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Crawlspace insulation advice

user-917153 | Posted in Energy Efficiency and Durability on

An acquaintance got an estimate to insulate her 16’x18′ crawlspace. Although I haven’t seen it personally, here’s the scenario:

House is in WI, 7600 degree days, concrete block foundation (no vents to outside) with bare soil in crawlspace. However, the soil is dug down quite a bit deeper next to the foundation, so the soil slopes up towards the center of the crawlspace (about 2.5 ft. high in the middle). Forced air ducts run through the crawlspace, and the access to the crawlspace is a former basement window, which is open to the basement.

Contractor is proposing putting 8 mil poly on the soil, and sealing it to the foundation (not sure how high up they plan to lap the poly). Then they would spray foam from sill box all the way down to the soil.

The owner asked about insulating the soil as well, so contractor gave the option of NOT using poly and putting down 2″ Thermax (only after owner paid someone else to level out the soil), taping the seams. The reason he gave for no poly was the possibility of condensation on the foil on the bottom side of the Thermax. Extra charge for the Thermax option is $1000. The reason for using 2″ Thermax is the owner was concerned about the rigid foam cracking if anyone ever had to crawl across it.

The contractor is skeptical of the benefit of doing the Thermax option (as am I), but I said I’d ask for advice via this forum.


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  1. stuccofirst | | #1

    Lay down the poly for a vapor barrier, 6" up the wall and affixed. Insulate the underfloor and rim. Mastic seal the ducts and insulate them as well. Seal around all duct "boot penetrations" going up into the house.

  2. Foamer | | #2


    Our climate here is a little milder than Wisconsin but we get excellent feed-back on all the conditioned crawlspaces we create using the method your contractor suggested. Of all the many types of projects we do, these are the ones with the most immediate positive customer response.

    As a minimum, we spray the foam down to overlap the vapor barrier but if the floor is less than 2 feet below grade (deeper for you), we extend it out horizontally by the distance we don't have vertically. This prevents the exterior cold from wrapping around under the footer and cooling the crawlspace.

    The uninsulated ducts will provide a little conditioning to the space. The floor above will be warm.
    Insulating the floor may save a bit of energy but is not necessary for comfort.

  3. user-963050 | | #3

    You would never insulate the dirt floor in this case.

    Either leave as a vented crawl space and insulate the under floors or covert it to a conditioned crawl and supply some HVAC air and insulate the side walls.

    In either of the two options, the floor should be covered with poly and sealed to the walls as well as sealing up the ribbon/band boards.

  4. GBA Editor
    Martin Holladay | | #4

    I advise following the contractor's proposal, without the Thermax. There is little benefit from insulating the floor, and in any case the Thermax wouldn't last long if anyone had to enter the crawlspace for maintenance work.

    If the owner really wants to insulate the dirt floor, here's the right way to do it:
    1. Level the dirt.
    2. Install a layer of crushed stone with perforated pipe for possible radon remediation in the future.
    3. Install a continuous horixontal layer of Type IX EPS or XPS on top of the crushed stone.
    4. Install poly on top of the rigid foam.
    5. Install a thin concrete slab (a rat slab) over the foam.

    It's not cheap, but that's the right way to do it.

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