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Cellulose insulation thickness

Hyeon Ju Son | Posted in General Questions on

– Is it o.k to use 3″ thick cellulose insulation in 6″ or 8″ metal stud wall?

It seems that cavity wall should be filled with cellulose insulation on your instruction.
We are not filling cavity to reduce cost and reduce R value inside cavity to locate condensing point outside of metal stud.

– Does it stable during construction and after construction if insulation fills only half of stud wall?

– Is 3″ good enough to act as vapor barrier?

Please advise.

Thank you

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  1. GBA Editor
    Martin Holladay | | #1

    1. Cellulose is not self-supporting. If you use cellulose, you need to fill the entire stud cavity.

    2. Steel studs are highly conductive, so installing any insulation between steel studs is a waste of insulation.

    3. With a steel-stud wall, all of the insulation should be installed on the exterior side of the steel studs (which usually means on the exterior side of the sheathing). Usually, this means installing a thick layer of continuous rigid foam.

    -- Martin Holladay

  2. BillDietze | | #2

    And... cellulose is not a vapor barrier.

  3. Hyeon Ju Son | | #3

    Thank you Martin,

    is 6" dense pack cellulose insulation to be considered as a vapor barrier?
    So that we can eliminate plastic vapor barrier from inside.

    By the way we are providing 2" continuous insulation on exterior sheathing. This cavity cellulose insulation is for more R value.

  4. GBA Editor
    Martin Holladay | | #4

    1. Cellulose is vapor-permeable. Six inches of dense-packed cellulose probably has a permeance in the range of 20 to 30 perms. So it is definitely not a vapor barrier.

    2. If you are installing a layer of exterior rigid foam, you don't need an interior vapor barrier.

    3. I don't know where you are building (your geographical location or climate zone), but in most climate zones, a building with steel studs needs much more than 2 inches of exterior foam. Remember, the insulation you provide between the studs is basically worthless, because of the very high conductivity of the steel studs.

    4. I advise you to abandon the idea of installing any type of insulation between the steel studs.

    -- Martin Holladay

  5. Hyeon Ju Son | | #5

    The project is located in Farmington in CT. I attached one image showing a typical wall assembly for your review and comment. We are not providing a vapor barrier inside face of the wall. I thought that is because 6" cellulose insulation acts as a vapor barrier. it seems you have different ideas. Please review and let me know this wall assembly makes sense to you.

    Thank you for you help.

  6. Hyeon Ju Son | | #6

    It doesn't show my image at this post. so I noted below my wall assembly.

    Wall assembly

    -5/8" GWB BOARD

  7. GBA Editor
    Martin Holladay | | #7

    My answer is the same: the stack-up you propose is no good.

    You are in Climate Zone 5, where R-20 walls are required by code. I advise you to install R-20 rigid foam on the exterior side of your wall sheathing. That would be about 4 inches of polyiso or 5 inches of EPS. You could use an R-20 nailbase panel if you wanted; for more information on that approach, see Nailbase Panels for Walls.

    -- Martin Holladay

  8. brendanalbano | | #8

    IRC 2015 requires U-0.060 for frame walls, right? (there's not metal-frame column in the R-values chart). That's a total wall R-value of 16.67.

    - Say the 2" of foam is R-8 or R-10
    - The 6" steel stud with 6" of cellulose is probably in the R-6 or R-7 neighborhood I believe, accounting for thermal bridging.
    - Depending on the foam and how all the little bits of sheathing and air films and whatnot add up, you might get to R-16.67.

    So this wall assembly is borderline code-minimum. The exterior continuous insulation (2" of foam) is probably adequate to control any condensation risks (per R702.7.1) with your interior latex paint serving as a class III vapor retarder.

    It sounds like it will probably work "ok". But it's not a "good" wall. Martin's recommendation to beef up the exterior insulation to 4+ inches will result in a much better wall if that is possible.

    Adding furring under your shingles and to create a rain screen gap will also help take your wall beyond just "ok" if that's of interest to you, although I'm not sure how that would interact with your fire rated panel.

  9. GBA Editor
    Martin Holladay | | #9

    Thanks for your nuanced and accurate answer.

    -- Martin Holladay

  10. Expert Member
    Dana Dorsett | | #10

    To get a better handle on the actual U-factor, are the metal studs a true 6", or are the metal 2x6 (5.5" deep)?

    What is the stud spacing?

    A 2x4 (3.5" deep) /R13 16" o.c. studwall comes in at about R6, so it's probably going to make it depending on the published R-value of your 5/8" plywood + foam nailbase panels. If it's EPS you're probably looking at R8.2 for the foam, and another R0.6 for the plywood skin of the panel brings it up to R8.8-ish. That's sufficient dew point control for 5.5" (R20) of cellulose in a CT climate, and still enough (if a bit marginal) for a true 6" (R22).

    If it's polyiso it will be more like R11.5-R12.

    The 2x6/fluff layer, which should be good for ~R7 if it's a true 6", 16" o.c. ~R9 if 24" o.c., when the 5/8" structural sheathing and half-inch gypsum & air films are added in.

    With an EPS nailbase panel and 16" o.c. studs you will be under R16 whole wall, just shy of code min performance (U0.060 == R16.67 whole-wall), but if it's polyiso &/or 24" o.c. it probably makes it even if the studs are only 5.5" deep.

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