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Community and Q&A

Options to Seal Above Grade Plywood Crawl Space at the Ground

Lindaloowho | Posted in General Questions on

We have an insulated plywood skirted crawlspace around piers. The entire space is above grade and we are looking at ways to seal it at the ground. 

We have dug a trench to put metal lath around the outside perimeter of the skirt to keep out the critters. Then, overtop the lath in the trench there will be a 12” wide x 12” deep surround of 3/4” stone. 

Since we have been advised to keep the plywood skirt edge out of the earth (we know it’s treated, but not sure if it’s PWF wood) we have looked at a few options to close the gap from the bottom of the skirt to the ground, which are within our budget and among our available materials. 

1) Taping EPS foam boards together to form 8 foot long blocks and then wrapping them with vapour barrier using sealant. Digging down several inches on the inside perimeter, sealing the blocks under the interior edge of the foam board insulated skirt (blocks would then also extend into the crawl floor space somewhat) and backfilling from the inside of the crawl until we can no longer see daylight. (We are going to put vapour barrier on the ground next). Would you recommend digging and insulating 12” down on the inside to stop air coming from below the lathed trench of stone  on the outside of the skirt, or would going down only a few inches, but extending horizontally into the interior of the crawl 10-12 inches be sufficient? 

2) Dig down on the inside perimeter, and attach dimple membrane to the interior perimeter edge of the skirt. (How deep should we go with this?  As deep as the rock perimeter on the exterior of the skirt?) 

3) A combination of 1) and 2)?

We know the best practice would be to pull our the skirt and insulate the floor assembly, but with a wood shortage / inflated wood price, as well as the amount of material it would take to insulate 1000 square feet of floor space, it’s not possible for us right now.  Appreciate your input! 

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  1. Lindaloowho | | #1

    I think I’ve found a few answers here under a question previously asked.

    “How to temporarily air seal (and insulate?) a leaky mesh wall / crawl space?”

  2. Expert Member
    ARMANDO COBO | | #2

    You don’t say where you live, but digging around the perimeter would need to be below your frost line. You also don’t say how tall is your skirt, so it could be a cinch to insulate a small wall or a big deal if it’s a tall wall. Perhaps, an effective, easy and less expensive option is to insulate under the floor. That depends how deep the floor members are. Probably it would be best to fill the cavity with permeable insulation and install reclaimed rigid foam underneath, attached to the framing members, and making sure you seal all edges.
    An additional thought... To insulate the skirt, one needs to either use impermeable board insulation attached to it or ccSPF, or if using impermeable insulation, you would need to install a sealed air barrier behind it. I would think both options could cost more money.

  3. Lindaloowho | | #3

    Thanks Armando,

    Sorry, we live in Southern Ontario...Range from frigid winters to humid summers. The wall is about 30” tall and is currently insulated on the interior.

  4. Expert Member
    ARMANDO COBO | | #4

    Frost line: "Part 9 of the Ontario Building Code states 1.2m which is 3.927 feet". Even though my design/build experience is the southern-half US, and CZ5 in the mountains of the Southwest, I still think my advise above applies. Southern Ontario CZ5 is around 3k-4k HDD, which is similar to US CZ5.

  5. Lindaloowho | | #5

    Would insulating at the base of the skirt on the ground at the perimeter have the effect of “raising the frost line”?

  6. Expert Member
    ARMANDO COBO | | #6

    No. The laws of Physics and thermodynamics are the same for you location, regardless of interpretations, budgets or methods.
    Here is a thermal image I found on Google, of a basement and I don't know where, but the point is that the "blue" cold weather goes quite a bit down the wall. The deeper your frost line is, the deeper you need to excavate to fix your issues.
    Perhaps some body in CA will have an actual image for your climate zone.

  7. Lindaloowho | | #7

    Thanks so much

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