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Community and Q&A

Sealing and insulating a crawl space

jlafferty | Posted in General Questions on

I am in central Virginia, Climate Zone 4, and have two clients with moisture issue in their crawl space.  One house has a fairly a serious mold issue and the other with a minor mold problem.  Both houses are vented crawlspaces, currently have R-19 Fiberglass insulation installed between the floor joists and a loose, unsealed, inadequate vapor barrier on the floor of the dirt crawl space floor but nothing on the walls.  Both houses have unsealed duct work in the crawl spaces.  Water has gotten into both of these crawl spaces from improper exterior grading in the rear of the house, through one of the crawl space vents.  I believe that the moisture/mold problem was exacerbated  by the water sitting on top of the loose vapor barrier and not being absorbed into the ground.

My plan is to:
-Rectify the water intrusion issue through proper grading/drainage
-Remove the fiberglass batt insulation,  seal existing crawl space vents and have the mold remediated.
-Install 2″ Rmax Thermasheath R-13.1 Polyios to the walls, (leaving a 3″ termite inspection area to the plate.
-Cut and cobble the 2″ Rmax between the joist at the rim joist, outer most floor joist and seal with canned spray foam
-Install 12mil poly sealed at the top to the walls and 20 mil poly on the floor with all joints, lapped and taped.
-Install a dedicated crawl space dehumidifier

My question are:

Is the R-13.1 rigid foam on the walls and rim joist enough insulation without the fiberglass batts between the joists?

Would you expect that the floor would be colder in the winter without the fiberglass batts between the joists and only the walls and rim joists insulated?

I have seen three different wall sections for crawl space wall insulation.  On, the rigid foam installed on the crawl space wall board goes down to the soil.  Another website has the rigid foam board wall insulation held above the floor several inches to prevent water from wicking up (although they had the poly against the foundation and the foam board on top of the poly).  And yet another site had a detail that showed the rigid foam board on the wall and an approximately 2′ wide section of rigid foam board on the dirt, insulating the floor, following the entire interior perimeter of the crawl space.  Do either of the crawl space insulation methods have merit worth considering?

I would appreciate any suggestions or feedback on the proper way to address these issue as  I want to ensure that they are addressed correctly and thoroughly the first time.

Thank you for your assistance,


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  1. Expert Member
    BILL WICHERS | | #1

    Fix the water problems first, and verify that you fixed everything. You don’t want any hidden leaks after you’ve completed your encapsulation project. Removing insulation after you’ve had mold problems is a good idea since there is probably a lot of mold hiding in the insulated cavities.

    Code requires R10 continuous insulation on basement walls in your climate zone, so the 2” polyiso is good there. Since you have had water issues already, I’d leave an inch or so gap between the bottom edge of the polyiso and the “floor” formed by the poly on the ground. The gap is to help to prevent the polyiso from sitting in water since polyiso will wick water.

    I would use the same poly for the floor and the walls. There is no reason you can’t use different poly for the walls than for the floors, but you’ll have more seams that way and more work. I would also try to minimize any taped seams in corners — try to form the corners instead of taping. Tape will hold better, and will be easier to apply, when working with flat seams.

    I would spray foam the rim joist instead of using cut and cobble. Spray foam is less work, and more likely to fully seal everything.

    I don’t see any advantage to putting the insulation on the floor like you mention some sites show. I doubt doing that would provide enough benefit to be worthwhile.

    The only step I don’t see detailed out is that you want to remove big chunks of things on the ground that might puncture the poly. Use a garden rake to even out the ground prior to installing the poly, you want a nice, flat, even surface without anything that might damage the poly sheet. I don’t see any other issues with your plan.

    I’d leave out the floor insulation and see if you are ok with the floor temperatures with only the insulated foundation wall. You can always go back and add floor insulation later if you want to.


    1. jlafferty | | #2

      Thank you, Bill.

  2. Expert Member
    Peter Yost | | #3

    Hi James -

    Comprehensive guidance here, including for CZ 4A:


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