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For a better floor

fourforhome | Posted in Energy Efficiency and Durability on

I will have a contractor build the shell of a house (including windows and shingles) for my dear, old Mom.
I would greatly prefer to condition the crawl space, even in my zone 4 marine (Vancouver, WA), but our county is deemed radon hot and conditioned crawls are not allowed.
Spray foaming the floor (9 1/2″ TJI) to r-30 is out of the budget which leaves fiberglass batts, or a blown in product held up in place with some type of battens.
I could make the subfloor the air barrier by using Fast Flash or tape on the seams and filling and sealing penetrations and:
A. Have an insulator string up batts and hope for the best.
B. Gravity-fit rigid foam or OSB/plywood between the I-joist webs, on the flanges to prevent batts from sagging over time and to greatly reduce the loss of R-value when the air blows past the fiberglass.
C. Attach rigid foam or OSB/ply under the joists and blow in insulation, one section at a time.

Owens Corning just emailed that the Foamular isn’t designed for this and that it might need a 15-minute thermal barrier such as sheetrock. Avoiding XPS is fine with me, but “What’s the Better Way?”

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  1. Expert Member
    Dana Dorsett | | #1

    Slab on grade is allowed, but conditioned crawlspaces aren't?

    How about a conditioned crawl vented with an HRV? (A ~20cfm Lunos E2 would purge radon better than typical open crawlspace vents, and you would not need to move air from the fully conditioned space upstairs through the crawl space.) That would allow you to run ducts & plumbing in the crawl space in an energy-efficient and freeze-protected way.

    Assuming you're stuck with a vented crawlspace, and have to insulate the TJI floor:

    A or B: Don't! Batts and TJIs are pretty much incompatible, since batt widths are designed for 1.5" milled lumber, not ~0.5" TJI webs.

    C (modified): Use 3/4" asphalted fiberboard on the underside of the joists. It costs about the same money as half-inch OSB or CDX, and the TJI bays can be blown full of cellulose (or fiberglass). Asphalted fiberboard sheathing at 3/4" would add about R1.5 to the stackup (not that it matters much), but would be far more moisture resilient than OSB/plywood, due to the inherent tolerant to moisture of the material itself, and the comparatively high water vapor permeance which allows the assembly to dry into the vented crawlspace. In the PNW the outdoor dew point averages are reliably above the deeps subsoil temps, and well below air-conditioning temperatures, so there is no need to make the bottom side cladding as vapor retardent as OSB or plywood.

  2. GBA Editor
    Martin Holladay | | #2

    My favorite specifications for exposed floors like yours: Install a continuous layer of rigid foam on the underside of the joists -- foil-faced polyiso is the easiest type of foam to tape -- and blow cellulose between the joists.

    You can protect the underside of the rigid foam with drywall if required by code, or with OSB if your code official allows that option.

    Ordinary subflooring (usually tongue-and-groove OSB or plywood) is an adequate air barrier when installed with construction adhesive (as it almost always is). You don't need to tape the seams. That said, it's a good idea to tape the seams of the rigid foam (if you follow my advice).

    -- Martin Holladay

  3. fourforhome | | #3

    The local plans examiner stated that using an HRV and insulating the crawlspace creates a conditioned crawl and "Information that I got from a local radon company is that they are finding that by encapsulating the space (conditioning) that they are coming in and having to do odor reduction (a cat urine smell) from leaks in the vapor barrier."
    So, moving on.
    Thank you, Dana. I wanted the HRV option to work.
    Thank you, Martin. Rigid foam and cellulose are in order.

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