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Community and Q&A

Why is mold/mildew growing behind my rigid foam?

jay443 | Posted in General Questions on

So I had started to finish my basement area and read all of the articles on GBA. I was using 2” of XPS foam on the cinder block walls and recently had to remove a bunch to access an area for the egress window. When I pulled it off, I was surprised and scared to see that I had black stuff growing on the cinder block. I don’t know if it’s mold or mildew, but it can’t be good, right? You can even see how it’s growing around the foam adhesive that I used to attach it to the wall.

Not sure why this would be the case. I keep my basement at less than 50% RH and there is no issue with water. In fact, I’ve kept cardboard boxes on the slab floor for years without any issues.

What should I do? Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    The mold is normal and harmless. Don't panic.

    First of all, the soil around your house is always damp. It's already full of mold. It's outdoors. Don't worry about it.

    We use rigid insulation in this case to create a thermal boundary or envelope for you house. Everything on the exterior side of the thermal boundary -- in this case, on the exterior side of the rigid insulation -- is outdoors. Everything on the interior side of the rigid foam is indoors.

    Ideally, you will install the rigid foam in a continuous layer, striving for an airtight insulation. Once it's installed, resist the temptation to peek behind it. (Unless, of course, there is a compelling construction-related reason to peek.)

    Your concrete blocks are damp because they are in contact with damp soil. They will always be damp.

    If your basement has no water entry problems, the fact that the soil is damp and the concrete blocks are damp is OK.

    The interior side of your rigid foam should stay dry and mold-free. Make sure that you install a layer of gypsum drywall on the interior side of the rigid foam for fire protection. As long as the drywall is mold-free, don't worry.

    For years, the inside surface of your concrete-block wall has been contributing moisture to your house. The water has been evaporating (which is why the concrete blocks feel dry). By installing the rigid foam, you will be cutting off this moisture flow from the concrete blocks to the interior of your home, improving the indoor air quality. That's good.

  2. jay443 | | #2

    Thanks for your responses.

    Martin, I appreciate your detailed response. My interpretation from you is to "don't worry about it" as long as I take care to properly seal for an airtight layer.

    I will be using EPS for the floor insulation as I read about how XPS is damaging to the environment. Since I have the XPS off the walls now, would it be worth painting it with a mold killing paint? Or instead of XPS on the walls, should I move to EPS to promote more drying?


  3. GBA Editor
    Martin Holladay | | #3

    If you've already bought the XPS, use the XPS. You don't need your foundation wall to dry inward. (That only adds a moisture burden to your house, which you don't want.)

    I wouldn't bother to try to kill the mold. Just put the XPS back and stop thinking about it.

  4. Expert Member
    Dana Dorsett | | #4

    The patterning on the wall appears to follow the mortar joints and possible seepage at the mortar seam 2.5 blocks above the slab. It's worth taking look on the exterior above grade at that point to see if there isn't something going on near that potential seepage point, such as a low spot in the surface grading, or grading that tips toward the foundation rather than away from it, a broken dripping spot on the gutters, or anything that would make the ground damper there than elsewhere.

    Even though it doesn't look like enough seepage to be a real problem, the less bulk water that makes contact with the foundation, the better.

    As long as you have a least foot or more of above grade exposure on the exterior it's unlikely that enough moisture will wick up to the foundation sill to cause a problem. But if you have chronic bulk water wetting of the foundation it needs more drying capacity. Painting the exterior of the CMU above grade can be a mistake if there isn't a good capillary break at the foundation sill. I've read of a house with painted CMU walls for the first floor and a wood framed second floor, where the moisture wicked all the way to the second floor after the foundation was insulated, in part due to polyethylene vapor barriers in the finish wall on the first floor too. Prior to insulating the basement it had been fine. Had the bulk water been better managed or the CMU left unpainted on the exterior, it probably would have continued to be fine.

  5. jay443 | | #5

    Got it. Thanks everyone!

  6. Jon_R | | #6

    Was it only in the lower parts?

    You can try cleaning the wall with bleach+water, applying a paint such as Zinsser Watertite/Mildew and using unfaced EPS (it has higher permeability than XPS and will allow more drying to the interior). Seal it such that there is no chance of any air flowing behind it.

    Don't be surprised if you smell mold (air sealing is never perfect) and need more extreme methods. I have a section of basement wall where the lower 1/3 can't be covered - doing so causes less drying and occasional water on the floor. Digging down to the footing drains to fix the drainage just isn't cost effective.

  7. john757 | | #7

    I'm glad I found this discussion, as I just started to finish my basement for a 20 x 20 living space in my 30-year old house. My walls are cinder block painted with moisture resistant Drylock. In a few areas, some dusty effervescence still seeps through but no water, and I scrape and repaint periodically for appearances. I have a 2-inch perimeter interior french drain actually (1-7/8" wide) with a sump pump that goes on during heavy rains and does its job.

    I plan to first install 2-inch XPS rigid foam on the walls and build 2x4 stud walls next to the foam panels. The rigid foam will sit on top of the drain which will still get air flow from the unfinished basement area to address dampness. I'm wondering if I should install a vapor barrier underneath the lower portion of the rigid foam and the 2x4 base plate on the slab to further isolate any moisture in the perimeter drain. Would like any advice. Thanks.

    1. Expert Member
      Dana Dorsett | | #8

      If it's not too late, change the XPS to EPS, or better yet, polyisocynurate. XPS is blown with HFCs that are extremely powerful greenhouse gases (>1000x CO2) soon to be banned for that application in the Kigali amendment to the Montreal Protocol. Both EPS and polyiso are blown with hydrocarbons, predominantly variants of pentane, at about 7x CO2. In the case of EPS most of the HC leaves the foam before it leaves the factory, and is recaptured.

      The performance of XPS declines over time as the HFCs escape. The manufacturers only warranty 90% of the labeled performance (which would be R9 at 2"), but at full depletion it's performance drops to that of EPS of similar density.

      If the foundation is seasonally pretty damp or has a history of leaking liquid water, using a dimple mat between the wall foam and foundation is prudent, to allow any bulk water to leave quickly to your perimeter drain.

      If using polyiso it's important to keep the cut bottom edge above the high-tide mark of any flooding, and not resting on the slab where it might wick ground moisture over time.

  8. john757 | | #9

    Thanks, Dana for your analysis. I did buy a few XPS panels just to check out the particulars of the job. I am leaning toward the XPS after viewing some comparisons of the EPS and XPS. One source says XPS performs better because of its "closed cell structure" that prevents less water absorption than EPS. For me, this factor has a higher priority, at this moment, than possible HFC emissions of XPS. However, I haven't ruled either out. EPS is less expensive and I like that. So I'll do some more thinking.

  9. Expert Member
    Dana Dorsett | | #10

    EPS is closed cell foam too. The primary difference it the interstitial spaces between the macroscopic beading of the foam, not the cell structure. Even under slabs EPS holds it's own. The only time there's an appreciable differences is when buried in potentially saturated soil.

    Foil faced polyiso is waterproof except at the edges, and is also cheaper per R than XPS. Unless you're anticipating a flooded basement polyiso (even fiber faced roofing polyiso) is fine, and has a higher R/inch.

  10. john757 | | #11

    Dana, I won't need XPS/EPS foam after all. I should have first checked with my township inspector who told me I just need fiberglass batts in the stud walls installed a few inches from the cinder block walls without rigid foam. This gives me R-13 basement walls which they require. Also it is simpler and saves a lot of money.

    Also, for anyone interested. In the Philadelphia area, I could not find EPS 2"X4X8 foam at Home Depot, and Lowes has discontinued it. I found it was available at Menards in the mid west for a great price $12.
    Thanks again for all the great information on this web site.

    1. Expert Member
      BILL WICHERS | | #12

      It’s risky to insulate basement walls with batts, especially when you already know you have moisture on your walls. Rigid foam is a much better choice regardless of if batts are permitted or not.


    2. mackstann | | #13

      Code is just a minimum. And in this case it's woefully lacking. Insulating a basement wall without foam is well understood to be a risky choice:

    3. Expert Member
      Dana Dorsett | | #14

      Batts in a 2x4 wall can easily become a mold farm. With no air barrier between the foundation side of the batts facing the "few inches" gap it won't even perform to spec. The inspector might buy it, but I wouldn't. YMMV.

      Even half-inch polyiso or 3/4" or 1" foil or vinyl faced Type I EPS (commonly found at box stores) held against the wall by the 2x4 studwall would be enough for dew point control on 2x4/R13, providing the necessary air-barrier on the foundation side of the batts, and puts a moisture barrier between the foundation and the studs/batts. The faced 1" EPS can be had in almost all Philly area box stores for about $15-16 for a 4' x 8' sheet, half inch polyiso for about $13-14/sheet. (Just checked- it's out there!) With the facers EPS is very low permeance and can't accumulate liquid moisture in the interstitial spaces between the beads unless submerged for days.

      A layer of used 2lb roofing polyiso 2" thick strapped to the wall with furring would meet code-min for zone 4A with none of the moisture or performance issues of a no-foam 2x4 batt wall, and would be cheaper, typically $10-15 per sheet (about the same prices as R3-ish box store foam.)

      Repurposed Materials in Marcus Hook PA often has pallets of used roofing foam for dirt cheap (WAY cheaper per R than box-store batts.)

      They're not showing any 2" polyiso in inventory this week:

      They DO have 30 sheets 4" thick x 2' x 4' (240 square feet) of concrete faced roofing XPS which might work in somebody's basement or crawlspace, but probably not yours.

      If the project can wait it's worth checking in from time to time.

  11. john757 | | #15

    Dana, Zephyr7, Nick: I agree with each of you. Thanks for your research. I was surprised to hear the inspector give me only minimum requirements but maybe I misunderstood. A contractor also gave me similar advice.

    I do have the white efflorescence deposits appearing on about 10% of the wall areas. Cleaning and applying Drylock over the years helps appearance but it always returns. I do like the idea of rigid foam against the walls then stud wall batts to bring it up to R13. The 2" R10 XPS I intended to use is $30/4' sheet x 6 stud walls or $360. Prefer not to use 1" because of the 2" perimeter french drain. Cost is not a problem but I will check into the other less expensive materials since there's no hurry. I'm taking my plan and permit application to the township next week. Thanks for the advice and tips!

  12. Expert Member
    BILL WICHERS | | #16

    2” thick polyiso will give you R13 in one step, and that’s continuous insulation without thermal breaks from studs so it will perform better than a stud wall with R13 batts in the bays. If you go with a product like Dow thermax that has a side that is rated to be left exposed (and they make a version that is white), then you can leave that polyiso as your wall and be done.

    If you want the stud wall for other purposes like utilities, you have some other options. You can lay the studs over the polyiso, or build the wall and not insulate it since the polyiso would already be doing all the “work” of insulating. If you do end up using batts in any way, I’d use mineral wool instead of fiberglass for better moisture tolerance.


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