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Installing Rigid Foam Between Floor Joists Above Crawlspace

Mike from Lafayette | Posted in Energy Efficiency and Durability on

I have had to remove fiberglass batting from my crawl space due to the intrusion of rodents. This has left the underside of our floors uninsulated. I am thinking of replacing with 4″ polyIso foam board. However, I am not sure what is the best way to install it:

Should I “cut and cobble” between the floor joists? This will be time consuming and will lend itself to thermal bridging. Or should I secure to the bottom of the floor joists? Doing the latter will leave a 6″ gap between the rigid foam insulation and the sub floor. Is that too much?

I live just outside of San Francisco, so weather is moderate but certainly not warm in the winter.

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  1. Expert Member
    Dana Dorsett | | #1

    Don't cut'n'cobble- securing it to the bottom of the joists. The gap to the subfloor doesn't really matter, but if you wanted to, installing fiber insulation in there it would improve the overall performance. In US climate zone 5 and higher the 4" of polyiso alone would quite hit code minimum even on a U-factor basis.

    Air sealing the band joists & foundation sills is paramount for preventing the joist bays from becoming thermal bypass paths. But even then filling them with fiber insulation (any density) will at least slow down any bypass air currents. Using a more air-retardent fiber such as cellulose would be better than low-density fiberglass, but even crummy R19s would be better than nothing.

  2. GBA Editor
    Martin Holladay | | #2

    Dana gave you good advice. A continuous exterior layer of rigid foam always performs better than the same foam cut into narrow rectangles and inserted between the joists.

    Step one is to perform air sealing work at the perimeter of the floor (the rim joists areas). This is absolutely necessary whether or not you fill the joists with fluffy insulation (mineral wool, fiberglass, or cellulose).

    Step two is optional: Fill the joist bays with fluffy insulation. If you do this, you want to choose batts that fill the entire depth of the joists bays (or use blow-in insulation).

    Step three: Install a continuous layer of rigid foam on the underside of the the joists. Pay attention to airtightness -- you will want to seal the seams of the rigid foam with high-quality tape.

    Step four: Install a layer of OSB or plywood to protect the rigid foam from creatures.

    Here is a link to an article that describes the necessary work: How to Insulate a Cold Floor.

  3. gusfhb | | #3

    Speaking from direct experience, foam on the bottom of the joists with fiberglass above is rodent heaven. I would suggest closed cell spray foam. While rodents can easily eat it, you have given them little place to stand and run around thus it is less attractive to them.

  4. Mike from Lafayette | | #4

    Yeah, I definitely don't want to fiberglass - just removed it all because it was one large rodent nest.

    Do I need to fill the space at all? Or will 4" of polyiso below the joists be good enough?

    1. Marc1021 | | #10


      I live in Lafayette, too, and was considering a similar project. May I ask which approach and materials you chose—and for the before/after difference you noted?

      I am tentatively planning on 2" polyiso across the joists, air sealed with tape at seams and foam at edges, followed by blowing an all-borate cellulose on top. (Unsure how to get the cellulose evenly distributed in that area!)

      It's a lot of work. I'd very much appreciate learning about your experience before marching down that path.


  5. GBA Editor
    Martin Holladay | | #5

    1. Let's revisit this question. It's better to install insulation on crawl space walls, and to leave the crawl space ceiling uninsulated, than it is to insulate a crawl space ceiling. For more information on this approach -- what I would call the standard approach -- see this article: Building an Unvented Crawl Space.

    2. In your climate zone -- Zone 3 -- most codes call for floor insulation to have a minimum R-value of R-38. Four inches of polyiso has an R-value of about R-16 -- less than minimum code requirements.

    3. That said, I already answered your question about the fiberglass batts in Comment #4. The batts are optional. That means that, as long as you've done a perfect job air-sealing the rim joist, you can omit fluffy insulation between the joists (assuming that you would be satisfied with R-16 performance).

    4. If you don't want to use fiberglass, remember, too, that you could substitute mineral wool batts or cellulose for the fiberglass.

    1. Owen76 | | #6

      I had a similar question, but my crawl space is very uneven with large boulders, so making the poly air tight and sealing it would be hard to do. What would you suggest as alternative to insulate the floor joists in an open unsealed crawl space?

      1. GBA Editor
        Martin Holladay | | #7
        1. Owen76 | | #8

          Thanks for that Martin, however I have a few concerns with being able to effectively seal the joists as there is a lot of plumping pipes and wiring under within the floor joists.
          My crawl space is dirt and uneven boulders (and a couple of old stumps). It has two open vents covered with old wire. Currently there is only fiberglass insulation in the floor joists that is infected with rodents, and some poorly covered poly along the ground. There is no insulation on the rim joists. If you were going to tackle the job of re-insulating such a space, what materials would you use, and where would you apply them? thanks

  6. MaxQ | | #9

    For zone 3 floor code is R19 for a ceiling its R38. Just in case someone in zone 3 is reading this in 2020

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