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Rockwool Exterior Insulation for New Home Walls

Dan Showalter | Posted in Green Building Techniques on

Starting a new home build in Zone 5. I have a questions regarding our proposed wall assembly. Here is what we are thinking about from inside out:
1-Drywall
2-Intello-Plus (Taped)
3-Rockwool Comfortbatt R23 Batt
4-Zip Sheathing (Taped)
5-Rockwool 1.5” Comfortboard R6 Board
6-1×4 or 2×4 Furring Strap 
7- LP Vertical Board and Batten Cladding

Here are my proposed questions:
1. We are debating on adding a service cavity on the outside of the Intello layer, thinking 2x4s for that. (Potentially adding an additional layer of batt insulation with this option if we added it, and then moving Intello outside of the second batt layer)

2. With our exterior insulation, we were originally thinking one layer of 1.5” R6. But after further research, I am unsure if that will be enough exterior insulation to keep our sheathing warm enough to prevent condensation issues. Is this correct? If it is, we are thinking of going with two layers of 1.5” R6 for a 3” R12 total. Would this be a better choice?

Thank you

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Replies

  1. Jon R | | #1

    I'd use Table 2 here. With your Intello, rainscreen and verified air sealing, it should be more than adequately conservative.

  2. Dan Showalter | | #2

    Jon R,
    Thank you for the response. From my reading and understanding of Table 2. Which I attached a screen shot of, I am thinking that either my R6 or R12 exterior insulation over my R23 batt insulation would be fine. Am I reading that correctly?

    1. Jon R | | #3

      Yep, .2 x 23 = 4.6 which is < R6. It even works if you conservatively use Zone 6.

      1. Dan Showalter | | #4

        Jon R,
        Appreciate your help, glad I am reading things correctly.
        Thank you

        1. Expert Member
          Malcolm Taylor | | #5

          Dan,

          If you are considering a service cavity I'd suggest drawing out each wall with what services will be in it. My experience has been that most of these walls end up having little or no services run in them. Perhaps you could use them just where needed?

          If you want the extra depth for more insulation, that's probably best thought through from that persecutive, and may include other options for how the whole exterior walls are built.

          1. Dan Showalter | | #6

            Malcolm,
            I appreciate the suggestion. I think drawing out those walls and seeing how many services will be in them is a great idea.

            A question from your second point, can you explain this a little more:

            "If you want the extra depth for more insulation, that's probably best thought through from that persecutive, and may include other options for how the whole exterior walls are built."

          2. Expert Member
            Malcolm Taylor | | #7

            Dan,

            I you were including the second stud wall primarily for the extra insulation, you may want to make the exterior walls as a double-stud wall both framed with 2"x4"s, or add more exterior insulation, or add a layer to it inside, etc.

            i was suggesting the decision to insulate the service cavity shouldn't be looked at in isolation. There may be better alternatives if increasing the r-value of the walls is the main goal.

          3. Dan Showalter | | #12

            Malcolm,
            Your explanation to my question makes a lot of sense. We have some things to consider on our wall assembly, that is certain.

        2. Expert Member
          Zephyr7 | | #8

          You could potentially make "mini" service cavities if you only need a few. You could use 2x2s to "let in" the intello, with the 2x2s tacked onto the inside edge of the studs on either side of the stud bay you need a service cavity in. Use the 2x2s to press the intello back to give you 1.5" of clearance for services, and use R15 mineral wool batts behind the intello in only those stud bays. This is like "furring in" a wall instead of the usual "furring out" a wall :-)

          The 2x2s used this way will shrink the depth of a 2x6 wall to that of a 2x4 wall if used this way. If you only do this in a spot or two, you don't take a huge performance hit on your overall wall, but you save a lot of effort compared to building a service cavity everywhere. You could always fill in the remaining space in the service cavity with sliced slabs of mineral wool batts to bring the insulating value back up.

          1.5" will let you use 4" square electrical boxes and mud rings, which is a nice way to wire things. Just make sure your electricians are careful if they install any ground screws in the back of the boxes so that you don't puncture the intello. The same goes for screws on cable clamps used on those boxes (which can often be installed sideways to avoid this issue with the screws).

          Bill

          1. Dan Showalter | | #9

            Bill,
            I had not considered this at all. I appreciate this. I may have to draw some things up and ask you more questions as we move forward.
            Thank you sir

  3. Brad Hardie | | #10

    Consider using CDX instead of ZIP to avoid any wetting that might occur from breaking down the ZIP OSB. Then just use Adhero on the exterior....

    1. Dan Showalter | | #11

      Brad,
      Thank you for the reply. Am I mistaking, or would the Adhero on CDX be accomplishing the same goal as the WRB layer adhered on the ZIP OSB?

      1. Brad Hardie | | #15

        Dan,

        Yes, the Adhero would be accomplishing the same goal, but better. I'm also not a salesman for it either! Having used Zip, Zip Tape, Prosoco R-Guard, Adhero, Henry Blueskin, HydroGap, RainSlicker and your other usual suspects over the years, the Adhero is by far the absolute best.

        It can be counter flashed, has a tenacious bond, and doesn't swell like Zip. It can be installed down to cold temps without a primer.

        Now Zip can delaminate and swell MUCH, MUCH easier than CDX. Because CDX is comprised of Doug Fir it also is more resistant to rot.

        When we use Zip we tape every seam, roll it, and then caulk every nail hole. Even doing that though, I've found that the Zip tape can allow leaks on the leading horizontal edge. It is more prone to this when you are doing an exterior insulation strategy where you don't have conventional overhangs. It is still very irksome. We caulk the nail holes and sometimes even the leading edge of the tape , not for air infiltration but for swelling and delamination and leaking.

    2. Andy S | | #13

      Have you worked with Zip? Have you worked with CDX lately?
      The only moisture related issues I've encountered over the last couple years have been with plywood getting moldy or delaminating. Zip had no problems in those same builds.
      I know it's counterintuitive to recommend an OSB product over plywood for moisture resistance, but that's the reality I've been facing here in the PNW.
      Don't get me wrong...you can't let the edges of OSB soak as it will swell, but outside of massive bulk water during framing the OSB has been more mold resistant and held up better than the plywood. I just don't think the plywood we have today is as good as a generation ago.

      1. Expert Member
        Malcolm Taylor | | #14

        Andy,

        A big difference been Zip and plywood is their relative permeability. One of the advantages of using rock-wool on the exterior rather than foam is to keep the wall vapour-open. If you use Zip you lose that.

      2. Brad Hardie | | #16

        Andy,

        Yes, and Yes. Thousands and thousands of sq ft of both. I can't tell you the last time I saw delaminated CDX, that wasn't a manufacturer error. ZIP is OSB, OSB is the cheapest sheet good. CDX has inherent rot resistance. Mold forming on CDX isn't a function of the wood as it is the environment you are building in. I've seen mold on both ZIP and CDX.

        I've had a few sheets of Advantech (which I know isn't CDX or OSB) sitting under an eave for literally three years. No delamination, no rot, very little swelling. I'll tell you what, if Huber started putting the ZIP paper and WRB on Advantech, they would have a true winner. Lots of builders blowing cellulose won't blow against ZIP because of the delaminating tendency of it paired with the celluloses capacity to hold (buffer) moisture.

        1. Tyler Keniston | | #17

          Brad, do you mean the concern with cellulose paired with zip is that the cellulose can make the zip wetter than otherwise, causing delamination?

          1. Brad Hardie | | #18

            Tyler,

            Yes. OSB in direct contact with Cellulose is questionable practice. Especially when we air seal the inside, and then the homeowner punctures the membranes we so delicately seal up.

            If you really think about it, how can you air seal the interior surface of the walls at the stud interface except when you seal the drywall, walls, and ceilings the first time? At least on the exterior you have self sealing membranes, and the likelihood of penetrating an exterior membrane in the future and having it not be sealed back up is far less likely than an interior membrane. Pictures, art, bookcases, anti-tip devices, shelves, trim, lighting changes, added switches and cables, etc all add up to anchor penetrations that will certainly occur during the lifetime of an interior wall.

            This means that the cellulose as it ages and the interior wall cavity becomes more and more dangerous.

            This is just food for thought, and I'm sure many will shrug this line of thinking off, but shouldn't we be building houses that are designed to work for the long haul, and be resistant to human error or just common living practices?

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