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Community and Q&A

Closed cell spray foam wall and dirt floor crawlspace

A.J. | Posted in Energy Efficiency and Durability on

I have a house in Central NY that the foundation was built like a trench. They dug a square trench in the ground, put a footer and 3 rows of blocks with one row of blocks above ground. I’ve added an addition on the house with a poured wall cellar(7’) and have access to this crawlspace. There is no real access to this area accept from the “new” cellar. I’m digging out the crawlspace to add more clearance (from as little at 6” to about 30”) to gain access to add support and jack up the old house where it has sagged.

What is your opinion on close cell spray foaming the dirt ground and the walls (block) to seal out the moisture?

I’ve thought about spraying the floor an joists under the house but I will be doing some re-wiring and changing the duct work in this crawlspace in the future.

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  1. Dana1 | | #1

    Closed cell foam in contact with the dirt doesn't always work out so well, bu ton the walls it's fine. Use an EPDM (or heavy polyethylene- 10mils minimum) vapor barrier over the floor, lapped up the CMU by 12", then spray 2-3" of closed cell wall overlapping the vapor barrier all the way up to the subfloor, covering the foundation sill & band joist.

    If the wall is reasonably flat it can be cheaper & greener to use rigid foam strapped to the wall with 1x furring through-screwed to the foundation, with cut'n'cobbled rigid board for the foundation sill & band joist, sealed in place with can-foam. If you use reclaimed roofing foam it can be exceptionally cheap- 50-75 cents per square foot for 3" reclaimed roofing polyiso instead of $2-3 per square foot for 2lb closed cell spray polyurethane. Polyiso should not be in direct contact with the soil, but against the walls it's fine. EPS & XPS is fine for soil contact. Installing a couple inches of EPS under the vapor barrier can be "worth it" if you are installing ducts in there.

  2. GBA Editor
    Martin Holladay | | #2

    Dana Dorsett gave you good advice. Most homeowners don't insulate a crawl space floor, because the return on investment is small. (As Dana noted, the least expensive way to insulate your crawl space floor would be to install recycled EPS or XPS.)

    That said, plenty of people have successfully sprayed closed-cell spray foam directly on a dirt floor. If you want to take that approach, and you can find a contractor who is willing to do it, that approach works.

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