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Found a surprise in the wall on Day 1 of basement reno

ddgrant | Posted in Energy Efficiency and Durability on

Hi all,
We live in Atlantic Canada in a split-entry home with 4 foot high concrete foundation walls in the finished basement (creates a ledge all around the basement). We’ve had to keep a dehumidifier going in the summer to control humidity and mold growth on baseboards, so I anticipated the wall construction to be traditional batt and poly over the concrete. I planned to counter some of that migrating moisture by adding either closed cell spray foam or XPS behind the existing studs, depending on what I found for available space. I broke open a small hole between studs in the finished wall in our basement today and was elated to also find 1″ EPS behind the studs! However, I know that proper installation is to SEAL all of the insulation seams and edges to prevent moisture migration, so my question is… Do I really need to remove all of my drywall just to seal the seams and edges? Should the poly be removed or is it beneficial enough to prevent the need to tape up the seams?

Also, I lifted an edge of the carpet to find the 1/2″ underlay glued directly to concrete (expected), but the only evidence of moisture was the slight rusting of the nails in the carpet nailing strips, no sign of mold. The wood strips were not even discolored. I had also planned to install insulation and plywood over the concrete slab because I’m not confident there is a vapor barrier under the slab, but I must be feeling lazy because I’m now questioning the need for it. Would the lack of vapor barrier under the slab be the cause of high basement mold in summer, or could that simply be from the open screen door which is only a half-flight of stairs away? (The basement is open to the stairs and is not well sealed to the adjacent garage). Any advice?

Thanks in advance,
Darcy Grant

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  1. KeithH | | #1

    I have a few ideas for you to consider. I'm sure Martin will post some references to articles he has written.

    1. 1" of EPS is probably < R-4. Not impressive. You could get condensation in a lot of conditions on the interior side of the EPS I'd guess. (I'm not very experienced with your climate)

    2. If the edges of the EPS aren't sealed well, what's to prevent that high humidity area from getting behind the EPS and condensing between the EPS and concrete wall?

    3. Have you performed the plastic square test for humidity on your basement floor? ie is the humidity coming through the slab?

    4. Carpet on a basement floor, especially with an iffy vapor barrier situation underneath, is pretty inadvisable. Remember that it isn't just mold that you want to avoid providing a high humidity environment but also dust mites which love carpet with a few years of cruft ground down in.

    So a few ideas for you:
    - Insulate your basement wall from the outside. (hard work, $$$)
    - Rip out the finished surface and fir out to permit R-20 or so against the basement wall (demoralizing, $$)
    - Insulate your slab with foam board and such as described elsewhere
    - Put a vapor barrier down directly on concrete floor and reinstall flooring (preferably not carpet) over that.
    - Stain the concrete and apply a sealant.

    I know this project well as I have a split level partial basement house that had drywall firred directly to uninsulated concrete. We opted, against Martin's advice, to install batts directly against the concrete with a smart vapor barrier between the insulation and the interior. I probably wouldn't do this in your climate. I'm in a much much drier climate.

    We had good results with taped vapor impermeable laminate flooring and underlayment for awhile. Problematic if you have a water or sewer backup ...

    Well, hope that helps a bit.

  2. GBA Editor
    Martin Holladay | | #2

    I don't recommend that you rip out all of the existing drywall and fiberglass batts. There is no evidence (to me) that your walls have problems.

    The easiest way to assess what's under your slab is to drill through the concrete and take a look. While drilling concrete takes time, it's easier than installing an unnecessary layer of rigid foam.

    It sounds as if your only symptom is high indoor humidity. The first step with any damp basement is to do the needed exterior work: check the exterior grade -- make sure that it slopes away from the house on all four sides -- and check your roof gutters and conductor pipes. Where is your roof water going now?

  3. ddgrant | | #3

    Well, there is a very narrow line of discoloration where the batts meet the seams of the EPS, so that is evidence (albeit minor evidence) that moisture is getting through the seams. But no sign of bulk running water at all.

    Martin, I agree that drilling through the slab to see if there is a vapor barrier is the way to know for sure. So, devil's in the details here but, assuming there IS a vapor barrier, when I go to seal that hole up again would I seal it with some spray foam at the bottom of the hole, before filling the remainder of the hole with concrete/grout?

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Assuming there IS a vapor barrier, when I go to seal that hole up again would I seal it with some spray foam at the bottom of the hole, before filling the remainder of the hole with concrete/grout?"

    A. That sounds like a good plan. Remember, the concrete slab is your air barrier, and vapor barriers work very well even when they have holes in them. So don't worry too much about this issue.

  5. charlie_sullivan | | #5

    I think the open path to the screen door is likely a big part of the problem. If you run a dehumidifier in a well-sealed space, it runs until the moisture is removed, and then (assuming it has a humidistat), doesn't need to run anymore. If you close up that route for outside humid air to get into the basement (e.g. add a weatherstripped door at the top or bottom of the stairs and seal the door to the garage better.), you might be able to get to lower humidity running the dehumidifier much less, unless much of it is coming up through the slab. So I agree that finding out whether there is moisture coming up through the slab is also key. Improving exterior drainage could include some low hanging fruit. Gutters, grade as Martin says. Adding or fixing the footer drain could also be necessary, though that is expensive. Another option is to dig down a foot and install a sloping plastic or metal layer to divert water away from the foundation after it seeps into the soil.

    But once you have done those things, adding underfloor insulation can also help, because the floor surface will be warmer and there will be less potential for condensation there.

  6. GBA Editor
    Martin Holladay | | #6

    Charlie is right that it makes no sense to operate a dehumidifier if you leave an exterior door open. For more information on dehumidifiers, see All About Dehumidifiers.

    For more information on damp basements, see Fixing a Wet Basement.

  7. ddgrant | | #7

    We don't have air conditioning, so it's pretty hard to leave the doors and windows closed when it's hot outside. I try to 'encourage' my wife to leave the doors and windows closed until the outside temp goes above the indoor temps too. Plus, I have a hygrometer in the basement that I check almost daily and when the humidity rises to 60% I'll turn on the dehumidifier at night to drop the humidity as much as possible while all of the doors and windows are closed. The humidity might stay at 45 or 50% for 4 or 5 days in the summer (all with windows/doors open), but then spike to 70% one day. So it doesn't actually run continuously.

    As I think about it now then, the rise in basement humidity seems to follow weather patterns more than anything else. I'll get the slab core drilled this evening to confirm the vapor barrier either way, but could this signify that the humidity in the basement is affected more by weather than from migration through the concrete from the ground?

    So, based on the above, apart from getting AC so we can keep the windows/doors closed, I don't really have a hope of preventing the humidity from rising. I just have to install systems to keep the slab/walls warmer than all possible dewpoint temperatures. Sounds like floor insulation is almost a requirement then!

  8. Richard Beyer | | #8

    This link should help you with insulating the basement correctly.

  9. GBA Editor
    Martin Holladay | | #9

    The document that you linked to on this page has information relevant to a discussion on a different Q&A thread (Insulating basement rim joist):

    "Goldberg and Huelman (2000) performed a series of experiments on basement insulation systems at the Cloquet Residential Research Facility ... One study was limited to testing fiberglass batt insulation, both unfaced and Kraft faced, with various combinations of wall side and interior vapor barriers. ... Their work clearly showed that placing batt insulation in the rim joist area with or without an interior vapor barrier results in condensation within the rim joist area."

    I'm quoting this, Richard, because you note that it is a helpful document. Of course, I agree with Goldberg and Huelman.

  10. ddgrant | | #10

    Thanks for the link. I've read that before, and the recommendation that a basement wall with EPS and batts NOT have a vapor barrier is well understood. However, it turns out there is very minimal mold growth in my basement walls that have been finished with a vapor barrier for over 24 years. I've been seeing mold slowly growing only near the floor (on the baseboards) when RH gets too high in the summer, which is why I thought we might have a major problem, specifically under the carpets. Turns out we don't seem to have a major problem, but I don't want to ignore the building science that I've learned on this excellent website unless others here think it might be okay as is.

    I'm willing to swallow the expense and do the work, but not if it isn't necessary. Appreciate all the info folks. I'll let you know if the slab has a vapor barrier soon.

  11. charlie_sullivan | | #11

    I wasn't recommending keeping all the windows and doors in the house closed--just finding a way to close off the basement from the open doors and windows above.

  12. ddgrant | | #12

    Jack hammered a hole in the slab last night and found there was no vapor barrier under the slab. Looks like I'm off to the box store for insulated subfloor panels. I'm looking at the Barricade subfloor panels. Any thoughts on this product choice?

    The rotten part about having to add insulation and plywood (or insulated panels) - since I also have a 3-piece bath and laundry room in the basement, I'll have to temporarily remove my cabinets, toilet, shower base and the appliances in order to install my insulated floor panels (will also have to extend my plumbing).

  13. GBA Editor
    Martin Holladay | | #13

    There are at least three brands of subfloor panels designed for basement slabs: AmDry, DriCore, and Barricade. Here is a link to a product review that mentions all three products: New Green Building Products — June 2013.

    Here is a link to a Q&A thread that discusses a variety of options for installing insulation above a basement slab: Basement subfloor retrofit insulation options.

    Good luck.

  14. Richard Beyer | | #14


    Back at you my friend... ;)~
    Some day when your down by the shore maybe we can go out for lunch and share a fortune cookie.;)

    Foundation Insulation Effectiveness: Basement building science

    Pat Huelman, Cold Climate Housing Coordinator with the University of Minnesota Extension discusses the issues associated with insulated foundation walls.

  15. ddgrant | | #15

    Mr. Huelman covers my scenario with 1" EPS and framed batts. He essentially reflects many of the principals noted by BSC and GBA, and specifically recommends that any rigid insulation installed against the interior of the foundation walls be sealed at the top and bottom (and all seams). However, I do not have any sealant anywhere and I would be hesitant to leave it unsealed, except that it's been like this for 24 years without much issue. Could this be a case of science suggesting the best possible approach for MOST basement applications and our basement's 'one step better than bad' construction has us close enough to the limit of suggested construction? Or maybe we've just gotten lucky!?

  16. GBA Editor
    Martin Holladay | | #16

    Fortunately, one or two sloppy details do not doom a house.

    If all it took to doom a house was a few untaped seams between adjacent pieces of rigid foam, there would be a lot more failed houses than we already have.

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