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Conditioned or not? What’s the best way to insulate a 1950s home’s crawl space?

user-509810 | Posted in Energy Efficiency and Durability on

We have a 1950s home with a small basement and the rest crawl space in Zone 4A. I need to figure out the best way to insulate the crawl space and maybe the basement. The house is heated with propane and cooled with central AC. The walls are block in both spaces. There are no mechanical systems or ducts in the crawl space. The sewer, water and pipes from the propane boiler run through the crawl space. Currently the crawl space is vented, uninsulated and has no vapor barrier.

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  1. Expert Member
    Dana Dorsett | | #1

    A ground vapor retarder and 1.5-2" of EPS covered by a 1-2" rat-slab, with 2-3" of EPS sprayed with intumescent paint or 1.5-2" of fire-rated polyiso works on the crawlspace walls, sealed to the foundation sill & band-joist with 3-4" of open cell foam works.

    In the basement portion you can insulate the walls with 2" of fire rated polyiso, or if there's space, an inch of EPS and a 2x4 studwall with rock wool or fiberglass batts (unfaced only), and no interior side vapor barrier. The bottom plate of the studwall and the bottom edge polyiso should be separated from the slab with an inch or two of EPS as a capillary break. (Polyiso can wick moisture on it's cut edges, EPS won't.) Compressive strength of the foam under the bottom plate isn't an issue- it's not holding up the house, only the interior gypsum. Sealing and insulating the foundation sill & band joist to the wall insulation (and studwall, if that approach is taken) with 4-6" of open cell foam is better in the full basement section.

    If the floor in the basement is also dirt, it's worth digging down for a 4" layer of clean gravel, with 1.5-2" of EPS under a 2" rat-slab or a full-thickness slab there too.

  2. GBA Editor
    Martin Holladay | | #2

    Everything you need to know is explained in this article: Building an Unvented Crawl Space.

  3. user-509810 | | #3

    I have read it but it seemed more geared to new rather than existing construction. Was looking for some specifics ie types and thickness of vapor barrier, use of foam vs wall wrap, etc.

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Was looking for some specifics, i.e. types and thickness of vapor barrier."

    A. Quoting from the article I linked to: "Install a durable vapor barrier — for example, a 20-mil pool liner or Tu-Tuf poly — over the floor and extending up the crawl space walls, to within 3 inches of the top of the wall."

    Q. "Use of foam vs wall wrap?"

    A. I'm not sure what you mean by "wall wrap." If you are asking about what type of insulation to use on your crawl space walls, here's what I wrote in my article: "Insulate the interior of the walls and rim-joists with rigid foam — many builders use Thermax, a polyisocyanurate foam that does not require a thermal barrier or ignition barrier — or spray polyurethane foam. Another option: insulate the exterior of the foundation walls. If your crawl space has stone-and-mortar walls, you can’t insulate the walls with rigid foam; the only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam. Install at least as much insulation as required by the 2012 IRC for basement walls, namely R-5 for climate zone 3, R-10 for climate zone 4 (except Marine Zone 4), and R-15 for Marine Zone 4 and climate zones 5, 6, 7, and 8."

  5. user-509810 | | #5

    Thx for pointing that out.

  6. user-509810 | | #6

    This is what they are calling "wall wrap" on our quote. "It is FSK faced R-11 Wall wrap from Service Partners of Virginia. It comes in 50’x4.5’ rolls" They want to use this with vapor barrier and 2 inches of closed cell foam on the rim and band joists and box sill.

  7. GBA Editor
    Martin Holladay | | #7

    The product you are describing is ordinary fiberglass insulation wrapped in polyethylene. Some versions of this product use unperforated polyethylene, while others use perforated polyethylene. Although contractors have used the product to insulate crawl space walls for years, I don't recommend it. Once it gets damp, it's useless.

    Follow the advice in my article and choose either rigid foam insulation or spray foam insulation.

  8. user-509810 | | #8

    Thx Martin for your advice...

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