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Community and Q&A

Exterior wall profile – Mold on insulation

metal7654 | Posted in General Questions on

Hey All

I live in Oklahoma, east of Oklahoma City about 90 miles, so I’m in the brown 3A climate zone.
I think I mostly have my issue figured out, but need some clarification. I started a remodel in the back bedroom, removed the sheet rock, and found my pink insulation has mold spots. My exterior wall profile goes: exterior paint + 4×8 wood panel siding + black roof paper (I think, it’s like felt) + pink insulation + clear plastic interior sheet + sheet rock + interior paint.

When I pulled the sheet rock and discovered the mold, I left everything alone while I did research to not disturb it yet. We had a few rainy days during that time. I went back in to just look at everything, and there was condensation on the inside of the clear plastic sheet. I’ve made many calls to siding contractors, read many articles on this site + others, and even had a structural engineer take notice and put recommendations in his final report. And with that, I’m 100% certain the clear plastic interior sheet needs to go because it’s not letting the wall breathe and dry out. I went in the attic, looked at the eves and just all around, and don’t think mass water is getting in any other way except through condensation.

My question is this though – I’ve read the difference between vapor barriers and air barriers… if I pull the clear sheeting, that will leave me with the black felt paper for the exterior. Do I still need something in place of the clear plastic sheet? I’ve read conflicting articles on this.


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  1. GBA Editor
    Martin Holladay | | #1

    The interior plastic layer is called a polyethylene vapor barrier. In your climate zone (Zone 3), it is neither required nor recommended.

    Your wall has a couple of problems. The interior polyethylene is one of them; you are correct that it would be best to remove it. In most cases, a wall like yours would ordinarily dry to the exterior during the summer, preventing the build-up of mold; but your 4x8 sheets (probably T-111) may be so tight that exterior drying is reduced.

    Fiberglass batts aren't the best insulation. With your walls open, you have the opportunity to improve the insulation performance by switching to cellulose, mineral wool, or open-cell spray polyurethane foam.

    When it's time to install new interior drywall, pay attention to airtightness, especially at the electrical outlets. Your new wall (without polyethylene) should perform fine.

  2. charlie_sullivan | | #2

    Martin, did you mean to say a wall like this would normally dry to the outside during *winter* ?

    John, Martin's recommendation is good and straightforward for the room you are remodeling anyway. For the rest, you have to decide whether to ignore the problem and hope it's not too bad, do something similar, or work on it from the outside. Which you do might depend on whether you are considering new siding at some point, or other interior remodeling at some point.

  3. GBA Editor
    Martin Holladay | | #3

    Most conventional wood-framed walls in the U.S. (those without any exterior rigid foam) accumulate moisture during the winter, and dry out to the exterior during the summer.

  4. charlie_sullivan | | #4


    I agree, for many wall types in many climates. But with poly on the inside, in zone 3A, there could be moisture accumulation against the poly when air-conditioning is on, and, if the the air sealing and flashing are both done well (two big ifs), there wouldn't be much moisture accumulation in the winter.

  5. GBA Editor
    Martin Holladay | | #5

    I pointed out that the type of wall described by John Howk would normally be expected to dry to the exterior during the summer. I stand by that statement.

    Clearly, the wall can't dry to the interior in any season because it has interior polyethylene.

    Drying to the exterior is likely to start in March or April, and would continue (in a typical wall like John's) until the winter moisture had dissipated.

    Wall sheathing accumulates moisture during the winter because it is cold. Even if a wall has interior polyethylene (a layer which would limit outward vapor diffusion during the winter), the wall sheathing can still take on moisture (from the exterior) during the winter.

  6. metal7654 | | #6

    Thanks for the quick response guys. It's clear I just need to remove the poly. And I hadn't considered taking that opportunity to upgrade instead of replace insulation, so good point there. Since I'm only able to work on a little at a time due to $$$, my plan was to stage the remodel in sections (moving from one end of the house to the next) and fix things as I go. Then save up enough to tackle the exterior with contractors for roofing and siding.

    Now to saddle up in a Tyvek suit, respirator, bust out a high end shop vac with Hepa filter, create some negative air pressure and tackle (not physically) the moldy insulation.

    Thanks again

  7. charlie_sullivan | | #7


    Sounds like you have a good plan. When you get to siding, you'll have an opportunity to add a layer of foam between the old siding/sheathing and the new siding, which would be a bad idea with the poly on the inside, because it could trap moisture, but a good idea once the poly is gone. I'd recommend polyisocyanurate insulation there, although EPS would also work well. Avoid XPS because of its high global warming impact, unless they fix that problem before you get to it.

    Martin, yes, in most US climates, the sheathing takes on moisture during the winter and dries in the spring. But in many climates, the summer humidity is higher than the spring humidity, and similar to the winter humidity, so moisture content of the sheathing can go back up in the summer. But that depends very much on the particulars of the climate. It's a little mind bending to think of spring as a time of low humidity, particular for New-Englanders who think of it as mud season, but it's a season in which the ground temperature is lower than the air temperature, which is conducive to having the ground damper than the air. If you look at weather data, spring is our lowest relative humidity season.

    In a wall with interior poly, the sheathing moisture content isn't the main thing I'd be concerned about--I'd be concerned about the moisture on the fiberglass side of the poly, where the problems typically occur, as in John's example. The moisture content there will typically rise through the spring and summer, reaching a peak near the end of the summer, and then gradually dry over the fall and winter.

    Anyway, we agree about the solution for John's situation, and about when sheathing typically gets damp and dries. I'm just debating this minor point for the fun of it.

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