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Is my wall assembly safe in zone 6

wiscoguy | Posted in Green Building Techniques on

So I think I have settled on my final wall and it will be a standard 2×6 wall with rockwool r23 and then zip or plywood with a self adhered web with 4” of polyiso. Either drywall or smart membrane for interior walls. I live in zone 6 it gets cold here. 

Just wanted double check my thoughts that this is a safe wall with minimal condensation concerns because of 50% of my insulation being on the exterior walls. Appreciate any thoughts and if this should be successful.

 

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Replies

  1. wiscoguy | | #1

    Anyone have any thoughts just checking to see if my final wall will be safe. Appreciate any thoughts or criticisms.

  2. Expert Member
    AKOS TOTH | | #2

    Your wall will work great. With that much exterior rigid, you can also skip the interior vapor retarder but sometimes it is easier to have that as your secondary air barrier.

    5" of rigid is not easy to work with, unless you are trying to hit an energy target, 2.5" of polyiso is enough for condensation control and much easier to install. There overall energy use difference between the two walls is not all that much.

    1. wiscoguy | | #3

      I can get recycled or second on polyiso local for cheap. I thought the rule of thumb was 50% on the exterior to avoid condensation in deffinetely open to using less. Was pla. I have on 2 sheets of 2”. Thank you for the reply.

      1. Expert Member
        Dana Dorsett | | #11

        The IRC calls out R11.25 minimum for dew point control on 2x6 walls. See:

        https://up.codes/viewer/connecticut/irc-2015/chapter/7/wall-covering#R702.7.1

        Even derated to R4/inch for age & temperature the 4" of reclaimed polyiso would be good for at least R16, which is plenty of margin. A single layer of 2" roofing polyiso (typically labeled R11.3- R11.4) would be cutting it close even if it were virgin stock goods.

    2. creativedestruction | | #4

      Agreed with Akos, you're on the right track. If you use a self-adhered air/weather barrier (awesome) you won't need zip. Regular ply or osb will work fine. Details can get a bit fussy with exterior insulation but worth it IMO. Makes the interior 'smart' membrane unnecessary.

  3. Expert Member
    AKOS TOTH | | #5

    For walls with no vapor retarder is 35% in zone 6. You can read the details here:

    https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies

    With a vapor retader, you can use even less if you want to. For example a common assembly around me (Zone 5) is 2x6 walls with interior poly and R5 rigid on the outside.

    P.S. I generally prefer to go with taped sheathing and standard house wrap. I usually put the house wrap over the foam and go with outie windows (you need to make some small window bucks for this). This makes window install much easier and requires no flashing tape origami as everything is in plane. It also makes the window sill much deeper which is great for pots and plants.

    1. wiscoguy | | #6

      Thanks for the thoughts and considerations

    2. ELIZABETH DISALVO | | #7

      This sounds pretty dangerous. Interior poly + exterior foam = vapor sandwich. Be careful! We never use interior poly in that situation.

      Our preferred method is 2x6 with fluffy insulation (Cellulose or Rockwool) and GWB inside.

      Outside plywood or zip with weather barrier there because it is much easier and more consistent to apply the WRB to that layer than it is to foam board (think top and bottom of boards. Think wrap it up and over the roof sheathing. Think connect it to the foundation WRB layer. Much easier to do a good job at the sheathing layer.)

      We put the face of windows flush with the sheathing tape it there. OR we build an outie buck and prosecco the heck out of it. Either.

      Then we put exterior board insulation. We tend to use only Comfort Board (Rockwool) or Steico / Gutex now. But ok- Polyiso. Then 3/4battens then siding.

      This way the whole wall breathes in to the interior and out to the exterior from the sheathing/ WRB layer. If you use Polyiso then the whole set up WRB + Polyiso acts as that middle Air barrier layer.

      No vapor sandwiches.

      1. wiscoguy | | #8

        I would not use poly in my wall but possibly a smart vapor barrier such as membrane or just plane drywall. In this case would you think this is a safe assembly. Appreciate you voicing your concerns. I believe he was referring to what code is in the area not what I should do in regards to the poly on the inside of the wall.

      2. Expert Member
        AKOS TOTH | | #9

        Double vapour barriers are not "dangerous", they seem to work well enough in our colder climate. The walls are not as robust when it comes to bulk water ingress, they do take much longer to dry than a permeable assembly, so if possible always go for a permeable assembly.

        My in-between is to use fiber faced polyiso for insulation which allows for a bit of drying to the outside.

        You can read more about double vapour barriers here:
        https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-092-doubling-down%E2%80%94how-come-double-vapor-barriers

        1. wiscoguy | | #10

          I was looking for fiberglass faced polyiso and haven’t been very successful in finding any do you have a source.

          Also my wall is vapor open to the inside wether I just use drywall or if I use a smart vapor barrier it would still be able to breathe. If I’m thinking about this correctly.

          1. Expert Member
            Dana Dorsett | | #12

            You ARE thinking about this correctly.

            Standard latex paint on drywall comes in at about 3-5 perms- it's a class-III vapor retarder. Something like (comparatively) cheap 2 mil nylon (Certanteed MemBrain) is normally much tighter than interior latex- when the proximate air is dry it's less then 1-perm (Class-II vapor retarder), severely restricting wetting the wall cavity via vapor diffusion. But when there is enough moisture in the cavity to support mold it becomes more vapor open than the latex paint, so it doesn't restrict drying toward the interior by much.

            But with R16+ of foam on the exterior it hardly matters- the wall sheathing doesn't dwell at low temps long enough to load up with significant moisture over the course of a winter, even with latex paint as the only interior side vapor retarder.

            In general air tightness is far more important than vapor retardency, but at your exterior-R levels the assembly would also be fairly tolerant of air leaks to the interior.

          2. Expert Member
            AKOS TOTH | | #13

            Fiberglass faced polyiso is common for commercial flat roofs, check your local roofing supply. Relatively cheap around here.

            With fiber facing your WRB must be outside the foam. This stuff doesn't like to get wet (tends to taco), so make sure to protect it as soon as it is installed.

          3. Expert Member
            BILL WICHERS | | #14

            I've seen fiberglass mat faced polyiso referred to as "kraft faced" before. Kraft facing would normally mean kraft paper, but I've seen people mix things up. Either is vapor open though.

            Bill

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