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Mold Within 2″ Rigid Thermax–Polyiso

TDSCC | Posted in Energy Efficiency and Durability on

Three years ago we remodeled a basement in a 1920’s vintage house. The foundation is poured concrete. In all but two little 8’x8′ areas we sprayed 2-1/2″ of a closed cell urethane foam over Platon drainage matt which was installed directly to the concrete wall. The Platon was channelled into a subslab drain tile that we cut into the floor. We then installed 2×4 walls, uninsulated and drywalled. Since the two little rooms were not finished with drywall and too difficult to install drain tile we decided to simply install 2″ of Poly-iso (Thermax) with taped seams, glued directly over the concrete wall. Now three years later, we are adding a small kitchen addition with a full basement that connects to one of the 8’x 8′ areas. When we pulled off the poly-iso to cut in the opening to the new basement, the foil face stayed on the wall and we found black mold between the foil that is in contact with the concrete and the foam itself, down the entire length of the poly-iso . The owner has never seen any bulk water at the floor so my only conclusion could be that this is an extreme condensation event perhaps as a result of vapor drives through nail punctures on the interior foil face and/or from the top and bottom of the Thermax where it meets floor and mud sill. Would adding Platon over the concrete then installing new Thermax over that prevent this from happening again even though the Platon would not be channeled into a subslab drain tile? We would also be much more diligent in sealing all nail holes, seams, etc. By the way, we did not open up the areas that were spray-foamed over Platon–(That worries me too!)

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  1. jbmoyer | | #1


    My guess is that you had minor moisture leakage in the foundation wall prior to installing the Thermax, but you didn't notice anything because it dried to the interior.

    Now that there is impermeable foil-faced foam boards on the inside surface of the foundation wall, the moisture entering the structure can't dry to the interior (I don't think the nail holes are the problem).

    Again, without seeing the home it's difficult to know exactly whats going on, so this is just a guess.

  2. TDSCC | | #2

    Brett, Thanks for your response. If you are correct, then adding the Platon membrane would at least keep the moisture from entering the Thermax but if there is no air movement between the Platon and the foundation wall how does it dry out? Would it make sense to keep the top and bottom of the Platon "open" for air flow?

  3. jklingel | | #3

    "Would it make sense to keep the top and bottom of the Platon "open" for air flow?" Wouldn't that negate the ability of the insulation to do its job?

  4. TDSCC | | #4

    John, I beleive you are partially correct. It would definitely reduce the thermal capacity and effectiveness of the insulation but the question is would it mitigate the moisture concern? Sometimes you have to pick your poison. Bottom line is, I am still not sure.................

  5. cascadian76 | | #5

    I was thinking that gluing Thermax to the concrete walls of a basement would be a good way to insulate an unfinished basement. Perhaps not, if it would trap moisture and support mold (and lose R-value with the moisture).

    If building new, would it make more sense to put rigid fiberglass insulation (like Warm-n-Dri) or rockwool (like DrainBoard) on the outside of the foundation?

  6. albertrooks | | #6


    I hope this post finds it's way to you.

    A simliar application just came up at The Small Planet Workshop and I wanted to share what we found:

    Insulating a basement of a proposed Passive House project in the Pacific Northwest. The layers are: Concrete wall, foil faced poly iso, fiberglass, drywall. The WUFI Model was set for a south facing wall, above grade, in the Seattle WA climate.

    The Application failed. The wall kept getting wetter every year. You can see the WUFI Model Attached. Note that the foil facing is represented by the 2 layers of vapor retarder at 0.1perm.

    it seemed to me at the time of your post that the issue is the water is coming from concrete and not vapor laden air. If that's the case, then airsealing will not cure the issue. For the application to be successful, the concrete has to be able to dry inwards. To me that means the inboard layers from the concrete wall will need to be vapor permeable to a level that will allow enough vapor to exit the assembly. if your concrete wall is below grade and even has an exterior vapor control layer, I think the results would be near the same.

    This is a great case for running a WUFI exam on the proposed assembly. It's really helpful to have the ability to be able to make use of WUFI on these tricky applications. You can find the free version here: If your not comfortable running or interpreting it yourself them our WUFI guy can do it for you at The Small Planet Workshop.

    Further to the diffusion open assembly: Building America Best Practice guide for Marine Climate (Chapter 8) suggests that interior basement insulation should be vapor-open, which foil-faced polyiso is not.

    Perhaps this post will reach you at a point that it can still be helpful. I wanted to comment earlier, had the assumption that the interior layers would need to be diffusion open, but could not be sure enough of the facts to post a comment until now.

    Best of luck

  7. pSpoW2tSEw | | #7


    I am very interested in understanding more about this application. When you removed the Thermax, you said you observed black mold. Did you have it tested and determine it was black mold? I ask because the growth of mold requires moisture and an organic material. Since the Thermax and the concrete do not contain organic materials, I'm wondering where the mold is getting its "food". The aluminum facer on Thermax will turn black with a chalky oxidation film if left exposed to moisture for long periods of time. Could this be oxidation rather than mold? Also, it would be instructive to understand what climate your house is in.

    Scott Cummings
    Dow Building Solutions

  8. user-626934 | | #8

    Albert -

    I don't think WUFI is (currently) up to the task of modeling the hygrothermal dynamics of a below grade wall.

  9. GBA Editor
    Martin Holladay | | #9

    You report that there is mold between the rigid foam and your concrete wall. Others have complained that they are worried about mold growing under the poly installed on a crawl space floor.

    These mold problems don't bother me, however, because they are outside of the air barrier of the house. After all, there is probably always mold in the dirt outside my concrete foundation. Who cares? The mold is outside and I'm inside. If you seal your air barrier well -- either the rigid foam on your concrete wall, or the poly on the floor of your crawl space -- then mold that is outside of your house is of no concern.

    After all, there are also snakes and spiders and bears out there. But I don't care, because I'm indoors.

  10. albertrooks | | #10


    "Could this be oxidation rather than mold? Also, it would be instructive to understand what climate your house is in." We had not considered this. I could certainly see where the aluminum face could/would oxidize. It sure would be nice to know if it was mold or oxidation...


    "I don't think WUFI is (currently) up to the task of modeling the hygrothermal dynamics of a below grade wall." Your most likely right. We have our doubts but are really-really curious to see how it models when faced with a real world example that has already failed ( if in fact it HAS failed??) If we were involved, this would be more of a "pro bono" excursion to test the limits of both WUFI and more likely... our understanding of it. Certainly the above grade and below grade would differ. How to approach it is the interesting part. We've got a Fraunhoffer guy coming up in September and hope to see if we can "play in the dirt" with any degree of reliability.

    Personally my "clearly unscientific" model is my wood shop this past winter: Picture a former (never been fired up) Nuclear Reactor with 5 foot thick concrete walls. My shop is above grade, but the structure goes 2 large stories below grade. We had a very wet and mild winter in the Pacific Northwest. The walls of my unheated (like I could really heat it...) shop were sweating water on the surface and anything near the walls got wet too. It's hard to say. Perhaps the moisture laden air was allowing the water to condense on the surface. It did not seem that way to me. It did seem that the porous concrete was drawing up liquid water like a wet basement and it condensed on the concrete face. I've not seen a better case to illustrate the benefits of a diffusion open interior to face a concrete wall. It would just be nice to get WUFI to agree.


    Lions and Tigers and Bears... Oh my!

    Ok, if it's out of the breathable envelop, then sure... Who cares?? I guess I'm stuck on advocating for practices that would understand and prevent it within a basement. Air barriers can degrade and mold is pretty unhealthy. We are beginning to work with foil faced vacuum panels with the idea of seeing what can be done with super insulating existing basements. If we were promoting unhealthy practices, that would certainly be counter productive. To quote one of my favorite "30 Rock" episodes: Mold "is a Deal Breaker!"

    It sure would be nice to hear more of the specifics from Sam...

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