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Crawlspace insulation

Ira Broussard | Posted in General Questions on

Zone 2, southeast Texas, 10 miles from the coast, new home construction…

The house will have a slab on grade foundation, with the foundation floor about 15″-18″ above natural ground level. The foundation floor will actually be the crawlspace floor, as we are building a CMU perimeter stem wall on the foundation and interior pony walls to raise the living area first floor up to almost four feet above the foundation floor. The CMU stem wall will be 32″ high (four courses), with a wood framed first floor built on top of it. There CMU stem wall will be covered by brick veneer (with at least a 1″ air gap between the two). The crawlspace will contain a dedicated dehumidifier.

The building envelope will consist of the crawlspace, living area, and attic.

Question… since the CMU stem wall and the crawl space wall are both quite a bit above ground level, does spraying 2″ of closed cell foam on the interior wall of the stem wall have much benefit? Since it’s above ground, is the added insulation value of the CC foam helping very much? Are there other solutions that would be equally effective, but less expensive, or does anything need to be done for the CMU stem wall?


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Ira.

    Can you tell me if I have this right? You'll be installing a slab-on-grade foundation. On top of that will be a 32 inch CMU wall, then a wood framed pony wall on top of which the first floor will be framed. So, this entire crawlspace below the first floor will be above grade. Correct?

  2. Ira Broussard | | #2

    You've got it right. The wood framed pony walls will be on the foundation floor to support the first floor joists between the stem walls (distance from front to back of the stem walls is about 35')..

    So, yes... the entire crawlspace below the first floor will be above grade by at least 15". We don't know what the FEMA flood zone requirement is.

    The foundation is a true, complete slab-on-grade foundation. I could actually completely do away with the crawlspace and build directly on the foundation if I wanted to.


  3. Ira Broussard | | #3

    Nobody has any comments/suggestions on this? Is it that weird?

  4. GBA Editor
    Brian Pontolilo | | #4

    Hi Ira.

    In your Climate Zone, I don't believe the International Residential Code calls for any insulation on crawlspace walls. However, if you intend to build a conditioned crawlspace, you'll need to find a way to air seal the floor, CMU walls, the rim, etc. There are a number of ways to do that work.

    A couple of inches of closed-cell foam would air seal and add R-value to the walls and rim joist area, but at a financial and environmental cost. EPS rigid foam is probably more affordable and definitely more environmentally friendly.

    In addition to dehumidification, you either need supply and return registers from the furnace or an exhaust fan.

    For more info on how to do this right, I suggest you read this: Building an Unvented Crawlspace

    And, if it turns out that you are in a floodplain, a vented crawlspace may be required and advantageous.

    1. Expert Member
      Dana Dorsett | | #6

      >"In your Climate Zone, I don't believe the International Residential Code calls for any insulation on crawlspace walls."

      That's only if it's an UNCONDITIONED, VENTED, and at least partly BELOW GRADE, crawl space. Only crawlspaces that are below grade and/or have the code-minimum R13 under the subfloor above can skip the wall insulation.

      For a conditioned, unvented, above grade crawl space in Zone 2 the IRC calls out R6 c.i. minimum if the insulation on the interior of a "mass wall", R4 if it's on the exterior.

      But don't use closed cell spray foam. R-for-R 1lb polyiso has half the CO2e impact of HFO blown closed cell foam, about 1/4 the impact of HFC blown closed cell foam. Type-I EPS is only slightly worse than 1lb polyiso, and is more flood tolerant. Most foil faced sheathing polyiso is R13 @ 2"- very comparable to closed cell polyurethane but half the polymer, and like EPS uses primarily low-impact hydrocarbon blowing agents.

      If it's in a location with at flood risk, go with 1.5" (min, for R5.8, R6 with credit for the facer) to 2.5" foil faced Type-I EPS (rather than polyiso) which won't hold onto the water as long after the tide goes out. If its high and dry, 1" polyiso meets code-min, but 2" isn't insane. It's pretty easy to make foil facers air tight using a quality temperature rated aluminum HVAC tape. With polyiso leave a 1/4" air gap to the slab or 1" of EPS at the cut bottom edge of the polyiso as a capillary break to keep the polyiso from wicking any incidental moisture from the slab.

  5. Walter Ahlgrim | | #5

    In my opinion spray foam is a product of last resort when it becomes necessary someone has made some poor choices put themselves in a bad spot. Recycled used foam board sounds like a much better idea.

    Even when code enforcement does not require you to it is a good idea to cover insulation foam with a fire barrier to give you the time needed to escape before the fumes kill you.

    Had you found and read the relevant articles on this web site?

    “The building envelope will consist of the crawlspace, living area, and attic.” Are you going to condition the attic and crawlspace? I like the idea of a conditioned crawl space containing all your HVAC equipment and ductwork. This would avoid very common and exceptionally stupid Texas practice of putting all the HVAC ducts and equipment in the attic.


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