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Community and Q&A

Mold on the inside of the basement wall?

Steve Mackay | Posted in Green Building Techniques on

Some time back I put some thought into the insulation strategy for my basement wall, see this link:

https://www.greenbuildingadvisor.com/community/forum/general-questions/105069/basement-insulation-strategies

As a result of that discussion I was planning a mix of 1.5″ of polyiso rigid foam and blown in fiberglass into a 2×4 stud cavity.

We’re building a new home in climate zone 6A, steep uphill sloped lot in mountainous snow country.

My insulation guy is recommending I don’t use rigid foam as it becomes a problem for any moisture trapped between concrete and the foam With nowhere to dry to this trapped moisture will mold, he said. My contractor said that in many years time the exterior waterproofing of the concrete is likely to fail and so you need to plan for that situation and allow drying.

The first answer in my old thread by Michael Maines said that concrete doesn’t really need to dry at all and is not affected by moisture which makes sense to me.

Per Joe Lstiburek “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”

Okay so it sounds like it is okay for concrete to be wet but how does mold not form?

Put another way my question is will I have a mold problem if moisture is trapped between the foundation wall and my foam?

There was a good reply from Dana in that previous thread explaining a few techniques I could use to allow the concrete to dry to the interior (dimple mat, stop foam short from slab, more vapor permeable foam). I’m trying to decide if an interior drying strategy is necessary or not.

Thanks,

Steve

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Steve,
    If you haven't seen it yet, you might want to read this article: "Joe Lstiburek Discusses Basement Insulation and Vapor Retarders."

    Q. "Will I have a mold problem if moisture is trapped between the foundation wall and my foam?"

    A. No, you won't have a mold problem. Now I'll answer a different question -- one that focuses not on a mold problem, but that focuses on mold. Here is the question: Will you have mold between the rigid foam and the basement wall?

    The answer to this second question is: Perhaps, but not much. And even if there is a little mold there, who cares? It's basically outdoors -- on the exterior side of your air barrier and insulation layer -- and will cause no more problems than the abundant mold in the soil adjacent to your basement wall.

    As this article confirms, mold requires oxygen to grow, and the amount of oxygen behind the rigid foam layer is limited.

    But, as I said, there might be a little mold there, just as there is mold in the soil. But we don't usually worry much about outdoor mold. You won't see it and it won't hurt you.

  2. Steve Mackay | | #2

    Martin,

    My primary air barrier in the basement will be the concrete to the sill plate and then to the sheathing ontop of the sill plate. So technically the mold will be inside my air barrier, no?

    I'm having a polish concrete floor in the basement. My secondary air barrier will be the sheet rock. How do I seal the sheet rock to the polished concrete floor? Caulk?

    If I can seal the sheet rock to the concrete slab then I agree with you that it is behind my air barrier.

    I suppose any moisture that gets inside of the rigid foam will dry to the inside. It's the stuff between the concrete and foam that I think my contractor is concerned with. If we seal the sheet rock to the slab then I guess it's not an issue.

    Steve

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Steve,
    If you plan to install polyiso on the interior side of your basement wall, it's essential to install the rigid foam in an airtight manner. That usually means that you will be taping the seams of the polyiso.

    Why? To prevent moisture-laden interior air from contacting the cold concrete.

    So this insulation approach results in an air barrier on the interior side of the concrete.

    It's generally a good idea to keep the bottom of the polyiso above the slab -- leaving a 1/2 inch gap to prevent wicking. This 1/2 inch gap can be sealed with a high-quality tape from Pro Clima or Siga.

  4. Steve Mackay | | #4

    Martin,

    Your advice on ensuring the Polyiso is installed in an air tight manner is the same for if it was just blown in fiberglass (obviously the fiberglass has is not allowed to touch the concrete foundation wall) is it not?. You'd still have to ensure air tight sheet rock to prevent moisture laden air contacting the concrete.

    In my case installing 1.5" of Polyiso directly over the concrete and taping the seams then having a 2x4 framing inside of the foam with blown in fiberglass and air tight sheet rock must be an improvement over using only 2x4 framing and fiberglass with respect to air leakage towards the concrete?

    You mentionedabout tape sealing the bottom of the foam, how do you seal the top side on the rim joist and any needed fire breaks? I assume spray foam?

    Going the sealed foil faced polyiso route like I'm planning why is this not just as bad for moisture as putting visqueen against the concrete which is a big no no?

    Is it better to use XPS instead of foil faced polyiso in the basement?

    Steve

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Steve,
    Q. "Your advice on ensuring the polyiso is installed in an airtight manner is the same for if it was just blown-in fiberglass (obviously the fiberglass has is not allowed to touch the concrete foundation wall), is it not?. You'd still have to ensure airtight Sheetrock to prevent moisture-laden air contacting the concrete."

    A. With or without blown-in fiberglass, the air barrier at the rigid foam layer is the most important. Once you have installed the rigid foam in an airtight manner, there isn't any worry about making the drywall airtight, since the cold concrete is now sealed off and inaccessible.

    Q. "Installing 1.5 inch of polyiso directly over the concrete and taping the seams then having a 2x4 framing inside of the foam with blown in fiberglass and airtight Sheetrock must be an improvement over using only 2x4 framing and fiberglass with respect to air leakage towards the concrete?"

    A. Of course. But you are comparing an approved, recommended approach with a discredited approach that no one should be using -- so I'm not sure what your point is.

    Q. "How do you seal the top side on the rim joist and any needed fire breaks? I assume spray foam?"

    A. Either spray foam or rigid foam installed in an airtight manner will work. For more information, see "Insulating Rim Joists."

    Q. "Going the sealed foil-faced polyiso route like I'm planning why is this not just as bad for moisture as putting visqueen [polyethylene] against the concrete which is a big no-no?"

    A. Polyethylene won't work because it has no R-value. It is just as cold as the concrete, without any insulating value, so when moisture-laden interior air contacts the polyethylene, the moisture in the air condenses.

    In contrast, the foil-faced polyiso has R-value. It is a type of insulation. It changes the temperature of the first surface that the moisture encounters. Instead of encountering a cold surface (the concrete or the polyethylene), the moisture now encounters a warm surface where condensation can't occur.

    Q. "Is it better to use XPS instead of foil-faced polyiso in the basement?"

    A. No. Green builders avoid the use of XPS, because XPS is manufactured with a blowing agent that has a high global warming potential. For more information, see "Choosing Rigid Foam."

  6. Steve Mackay | | #6

    Martin,

    Thanks for your detailed response once again. I really appreciate your expertise.

    With respect to my question:

    Q. "Going the sealed foil-faced polyiso route like I'm planning why is this not just as bad for moisture as putting visqueen [polyethylene] against the concrete which is a big no-no?"

    I was thinking more from a moisture coming in through the concrete wall (in years time when the concrete is cracked and waterproofing is failing) and being trapped between the insulation issue rather than moisture laden air hitting the cold concrete surface.

    Like has been mentioned concrete doesn't care if it's wet and if the polyiso is sealed well enough there should be no mold issue so I suppose any water will just sit there, not being able to dry.

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Steve,
    Q. "I was thinking more of moisture coming in through the concrete wall (in years time when the concrete is cracked and waterproofing is failing) and being trapped between the insulation issue rather than moisture-laden air hitting the cold concrete surface."

    A. If the concrete is wet -- it almost always is -- the moisture isn't trapped. It just is. There has always been moisture in the soil, and foundation walls have always been damp. The moisture never gets anxious, and never feels trapped. It is relaxed. It just stays there, hanging out.

  8. Steve Mackay | | #8

    Ha ha, Okay. I'm trying to do the same and just chill but there is a logic issue.
    Moisture from the outside of the concrete to the inside of the concrete is okay it just hangs out and is not a problem but moisture from the inside of the home that touches the inside of the concrete is the nasty bad stuff. See why I am confused?

    Is it a matter of quantity? Is it a matter of that moisture from the inside means you have a bad design or bad implementation? Even if the polyiso is not taped and sealed very well won't the condensed moisture just chill and hang out with its brother, the moisture that came in from the outside of the concrete?

  9. Jon R | | #9

    Moisture, from either source, only "hangs out" up to a certain quantity - then it runs down the wall and out onto the floor.

    EPS insulation allows some inward drying, causing dryer concrete. Dry concrete can absorb a little more occasional moisture from the exterior before the above occurs. There is also less mold, which is relevant for less than perfect air sealing.

  10. User avatar GBA Editor
    Martin Holladay | | #10

    Steve,
    Q. "Moisture from the outside of the concrete to the inside of the concrete is okay it just hangs out and is not a problem but moisture from the inside of the home that touches the inside of the concrete is the nasty bad stuff. See why I am confused?"

    A. If we are talking about two cases --(1) a damp concrete wall with fiberglass batts on the interior, and (2) a damp concrete wall with rigid foam installed in an airtight manner on the interior -- we note two differences in these cases.

    One difference, as Jon R correctly noted, is the quantity of the moisture on the interior side of the concrete. In case (1), you can indeed see so much moisture that it flows to the bottom of the wall and creates puddles. That's a problem.

    The second difference is that there is an air connection between the damp area and the interior in case (1), but there is an air barrier between the damp area and the interior in case (2). In case (1), mold is on the interior. In case (2), a small amount of mold isn't very concerning, because it is on the exterior of the air barrier.

    If you think that these are small semantic differences that don't distinguish between important factors, you're wrong. Case (1) often results in a smelly, wet mess. Case (2) results in a pleasant, dry basement. Huge difference.

    In all cases, we need good drainage details on the exterior to prevent the entry of bulk water. That's the job of your dimple mat and your footing drain.

    Jon R and I have in the past discussed whether it's a good idea to come up with details that allow a basement wall to dry to the interior. Jon believes that inward drying is good. I disagree. After past discussions, I've concluded that Jon and I will just have to "agree to disagree" on this point.

  11. Jon R | | #11

    If you don't like the idea of "perhaps, but not much" mold in vapor barrier foam walls, you could discourage it by painting the concrete with mold killing paint.

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