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I am having a house built and have some questions about the crawl space.

user-3070375 | Posted in General Questions on

This is a northern Idaho climate and the house was designed to be at garden level. There is a crawl space and the floor joists were originally designed to be hung with Simpson hangers below grade. Instead, there was a short support wall placed on the footings and the floor joists are sitting on the short wall. We intend on making the crawl space conditioned/mechanically ventilated. The short wall was installed within 1/8 of an inch of the foundation wall. If we enclose the short wall and foundation with foam insulation, will we be risking decomposition/rot of the vertical 2×4 supports in the short wall. In addition, the vapor barrier will be also covering the insulation and the wall. If there is moisture from the foundation wall, it seems we will have problems if we seal it all up .

Thanks for your help


  1. GBA Editor
    Martin Holladay | | #1

    It sounds like the wall studs at the perimeter of your crawl space were installed too close to the concrete foundation. I like to see a gap of at least one inch in cases like this.

    You have several options. You can reframe the walls properly, maintaining a gap of one inch. You can reframe the walls with pressure-treated lumber. Or you can leave the studs the way they are, and cross your fingers.

    Remember, you can't insulate these walls with a fluffy insulation like fiberglass. You need to use either closed-cell spray foam (again, with a gap of at least 1 inch between the concrete and the studs) or rigid foam (ditto on the gap).

    Finally, you don't want to install any polyethylene on the interior side of these stud walls.

    For more information, see these two articles:

    Building an Unvented Crawl Space

    How to Insulate a Basement Wall

  2. user-3070375 | | #2

    Thank you for your reply Martin. It seems like placing some pressure treated studs alongside the existing suds may be the easiest? There is PT on the bottom of the wall over the footing. possibly we could place PT studs and leave the old ones. Is there a code reference for the one inch gap?
    In the third paragraph are you saying that the spray or rigid foam should have a one inch gap between the insulation and the concrete? If not, should we try and spray behind the studs? So you would prefer that the studs to be exposed on their skinny sides on the interior of the crawl space. No vapor barrier over the studs just up to the footing then correct? I read the two articles, thanks!!

  3. GBA Editor
    Martin Holladay | | #3

    If you are going to the trouble of installing new pressure-treated studs, why not take out the existing studs as you are doing the work? Get a hand sledge and knock out one stud at a time -- tap the stud sideways, once at the bottom and once at the top. Cut the protruding nails with a Sawzall if you want, or just leave the nails in place. Then install a new stud (maintaining the 1-inch gap, of course).

    Move on to the next stud, and repeat.

    As far as I know, there is no mention of the need for a 1-inch gap in the building code.

    If you are insulating with spray foam, you want to make sure that you get spray foam in the 1-inch gap behind each stud.

    Yes, the correct orientation of the studs is to have the dimension measuring 1 1/2 inch facing the interior of the crawl space.

    Consult your local building department to determine whether the spray foam needs to be protected by a layer of 1/2 inch drywall.

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