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Community and Q&A

Condensation & Mold above insulated crawlspace

tomrgon | Posted in Green Building Techniques on

Doing a phased renovation / addition project on a 2,400sf cabin with a dirt floor block crawlspace ~4′-0″ high. In summer client reports mold blooms on the wood floors above – they had insulation installed in the crawlspace 10 years ago, batt stapled to the underside of the floor boards between the joists, and then onto the walls of the crawlspace. The cabin above is unconditioned log, and gets quite warm in the summer, and retains the heat and humidity. My hypothesis is that condensation is forming in the wood, or on the wood floors, when the outside temperature drops, or when it rains and moisture accumulates in the crawlspace. I plan to remove the insulation and clean the crawlspace, but I wanted some advice. The client is planning for an addition to the cabin in the near-mid future, which would possibly include geothermal wells for conditioning the space (currently, it has electric baseboard heat and nothing else.) Would I be well-served recommending that they pour a slab on grade in the crawlspace, and insulate the block walls with rigid, or will that not be sufficient? My thought is that, when the client builds the addition, this newly insulated and floored crawlspace could act as a plenum space, or at minimum, a mechanical space for duct runs or plumbing. What are your thoughts? Does the slab need to be insulated? The crawlspace is ventilated, and the vents could potentially be sealed, or at least closed in the colder months to maintain the internal cabin temperature. I would appreciate any thoughts and suggestions.

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  1. Expert Member
    Dana Dorsett | | #1

    A dirt floor is constantly emitting water vapor, and even if the crawlspace is vented the venting doesn't solve the problem, especially in locations where summertime outdoor dew points are high.

    At a minimum installing continuous ground vapor retarder would be necessary to block that moisture flow, whether the crawlspace gets insulated or not. A 2" rat-slab on it's own doesn't do much for blocking the moisture flow (only the rodent & insect flow :-) ) - a slab would still need a vapor barrier under it.

    If converting the crawl space into a sealed insulated conditioned space that is also used as a plenum the foam board would need to be covered with a timed thermal barrier against ignition, such as half-inch wallboard. The slab would serve as the thermal barrier if insulated with foam below.

    Whether or not the slab needs insulation, and how much foundation wall insulation is needed depends on the location. Where is this house?

    1. tomrgon | | #3

      It's located in rural Wisconsin, thanks for your reply!

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Tomrgon,

    I'm not sure that you need to pour a concrete slab. As Dana pointed out, even if you do, you need to install a vapor barrier on the floor and onto the walls. I recommend that you read this article and consider removing the existing insulation and converting to a sealed crawl space.

    1. tomrgon | | #4

      Thanks for the information and the article, I look forward to reading it!

    2. tomrgon | | #5

      The main issue I see with an unvented crawlspace is that the building is not conditioned, so it would be difficult to keep the sealed space dry (the air being supplied as the space is exhausted would be just as humid as the air from the exterior.)

      1. Expert Member
        Dana Dorsett | | #6

        >"...the air being supplied as the space is exhausted would be just as humid as the air from the exterior."

        That would be an improvement from the current situation, where the air in the crawlspace is also burdened with ground moisture on top of outdoor air moisture, due to a the absence of a ground vapor barrier.

        We still don't know where (what climate) this place is located in, which will make some difference in how the crawlspace would be insulated.

        How often and what season(s) is the place in active use?

        1. Expert Member
          Peter Engle | | #7


          He did say that the cabin is located in Wisconsin. That means relatively low average ground temperatures, certainly below summer dewpoints. So the outside air coming through the vents is pretty much guaranteed to condense in the crawl space.

          I would go with insulated floor and walls, vapor retarder, and then a rat slab. I spend a lot of time in crawlspaces, and the ones with only a vapor retarder get beat up pretty fast, even with an extra heavy duty one. The rat slab makes a smooth, dry surface for scooting around on for maintenance, and is usually dry enough for storage as well.

          You're still going to have some moisture/mold issues until you can get the cabin and crawlspace conditioned, but not as bad. That's just the way it is in humid areas.

          1. Expert Member
            Dana Dorsett | | #9

            >"He did say that the cabin is located in Wisconsin. That means relatively low average ground temperatures, certainly below summer dewpoints."

            Right, on all counts!

            An inch of EPS under the rat slab with a vapor barrier between the EPS and slab will reduce the amount of summertime moisture accumulation in the crawlspace whether vented or unvented. If it is to be later used as an HVAC plenum make it 2" of EPS (R8.4).

            IRC 2018 code minimum for foundation walls in zones 5 or higher (southern WI is zone 6, northern WI is zone 7) is R15 continuous insulation, which could be 2.5" of foil faced polyiso, seams taped with foil HVAC tape, strapped to the wall with 1x4 furring through screwed to the masonry with 4" masonry screws 16" o.c., with half-inch gypsum board mounted on the furring as the necessary thermal barrier against ignition. If the building is going to remain unconditioned much of the year, use exterior grade fiberglass faced gypsum, (eg GP DensGlass).

  3. Jon_R | | #8

    While it is vented, also increase ventilation to reduce moisture.

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