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Crawl space: open-cell spray foam?

charlie_sullivan | Posted in General Questions on

We are planning on encapsulating the crawl space in a house on the border of zones 4 and 5 in NJ.  It’s been vented for 58 years but it’s time to fix that. The plan is to close the vents, put polyethylene on the dirt floor, and insulate the foundation walls.  The question is what to use for insulation.  We are lucky to have a highly recommended contractor who is fluent in building science and home performance.  He recommends open-cell spray foam for the foundation wall and the rim joist area.  I’m nervous about open-cell for the below-grade portion and would be more inclined to use EPS for at least some of that.  I’d like to get your opinions on whether open cell would actually be fine, or whether it’s important to use EPS or similar for the lower section.

It a CMU block foundation wall, about 3.5 feet from the dirt floor to the joists.  About 3 feet of that is below grade and about 6″ above.  Last summer’s torrential rains resulted in some water coming in the vents and collecting inside, but that’s being fixed with some re-grading and by  closing off the vents.

There’s no capillary break on the top of the block wall, so the motivation for the open-cell foam is to allow the CMU to dry inward and avoid having water wick up and keep the sill plate wet.  The following BSC information sheet recommends EPS or open-cell foam inside crawl spaces as two options that allow drying to the inside.

But their drawing shows a crawl space with the dir floor level above grade, vs. this house which has most of the crawl space below grade.  So it seems that opting for the lower-permeability EPS could be safer than the higher permeability open cell foam–I worry that it would become saturated with moisture, both from moisture wicking in through the concrete from the soil, and moisture from the interior air condensing against the cold concrete near the bottom of the wall. 

Which of these options are acceptable and which is best?

1. EPS for the whole foundation wall and OCSF for the rim joist area.

2. EPS for the bottom two feet of the foundation wall and OCSF above that.

3. OCSF only.

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  1. Expert Member
    ARMANDO COBO | | #1

    I think a reread is warranted... "Crawlspace walls should be insulated with non-water sensitive insulation that prevents interior air from contacting cold basement surfaces—the concrete structural elements and the rim joist framing. Allowing interior air (that is usually full of moisture, especially in the humid summer months) to touch cold surfaces will cause condensation and wetting, rather than the desired drying. The structural elements of below grade walls are cold (concrete is in direct contact with the ground)—especially when insulated on the interior. Of particular concern are rim joist areas—which are cold not only during the summer but also during the winter. This is why it is important that interior insulation assemblies be constructed as airtight as possible."
    The 2021 IRC Table N1102.1.3(R402.1.3) requires R15 ci (3" R5 or 2.5" R6.7 iso). My detail is for CZ3... we been installing 1" Thermax or Styrofoam on the wall, but now moving to 2" R10 in the new code.

    1. charlie_sullivan | | #2

      Thanks for that comment, and for that really interesting detail.

  2. charlie_sullivan | | #3

    I'd appreciate any other comments. If option 3 is a bad idea, any preference between 1 and 2?

  3. b_coplin | | #4

    Peter Yost used open cell on his basement, and might comment on the relative risks of the approach:

    Not clear from the article how he insulated a previously-vented crawlspace on the same project.

    I have no data or experience to back this up, but using open cell in a basement seems less risky than a crawlspace--people visit basements pretty regularly, so problems are more likely to be found in a timely fashion.

    1. charlie_sullivan | | #6

      Thanks--interesting to see that he did that, and it would be even more interesting to see how it has done over the last 12 years. It does seem like EPS, at least for the lower part would be lower risk, even if the ocSF is acceptable.

  4. Expert Member
    ARMANDO COBO | | #5

    Here is what Joe recommends in his BCS article: "The best insulations to use are foam based and should allow the foundation wall assembly to dry inwards. The foam insulation layer should generally be vapor semi impermeable (greater than 0.1 perm), vapor semi permeable (greater than 1.0 perm) or vapor permeable (greater than 10 perm) (Lstiburek, 2004). The greater the permeance the greater the inward drying and therefore the lower the risk of excessive moisture accumulation.
    Up to two inches of unfaced extruded polystyrene (R-10), four inches of unfaced expanded polystyrene (R-15), three inches of closed cell medium density spray polyurethane foam (R-18) and ten inches of open cell low density spray foam (R-35) meet these permeability requirements."
    A good litmus test is to know what those guys do in their own house. I don't know how much ocSPF Peter used in his crawl space, but I've been at Joe's famous crawl space wine parties during Summer Camp, where he encapsulated it with ccSPF. I've also designed/built many basements in CZ4-5, and I've always used 2" polyiso (sealed and taped), and then 2x4 walls filled with batts, DPC or rockwool. I do spray ocSPF at the rim joists (trusses) because I always install rigid foam boards on the outside, and I don't like to sandwich the sheathing with 2 cc foams. Here is a detail I've used in CZ 4-5.
    Check out Energy Vanguard's blog, Step 3:

    1. charlie_sullivan | | #7

      Thanks. Joe's crawl space parties are a good inspiration for how good it can be! It seems like aiming for around 1 to 5 perms might be ideal. Given that EPS at 4" is around 0.5 to 1, and open cell is likely closer to 10 to 15 for that thickness, either is likely OK, and doing the bottom half with EPS and the top with open-cell might be ideal.

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