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Community and Q&A

Seeking Opinions on Insulation Package

joegstl | Posted in Energy Efficiency and Durability on

Hi All,

I have poured a lot of time into planning our insulation with a little bit of my own experience, a lot of reading and some input from our insulation contractor.  I’m just hoping for a bit of confirmation (or not) that what we have come up with is a solid and well-balanced insulation plan.

Background: Climate Zone 4 (St. Louis), 1.5 Story New Construction.  Materials – ZIP-R3 wall sheathing, ZIP roof sheathing), 2×6 wall framing, vented attic via soffits and ridge.  Builder is targeting 2ACH 50, will be verified by ASERusa blower door.   Full basement with 2in Type 9 EPS under slab.   Note the ZIP-R3 covers the rim joist.

Insulation summary:
– Accessible ceiling: R49 blown-in fiberglass
– Inaccessible ceiling: R49 kraft-faced fiberglass
– All framed exterior walls: R23 Blown-in fiberglass
– Rim joists: 2″ Closed Cell + R19 Unfaced Fiberglass
– Finished Basement walls: 1.5″ Closed Cell + R13 Unfaced Fiberglass
– Unfinished Basement walls: R19 Basement Batts
– Top plates: 1″ closed cell with top plate gasket

Thanks all for your review!

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  1. Expert Member
    ARMANDO COBO | | #1

    Insulation summary: A quick Google search for Codes will give you these answers.
    – Accessible ceiling: R49 blown-in fiberglass. I hope you have heel truss or heel framing for continuous R49 insulation.
    – Inaccessible ceiling: R49 kraft-faced fiberglass. If it’s stapled to the bottom of the joist, you maybe ok. If it’s a vaulted ceiling and against the roof decking, it’s not to code, see R806.5.
    – All framed exterior walls: R23 Blown-in fiberglass. Wall ZipR3 in CZ4 needs to be 1” R5 min. on a 2x4 wall and 1 ½” R8 on 2x6 walls to meet code. See Table R402.1.2
    – Rim joists: 2″ Closed Cell + R19 Unfaced Fiberglass. I would install 5.5” R21 ocSPF instead of 2” ccSPF, and no unfaced batts, for less money and better performance.
    – Finished Basement walls: 1.5″ Closed Cell + R13 Unfaced Fiberglass. To meet code, you need 2” R10 min. (rigid foam would be easier and cheaper)+ R13 batts.
    – Unfinished Basement walls: R19 Basement Batts. I would do same as above. Batts against concrete will mold.
    – Top plates: 1″ closed cell with top plate gasket. Say what? Please explain.

    1. joegstl | | #2

      Thanks Armando. A few clarifications:

      - Accessible ceiling: Yes, we do have energy heels.
      - Inaccessible: No vaulted ceilings, so we'll have room.
      - Framed Exterior walls: I'm not sure I am following you. Isn't code R20 continuous? We are doing R23 + R3.
      - Finished basement walls: Thanks.
      - Unfinished basement walls: OK - I was concerned on this. These basement batts are in vinyl, but still wasn't sure.
      - Top plates: This is part of the air sealing package - they were going to flash closed cell at the top plates in the unfinished attic.

    2. Patrick_OSullivan | | #3

      > – Finished Basement walls: 1.5″ Closed Cell + R13 Unfaced Fiberglass. To meet code, you need 2” R10 min. (rigid foam would be easier and cheaper)+ R13 batts.

      While I wouldn't put batts only in a basement, code allows for R-13 batts alone in a CZ4 basement. 1.5" ccSPF + R-13 batts sounds PERFECTLY safe to me, especially since this is above and beyond the requirement for an above grade wall in CZ4 and the delta-T will be less in the basement.

      (That being said, if it were me, I wouldn't necessarily choose this option, but it seems absolutely fine.)

  2. Expert Member
    ARMANDO COBO | | #4

    Continuous insulation means that there is no break on the insulation by the studs. If you use ZipR3 + BIB R23, the R3 is the continuous insulation and not the R23.
    On the top plates, if that is part the rim joist, see above.

  3. Expert Member
    BILL WICHERS | | #5

    Armando pretty much covered everything but I’ll add another vote for rigid foam in the basement instead of spray foam. Rigid foam will likely be cheaper, and it’s going to be easier to frame a wall over since it will have a nice, flat surface.

    You need more than R3 exterior foam in your climate zone. “Continuous insulation” means something without thermal bridges. For a simple example:
    R5 exterior rigid foam, installed as full sheets OUTSIDE the studs with seams taped, is “continuous” insulation — it covers everything, so even the edges of the studs have R5 if insulation over them.
    R5 rigid foam, installed “cut and cobble” BETWEEN the studs is NOT continuous insulation since it doesn’t cover the studs. You have R5 in the stud bays this way, but the studs are thermal bridges every 16 inches or so between the cut sheets of foam.
    That’s the difference between “continuous” and “regular” insulation. The exact same product can be installed either way, it’s HOW it’s installed that makes the difference here.


    1. joegstl | | #6

      Thanks All. Makes sense. I will adjust our plans in the basement for rigid foam. For the main floor exterior walls, we were planning R3 (exterior ZIP-R) plus R23 Blown-in Fiberglass in the 2x6 stud wall cavity. Code for CZ4 reads R20 or 13+5. Aren't we good for code then with our configuration?

      1. b_coplin | | #7

        Code compliance is not necessarily the same thing as a robust assembly. Bill is referring to minimum thickness of foam sheathing to avoid more stringent forms of vapor control:

        Note the upcoming 2021 code calls for 20+5 and 13+10 in CZ4 and 5.

        I do not see the point in using a premium product like ZIP-R to get an R3 thermal break. This does not even cut framing losses in half. Bump up to R6 ZIP-R, or spend the difference between ZIP and ZIP-R on better doors and windows, or perhaps better mechanicals and/or a small pv array.

        Energy modeling could tell you which investment saves more energy.

        1. joegstl | | #8

          Got it. Thanks for the reply Bryan. Much appreciated.

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