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Rockwool Comfortboard vs. ZIP-R

dgotsch | Posted in Energy Efficiency and Durability on

I’m in the process of re-sheathing the house in central Texas because my brick has only painted OSB behind it (no tar paper or house wrap) and as you can imagine the mortar bridges have caused water damage to the sheathing. Since I’m removing the brick I have the option of adding several inches of continuous exterior insulation and using a thinner rain screen such as Nichiha fiber cement. I’m trying to decide between ZIP with Rockwool Comfortboard and ZIP-R or possibly insulated MgO panels. The ZIP+Comfortboard combo should allow the exterior of the sheathing to dry to the outside but I’m concerned it could cause condensation on the exterior of the sheathing in the summer months. On the other hand, ZIP-R only goes up to 2.5″ so I might still have to do exterior insulation anyway. Matt Risinger put exterior polyiso insulation on his house which maybe is the right option for me too, but couldn’t it trap water between the insulation and sheathing?

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Replies

  1. Expert Member
    KYLE WINSTON BENTLEY | | #1

    I think a lot of it depends on how much you want to spend. Zip-R is a very premium product, and only at that it is a compromise on insulation, shear, and nailbase. The insulated MgO panels are also very niche right now.

    A good wall can be had with commodity materials - regular old 7/16" OSB, taped seams, typar or tyvek, or any of the similar, a 2" layer of polyiso, 3/4" furring strips, and the fiber cement siding. That should just about get you to the same depth as your brick use to have with the air gap, and be a good durable, easily sourced wall.

    With a properly constructed rain screen, taped polyiso seams are essentially impervious to vapor and liquid water. Even if they made it past, you have the building wrap, and if they make it past that, you've got some drying to the interior with normal drywall. I would say that if bulk water has gotten that far you've got larger problems as is.

    1. cedarknoll | | #2

      Your wall assembly example is exactly what I want to do. I am concerned about carpenter ants in the polyiso though. I had some 4" polyiso stored in my shop and the ants went to town in it, I ended up just trucking it all to the dump.
      What kind of ant defense would you add to that wall assembly? Thanks for your time!

      1. dgotsch | | #4

        This is my concern as well, I'd rather my wall assembly was resilient where a single failure in the outer layers doesn't create a potential catastrophic outcome such as ants/termites taking over the polyiso insulation and getting to the sheathing+framing.

      2. Expert Member
        MALCOLM TAYLOR | | #5

        CedarKnoll,

        I've dealt with quite a few Carpenter Ant infestations. Some have been real nightmares. Last year I turned down a renovation when the owner refused to see the thick train of hundreds of ants coming and going from the corner of his house as a problem that needed addressing, and this summer (lucky me) I'm removing an un-vented roof in which the foam is completely riddled with tunnels.

        My experience is that almost no building assemblies will keep them out. You need to either use materials they aren't attracted to as nest sites, or maintain a perimeter outside the house you can easily monitor, and quickly stop them establishing a nest.

        1. cedarknoll | | #8

          I really appreciate the idea of a maintained perimeter that you can easily monitor. Seems like doing your absolute best address the concern during construction is important but this seems like a really important step, especially hen building your own home where you know you will be the one monitoring.

          1. Expert Member
            MALCOLM TAYLOR | | #12

            CedarKnoll,

            My own house is almost completely surrounded by concrete patios and walkways. On the only area where the flower beds abut the foundation I made a small gravel separation - just to keep the insects out.

          2. cedarknoll | | #16

            >My own house is almost completely surrounded by concrete patios and walkways. On the only area where the flower beds abut the foundation I made a small gravel separation - just to keep the insects out.

            Malcolm Taylor,
            That's great thanks for the picture. I'm in the process of design my house I might have to do something like this. A monitorable barrier seems like a great defense layer. Looks good too!

      3. Expert Member
        KYLE WINSTON BENTLEY | | #7

        Just to ask, was the polyiso 'edge down' and was it damp? Moisture is one of the things that attract them in the first place.

        Behind a wall on a vertical surface / flashing / control should be a different matter. It must work, because so many commercial buildings are using EIFS or similar without apparent damage.

        1. cedarknoll | | #9

          It was edge down, but it seemed dry and was in a shipping container with what I thought was a tightly sealed door. Although it is entirely possible that the ants were already in the polyiso, I bought it used from a roofing company, maybe it just took a couple months to notice them. I guess that is one draw back of buying used polyiso. We do have a lot of carpenter ants in my area though, they seem to love the western red cedar.

  2. Expert Member
    ARMANDO COBO | | #3

    You could use stainless steel screen. Buy 36" rolls and cut them in 9"-12" sections, so you'll end up with 3-4 rolls. Make a "U" at the top and bottom 2'-3" behind the taped OSB and wrap forward over the rain screen and behind the siding. See a detail I have.

    1. Expert Member
      KYLE WINSTON BENTLEY | | #6

      I second this approach. I used it with mineral wool, and it's fairly quick. Use a hammer stapler and it doesn't take all that long.

      I'll also add that a good chemical treatment is probably necessary. You can fight them physically, but I wouldn't hesitate to go further with a good pest maintenance schedule.

    2. cedarknoll | | #10

      this is great. thank you!

    3. dgotsch | | #14

      What's the reason you wouldn't wrap the entire insulation board? Are you using foil faced polyiso?

      1. Expert Member
        BILL WICHERS | | #17

        Wrapping the entire panel takes a LOT more screen, and screening just the edge usually is enough to keep critters out. I think we're all assuming you have some siding or other reasonably critter resistant material on outer side of the foamboard, so the critters can only get in through the top and bottom edges, and maybe at the edge on a corner too depending on your details. You only need to screen the exposed edges for a bug barrier if the siding itself doesn't have open gaps.

        Bill

        1. dgotsch | | #19

          Okay so the bug screen is attached to the back of the siding - that's a detail I missed. Thanks for explaining.

  3. Patrick_OSullivan | | #11

    > On the other hand, ZIP-R only goes up to 2.5″ so I might still have to do exterior insulation anyway.

    Seems like a lot of exterior insulation for Texas, unless I'm missing something.

    1. dgotsch | | #13

      I guess so, I do have about 4-5" to fill out though once the brick is removed.

  4. Patrick_OSullivan | | #15

    > I guess so, I do have about 4-5" to fill out though once the brick is removed.

    Because of how the brick shelf sits, because the windows are bucked out closer to face of brick, or something else?

    1. dgotsch | | #18

      Yes, I'd like to bring the siding out flush with the brick shelf. I'm attaching a picture of the situation near the garage door were stud to front of brick is 6.5" although I think other parts of the house it might be closer to 5-5.5"

      1. Expert Member
        KYLE WINSTON BENTLEY | | #20

        You may just need to make an L shaped furring strip out of a 2x4 and a 2x3, or similar. Heck, a 2x3 and 2x6 would even let you put a 3.5” batt in that space, and leave room for an air gap.

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