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Basement insulation

Matthew Adkins | Posted in General Questions on

Building a new home in zone 6 (NE Ohio). The basement will be poured concrete walls and have approximately 16-24” exposed. We will be using Tremco Warm and Dry (R-10) below grade. Eventually, plan to finish basement with 2” rigid on the inside then 2×4 walls. What can I do for the exposed portion of the foundation? Is anything needed? Note: Will be using a continuous insulation system from the rim joist up (R-5)

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Replies

  1. Expert Member
    Michael Maines | | #1

    User-7463728 (would you mind sharing a name?)

    Cutting exterior insulation off at grade is a common practice, and usually a mistake in cold climates--the above-grade portion is actually the part that most needs insulation. This article covers the basics of how to finish the above-grade insulation: https://www.greenbuildingadvisor.com/article/how-to-finish-exterior-foundation-insulation.

    On the other hand, if you really are going to install rigid insulation on the interior, and it will extend to the top of the wall, you can just leave the exterior concrete exposed. It should be air-entrained to prevent spalling, but that should be common practice anyway.

    If you insulate the interior, I would urge you to either find a source for used XPS, or use another type of foam, because the blowing agents in XPS are extremely potent global warming gasses. I generally spec polyiso on the interior and don't bother insulating the exterior.

  2. Expert Member
    Dana Dorsett | | #2

    What Michael said.

    Then...

    There are no locations in NE OH that are DOE climate zone 6. It's all zone 5A.

    In NE Ohio there are multiple sources of used EPS at a fraction of the cost of virgin-stock goods. Many of those vendors advertise here:

    https://cleveland.craigslist.org/search/sss?query=rigid+insulation

    Most roofing EPS is Type-VIII (1.25lbs per cubic foot nominal density) , which runs about R4.15 per inch, which is comparable to Tremco Warm'n'Dri , and would be fine in an above-grade application (or even below grade if using a well draining select backfill.)

    Do NOT substitute polyiso for any foam on the exterior side of the assembly (even above grade) as it is likely to wick up moisture over time. It's OK for exterior sheathing that has a standard siding on it, but the cementicious EIFS solutions aren't sufficiently protective of that moisture wicking potential. Polyiso is fine on the interior side though, as long as the cut bottom edge has a capillary break (like 1/4" of air space, or EPDM flashing tape) between it and the slab.

    IRC code minimum for zones 5 & higher is R15, so the R10 Warm'n'Dri isn't going to cut it on it's own. But dropping it to R5 would be sufficient dew point control for using a 2x4/R13 studwall tight to a 6 mil polyethylene vapor barrier the foundation on the interior side of the foundation, which would have the same thermal performance (assuming there was also R5 continuing above grade on the exterior side.)

  3. Matthew Adkins | | #3

    Dana, you are correct, the zone is 5A

    If i use XPS on the inside and only insulate the exterior below grade on the outside it should be ok?

    Really appreciate the info

    1. Expert Member
      Dana Dorsett | | #5

      >"If i use XPS on the inside and only insulate the exterior below grade on the outside it should be ok?"

      No, it's not OK, on a couple of counts.

      XPS is the LEAST green form of insulation in common use in the US, primarily due to the extreme global warming potential of the HFC soup (primarily HFC134a mixed with others) used as the blowing agent. HFC134a has a global warming potential at 100 years nearly 1400x CO2. As the HFCs diffuse out over a few decades the performance drops, and is only warranteed to 90% of the labeled R. But after 50 years it's likely to be closer to 85% of the labeled-R, the same performance of EPS of similar density. (Care to test it and submit the warranty claim? I didn't think so, and neither do the vendors.)
      EPS and polyiso are blown with much lower impact hydrocarbons, predominantly a variant of pentane at about 7x CO2. In the case of EPS nearly all of the blowing agent escapes the foam at the factory where it is recovered, not vented into the environment, and performance is stable over time.

      See:

      https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.pn

      To meet the letter of code it would need at least 3" of XPS on the interior, even if the exterior side has R10 below grade, and as stated above, over time performance will drop to sub-code. The R15 has to be continuous- it can't just be applied to the above-grade section, with R5 on the interior below grade. The concrete wall is very low, and would conduct heat between the exposed above grade portion to the under-insulated R5 below grade section quite readily.

      Instead of XPS, installing 3" of reclaimed roofing polyiso on the interior would be about R17, and would thus meet code, both now and 50 years from now.

  4. Matthew Adkins | | #4

    Not sure why they posted as User... Name is Mike

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