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Suggestions for insulation on ceiling of wet crawlspace?

user-1140356 | Posted in Energy Efficiency and Durability on

For a super insulated house being extensively retrofitted in South Jersey(r40 walls, r70 attic, triple pane windows) with a permanently wet crawlspace (riverfront, high water table), I am considering insulating the ceiling of the crawlspace. Possibilities include 1 inch spray foam (not the first choice due to high cost and high ), 1 inch of polyisocyanurate (Thermax) stapled to floor joists, taped and 6 inch of unfaced fiberglass batts in the floor cavity, fanfold (High-density EPS?) stapled to floor joists with 10 inches unfaced fiberglass, sheet plastic stapled to floor joists with 10 inches unfaced fiberglass.

Every effort has been made to contain the water problem and it is not as severe as an underground stream, but the house will always have some water penetration. In the past (before I bought the house), the water created some mold in the basement (though not severe), and ruined the fiberglass stuffed in the joists.

Though the building inspector recommends against it, I will probably seal the crawlspace vents in the summertime.

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  1. GBA Editor
    Martin Holladay | | #1

    Skip the fiberglass. Your experience with "ruined fiberglass" should be enough to convince you that fiberglass batts don't belong in a damp crawl space.

    I suggest that you install at least 2 inches of foil-faced polyiso, installed in a continuous layer on the underside of the floor joists. (I am suggesting foil-faced polyiso because it is the easiest type of rigid foam to tape.)

    Do a careful job of air sealing at the perimeter of the ceiling, using canned spray foam or a high quality European tape. Tape the seams of the polyiso.

    Even better would be to install two layers of 2-inch polyiso (a total of 4 inches), installed with staggered seams.

    Remember: no fiberglass between the joists.

    If you ever win the lottery, hire a backhoe to excavate the exterior of your foundation down to the footings, and install new perforated drainage pipe leading to daylight. Then install a new waterproofing system and a dimple mat before backfilling with coarse granular material.

  2. user-1140356 | | #2


    Thanks for your advice. I will leave out the fiberglass completely. I had thought that the polyiso would seal the fiberglass well enough to stop mold growth. But the ruined fiberglass is pretty persuasive.

    I do hope to win the lottery but, in the meantime, I will probably dig out the foundation by hand and install waterproofing. However, the water comes up through the floor as well, so, unless I remove the rat slab, I doubt I can stop the water.

    Should I cover the vents in the summer? winter?

  3. GBA Editor
    Martin Holladay | | #3

    If you can install perforated footing drains with the bottom of the pipe below the level of your crawl space floor, and if you can run that pipe downhill to daylight, you will never get water in your crawl space.

    I'm in favor of sealed crawl spaces, especially east of the Great Plains. More information here: Building an Unvented Crawl Space.

  4. user-1140356 | | #4

    Thanks again, Martin. Though occasional tides ( and recent hurricanes) push the river water higher than the footings, I suppose I could add one-way valves to the drains -- or use a pump? Still, I wish I had a slab instead (of an upside down swimming pool).

    I will seal the vents after the local official goes home to his musty basement or crawlspace.

    thanks again, Mike

  5. GBA Editor
    Martin Holladay | | #5

    Since you are planning to win the lottery, here's what you can do: jack up the house; put it on cribbing or temporary steel beams; fill in the crawl space with gravel; pour a slab; and lower the house onto your new slab.

  6. user-1140356 | | #6

    Can't I just back the concrete trucks up and start filling. The crawlspace is three foot. I would get about r30 from a 3 foot slab and have no need for the polyiso.

  7. Expert Member
    Dana Dorsett | | #7

    My math sez you get about R3 from a three foot slab, not R30. (Just sayin'...)

    If you take the rigid foam under the joists approach, before putting up the rigid board, seal and insulate the band joist & foundation sill with at least a 1" shot of closed cell foam.

    But the sealed unvented crawlspace approach is usually higher performance, and sometimes even cheaper. With 3-4" of EPS on the walls and 2-3" of EPS under a rat-slab an unvented crawlspace would be flood tolerant and performs quite well.

  8. user-1140356 | | #8

    Thank goodness for your quick response. I was able to stop the cement truck in time ;^)

    A 3 foot slab would be something to hold the house down in the next hurricane, but it is clearly a poor insulation material.

    Now I will be forced to lift the house and pour a slab as Martin suggests.

    I will definitely seal the rim joist. I would like to seal and insulate the crawlspace but the water intrusion,without the drain system Martin suggested, might still be a problem -- and the drain is a costly endeavor. I might one day add the drains as my teen-age boys are getting old enough to dig ditches now.


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