[Editor's note: This is the second part of a two-part story; Part I appeared in the May issue.]
Once all the inner 2x4 bearing walls were framed, we moved on to frame the roof, using mono-pitched raised-heel trusses with 3-foot tails on the south side. We used typical vent chutes and soffit venting at both eaves and roofed with asphalt shingles.
Before setting the trusses, we installed a 6-inch-wide strip of OSB around the second-story top plates, letting it project to the interior. Over this OSB strip, we applied Grace Ice & Water Shield to seal the outside wall corner, then for good measure, taped the membrane to the sheathing with 3M's 8067 flashing tape. The inward-projecting flange would give us a practical way to seal the wall air barrier - the OSB sheathing - to the ceiling air barrier, also OSB, which we fastened to the bottom chords of the trusses. (When it was time to drywall that ceiling, I supervised like a nervous mother, watching to make sure every screw hit wood. I didn't want any random holes in the air barrier.)
With the shell dried in, we began the process of sealing all the joints and penetrations in the OSB - both the structural wall sheathing and the interior subceiling - with 8067 tape and Grace Vycor flashing tape. At the wall base, we taped the subslab vapor barrier to the bottom edge of the OSB, and also sealed window and door bucks and other penetrations.
Outer wall assembly. In the completed shell, the OSB sheathing is actually buried a foot deep inside the wall, behind an outer layer of wood I-joists, whose main purpose is to hold cellulose insulation. Wherever we had to install an I-joist, we made sure to apply Vycor, so that there would be no air leaks around the screws. We fastened each I-joist through the sheathing to the interior stud using 5/16-by-4-inch RSS lag screws (grkfasteners.com) at 2 feet on-center. For an exterior sheathing - needed to retain the dense-pack cellulose - we nailed up a vapor-permeable (37 perms) asphalt-faced fiberboard sheathing from BP called BH900.
High-Performance Windows
Top-performing windows are vital to a Passive House. The program has a separate window standard that sets a maximum whole-window U-value of about 0.14. You can pass the standard with less-efficient windows as long as you satisfy the whole-house criteria for energy use and airtightness, but it's not the best approach, especially in a heating climate. I spent a long time shopping for windows and entry doors for this house and ended up with units manufactured in Lithuania by Intus (intuswindows.com). The windows I bought aren't actually certified by the Passive House organization (certification is expensive and raises the price of windows), but Intus's test data show that they perform well enough to meet the program's specs.
Good windows enhance comfort and simplify space conditioning. Compliant windows have such high R-values that the inner glass surface stays within 5.5°F or so of indoor ambient temperature, which prevents cold convection currents. And because most of this home's windows face south, they contribute a lot of space heating in the winter. In summer, the first-story windows are shaded by a wall-mounted solar array, and the second-story windows by the roof overhang.
According to computer models, windows should be positioned in the "R-value center" of the wall, because this makes for the least heat conduction; it seems like a minor point, but succeeding with Passive House requires a lot of fine-tuning to make sure every Btu is conserved. So we packed out our window openings with engineered-wood window bucks that held the windows at dead center in the wall plane. We applied flashing tape to the window sills and jambs before setting the windows, then - when the windows were in place - foamed the gaps around the frames and integrated the windows into the Tyvek drainage plane with flashing tape.
We covered the outer edges of the window bucks with EPS insulation before we applied the trim, in an effort to eliminate thermal bridging around this vulnerable opening. With the extra insulation, the majority of the window frame performs just as well as a clear wall section.
Unlike the windows, the inswinging door units were installed on the inside face of the wall. A layer of EPS completely covers the plywood bucks.
The exterior is clad with wood siding installed as a vented rainscreen. We first applied housewrap over the fiberboard, then nailed up vertical furring strips for the clapboards. At the base of the wall, we also ran horizontal furring for cedar shakes.
