The wall system shown here is an improved version of the system I used for the house I built in Knox, Maine, in 2011 (see "An Affordable Passive House," JLC May/12 and Jun/12). Since building that prototype house, I've switched to some advanced European materials for air-sealing and moisture management that weren't on the U.S. market back then.

We start with an inner 2x4 stud wall, sheathed on the exterior side with OSB (A); all joints and nail lines are sealed with Pro Clima Tescon Vana tape (foursevenfive). The OSB layer is airtight, and also braces the structure. Next, we fasten 11 7/8-inch wood I-joists to the outside of the walls, driving 4-inch GRK RSS structural screws through the flange and OSB sheathing into the studs. As a weather barrier over the I-joists, we apply a vapor-open waterproof fabric from Pro Clima called Solotex Mento Plus.

EcoCor’s wall system is based on a simple stick frame with OSB sheathing, which then gets wrapped in an outer blanket of wood I-joists. The inner frame’s sheathing is taped at joints and nail lines. After the house is framed, sheathed, and roofed, the I-joists are screwed into the wall studs. (We typically use 12-inch I-joists, but on some projects have upsized to 16-inch I-joists.) Both the I-joist cavities and the inner stud walls are filled with dense-blown cellulose insulation, for a total wall R-value of about 56 (not including membranes). The system is airtight at the plane of the OSB and is vapor-open to the inside or the outside.
EcoCor’s wall system is based on a simple stick frame with OSB sheathing, which then gets wrapped in an outer blanket of wood I-joists. The inner frame’s sheathing is taped at joints and nail lines. After the house is framed, sheathed, and roofed, the I-joists are screwed into the wall studs. (We typically use 12-inch I-joists, but on some projects have upsized to 16-inch I-joists.) Both the I-joist cavities and the inner stud walls are filled with dense-blown cellulose insulation, for a total wall R-value of about 56 (not including membranes). The system is airtight at the plane of the OSB and is vapor-open to the inside or the outside.

We double strap over the fabric to create an air space behind the siding. For vertical siding (barn boards or shiplap), we apply vertical strapping along the length of the I-joists, then horizontal strapping for nailing. But if the plan is for horizontal siding, we first nail horizontal strapping across the I-joist ribs, then apply vertical strapping for nailing. Finally, we fill both the inner 2x4 wall and the outer I-joist assembly with blown-in dense-pack cellulose insulation for a total wall R-value of 56.

Where the wall system meets the roof assembly, the airtight boundary runs across the wall plate to the underside of the truss ceiling. To accomplish this, the crew lays a strip of airtight fabric across the wall plate before setting trusses, taping the fabric to the OSB on the outside of the stud wall and to a layer of vapor-control Intello Plus fabric stapled to the underside face of the trusses on the ceiling inside. Self-sealing tape is applied over the staple lines, and strapping along the trusses secures the fabric. The ceiling is cross-strapped for drywall, and drywall screws are sized so they will not poke through the strapping or punch holes in the fabric. The roof is insulated to about R-150.
Where the wall system meets the roof assembly, the airtight boundary runs across the wall plate to the underside of the truss ceiling. To accomplish this, the crew lays a strip of airtight fabric across the wall plate before setting trusses, taping the fabric to the OSB on the outside of the stud wall and to a layer of vapor-control Intello Plus fabric stapled to the underside face of the trusses on the ceiling inside. Self-sealing tape is applied over the staple lines, and strapping along the trusses secures the fabric. The ceiling is cross-strapped for drywall, and drywall screws are sized so they will not poke through the strapping or punch holes in the fabric. The roof is insulated to about R-150.

Our roof system has also evolved. We're still using raised-heel trusses, aligning them over the wall studs and insulating the attic with several feet of loose-fill cellulose. But instead of skinning the underside of the trusses with OSB and tape to form the airtight second-story ceiling, we now staple Pro Clima Intello Plus, a fabric reinforced to support insulation, under the trusses and tape the seams, then strap the ceiling for drywall (B). To tie that under-ceiling fabric into the taped OSB sheathing that forms our wall's airtight control layer, we apply a strip of Pro Clima DA-S polypropylene fabric over the top of the wall plate before setting the trusses, and tape it to the OSB layer and to the ceiling fabric with Pro Clima Duplex double-sided tape.

The house shown here sits on a full basement foundation. The basement walls are poured using conventional formwork and then insulated with 6 inches of expanded polystyrene (EPS) rigid foam. The insulated I-joist wall cavities meet up with the foundation insulation. To complete the airtight boundary from the wall to the foundation, the OSB sheathing extends down past the floor system rim board to the concrete sill, and tape is applied to the joint between the sheathing and the waterproofed sill
The house shown here sits on a full basement foundation. The basement walls are poured using conventional formwork and then insulated with 6 inches of expanded polystyrene (EPS) rigid foam. The insulated I-joist wall cavities meet up with the foundation insulation. To complete the airtight boundary from the wall to the foundation, the OSB sheathing extends down past the floor system rim board to the concrete sill, and tape is applied to the joint between the sheathing and the waterproofed sill

Outboard of the concrete foundation wall, there are 6 inches of Type IX EPS insulation, which matches up with the insulated cavity of the outer I-joist assembly (C). The top of the concrete sill is sealed with Prosoco R-Guard FastFlash. To connect our airtight wall to the foundation sill, we tape the OSB sheathing layer to the sealed concrete with Pro Clima Budax butyl adhesive tape.