | | #1

    Even though your description is fairly complete, it's hard to visualize every aspect of this wall.

    The bulging Saran Wrap is a key clue. Your wall assembly is not airtight. You have airflow through the wall assembly, probably driven by the stack effect (or possibly by building pressurization). So one way to reduce the problems you note is to track down the air leaks and seal them.

    I agree with you that this sounds like a condensation problem (rather than a rain entry problem). Assuming that's so, one of the factors that's allowing the condensation to form is a cold surface. Is this cold top plate directly under the parapet? If so, perhaps there is an insulation defect directly over the top plate.

  2. robinasu | | #2

    Hi Martin, thanks for the reply. Yes, the top plates are below the parapet. The hole I opened to inspect a ceiling joist cavity allows me to feel over to the top plate. That joist cavity seemed to have fiberglass batt installed over the top plate. I may need to explore that more. In that joist cavity there was no evidence of moisture at all. I should probably look at the joist cavity above the saran wrapped cavity at some point. A detail I just recalled: One of the stud cavities in the middle of the group was dry when I opened it. During construction, it seems, somebody drilled a 1/2" hole through the top plates in that bay. The floor plate construction seems normal, unless there are holes drilled in every cavity's floor plate. The consistency of moisture in each cavity makes me wonder. The cavity with the Saran Wrap installed was the worst by far. I wonder if there is a hole in that bay sill plate.

    Does anyone have some guidance on how long moisture can take to dry out in a well sealed wall cavity?

  3. robinasu | | #3

    Update: I had a roofer look at our roof and the flashing between the problem wall and the shorter next door building. The roof and flashing is fine - no problems. I was hoping it was going to be something easy like the siding dipping into a pool created by the flashing, but that is not the case.

    After the bad news from the roofer, I accessed the interior of problem wall from the interior of the building. I cut out a square of the exterior side gypsum board which revealed a woven while plastic sheeting. Moving further outward, the next layer is plywood, then I think the tyvek wrap, then fiber cement board. It's unclear if there are more layers between the plywood and tyvek wrap, but based on photos from the fiber cement board install, it appears the plywood was the original building siding and the tyvek and fiber cement board was added on top. So, from inside of building out we have:

    Interior drywall, 2x6 studs with FG insulation, exterior gypsum board, white woven plastic sheet, plywood, tyvek wrap, fiber cement board.

    My cheap moisture probe said the plywood moisture % was 40%.

    At this point, I think there are three primary possibilities for the moisture accumulation:

    1. Before the fiber cement board and tyvec was added, a lot of moisture was trapped in the wall and it never dried out.

    2. The two layers of moisture barrier is retarding the walls ability to dry

    3. A mixture of the two

    Question: How bad is having the plywood encased on either side by moisture barriers? I understand moisture barriers allow vapor transmission, but could two layers be a bad idea in a marine environment such as San Francisco? My instincts say yes.

  4. GBA Editor
    Martin Holladay | | #4

    I'm sorry, but this is the kind of moisture problem that can't be diagnosed over the internet.

    If the material you describe as a "white woven sheet" is Tyvek or a similar housewrap, it shouldn't be causing the problem. The materials on the exterior side of your studs all sound vapor-permeable, and if the fiber-cement siding is exposed to outdoor air and sunlight, these exterior layers should dry readily.

    If the "white woven sheet" is some kind of vapor barrier, then that could be causing the problem or contributing to the problem.

  5. robinasu | | #5

    Hi Martin, thanks for the response. Do you or others know a good moisture specialist in the San Francisco area?

  6. GBA Editor
    Martin Holladay | | #6

    I would start by contacting the Building Science Corporation in Massachusetts. They might be willing to refer you to a consultant in California.

    Building Science Corporation
    70 Main St.
    Westford, MA 01886

  7. robinasu | | #7

    Will do, thanks!

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