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Community and Q&A

How Risky is this Assembly?

Bbeaulieu | Posted in Green Building Techniques on

On a scale of 1 to 10 how risky is the following assembly? 1 indicating that the assembly is safe, and will remain dry and mold free for several years and 10 indicating that the sheathing will be soaking wet and we’ll have mold in our walls in no time. We’re in zone 6A, 30-40″ of avg rainfall.

Double stud wall, from the inside out: 1/2″ drywall painted with latex, 2×4 interior stud wall insulated with Roxul, Smart Vapor Retarder at exterior of interior wall caulked and taped seams, 5.5″gap insulated with Roxil, 2×4 exterior wall insulated with Roxul, plywood taped seams, Benjamin Obdyke Hydrogap WRB followed by cement board horizontal lap siding. The house is 2500sqft with three in the residence. We have an HRV. 2ft roof overhangs.

Unfortunately this assembly is built so I’m trying to find out if I can sleep at night or if I should start demo-ing. The contractor was meticulous with flashing details. From what Ive read on this site I wish we would have used cellulose and added a larger rain screen than the hydrogap. Should we peal back some siding in 5 years and see if the sheathing is wet?

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  1. user-1140531 | | #1


    What is it that worries you about the assembly? Why do you wish you would have used cellulose?

    Are you confident that your flashing details will prevent rain penetration? Why do you think you need a larger rain screen?

    It sounds like you are mainly concerened about the sheathing getting wet. Since you have an air barrier, smart vapor retarder, and good flashing, why do you anticipate wet sheathing?

  2. Bbeaulieu | | #2

    I've read about the cold sheathing issue with dbl stud walls and condensation building up at the back of the plywood. I also read about cement board being a reservoir material and should have a rain screen to help drying to the exterior. My major concern is infact the sheathing not being able to dry to the outside and drying slowly to the inside given the thick assembly and several layers of latex paint thatll be applied over the years.

    Hopefully I'm worrying for nothing.

  3. GBA Editor
    Martin Holladay | | #3

    I think you should relax.

  4. user-1140531 | | #4


    I don’t believe that you will get moisture inside of the wall, so there will be no need to provide a means for moisture to dry out of the wall. In fact, that would be my objective in building a double stud wall. Your smart vapor retarder will stop outward vapor drive by air flow or by diffusion. It sounds like you have installed it with good attention to sealing detail. So there will not be a “cold sheathing problem” due to outward vapor drive condensing on the inside of the sheathing.

    Regarding the issue of reservoir cladding: The cement board siding is not much of a reservoir in terms of capacity. But more importantly, it should not be able to absorb water easily. Your roof overhang will also tend to keep a lot of rain off of the siding as a degree of back up insurance.

  5. dickrussell | | #5

    I, too, would not be very worried about that wall assembly. Frankly, I don't really understand the "double wall cold sheathing" concern. Perhaps the comparison is between a single stud wall having exterior insulation vs double stud wall without exterior insulation. In the former case, the sheathing definitely will be substantially warmer, due to the insulation outside it.

    Without exterior insulation, there isn't much difference, because the sheathing is very close to the outside air in either wall. Let's quantify the difference. Suppose the single stud wall gives R20 (center of cavity), that the double wall gives R40 (likely whole wall), that the sheathing and siding in each case contribute R1 to the total, and that design delta T is 70 degrees F. The inside surface of the sheathing will be less than 2 degrees colder in the double wall (70/20 -70/40 = 1.75). That's less than the daily or even hourly variability of the outside temperature and of little importance from a fail/no fail point of view.

  6. user-1140531 | | #6

    The cold sheathing problem arises from water coming from four different sources. I do not understand the significance of the distinction between single and double stud walls in contributing to the cold sheathing problem. One of the water sources is rain, and I do not understand what that has to do with cold sheathing.

  7. user-1137156 | | #7

    As Martin said, RELAX. If the flashing details and air sealing details are done competently , and it sure sounds like they were, you have nothing to worry about! With the 16 perm ventilated house wrap your sheathing has good drying to the outside and it' is plywood which is tolerant of occasional moisture.

  8. LucyF | | #8

    This sounds like a great wall certainly way less at risk for wet sheathing than a house built to code (sc code anyway).

    Good job.

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