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Community and Q&A

Foundation Wall Condensation

jnarchitects | Posted in Energy Efficiency and Durability on

I’m working on a project in Climate Zone 5. The basement is a walkout configuration. The interior walls of the basement are insulated with 2″ polyiso w/ taped seams. The box is insulated w/ 2-3″ closed cell foam. The floor (bsmt. ceiing) is insulated with fiberglass batts.

The are no mechanical systems in the bsmt., only insulated duct work. The walkout bsmt. wall also has 5 flood vents spaced along the bottom of the wall. These have not been sealed, but will be after the final inspection. The basement is fairly cold (lower 40s), primarily because the flood vents have been partially open.

I noticed today in several corners of the basement that the fiberglass batts were wet. They were in contact with the top of the foundation wall. Also, there was considerable moisture on the top of the fdtn. wall. One corner in particular actually had frost forming on the top of the fdtn. wall and the sill plate was very wet.

I assuming that the condensation is a result of the cool moist air condensing on the colder fdtn. wall. We started a dehumidifer and will seal the flood vents to see if we can bring the temp up. Additionally we are considering bringing some conditioned air into the space.

I was planning on adding rigid along the top edge of the fdtn. wall, but had been saving that for a rainy day.

Any suggestions for further diagnosis and addressing the issue.

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  1. Riversong | | #1

    Most likely your ducts are leaking into the basement. There should be no other source of moisture sufficient to cause such condensation. Condensation requires warm air contacting a cooler surface.

  2. GBA Editor
    Martin Holladay | | #2

    You need to install some type of air barrier, as well as some insulation, to keep interior air away from the cold concrete. In those areas where you are able to inspect the concrete, air is reaching the concrete.

    Spray foam is the easiest solution. Wherever you see exposed concrete peaking out, cover the concrete with spray foam.

    If you don't want to use spray foam, you could use rigid foam and caulk, although the work will be trickier.

  3. Garth Sproule Zone 7B | | #3

    Are you saying that 40° F air is not warm enough to form condensation?

  4. jnarchitects | | #4

    The ducts have been tested at less than 6% leakage as part of the Energy Star inspection. The rater felt the ducts were very tight.

    Is the easiest solution to just have the spray foam installers back to spray from where they stopped in the box and cover the exposed part of the top of the fdtn. wall?

  5. GBA Editor
    Martin Holladay | | #5

    Yes. If spray-foam installers came to do the rim-joist area, it's a real shame that the installers didn't notice that the top of the concrete wall was bare and exposed. That exposed concrete needs to be covered.

  6. homedesign | | #6

    The box is insulated w/ 2-3" closed cell foam. The floor (bsmt. ceiing) is insulated with fiberglass batts.

    I don't know the term "box"
    what does it mean?
    Is there a finish wall in the basement?
    any chance to post a photo?

  7. jnarchitects | | #7

    I was referring to the rim joist area.

  8. jnarchitects | | #8

    No finished walls in the bsmt. Just rigid attached directly to the concrete fdtn. wall. I should add the fdtn. wall is full height around the entire perimeter even on the walkout side.

  9. Riversong | | #9

    Are you saying that 40° F air is not warm enough to form condensation?

    It depends on the relative humidity of the air. If it's interior air or heated duct air that's leaking into the basement, then it will have a very high dewpoint. But if it's outside air and warmed to 40°, it would have to have 73% RH to have a dew point at 32° (the foundation top must be at 32° or lower if there's frost on it).

    But if the basement is that humid and cold, then fiberglass is not the best insulation to have in the floor above.

    And I agree with Doug that it's almost always preferable to insulate a basement on the outside and keep the warm thermal mass on the inside.

  10. HDendy | | #10

    Have you tried removing a portion of the rigid insulation you already have in place, just to verify that the moisture is only occurring at the exposed portions of concrete? I think Martin's diagnosis is correct, but it wouldn't hurt to verify (through observation) before completely covering everything. It would also be a good idea to use a hygrometer in the space to get an RH reading to determine if there is excess moisture in the air as Robert said. If there is, then time to hunt down the source, in addition to eliminating the cold condensing surfaces (source and symptom).
    Let us know what you find out.

  11. user-723121 | | #11

    This is good example of why foundation insulation should be in some part (at least 1/2 of total R-value) on the outside (cold side) of the foundation. You can take care of the rim joist area by extending the foundation insulation up past the rim.

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