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Community and Q&A

Crawl space insulation, previously encapsulated

dhf0611 | Posted in General Questions on

Hello, I line in a townhome in northern NJ (zone 4) and I have a crawl space about 3 feet high. The crawl space was already encapsulated when I purchased the home, poly over the ground and up the walls to the sill plate, two sump pumps, a dehumidifier, and all but one external vent sealed (not sure why only one was left open, maybe soil gases?). My ducting runs down there as well as water pipes. There is R19 fiberglass insulation in the floor joists and the ducting runs off the main trunk are wrapped with insulation. There’s also a vent in the ducting to I guess condition the crawl space, but it’s closed.

I recently had a small pipe burst, so some of the floor joist insulation got wet and now has to be replaced (job valued ~$600 by insurance company). I’m debating if I should replace the insulation with R30 fiberglass batts, which I could do myself, or bite the bullet and have the insulation done right by insulating the walls, sealing the rim joist and removing all of the floor joist insulation. This would require removing and then re-installing the encapsulation on the walls to install the foamboard and possibly having to do the same on the floor, which I imagine will be a pretty big job. Is this worth doing for the energy savings and general long term health of the home?

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  1. Expert Member
    BILL WICHERS | | #1

    You don’t need to remove the encapsulation to install the rigid foam insulation. Just install the rigid foam over the encapsulation material (usually a poly sheet). I like the plastic anchors made by Hilti for this purpose.

    I think you’re better off insulating the crawlspace walls and sealing the rim joist. The crawlspace encapsulation is most of the work, insulating is the easy part!


  2. Expert Member
    Peter Yost | | #2

    You have a crawlspace that is not detailed properly as either an unvented or vented crawlspace.

    Move your air and thermal control layers to the crawlspace perimeter walls, seal up that last vent, and make your crawl essentially a very short basement, especially since you have mechanical system components in the space.

    See this recent Q&A Spotlight blog on this topic:

    And pick and choose from the Related Articles sidebar in there as well.


  3. dhf0611 | | #3

    Thanks Bill and Peter for your responses. I wasn't sure about closing up the last vent due to soil gases but I guess that's not really an issue given the crawlspace is already encapsulated.

    Bill - is there no issue poking holes in the poly on the walls with the plastic anchors to attach the foamboard? Do I need to use a particular style of foamboard to retain the air and moisture seal with those holes? I was thinking basic blue or pink R10 XPS but maybe another style would be better?

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