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Can I dense pack cellulose insulate my crawl space?

Sarah Hayden | Posted in Energy Efficiency and Durability on

I have a small 1890s house in central Minnesota. The crawl space is ranges from 8 to 18” high… this would require excavating to gain access to the rim joists and foundation walls to spray foam them. I would love to just seal off and dense pack insulate the whole thing, since the house has a footprint of only 500sf – or at least dense pack the first 3 feet from the edge. The floor is dirt, will I be creating a moisture problem if I do this?
Thanks! Sarah

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I dense pack cellulose insulate my crawl space?"

    A. If you mean, "Can I fill the whole thing up with a tractor-trailer load of cellulose?", the answer is, "You can." If you follow up that question with another -- "Should I?" -- I would answer, "No."

    Q. "Will I be creating a moisture problem if I do this?"

    A. You sure will. Crawl spaces are damp, and you don't want cellulose insulation anywhere near a crawl space. If you try this experiment, I'd love to be there in 5 years with a video camera to film the crew shoveling out the science experiment you had created.

  2. Sarah Hayden | | #2

    I see... What would you recommend in place... excavating from inside and spray foaming the walls/joists, or excavating from outside and foam boarding? Something else altogether? It wouldn't be a tractor trailer load-- I have 22 inches deep in my attic of the same square footage.... brought that all home in one pickup load :). The average crawl space depth is less than 1 foot. Is there anything else I could fill the space with that wouldn't cause a moisture problem? Thanks for your response, Mr. Holladay!

  3. GBA Editor
    Martin Holladay | | #3

    Sarah,
    The GBA encyclopedia has lots of information on crawl spaces:
    https://www.greenbuildingadvisor.com/green-basics/crawl-spaces

    It's possible to excavate around the exterior of your house and install closed-cell spray polyurethane foam or extruded polystyrene (XPS) insulation on the exterior of your foundation. The above-grade portion of the insulation would need to be protected with a durable finish material like stucco.

    It's also possible to excavate the interior of your crawl space and insulate the interior side of the crawlspace walls with closed-cell spray polyurethane foam, EPS, XPS, or polyisocyanurate. Local building codes usually require interior foam to be protected with a thermal barrier like drywall or cement board.

  4. Riversong | | #4

    Sarah,

    If you can convince someone to crawl around in that tight space, the simplest approach might be to install 2" foil-faced polyisocyanurate insulation board (Thermax, e.g.) under the floor joists. That insulates the floor above from both heat loss and moisture gain and radiantly decouples the floor system from the cold ground below.

    This may be possible, even in your very tight spaces, if the foam board is glued with a compatible construction adhesive and propped up temporarily while the adhesive sets (PL Premium is a urethane quick-setting, high-tack adhesive, e.g.).

    This will make a dramatic improvement in the comfort and durability of the floor above. You should, however, check for existing rot before covering everything up. Then, with the floor insulated, you can vent the crawlspace to prevent moisture and soil gas accumulation.

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