We chose 5/8-inch-thick tongue-and-groove AdvanTech OSB sheathing (800/933-9220, huberwood.com) instead of typical 7/16-inch OSB for a few reasons. First, the thicker, stiffer sheathing helped flatten out any unevenness in the foam layer when we pulled it tight against the insulation. Second, we could biscuit-joint the butt edges of the sheathing, which - along with the tongue-and-groove profile on the long edge - also helped to create a flat wall plane, even on wall sections where we used small pieces of foam to fill in around openings. And finally, the thicker sheathing provided a strong substrate for the new fiber-cement siding.
To ensure good bite and prevent sagging, we had to hit the framing, so we laid out the stud locations on each sheet, then drilled countersunk screw holes 16 inches on-center (28 screws per full sheet of sheathing) with a 3/4-inch-diameter spade bit. The holes were just deep enough to prevent the screws' large washer heads from protruding beyond the plane of the sheathing. Along the panel ends, we also cut slots for large (#20) biscuits, also on 16-inch centers.
After snapping layout lines on the wall, we spread construction adhesive on the foam, again at a rate of a quart per five panels, then secured the sheathing with 9-inch HeadLok screws driven through the foam and sheathing into the studs. Even with the careful layout, we occasionally missed the framing and had to adjust the screw angle accordingly.
Wherever a butt joint fell on a stud, we countersunk the holes in place so that the washer-heads on the screws caught both sheets. It didn't matter if butt joints landed between studs because of the biscuits: As the screws pulled the sheathing tight to the foam, the biscuits held the panels flush to one another. After we were done, the sheathing layer was as flat and rugged as if it had been applied directly to the studs, and was ready for housewrap, flashing, window and door installation, and siding and trim.
Windows and Doors
When we removed the existing windows and doors, we built out the rough openings with bucks that would support the new windows and doors at the new sheathing plane. The window manufacturer (Loewen, 800/563-9367, loewen.com) was able to provide us with double-glazed U-0.28 units that closely matched the size and total glazing area of the originals, yet still fit snugly inside the bucks.
We built the bucks from 3„4-inch CDX plywood salvaged from the interior demo. They extend from the interior plane of the wall studs to the exterior plane of the wall insulation, and are sealed to the rigid foam with spray foam. When we installed the sheathing, we were able to screw the sheathing directly to the bucks, providing extra support for the windows and a solid surface for fastening the window and door nailing flanges.
The doors needed additional support. Under each opening, we bolted a section of 3/8-inch angle to the wall, either directly into framing or to anchor bolts cast into the new concrete radiant slab floor. Then we attached PT wood blocking and shims to the angles as needed to fully support the door sills, driving screws through small predrilled holes in the steel.
Finishing the Thermal Envelope
Once the foam and sheathing were in place, our insulation sub finished the thermal envelope from the inside. First, he used high-density closed-cell spray foam to fill in the shallow truss rafter bays over the exterior walls and to seal the soffits and other bypasses into the attic. He then topped off the existing R-19 fiberglass batts with another 18 inches of cellulose. In places where we'd removed drywall during the demolition phase, we pulled out the old fiberglass batts and reinsulated the stud bays with damp spray cellulose.
Before hanging drywall, we scheduled a blower-door test, which took place on a cold February day. Preliminary results showed a respectable air leakage rate of 500 cfm50, even before the technician toured the house with her IR camera to help us pinpoint air leaks in the envelope that we had missed. Even though we haven't done final testing, we anticipate that the home's air exchange rate will be well below 3 ACH50.
The IkoTherm panels we ordered through our local lumberyard were packed in 11-panel bundles, so we ordered 77 panels at $57 per panel to cover about 2,400 square feet of wall area. We allowed three man-hours of labor to install each panel and cover it with sheathing, for an installed cost of about $7.45 per square foot. By comparison, wrapping the house with a double layer of 2-inch blueboard and strapping would have cost about $8 per square foot, while we estimated that nailbase or SIP panels with an equivalent R-value would have cost at least $10 per square foot.
Mike Shepard works for Conner & Buck Builders in Bristol, Vt.