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Community and Q&A

Which wall system?

robhhes | Posted in Energy Efficiency and Durability on

Hello ,

My name is Rob B. I read your article in GBA about Roxul ComfortBoard being used as an exterior home insulation. It was very interesting and is the reason that I am reaching out to you.

In addition to being a Home Performance contractor I am a General Contractor. Currently I am building my personal home. It has been difficult to decide what exterior wall system to use on my home. Arguments are good both ways about using a polyiso board, a polystyrene or an product like Roxul ComfortBoard 80.
Being in the Home Performance business I know that exterior walls are tricky when it comes to “breathing” and being able to “dry” to the interior as well as exterior.

My original plan was to use a Polyiso board for it’s high R value, density, and maybe even it’s radiant benefits in the summer months. But I continue to read info that argues against a non vapor transmitting product like Polyiso or Dow board product.

You indicated in your article that a non vapor transmitting board in this application should not be a concern. I live in Western North Carolina where high humidity and rain are prevalent. Winter temps can get into the single digits but typically not for very long. Summers are hot and humid. I don’t want to run a risk of trapping moisture in my exterior walls. Can you please elaborate on why you think I should not be concerned if I use a polyiso or Dow board product.

I have a poured concrete (10″ thick ) wall foundation with a waterproof peal and stick membrane followed by a 2″ dow product. My walls are 2 x 6 with a Zip wall sheathing system. I will likely dense pack cellulose in my walls between the studs followed by 1/2 drywall. The house will be very well air sealed. Exterior cladding will be stone, stucco and LP composite siding.

Thank you for any input that you may have.

Kind Regards,

GBA Prime

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  1. brendanalbano | | #1

    It's a semantics thing, but it's better to think about "drying" than "breathing". Walls don't need to breathe. They do need to dry.

    In your wall, if you used a vapor impermeable insulation like foil-faced polyiso, you have two questions:
    - Is there a risk for condensation?
    - When (not if) water gets into the wall, can it dry out?

    As long is your interior finish is permeable, everything inward of the insulation (acting as a vapor barrier) can dry inwards, and everything outboard of the insulation can dry outward. As long as you have enough exterior insulation for your climate, there isn't a condensation risk.

    Now if you were to put an impermeable vinyl wall covering on your walls, or put a vapor barrier on the inside, then the wall between the interior finish and the foam would have no path for drying, which would be bad.

    If you use a permeable insulation, you dry in both directions all the time, and your zip sheathing is in the middle of your sandwich, which is a great place for an air barrier and vapor retarder.

    These Building Science Corporation articles have some good fundamentals on vapor barriers if you jive with their way of explaining things:


  2. robhhes | | #2

    Thanks Brendan! VERY helpful.

  3. GBA Editor
    Martin Holladay | | #3

    Every study of test huts and occupied houses that compared the moisture performance of a stud wall without exterior foam to the moisture performance of a stud wall with exterior foam has found that the wall sheathing stays dryer when walls have exterior rigid foam.

    Here are links to articles on GBA that address your questions:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    How to Install Rigid Foam Sheathing

    How to Design a Wall

  4. robhhes | | #4

    Martin, thank you for your response. I will read the articles that you and Brendan have suggested.

  5. Expert Member
    Dana Dorsett | | #5

    NC is primarily climate zones 3 & 4 except for the handful of counties from Yancy to Allegheny in the Smokies, which are zone 5. See:

    In zone 5 it takes a minimum of 2" of foam (any type) for dew point control at the sheathing for a fiber insulated 2x6 wall, but only 1.5" for a 2x4 wall (1" if polyiso.)

    In zone 4 it only takes 1" for a 2x6 wall, 3/4" for a 2x4 wall (a half-inch, if polyiso.)

    In zone 3 it doesn't much matter- the winters aren't cold enough long enough to build up moisture, as long as it can still dry to one direction or the other. In summer it can be useful to have a higher vapor retardency product like foil faced EPS/polyiso on the exterior if the house is going to be air conditioned to 75F or lower, but if you're happy air conditioning only down to 80F it doesn't much matter.

    As long as you've met or exceeded those minimums it would be code compliant without interior side vapor retarders, and at fairly low risk of moisture problems, even if it only has good drying capacity toward the interior. If you go even higher with the foam, it's that much more safety margin, making it tolerant of somewhat higher wintertime air moisture.

    From a general verde-tude point of view, polyiso & EPS are the lowest impact foams, but rock wool is lower impact still, but it's more expensive per R, and it's highly vapor permeable.

  6. iLikeDirt | | #6

    People often talk about drying towards the interior. Well, how vapor-permeable is that drywall once it's been painted with ten or fifteen layers of latex paint and primer over time? IMHO it is much safer to design for exterior drying even if you hope for interior drying. That makes external mineral wool all the more attractive.

  7. Reid Baldwin | | #7

    Rob mentions that exterior cladding will be stone, stucco and LP composite siding. Is exterior foam or mineral wool able to provide proper mechanical support for stone?

  8. robhhes | | #8

    Thanks all,

    After considering your comments and reading your recommended links to various articles, I will very likely
    opt for my original system utilizing 2" of polyiso board on exterior. As Reid states in his Answer I still need to address How and IF I can attach mesh, brown/scratch coats with thin stone (not manufactured stone) and stucco. Does anyone have a source for those applications?

  9. GBA Editor
    Martin Holladay | | #9

    For more information on installing stucco over rigid foam, see To Install Stucco Right, Include an Air Gap.

    In that article, I recommended:

    "If you’re installing stucco over foam, here’s how to create a ventilated rainscreen gap:
    Install 1x3 or 1x4 vertical strapping on top of the foam, screwed through the foam into the studs.
    Fasten paper-backed metal lath to the strapping, and proceed with a standard installation of three-coat stucco.

    "If your local building inspector raises questions about the number and size of the screws you plan to use to fasten your 1x4 strapping, you may need to get an engineer to review your details. As long as you have an engineer’s stamp, your local building department should be happy."

  10. JC72 | | #10


    Mineral wool and rigid foam do not lend any structural support but it's not needed. Stone/brick would require a brick ledge and stand offs. Siding manufacturers have their own requirements regarding fastener length and Building Science Corp has don't the research regarding assembly creep of siding attached to furring strips over foam.

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