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Community and Q&A

Rigid Foam Board in Crawl Space, Zone 3

ahiweather | Posted in Energy Efficiency and Durability on

I have a client in Zone 3 whose home is suffering from extensive air infiltration through the diagonally run dimensioned 1x subfloor that is covered with T&G hardwood flooring. The homeowner shows very little interest in encapsulating the vented crawl space and adding a dehu, and I cannot convince them to use SPF in their crawl space floor cavities either. They have read an article that suggests the installation of rigid foam board @ the bottoms of the floor joists. The floor joist cavities in the home currently have R-19 fiberglass batts installed in them that appear to be effectively in contact with the 1x subfloor material. I have reservations regarding the application of rigid foam board at the bottoms of the floor joists due to the air space void that will be created between the rigid foam board and the kraft paper faced fiberglass batt insulation that currently occupies the floor joist cavities.

I have seen and have installed rigid foam board within floor joist cavities in the past, but the rigid foam board was friction fit into the cavities to be in contact with the subfloor material. After its installation into the cavities, all of the edges of the rigid foam board were sealed with caulk or spray foam in order to eliminate any opportunity for air leakage to bypass the rigid foam board. Fiberglass batt insulation without a kraft paper vapor retarder was then installed in the floor joist cavities in order to increase the R-value provided. All of the air barrier and insulation components were in contact with each other, and there were no air spaces between them. I am confident that this application will work, but it is very time-consuming. Not to mention that the floor joist cavities in this homes crawl space have a lot of diagonal cross bracing within them, and removal of the bracing in order to friction fit install rigid foam board into the cavities would be a real pain and increase the time involved.

Any thoughts on the installation of rigid foam board @ the bottoms of the floor joists? Does the air space void that will be created between the rigid foam board and the kraft paper faced fiberglass batt insulation create the opportunity for an issue to occur?


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  1. Expert Member
    ARMANDO COBO | | #1

    The batts insulation this home has now is worthless, as the floor sheathing should have been sealed. Installing a Dehumidifier would make things worse as you would be drawing moisture from the ground and outside.
    You can install 2-1” layers of taped, sealed and staggered rigid foam under the floor joists (2-2” is better), or you could spray 2” closed cell foam. Make sure you seal/insulate all penetrations as well.

  2. GBA Editor
    Martin Holladay | | #2

    The basic problem is that the floor lacks an air barrier. A layer of rigid foam -- or, better yet, two layers of rigid foam with staggered joists, as Armando suggests -- will solve the problem, if the foam is properly installed.

    To create an air barrier, you have to first seal the perimeter of the floor assembly -- the rim joist areas -- with closed-cell spray foam or sealed rectangles of rigid foam. Once these areas have been carefully sealed (to limit air infiltration), you can install one or two layers of rigid foam under the floor joists.

    The easiest foam to tape is foil-faced polyiso, so that's the type I would use. Tape all of the seams with a high-quality tape. Then seal the perimeter (where the polyiso meets the crawl space walls) with caulk, canned spray foam, or a high-quality European tape from Siga or Pro Clima.

    An alternative approach -- one that your client has apparently rejected -- is to seal the crawl space vents and to insulate the crawl space walls. For more information on this approach, see Building an Unvented Crawl Space.

  3. bigrig | | #3

    Just thought I should mention an issue that comes to mind when I see this type of installation mentioned. Assuming the building is in an area where the NEC is enforced, how do you maintain the required access to any junstion boxes and splice points? I have yet to see an older crawlspace where there is not at least a few j-boxes mounted to the sides of the floor joists.

  4. GBA Editor
    Martin Holladay | | #4

    Good point. An electrician has to move all of the electrical boxes first.

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