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Bottom 6 inches of plywood sheathing is getting mold

kevinjm4 | Posted in Energy Efficiency and Durability on

I live in Snoqualmie WA, just 25 miles east of Seattle, zone 4c (I believe). This past summer I ripped out everything from the south exterior wall. Windows, siding, trim, gutter and sheathing. I’m in the final stages before siding. ONE THING is holding me up…

One reason I tore the wall apart in the first place was because the bottom 12″ or so of the sheathing was rotting and was covered in some sort of white mold, not really sure what it was exactly… pretty stinky musty stuff. I figured, must be because it’s that cheap-o OSB stuff so I replaced it with CDX exposure 1 plywood 1/2″ and put in all new windows and wrapped the house in Tyvek.

I installed the tyvek from top of sheathing (taped it there) down to 1″ below sill, fastened it with cap nails, tried to follow all tyvek protocol. My last row of cap nails was about 2″ from bottom. So as the housewrap began to fray a bit at the bottom i noticed something – the first 1″-2″ of sheathing is turning black.

The tyvek has been exposed for about 5 months. We get a lot of wind driven rain on that side of the house, and there wasn’t a gutter on that side of the house until about 1 month ago. I also dug a trench on that side of the house before all of this as well and plan to put a footer drain there when we get some extra cash, as I am going to do everything to direct moisture away from the house, especially that side.

some more details that might be helpful… I caulked the sill/foundation gap before i installed sheathing as there was no foam gasket there at all… My foundation wall is only about 5″ tall, and the footer is about 4″ tall. Before i tore that wall apart there was a gutter, drain rock, and maybe about 3″ between siding and ground. Tyvek was about 1″ below sill plate.

I have seen many detail drawings/videos regarding the bottom sheathing/sill plate/foundation area when it comes to how to flash, whether you use flashing tape, some sort of paste, or z flashing.

oh also, I sprayed the bottom 12″ or so with concrobium but i’m considering just removing and replacing that first 12″ of sheathing across the whole wall altogether so I have peace of mind going forward.

How do I prevent bottom of sheathing from getting moldy? Whats the correct, and/or best way to do this?

THANKS A LOT, and sorry for the length!

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  1. GBA Editor
    Martin Holladay | | #1

    You haven't explained whether you are seeing mold on the exterior side of the sheathing or on the interior side of the sheathing.

    If the mold is on the exterior side of the sheathing, then I suspect that the grade is too high -- too close to the lowest wooden components of your house -- and that the source of the moisture is splashback. Possible remedies would include (a) lowering the grade, or (b) installing gutters at the eaves of your roof.

    If the mold is occurring on the interior side of the sheathing, you should read this article: "Condensation on Wall Sheathing During Construction."

  2. kevinjm4 | | #2

    Yes, mold is on the outside. Lowering the grade is a good idea, agreed.

    And yes, I currently do now have gutters. And I need to reverse the slope on the grade as well as there was a slight slope toward the house. And then put a footer drain. (Any good details out there for that? I’m new here...)

    Also, I’m considering buying a vapor permeable flashing tape such as this one:
    To help cover the plywood from splash back, but also allowing it to breathe. Heard great things about it - don’t know if it’s nexessary. I’d then shingle the tyvek over the top of it and tape the tyvek to that, and possibly add a z flashing as well.

    (Martin, I came across that article AFTER I deduced what my problem was on the inside as I was also having mold on the inside sheathing wherever it was not sheetrocked, but insulated w/ mineral wool. So I’m CONSIDERING certainteed’s Membrain before install of Sheetrock because Mold scares me, but I’m not sure if it’s worth the cost, or if that’s what i’d Need or if drywall and paint is enough.)

    Thanks for the help!

  3. Expert Member


    The biggest thing you can do to protect the plywood from water damage from the outside is install rain-screen strips before you put the siding back on. Without addressing the problems of grading etc. you may get some mold or rot on the siding, but the wall underneath will be safer.

  4. walta100 | | #4

    Trimming back any trees and bushes will allow the wind and sun to dry the walls.

    If you have little or no overhanging roof you will need gutters to keep the water from splashing back onto the house. Even then this may turn into a maintenance chore to wash that part of your house every so often much like you wash a car but using bleach water with a shot of dish soap.

    Please note Tyvek can degrade if left exposed for too long I seem to recall 6 months was the manufacturer's stated limit. You may want to get it covered soon.


  5. kevinjm4 | | #5

    Is there a preferred detail regarding this area of the house? I’ve seen many variations...

    And would it be beneficial since that side of my house is cold and damp and close to grade, to replace the moldy portion of sheathing (the bottom 12” or so - across entirety of wall - with PT plywood? Or just treat the mold and move on.


  6. maine_tyler | | #6

    "I installed the tyvek from top of sheathing (taped it there) down to 1″ below sill"

    I'm about as far from an expert as you'll find on this site, but if the sheathing is also lapping or touching the concrete at all, and then tyvex is covering this, I would imagine moisture wicking from the concrete into the ply could be significant. Not sure if this is really the problem, but another thing to think about.

    I'm not sure how common place it is to lap the plywood over the concrete (I know it happens) or how common place it'd be for this to cause a problem... but I do know that when i removed some old t1-11 exposed to the elements for years, the only parts showing signs of mold and decay were the bottom couple inches that had been lapped over the concrete... it nearly formed a line there.

  7. kevinjm4 | | #7

    No, sorry, the plywood extends down to about 1/2” before the bottom of the sill plate/concrete joint.

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