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Walls, Moisture …. and Paint.

user-742626 | Posted in Energy Efficiency and Durability on

For a Pacific Northwest house, we’re recommending R-30 in the walls, and proposing to achieve it with: (Metal and/or wood siding) 1-1/4″ Extruded Polystyrene with taped joints, over plywood as required for structural shear, over conventional 2×6 framing, filled with blown-in cellulose, covered by 5/8″ gypsum wallboard. The house will be both heated and cooled (A client mandate). We’re assuming that the wall would dry to the inside, but… due to the proximity to a freeway, we’re also proposing to use QuietRock for acoustic isolation, and this product has a proprietary polymer in a gypsum sandwich, the vapor-permeability of which I haven’t yet learned. Even where we specify a conventional gypsum board, we would specify a vapor-permeable paint. Five years from now, the client will re-paint with who-knows-what or will hang wallpaper.
If we end up with an impermeable interior finish and an impermeable exterior insulation/air barrier, aren’t we doomed to trapping moisture in the cavity?

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  1. Riversong | | #1

    First, your wall section will yield no more than R-24 to R-25 whole wall, hardly R-30.

    Second, while some building scientists might consider your wall exterior to be only semi-impermeable (depending on what kind of metal siding), I would agree that you want the assembly to dry to the interior, particularly in that moist climate. And I suspect that QuietRock is impermeable.

    Why not use resilient channels or a staggered-stud wall for both better insulation and better STC?

  2. user-742626 | | #2

    Thanks Robert. We are constrained by an existing foundation, on which the wall must fit, and a tight lot coverage situation that makes even the added layer of rigid limited. So we can't do a staggered stud wall without furring inside and impinging on some already tight areas like stair cases. We are limited to our 2x6 wall at this point, I'm afraid. So any suggestions about a reasonably well insulated assembly that (with air sealing) could give us a higher performance wall than R-21 code would be helpful. We're also dealing with a slightly chemically sensitive client, so we had eliminated the idea of foam insulation per the past discussion on this website. The resilient channel suggestion regarding drying to the inside is a good thought, but it won't do anything for R value, would it?

  3. Riversong | | #3

    The resilient channels will thermally decouple the drywall from the studs (thermal bridges) to some extent. If you could do a closed-wall cellulose blow behind the furred out drywall, then you would fill the resilient channel spacing and add a little bit of thickness to the insulation.

  4. Doug McEvers | | #4

    I have never heard of 1 1/4" extruded, 3/4", 1', 1 1/2" but never 1 1/4". I would try for a 2x6 with R-10, 1 1/2" polyiso exterior foam. You will surprised at the sound deadening property of exterior foam . I think you can do without the sound channel.

  5. user-742626 | | #5

    Hi Doug, The contractor sent along the idea of Thermax Polyiso at 1.25, maybe a typo on his part. Interesting information about the acoustic properties of exterior rigid. Thanks.

  6. Justin Fink | | #6

    My math is coming up with a whole-wall r-value of around R-27... 5 1/2" of cellulose at R-4 per inch, and 1" of XPS at R-5 per inch. My only question is about the 1.25" XPS, which I've never heard of before.

  7. Riversong | | #7


    You're not going to get more than R-3.8/inch from dense-pack cellulose, and XPS is often assumed to have R-5/inch but tends to test at R-4.8.

    And Lydia stated that she was referring to Thermax polyiso rigid board, which comes in thickness increments of ¼" from ½" to 4¼".

  8. user-742626 | | #8

    The Thermax product (I just found the r-values online) states that it has an R-8 value for 1.25", that with the dense-pack cellulose at R 3.8 is closer to our target of R30. The team is considering a hybrid advanced framing method that could reduce studs and increase wall insulation in many key places (we are limited, again by the existing foundation). With a well air sealed envelope, the sound transmission is going to be greatly reduced as well. We could ditch the QuietRock and use two layers of gyp in the bedroom where the noise is the biggest issue.

    This leaves the one nagging question of relying on drying to the interior, with the possibility over time of repainting and destroying the drying this enough of a redflag to pursue a different assembly altogether?

  9. Doug McEvers | | #9


    A well built wall, regardless of the type should not have much moisture passing through it. In a marine climate the drainage plane details are critical. Most moisture problems are due to improper flashing around windows, doors, decks, dormers, chimneys, roofs into vertical walls, you get the picture. In my opinion, the wall you describe should do just fine from a durability standpoint, you are asking the right questions, make sure to execute the details.

  10. Garth Sproule | | #10

    If you are really serious about noise transmission issues, and are thinking of using two layers of wallboard in the bedrooms, then you should take a look at "Green Glue".
    Consider triple glazed windows for the bedroom as well...

  11. Riversong | | #11


    Most interior paints are quite vapor permeable, in the 3-5 perm range, even for multiple coats.

  12. Ted White | | #12

    Foams of any kind will certainly increase R-Value, but sound isolation will suffer. See test reports from the NRC of Canada. Unfortunately, humble fiberglass is superior to foams, both open and closed cell.

    Take care when blowing in insulation. Decoupling methods like resilient channel need to flex, and contact with dense insulation will impede this flex.

    Standard drywall and field applied damping compound (as Lydia suggested) will be less expensive and higher performance.

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