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Community and Q&A

Insulating two crawl spaces

svquv9Qkge | Posted in Energy Efficiency and Durability on

A friend is having some work done on his old house in upstate New York, Climate Zone 5/6. There are two crawlspace issues.

1) One crawl space area (25’ x 20’ x 2.5’ walls) is tall enough to crawl into, and accessible from the basement. Stone walls, but has holes where light comes through, so it is drafty. He’s gotten several proposals that include: 10mm plastic on the dirt ground, and then sprayfoam the walls with 2 or 3 inches (closed cell, 2 pounds). Would doing this just alone suffice? Or should the floor area/joists (500 sq. ft.) be sprayed as well? If so, since the crawl space walls will be sprayfoamed, how many inches do you recommend for the floor? Apart from the cold, he’s also worried about condensation build up on the wood. There will be heat ducts running through the crawl space. No water pipes, but some drains.

2) The second “crawl space” (12’ x 11’) cannot fit a person as there is only maybe 1 foot height. The floors and subfloors will be removed to expose the floor joists and to reveal what is assumed to be just dirt underneath. One company is suggesting that they just put 5 inches of sprayfoam directly on the dirt ground and then sprayfoam the 1 foot walls of this area. This seems weird, but is this a decent solution? Any other options? (non-sprayfoam too).

He had a cellulose company come in and they said they weren’t the best option for these scenarios, which is why he went with the spray foam.

Any thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    The suggested solutions make sense. When creating a sealed (unvented) crawl space, it's important to seal air leaks and to insulate the crawl space walls. Usually, you don't want any insulation at the crawl space ceiling (that is, between the floor joists).

    It's perfectly OK to install closed-cell spray foam directly on dirt (or on polyethylene over dirt).

    The only question I would have concerns fire safety. I would check with the local building department to see whether an intumescent coating or some other type of thermal barrier or ignition barrier is needed on the interior side of the cured spray foam. Code interpretation for these requirements vary widely from jurisdiction to jurisdiction.

    Here are links to two articles with further information on these issues:

    Building an Unvented Crawl Space

    Thermal Barriers and Ignition Barriers for Spray Foam

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