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Community and Q&A

Sub Basement Crawl Space

Stockwell | Posted in Green Building Techniques on

In the planning stage for steep slope home in North Carolina. After the topo survey results came in today, it is obvious we will need a crawl space under the lower level(will have main floor, lower level, then crawl space below due to steepness of lot). The foundation will step down on the sides. The way the builder describes it, they generally leave dirt floors, then encapsulate and condition this space and spray foam the walls. Seems to me I would rather treat this crawl like a mini-basement. It might require another footing across the space with a stem wall, but then I could pour slabs as the foundation steps down, and could add foam board/vapor barrier beneath them like a proper slab. Then I could insulate the walls with foam board just as I would any conditioned basement space. At the downslope side, this “crawl” could be 8 or 10 feet tall. Am I missing something here? It seems like a large area beneath my house to leave with a dirt heat sink.

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  1. Stockwell | | #1

    I added a really bad "visual representation" of what I am talking about. Concrete in black, framing in red. The framing would be supporting the floor of the lower level.

  2. Stockwell | | #2

    Oops--here is the pic

  3. GBA Editor
    Martin Holladay | | #3

    If you can afford the work, adding horizontal rigid foam and slab floors will certainly make these spaces more usable and pleasant to visit.

    But from a moisture-management perspective, installing a heavy-duty layer of polyethylene over the dirt has the same effect as installing a concrete slab with polyethylene under the slab.

    In North Carolina, the installation of horizontal rigid foam on (or under) a crawl space floor will never save enough energy to justify the cost of installing the foam. But if you can afford to install the rigid foam, and you want to do that, you can.

    For more information on these issues, see Building an Unvented Crawl Space.

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