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Insulating Gap Between Sistered I-Joists

user-1119983478 | Posted in General Questions on

Insulating dual I-joist dead air space I am in the midst of construction of a 230 Sq foot addition over a ventilated crawl space.  During construction, I saw that because of the 20-foot span, the I-joists were doubled up, resulting in a 2″ space between the ribs of the I-joist.  The rib has a height of 9-1/4 inches.  There is a 12-inch space between the I-joist pairs. I got a quote of $2000 to inject non expanding Retrofoam between the dual joists, which seems too expensive and wasteful.  I am thinking of simply air sealing the space between the dual-joists so the air space is sealed, and then installing 2″ Polyiso board under the joists, which is only R-11.6, and 6″ of fiberglass in the 12″ spaces between the joist pairs.

Can I blow in cellulose insulation with a space of just 2″ between the joists?  

What do you suggest I do under the circumstance.  Denver Climate zone 5. Steve

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  1. freyr_design | | #1

    You might consider insulating and conditioning crawl, may be close to same price or cheaper.

    A couple of notes: I would use terrablock next time I encapsulate a crawl, haven’t used it yet but seems like a no brainer as it’s not that much more than standard vb. (At least a nice one).

    Install a perimeter perf pipe in case radon becomes an issue.

    If you are installing an erv hrv I would have an exhaust run connected to the crawl to help offset the conditioned air supply and reduce crawl air entrance to structure with less energy loss. I don’t know how tested this is but it just makes sense in my mind.

  2. wastl | | #2

    There is that.
    If the crawlspace has to be ventilated your proposal will work. The top gap HAS to be tight, the bottom one is a bit optional. If you have access, maybe apply some liquid flash on top before you roll out the subfloor?

  3. user-1119983478 | | #3

    Denver requires ventilated crawl spaces. Crazy, since it is so dry that moisture is never an issue with crawl spaces.

    1. freyr_design | | #4

      I imagine it is more of a radon issue and IAQ issue, but still seems wrongheaded as these can be mitigated.

      Also at least in Washington, you can still insulate crawlspace as long as you install an exhaust fan, perhaps this is available for you as well.

    2. Expert Member
      MALCOLM TAYLOR | | #6

      User ...478,

      I'm not that familiar with your code, but a quick look at the Denver Residential Code of 2021 seems to expressly allow un-vented crawlspaces at R408.3

      1. Expert Member
        Akos | | #7

        I think this falls into the same category as "heat pumps require back up heat". People quote that code calls for it but when you check it doesn't exist.

        Unless you are in a flood plane, you can do a sealed crawl which is cheaper, more energy efficient and more comfortable than insulating the floor joists. Insulated conditioned crawl space means much warmer floors, no amount of floor insulation in a vented crawl in zone 5 will get you warm floors.

        It does mean dealing with radon if an issue locally and conditioning the space. In most cases a dehumidifier or a small supply duct is all the conditioning you need.

  4. tim_william | | #5

    Just curious, were they doubled because a deeper single joist would be too tall?

    1. user-1119983478 | | #8

      Tim, Yes, the joist to span 20 feet would have been too tall.

  5. Expert Member
    Michael Maines | | #9

    I would probably drill holes in the web, following manufacturers' instructions, and fill the gap with dense-packed cellulose. If that's not available in your market, then I'd use blown fiberglass.

  6. Expert Member
    DCcontrarian | | #10

    Another vote for moving the insulation to the wall of the crawlspace.

    The plumber and the HVAC guy are going to want to run everything below the joists. Instead of fighting them make their job easier.

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