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Wood pony walls for closed crawl space foundation stem walls

Ira Broussard | Posted in General Questions on

We are designing a home in Zone 2, about 60 miles south of Houston, 10 miles from the coast. 99.9+% of the homes around here built after 1960 (and not within a mile of the beach) are built on some type of slab on grade foundation, but we want a closed crawl space, with the floor of the crawl space about one foot above natural grade, and the first floor height four feet above the crawl space floor (about five feet above natural grade.

One big problem we are having is that the local foundation subcontractors don’t have any experience with crawl space foundations (closed or vented), so estimates are very expensive for a crawl space foundation with poured concrete stem wall (I assume to cover lack of experience). The foundation engineer we are working with suggested wood pony/stem walls on a concrete perimeter beam (top of beam about 12″ above natural grade) to reduce costs. He’s from Houston, where crawl space foundations are becoming very popular, so he’s knowledgeable about them, although most of his work has been vented crawl spaces.

The original plan was to go with poured concrete or CMU stem walls, and insulate the inside of the stem walls with two inches of closed cell foam. What is the best practice if PT lumber is used for the stem wall? If the stem wall is still insulated on the inside with spray foam, is it still 2″ of closed cell? Does all of the lumber, i.e., the “rest of the exposed 2×6” need to also be insulated?

Any opinions on wood vs. CMU vs. poured concrete with regards to strength, longevity, suitability, cost, etc.

Thanks.

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Ira,

    if I'm understanding this correctly and the top of concrete footing is 12" above finished grade, then there is no reason not to just to frame and sheath pony walls up to the main floor. Except for the bottom plate I also don't see why those walls need to be pressure-treated.

    1. Ira Broussard | | #2

      That's correct. The top of the perimeter beam and the crawl space floor will be above finished grade (although probably 6" instead of 12" as I previously said. Good question as to why the stem wall lumber needs to be PT (other than the bottom plate which is in contact with concrete). I will ask the foundation engineer.

      So any opinions on the different types of crawl space walls, and how the lumber stem wall should be insulated? Does it really need to be done with closed cell foam if the stem walls will all be above finished grade and the exterior stem walls will be constructed the same as the first floor exterior walls (probably Zip system, open cell foam, etc.)?

      1. Expert Member
        Malcolm Taylor | | #3

        Ira,

        When we pour footing for crawlspaces we also include a short stem-wall on top. This has a number of useful functions.

        - It allows you to bury the footing so it isn't visible around the house.

        - It provides enough depth for the perimeter drains and rock.

        - If the footing is exposed, water that hits the top would move through the p0ny-walls into the crawlspace.

        - it protects the pony-walls from the surrounding fill, both inside and out.

        The pony-walls can then be insulated using batts, spray foam, or cellulose. Only a short piece of foam board is necessary to cover the concrete stem-wall. This can be placed either inside the crawlspace or on the exterior.

        The one disadvantage of having the footings near grade and using wood pony-walls, instead of excavating and using concrete stem walls, is that the first floor of the house ends up several feet above the surrounding yard. That may or may not be desirable.

  2. Expert Member
    Peter Yost | | #4

    A great resource for your neck of the woods and both design and specification for foundations is La House based out of the LSU Ag Center Extension Service: https://www.lsuagcenter.com/topics/family_home/home/lahouse.

    Here is a great example of their work: https://www.lsuagcenter.com/portals/communications/publications/publications_catalog/home%20improvement/construction/buildingyourhighperformancehomegulfregionhomeownersguide.

    Peter

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