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Is mold growing under an encapsulated crawlspace a concern?

talusscree | Posted in General Questions on

On a project we are doing , the home owner is very reluctant to encapsulate the crawlspace because he believes that mold will grow on the ground side of the vapor barrier and the spores will end up migrating into the house. We both know that having a completely airtight ground barrier is impossible so slight infiltration is expected. Has anyone documented this problem occurring? He wants to close the vents, install a loose fitting vapor barrier, and a dehumidifier. The original builder(home built in 1976) put fiberglass bats perpendicular to the joists and then laid the sheathing on top of that. With that situation, I can’t use the argument about bats not staying put in the crawl. He believes that it has performed well so is reluctant to change.

We are also converting the old garage to living space with an floor elevated to that of the house. It will have about 14″ of space between the bottom of the joists and the existing slab. I see no reason to remove the slab though I doubt that there is a vapor barrier below. Would a coating of Drylock or similar waterproofing masonry paint and urethane perimeter caulk take care of any potential moisture movement?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mike,
    I'm not sure whether any discussion that I have with you will convince the homeowner to make changes to the crawl space.

    In general, it's a good idea to use black polyethylene, not clear polyethylene, to cover the dirt floor of a crawl space. That way the homeowner can't see through the polyethylene to see what's underneath.

    The fact is, even without any poly, the soil is damp. It probably already includes mold spores, although you may not be able to see them. In many crawl spaces that I've visited, the soil also includes cat feces and raccoon feces.

    So the situation will certainly be improved by installing polyethylene. What's going on under the poly? Who cares? Don't look!

    Under the poly is the same nasty, damp, moldy soil that was already there. Remember, the reason that the crawl space floor needs poly is because the soil is nasty, damp, and moldy.

  2. GBA Editor
    Martin Holladay | | #2

    Mike,
    You wrote, "The original builder (home built in 1976) put fiberglass batts perpendicular to the joists and then laid the sheathing on top of that."

    I'm having a hard time visualizing this. You're talking about floor joists, right? When you write "sheathing," do you mean "subfloor"? If so, I can't imagine how that squishy sandwich would work. So you must mean something else.

  3. GBA Editor
    Martin Holladay | | #3

    Mike,
    You wrote, "He wants to close the vents, install a loose fitting vapor barrier, and a dehumidifier."

    That's not a bad plan, as long as the vapor barrier is installed on the dirt floor. Of course, it would be best if the seams of the vapor barrier were taped, but even without the tape it's still a pretty good plan. In most crawl spaces, a dehumidifier only needs to operate temporarily, until the RH stabilizes. At that point you can unplug the dehumidifier.

    It would also be a good idea to install rigid foam insulation on the interior of the crawl space walls.

  4. GBA Editor
    Martin Holladay | | #4

    Mike,
    Q. "We are also converting the old garage to living space with an floor elevated to that of the house. It will have about 14 inches of space between the bottom of the joists and the existing slab."

    A. This situation has been discussed at GBA several times. You should realize that a 14-inch-high crawl space is illegal in many jurisdictions; check with your local code inspector before going forward.

    This type of crawl space isn't high enough for human entry, and that's a problem. In most cases, it's better to install 10 to 18 inches of rigid foam (use recycled EPS -- it's cheap) and a new 4-inch concrete slab on top of the EPS.

    Another option is to install sand or crushed stone to raise the level of the floor, followed by EPS and a new concrete slab.

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