Drywall in the middle of a double wall system
Thanks in advance to all who take the time to read this!
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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