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Optimum insulation thickness

user-3258290 | Posted in General Questions on

I would like to seal up the crawlspace walls on my house (in zone 3B). I cannot locate a contractor willing to spray any of the low-GWP closed cell foams and there are too many obstacles to allow for a quality cut-and-cobble job. Wall assembly is full-dimension 2×4, (no sheathing), lap siding, housewrap, then cedar shingles. So, assuming my only option is high-GWP closed cell foam, what is the minimum I can safely specify? Is there any scenario where open cell spray foam would be reasonable? The crawlspace is dry, even in this 2.2x typical rainfall winter. (I know what the building code says, I am looking for a building science answer)

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  1. GBA Editor
    Martin Holladay | | #1

    Before answering your question about insulation levels, we need to know more about your crawl space details. Most crawl space walls are made of poured concrete, CMUs (concrete blocks), or stones and mortar. It sounds like your crawl space has 2x4 walls.

    That raises the question, "Are the walls in good shape? What are the bottom plates of the walls sitting on -- dirt or crushed stones? Are the bottom plates below grade? Are the bottom plates and studs made of pressure treated lumber? How close to grade is the lap siding? If the lap siding doesn't extend to grade, is there a big gap at the base of the wall?"

    And most important of all: "Is any of the wall framing or lap siding rotten?"

  2. user-3258290 | | #2

    Poured concrete foundation to about 6" above grade. Untreated 2 x 4 wall sitting on that with no capillary break. Wood siding is 2-5 inches above grade. All wood is in good condition, no rot. The concrete will not be insulated, there will be no area remaining after using a few vertical inches to attach the poly ground cover and a few inches for termite inspection gap.

  3. GBA Editor
    Martin Holladay | | #3

    You are describing above-grade wood-framed walls. These walls can be insulated the same way as any above-grade wood-framed wall; they don't have to be insulated like a concrete wall that is in contact with grade.

    Since your walls have no wall sheathing, you should follow the advice in this article: Insulating Walls in an Old House With No Sheathing.

  4. Expert Member
    Dana Dorsett | | #4

    The lap siding under the shingles is effectively sheathing, and with the housewrap separating the old siding from the shingles it's exterior drying performance should be pretty good, and the capillary draw pretty weak. It's probably good enough that (much cheaper and more breathable) open cell foam cavity fill is going to work. But a better description of the old lap siding would be useful for making that call.

    It's important that the window flashing is properly lapped to the housewrap no matter what you put in the wall cavities.

  5. user-3258290 | | #5

    What else would you need to know about the siding? It is 110 year-old redwood siding, about a 4" reveal, painted a bunch of times, then shingled over.

  6. GBA Editor
    Martin Holladay | | #6

    Since your wall has two layers of siding, Dana is right. The older siding acts somewhat like sheathing, so you don't have to use the techniques described in my article about how to insulate a wall with no sheathing.

    You have a wide variety of insulation materials to choose from when insulating these walls. Almost anything will work, as long as you pay attention to airtightness.

    -- Martin Holladay

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