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Drywall in the middle of a double wall system

Jim Roncace | Posted in General Questions on

Thanks in advance to all who take the time to read this!

Questions First…

In double wall construction…
1. Is having a layer of drywall, which would be the primary air barrier, positioned in the middle of a double wall assembly a problem?

2. Should I be concerned that the kraft paper of the insulation installed against the exterior side of the air barrier drywall is not stapled to the wall studs?

3. Should the insulation installed to the interior of the air barrier drywall have a vapor barrier?

Background Info… (sorry: this got pretty long)
I have been considering a modular home and recently toured a manufacturer’s facility to see the building process and discuss the goal of customizing their system to build a home with ~R40 exterior walls. 

In the manufacturer’s system, the exterior 2×6 walls are framed in a jig and the interior drywall is installed before the walls are attached to the floor assembly.  An elastomeric gasket is installed under the wall base plate to air seal the wall to the floor sheeting.  All electrical and plumbing is then added from the exterior side of the wall.  Penetrations through the drywall, and the electrical boxes themselves are sealed with spray foam.  Joints between drywall and perimeter of the framing, and between wall sections, are sealed with a spray adhesive.  Typically, if spray foam insulation isn’t specified, R21 faced fiberglass batt insulation is then installed from the exterior side of the wall with the kraft paper against the drywall. The kraft paper is not stapled or attached to the wall studs in any way; the batts just friction fit into the stud bays.  OSB and then vapor permeable house wrap are then installed, followed by, usually vinyl, siding.  Note that the house wrap is not taped at the seams, and penetrations for exterior electrical boxes, etc. through the OSB are not sealed, so in this construction assembly, the manufacturer is using the drywall as the primary air barrier.  

In discussions of ways to modify this construction system to build an ~R40, thermally broken wall, the method that the manufacturer preferred and recommended was to build double walls, installing a 2×4 wall interior to the 2×6 walls.  This was preferred because no modification to the methods of constructing the exterior 2×6 walls would be required.  The manufacturer still wants to install the drywall on the 2×6 wall.  The 2×4 wall would be built in the same way the factory builds interior partition walls.   Electrical and any plumbing would be run in the 2×4 wall.  With a 2″ offset between the interior surface of the drywall and the 2×4 interior wall, a second layer of R21 batt insulation would be installed, achieving a ~R42 for the insulation value (not accounting for framing factor).  This method would also provide a thermal break at all wall studs, headers and plates.   Building boxes for window and door penetrations would be unusual for the the factory, but otherwise this system would be fairly easy and relatively inexpensive for them to build.

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Replies

  1. Jon R | | #1

    Conventional wisdom is that an air barrier can be anywhere, but dig a bit deeper and you will find that it works somewhat better on the warm side and even better than that on both sides. Seems there is little data on how much difference it makes.

  2. Expert Member
    Michael Maines | | #2

    Jim,
    "1. Is having a layer of drywall, which would be the primary air barrier, positioned in the middle of a double wall assembly a problem?"
    No, as long as insulation ratios and construction details keep it safe from moisture accumulation. To be airtight, all seams will need to be taped.

    "2. Should I be concerned that the kraft paper of the insulation installed against the exterior side of the air barrier drywall is not stapled to the wall studs?"
    No, but it is hard to install fiberglass batts without voids. You could ask about upgrades to high-density and/or unfaced fiberglass batts, mineral wool batts or dense-packed cellulose or fiberglass. But there is nothing inherently bad about not stapling the kraft facing.

    "3. Should the insulation installed to the interior of the air barrier drywall have a vapor barrier?"
    What is your climate zone? In most places your wall will perform better with a vapor retarder (not a vapor barrier). It's not always required by code, but a good idea in highly insulated assemblies that are vulnerable to moisture accumulation. A variable permeance membrane is often the best because it keeps most water vapor from entering the assembly but allows the wall to dry to the interior if necessary. Siga Majrex, Pro Clima Intello and Certainteed Membrain are all variable permeance membranes.

    You could ask them to tape the sheathing to make it relatively airtight.

    1. Jim Roncace | | #3

      Michael,
      Thanks for the information.

      The house will be built in the Pittsburgh, PA area, which is climate zone 5.

      For the current wall plan, the insulation ratio will be 50/50 exterior/interior to the drywall air barrier, which I believe is acceptable to keep the dew point outside of the drywall surface for my climate zone. I'm basing this on the information I've read about exterior foam insulation thickness requirements; do these same ratios apply when all the insulation is fluffy?

      I understand your suggestion about using a smart vapor retarder on the interior of the inner wall before the final drywall is installed. The interior insulation needs to dry toward the inside of the house, so a vapor barrier would not be a good idea at this location.

      1. Expert Member
        Michael Maines | | #4

        Jim, yes 50/50 is a safe ratio for walls in CZ5, generally speaking, according to the IRC. The ratios apply regardless of material. (The exception is when discussing roof venting.) The only place in your house where you want a vapor barrier (≤0.1 perms) is below the foundation slab.

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