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Joist line insulation

greenwithinmeans | Posted in Energy Efficiency and Durability on

What I got: So I have a 20″ floor truss joists with 1 1/2″ blocking on end so that leaves me with a  4″ void to fill with insulation  to get flush with above and below 2×6 walls/plates.

What I plan:  Thinking of gap/crack filling around end floor truss blocking then gently placed R15 rockwool, and finally adding 1/2″ Rtech Rigid gently placed against the rockwool with gap/ crack foam around that.   This will give me a flush insulation line from below walls through floor truss cavity to upper and lower walls/plates.

For the above and below wall insulation I will probably just go with vapor barrier backed R21 batts and not rockwool due to costs.  Zone 5.

Question is will that be thick enough foam for a good vapor barrier?   Should I consider thicker foam or Rockwool that would extend past the 2×6 walls/ plates and into floor truss cavity or good enough?  Also open to any other thoughts ideas aside complete $pray foam.  TIA and great plethora here!

* Adding outside floor trusses and walls have 7/16″ osb and weather wrap…

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Replies

  1. Malcolm_Taylor | | #1

    greenwithinmeans,

    You don't need to worry about any difference in the thickness of the insulation at the rim-joists and the walls above and below. There is no relationship between the two.

    Unless you insulate on the outside, you need to use impermeable insulation, not rockwool. This article explains why:
    https://www.greenbuildingadvisor.com/article/insulating-rim-joists

  2. greenwithinmeans | | #2

    I've been leaning rigid or polyiso aside the thermal drift but 4" of either is hard to source along with costs. Maybe two pieces of 2" rigid or poly and gap foam around each piece?

    1. Malcolm_Taylor | | #3

      greenwithinmeans,

      In zone 5, you can use one 2" layer of foam and add batt insulation to the inside, or two 2" layers of foam.

      1. greenwithinmeans | | #4

        Malcolm, Thanks for the linkage above which I am almost done going through the comments.
        This comment from Martin:

        GBA Editor
        Martin Holladay | May 22, 2022 06:09am | #119

        Art,
        Malcolm is right. You could use rigid foam (with canned spray foam at the perimeter of each piece of rigid foam) if you prefer that method to using closed-cell spray foam.

        If you are asking about the rim joist between the first floor and the second floor of a multi-story building -- in other words, a rim joist that is not in the basement -- it's possible to insulate such a rim joist with mineral wool or fiberglass. Moisture problems in such rim joists are less likely than in basements, in part because there is usually a layer of drywall between the interior air and the rim joist.

        Since I am not in a basement do you see any problems just going rockwool only and I understand rigid or spray foam is be$t? Also, from reading the comments, do I understand the rockwool itself will be a similar moisture barrier to 1/2" drywall?

        The difference without rigid on my project is a lot of money. So $900 for 2" rigid with two layers =R20 or rockwool R21 for $600. This for the entire joist line only of a shop. I will also gap/crack spray the joist line then rockwool. I've been crunching these numbers too much apparently. I do know at least rockwool will not have any thermal drift compared to polyiso and eps. Thanks

        1. Malcolm_Taylor | | #5

          greenwithinmeans,

          Yes second floor rim-joists are safer than those on basements or crawlspaces as long as the joist space is enclosed, and there is a fairly effective air barrier, as Martin says.

          You also have this being a shop in your favour. Much of the moisture problems houses experience comes from the occupants activities, and they typically don't generate the same levels of humidity by showering or cooking in shops.

          Drywall is about 50 perms. With two coats of latex paint it drops to 5 perms. Four inches of rockwool is about 30 perms. So really only the painted drywall gives you any sort of vapour control.

      2. greenwithinmeans | | #9

        Malcolm,
        I found some 4" and 3" felt/fiberglass backed both sides roof polyiso locally and I am thinking about that for the 20" rim joist cavity as it would get me a good "R" value and be much easier to install and and foam sealing around the edges.

        It will be very difficult to seal the batts on the inside due to no backing or solid surface past the below wall line to attach properly. The batts would be floating if that makes sense? So with the rim blocking on wall the remaining cavity void is exactly 4" to flush with below 2x6 wall as well as the floor truss end 2x6 cripple.

        I am wondering if I can use this 3" or 4" thick polyiso inside the cavity and also if I could forgo the "vapor barrier" (6mm polyurethane) on the inside?

        The outside 2x6 wall has OSB (continued and tied through joist line and into second floor studs) and weather wrap skinned with metal panels.

        *Adding: Could I just paint the polyiso with the airless for the required membrane barrier?

        Having trouble loading picture (2.7mb)and I will try again later.
        Appreciate your thoughts

  3. greenwithinmeans | | #6

    Not sure when or if I will drywall the 1st floor lid. That said would a 4mm cover of visqueen suffice over the rockwool or should I go back to the original plan reversed with R16 rockwool and use 1/2" rigid foam first and directly against the rim exterior blocking. Drywall would be a bear for the joist compartment bays and redundant if I do drywall the lid. Appreciate the help and I hope I'm making sense and not overthinking this...

    1. Malcolm_Taylor | | #7

      greewithinmeans,

      The standard detail here for second floor rim-joists is still batt insulation covered by a square of 6 mil poly, sealed at the perimeter - and it does fine, but is quite a bit of work.

      1. greenwithinmeans | | #8

        Ok that's great info. I have a scissor lift for use indefinitely so that takes the ladder work part out of it! So flash and batt and don't sweat the rigid layer. Seems like no real reason to go rockwool either.
        Thanks

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