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Could a lack of air movement cause a disagreeable odor emanating from a closed crawlspace?

AGMcPhee | Posted in Interior Design on

In April, we moved into in a new, single-story, 2,000 sf home in Virginia. Early on, we became aware of an disagreeable odor coming from our closed crawlspace. Six months later, it has not gone away. The house is well-insulated (R-60 in the attic and foam and batt insulation in 6-inch walls) It’s also tightly sealed. The unvented, unventilated crawlspace is part of the envelope. The house’s only ventilation comes from windows and two Panasonic spot-ERVs in the bathrooms. Since heating and cooling come from four Fujitsu minisplits, there are no ducts in the crawlspace, although there is a dehumidifier. We don’t know what the crawlspace odor is from. We don’t think it’s from the foam-sprayed walls. Instead, we think it must be related to the lack of air-movement in the crawlspace. Because we have humid summers, we are reluctant to put exhaust fans in the crawlspace walls. Instead, we are thinking of having an after-the-fact whole house ventilation system installed. European Lumos HRVs sound like a less-expensive alternative, but they are made for thicker walls. We’re looking for suggestions on how to proceed. Art

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  1. user-2310254 | | #1

    Arthur. Is the home covered by a builder's warranty? Is the odor mold and mildew or something else?

    Post about odor problems in crawlspaces are pretty common. How is this one done? Does it have a concrete floor (rat slab) or a thick plastic cover? Are the foundation walls covered with rigid foam, and is the transition between the foundation and house covered with spray foam?

    Also... Panasonic does not allow you to install its ERV in baths as ventilators. It voids the warranty because the unit's circuit board will fail will exposed to moisture. You should have your builder relocate the units and install actual bath fans.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Could a lack of air movement cause a disagreeable odor emanating from a closed crawlspace?"
    A. No. If I sit on a boulder in the Rocky Mountains on a still day, without a breath of wind, the air will smell great. You don't need air movement for air to smell good.

    If your crawl space stinks, it's because your crawl space is stinky. It could be mold, or buried trash abandoned by the construction crew, or a dead mouse, or some oddball building material odor.

    The building code requires a sealed crawl space to have some mechanism of air exchange, and your builder should know that. If the crawl space doesn't meet code requirements, the builder is liable -- and the builder should fix it.

    You probably want to install an exhaust fan in your crawl space (for example, in a rim joist), and a grille in the floor above to allow upstairs air to enter the crawl space. More details are provided in this article: Building an Unvented Crawl Space.

  3. AGMcPhee | | #3

    Thanks, Steve. I did not know about the Panasonic ERV restriction; I will follow up on that.

    The crawl space does not have a rat slab. The vapor barrier is thick, well-installed, and extends up the walls to a termite inspection strip at the top.

    The perimeter foundation walls are sprayed with one-inch closed cell foam and over the foam are fiberglass battens. The transition is spray-foamed too.

    Before we installed the dehumidifier, the floor joists were damp and mildew had begun to appear. This did not surprise us, because we were told the area had much heavy rain when the construction was underway. The wood is now dry and no more mildew seems to be growing, although some of the dry, gray, powdery residue remains on the joists.

    There is a builder's warranty and the builder is very responsive.

  4. Expert Member
    Dana Dorsett | | #4

    A small exhaust fan constantly ventilating the crawlspace with conditioned space air should take care of it over time.

    Cleaning of the dusty residue (probably mold spores) with a mild bleach solution may reduce the stink.

    The lack of crawlspace floor insulation means that during the summer the vapor barrier will be below the dew point of the ventilation air, and would have some potential for growing mold. If it's in the budget an inch of EPS with a rat-slab on top would lower the overall mold potential.

    A Lunos would be an expensive solution for ventilating just the crawlspace, but would probably be a good idea for the house.

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