Our wall system forms part of a set of two-family and three-family designs that we developed for a 36-unit multifamily cohousing project in Belfast, Maine. The design is guided by the Passive House building standard, and the finished buildings meet the criteria for Passive House certification. (However, it's up to the individual homeowners to apply for formal Passive House status, and so far none of the homes are officially certified.)

The wall system consists of an inner stick-framed 2x4 stud wall, insulated to R-13 with dense-packed blown cellulose (A). Outboard of this wall, we apply 8 1/4-inch-thick, R-32 SIPs (structural insulated panels) supplied by R-Control (r-control.com). The entire wall system, including drywall and wood shingle siding, has an R-value of 46 (13+32+.45+.5).

The load-bearing 2x6 stud wall at the core of the GO Logic wall system supports the second floor and roof loads. After the walls are braced, crews fasten SIPs to the exterior one at a time using 10-inch R-Control wood screws. To maintain a continuous airtight vapor-impermeable boundary at the inner face of the SIPs, crews apply beads of R-Control Do-All-Ply adhesive sealant to the edge of each inner panel face before standing the next panel. Adhesive sealant is also applied to the edges of the panel foam cores in each joint, adjacent to the panel spline grooves.
The load-bearing 2x6 stud wall at the core of the GO Logic wall system supports the second floor and roof loads. After the walls are braced, crews fasten SIPs to the exterior one at a time using 10-inch R-Control wood screws. To maintain a continuous airtight vapor-impermeable boundary at the inner face of the SIPs, crews apply beads of R-Control Do-All-Ply adhesive sealant to the edge of each inner panel face before standing the next panel. Adhesive sealant is also applied to the edges of the panel foam cores in each joint, adjacent to the panel spline grooves.

The wall system is airtight at the exterior skin of the SIPs, where we seal the joints with tape. However, we also establish an airtight vapor-control boundary at the inboard face of the SIPs to prevent interior moisture from penetrating the panel joints and condensing near the cold exterior in winter.

Construction proceeds as follows: After the foundation slab is poured, we frame and brace the first-story stud walls, frame and deck the second floor, and frame and brace the second-story walls. Then we apply the SIPs to the outside of this stick frame. As we stand up and attach the SIPs one by one, we apply beads of R-Control adhesive sealant to the EPS foam and also to the edges of the inner OSB skin of the SIPs. We bed the panel bases in sealant and apply sealant to any penetrations or gaps.

At the roof, Zip sheathing applied to the underside of the truss bottom chords provides the air and vapor barrier. To integrate this sheathing with the wall system air and vapor barrier, crews rip a strip of Zip sheathing to cover the full thickness of both the framed and the SIP walls, and apply a strip of 3M 8067 tape onto the bottom side before fastening it to the top plate of the inner wall. The tape has split-release paper, so half of the tape is left flapping inside the building with the paper still on. After the roof trusses are set and the Zip sheathing nailed to the ceiling, crews peel the rest of the tape and seal it to the ceiling sheathing to complete the airtight joint.
At the roof, Zip sheathing applied to the underside of the truss bottom chords provides the air and vapor barrier. To integrate this sheathing with the wall system air and vapor barrier, crews rip a strip of Zip sheathing to cover the full thickness of both the framed and the SIP walls, and apply a strip of 3M 8067 tape onto the bottom side before fastening it to the top plate of the inner wall. The tape has split-release paper, so half of the tape is left flapping inside the building with the paper still on. After the roof trusses are set and the Zip sheathing nailed to the ceiling, crews peel the rest of the tape and seal it to the ceiling sheathing to complete the airtight joint.

At the top of the wall, we extend the ceiling air barrier (Zip sheathing) under the truss heel and seal it to the outer airtight skin of the SIPs using tape (B).To integrate the wall system air and vapor barrier into the slab foundation, we fold the heavy plastic sub-slab vapor barrier around a PT plywood sill under the SIPs panel and sandwich the end between the mudsill and the bottom plate of the inner wall (C). Then we seal the corner before setting the SIPs.

The foundations are reinforced concrete slabs-on-grade placed in an EPS insulating form system on top of a sub-base of compacted stone dust and self-leveling, non-shrinking “flowable fill.” To integrate the sub-slab moisture barrier with the wall system air and vapor barrier, crews lay a heavy plastic membrane across the slab perimeter insulation and double the plastic back over a protective sheet of preservative-treated plywood. Then they fold the plastic up and onto the PT mudsill, where it is covered and secured by the bottom plate of the inner wall.
The foundations are reinforced concrete slabs-on-grade placed in an EPS insulating form system on top of a sub-base of compacted stone dust and self-leveling, non-shrinking “flowable fill.” To integrate the sub-slab moisture barrier with the wall system air and vapor barrier, crews lay a heavy plastic membrane across the slab perimeter insulation and double the plastic back over a protective sheet of preservative-treated plywood. Then they fold the plastic up and onto the PT mudsill, where it is covered and secured by the bottom plate of the inner wall.

In the course of building out the 36-unit Belfast project, our methods have evolved. Most of the improvements to our system have come from our carpenters in the field—we send the details out to them, they start using them, and often they find better ways to accomplish the goals. With practice, our results have also improved: Our latest blower door test results came in at 0.21 ACH50.