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Should I spray foam the underside of the floor in my crawl space?

user-6458536 | Posted in Energy Efficiency and Durability on

I have a 1450 sq. ft. lake house in Alabama built in 1964. The crawlspace is not ventilated nor is it encapsulated. Its about 24-36″ high. There is only dirt under the house. A/C ducts and air handler are in this area. The crawlspace is not air tight but is enclosed on all sides. There is no insulation anywhere in the crawlspace. Also, no vapor barrier. We’ve had a problem with humidity inside the house during non-A/C times of year (October – April). Drainage is managed. There is no current problem with rot or mildew/mold in the crawlspace.

I am thinking about spraying closed cell under the wood floor decking to keep the moisture in crawlspace. However, someone else, said don’t do that – it will mess up the normal flow of moisture and the levels in the crawlspace will get higher and cause mold/mildew/rot.

Alternatively, I could just do a vapor barrier on the dirt – wouldn’t be 100% sealed but I suppose would keep most moisture from gravitating out of the ground.



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  1. GBA Editor
    Martin Holladay | | #1

    I suggest that you follow the advice in this article: Building an Unvented Crawl Space.

    The insulation belongs on the crawl space walls, not the crawl space ceiling. (This usually requires less spray foam -- the area of the walls is almost always less than the area of the ceiling.) Since it's always best if your ducts are inside your home's conditioned envelope, you really want to insulate your crawl space walls.

    This work may or may not help lower the relative humidity of the air upstairs. It's possible that your home would benefit from a dehumidifier.

    It is certainly a good idea to install a polyethylene vapor barrier over the dirt floor.

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