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Crawl space – Need advice

WEG | Posted in Energy Efficiency and Durability on

Climate zone 5; 1,900 sq. ft. unvented crawl space. 3 feet clearance. Poured concrete walls and floor. Foil faced fiberglass batts in poor shape on joists installed incorrectly. Recently installed very large dehumidifier.

Where should I insulate? Walls or floor? Only access through 2’x2′ hatch.

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  1. mfredericks | | #1

    Walter, assuming you want to keep this as an un-vented space, you'll probably want to remove the old fiberglass batts, then air seal and insulate the rim joist areas and walls. If it's a concrete floor that makes insulating the floor easy too. However 3' clearance might make this work pretty uncomfortable.

    Martin Holladay's article is a great resource that I followed when I sealed up and insulated my crawlspace. You might find more answers to your questions in this article:

  2. Expert Member
    Dana Dorsett | | #2

    A 2'x2' hatch is a barn door compared to what I had to crawl through to insulate my crawlspace! (I used 2" x 2' wide reclaimed XPS pre-cut to height, and it still scraped edges when slipped through the hatch on a diagonal.)

    What Mark said- you get far more bang/buck out of air sealing and insulating the walls, and it's nearly impossible to properly air seal the joist bays overhead. If the concrete doesn't have a ground vapor-barrier under the slab, putting one on top of the concrete is in order. If you have a sill gasket between the foundation and foundation sill you can staple the vapor barrier to the foundation sill, if not, run it ~12" up the wall and seal the edge to the concrete with duct-mastic, then glue the foam to the wall overlapping the vapor barrier. Seal all foam edges & seams with can-foam, and use some 2x furring through-screwed to the foundation with TapCons 24" o.c. as the permanent mechanical securing of the foam. You can either spray-paint the whole thing with an intumescent coating, or add a code-legal amount of insulation, wallboard or wood etc nailed to the furring as the ignition barrier. The IRC code legal list of ignition barriers for foam in crawlspaces includes:

    1. 1.5-inch-thick (38 mm) mineral fiber insulation;

    2. 0.25-inch-thick (6.4 mm) wood structural panels;

    3. 0.375-inch (9.5 mm) particleboard;

    4. 0.25-inch (6.4 mm) hardboard;

    5. 0.375-inch (9.5 mm) gypsum board; or

    6. Corrosion-resistant steel having a base metal thickness of 0.016 inch (0.41 mm).

    (clipped from IRC 2006, but it didn't change much for IRC 2009 or 2012.)

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