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Thread: Insulation where floor joists overhang foundation

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    DIY Senior Member pmayer's Avatar
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    Default Insulation where floor joists overhang foundation

    I am continuing my basement remodel and following the great advice that I have received here in the past regarding insulation. In fact I should mention that the previous basement room that I finished is by far the most comfortable room in the house thanks to you guys. In the area that I am currently finishing there are a couple places where the floor joists extend a couple feet past the foundation wall (one is for a bay window and the other is for a patio door bump-out thing. Currently there are just unfaced fiberglass bats in these cavities, installed with the bats laid flat apparently to allow some air flow and to make a nice comfy home for mice. Now that I am replacing all of the fiberglass that is up against the rim joist with Thermax (inspector won't allow unfaced XPS), but I won't be able to reach in far enough to install this on the protruding rim joist in these two areas. So, my question is, how should I insulate in these areas? Do I need to use spray foam? Or replace the unfaced bats with paper back ones? What about the air flow that appears to be currently in place; is this important?

    Also, on the exterior side of 2 of my foundation walls there is a layer of 6 mil plastic directly against the foundation, and 1" of XPS on the outside of that. Am I OK to install another 1" XPS against the inside of the foundation wall? The other wall that I am finishing had no XPS or plastic on the outside and I am adding 2" of XPS on the inside foundation wall as I did on my previous remodel.

    Thanks!

    Paul
    Last edited by pmayer; 01-01-2013 at 05:30 PM.

  2. #2
    In the trades Dana's Avatar
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    If you're blocking the cantilevered bays at the foundation with Thermax there is no rationale for a ventilation gap between the subfloor under the bay-window bump-out and the fiber insulation (if there ever WAS one.) A nominal 1" skim-coat shot of closed cell foam (you can use the 12 board-foot FrothPak kits from box stores, if it's a small enough area) on the subfloor would be sufficient vapor retarder to allow you to just fill up the space with fiber insulation, so long as the bottom sheathing of the exterior overhang doesn't have any strong vapor retarders like poly or foil.

    On the fiber insulation, both density, and snugness (preferabably a compression-fit) count. I personally prefer to use rock-wool, which is about the same cost & R value/inch as "cathedral ceiling" batts. It's probably not a huge area, so there's no real savings by to going with low-density batts, which are much better mouse-nests. Are they 2x10s, 2x12s or... ???. If it's more than arm's length it can be awkward shoe-horning in batts (especially if there are nail-heads interfering and catching). It's easier to get a tight fit with blown fiber, but it's probably not worth the hassle & expense of renting a blower.

    Then, just blocking it in with fire-rated Thermax and sealing the edges (with 1-part can-foam or FrothPak) works.

    Where there is poly on the exterior of the foundation, unless you have a metal-flashing or membrane type capillary break between the concrete and foundation sill it's better to use something more vapor-open than XPS on the interior. Using 1.5-2" of unfaced EPS (not the goods with poly or foil facers sold at some box stores, which are <1 perm) would give it 3-5x the drying capacity of 1" XPS for about the same or better R-value. If XPS is all you have available, hold the line at 1" (which runs ~0.8-1.2 perms, typ.).

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    DIY Senior Member pmayer's Avatar
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    Thanks, Dana. On the cantilevered bays, I believe that the joists are 2x10, and yes, it is a bit of a reach of erhaps 2 feet in each area (probably 8 or so total cavities to deal with). The reach is for sure too far to do anything nimble out at the rim joist. If I am interpreting correctly you are suggesting to spray the foam directly onto the rim joist, then fill the cavity in behind the foam with rock wool or blown fiber. But then are you suggesting putting Thermax between the joists right at the foundation, directly behind the rock wool or blown fiber? I'm trying to understand the benefits of foaming the rim joist and again at the foundation, sealing in the cantilevered area. Did I misinterpret this?

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    In the trades Dana's Avatar
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    The rim joist on the cantilever is way the heck out there, and the bulk of the moisture drive and heat loss is through the sub-floor of the bump-out, not from the basement out to the band joist. Spraying the foam on the sub-floor (not the cantilevered band joist) to air seal it and give it a reasonable vapor retardency keeps wintertime moisture from the conditioned space above the cantilever from accumulating in the cold under-side sheathing or band joist of the cantilever.

    Blocking air & moisture movement from the basement out to the cantilever at the foundation sill with the Thermax keeps wintertime moisture from the basement from accumulating there.

    |
    |...bay window cantilever area...
    |
    |
    |__________subfloor_________
    |^^^the foam only goes here^^^
    |
    |
    |<< band joist
    |
    |{fill space between foam & bottom sheathing with fiber}
    |
    |______bottom sheathing_____

    ...outdoor air...





    _____ground__________



    With a 2x10 bay and a nominal-inch of closed cell on the subfloor you have about 8.25" of space between the foamed subfloor and bottom side sheathing. Compressing a standard density 9.25" R30 batt in there would be the cheapest and may be the "right" option, and would perform at about R28. (Better R/inch at the compressed density, but fewer inches.) Unfaced R30s in 300 inch rolls are usually sub-$15 at box stores.

    R15HD or rock wool batts are about 3.5" and R23HD/rock wool are 5.5", which would give you a compression fit if stacked. It'll be a pain, but it's doable. You'll be into it for $75-80 and you may have a lot left over- I'm not sure if it's worth the extra expense if there is going to be much scrappage after it's all cut & fit.

    R30HD/ rock wool batts or high density fiberglass batts are nominally 7.25" inches. If the budget allows it might be better to go with 2" of foam and a snug fitting high density R30.

    The key to avoiding rot/mold is the spray foam air seal & vapor retarder. Since the bottom side sheathing never gets rained on, it can dry toward the exterior even in the winter. As long as it can dry to the outside air faster than the interior-side is delivering moisture via air-leaks or diffusion, it'll stay in great shape. If the previous configuration had the air space above the fiber and no vapor retarder, that was a much worse situation, since in winter the sheathing was constantly being supplied with air-transported moisture from the warm interior.
    Last edited by Dana; 01-02-2013 at 02:28 PM.

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    DIY Senior Member pmayer's Avatar
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    Dana,

    This is so helpful. I really appreciate your thorough response once again.

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    DIY Senior Member pmayer's Avatar
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    Here is how I ended up doing this after some input from a contractor friend. I cut XPS panels to fit the bottom of the subfloor, extending from the rim joist to the foundation block. Then I heavily foamed the perimeter of the XPS and pressed it into position so that the foam oozed and filled the gaps around the perimeter. Then I wedged scrap material against the XPS to hold it in position until the foam cured. Then I cut pieces of Thermax to seal the cavity to the basement and foamed that in as you suggested. Worked great! Thanks again for the advice, Dana.

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