Editor’s note: This is the second installment of a two-part story; “Building a High-Performance Shell” ran in the May 2010 issue. Our company focuses on energy- efficient retrofits and new construction. Last month, I described the advanced framing and foam-board sheathing that we used in building a new home in Concord, Mass. The house was designed by architect Betsy Pettit of Building Science Corp. for owners who wanted a sustainable home with monthly energy costs that would remain fairly constant from one season to the next. They also wanted a building that was ahead of its time, so that 30 years from now, should they decide to pass it on to their children or sell, they wouldn’t be handing off a dinosaur. These goals dovetailed perfectly with our own.
Windows and Doors
To terminate and seal the double layer of 2-inch foil-faced Tuff-R polyiso board around the rough window openings, we lined them with 1/2-inch plywood bucks that project 4 inches beyond the framing. The thickness of the sheathing ruled out installing the windows — Marvin Ultima aluminum-clad low-E argon tri-pane double-hungs (888/537-7828, marvin.com) — using their vinyl nailing fins. Instead, we attached Simpson ST9 strap ties (strongtie.com) to the windows’ side jambs, two per side, with 1/2-inch wood screws. After adjusting for an even gap around the jambs, we screwed the straps to the framing. The straps have enough flex that we’d be able to shim the jambs plumb later.
On the exterior, we added flat casings and subsill extensions, made and installed as preassembled units. For all exterior trim, we used TUF Board (800/452-2117, tufboard.net), a wood-PVC composite that doesn’t expand and contract nearly as much as some of the solid cellular vinyl trim we’ve tried. We used a complementary Azek molding under the window sills. The window trim is simply butted around the outside of the windows, installed over vertical strapping that vents both the trim and siding (there’s more discussion of the venting ahead). With the windows sealed against the foil-faced sheathing, any water that gets past the trim and siding will drain back out at the bottom of the wall. The painters caulked the joints between the trim and the window jambs but kept the bottom edges caulk-free for drainage.
When installing the exterior doors, we aligned the outside edge of the 5 9/16-inch jambs with the framing rather than with the foam, creating a 4-inch inset. If we had used a deep extension jamb on the interior, it would have prevented the doors from fully opening. Instead, we ordered the doors without exterior trim, applied exterior extension jambs and trim, and extended the sill with TUF Board. Over the rim joist below the door openings, we installed a single layer of foam with a 2x8 ledger bolted above it for threshold support. We then covered the ledger with 1/2-inch foam board and installed a self-adhering flashing pan in the rough opening.
Back-Vented Siding
The prefinished WeatherBoard (800/233- 8990, certainteed.com) fiber-cement siding chosen for this project is installed over vertical 1x3 strapping, installed at 24-inch centers over the studs. This strapping creates a venting and drying space behind the siding. The foam sheathing’s foil face sheds any water that may be driven past the siding.
We screwed the strapping through the 4-inch-thick sheathing using 6 1/2-inch HeadLok screws (800/518-3569, fastenmaster.com). At this length, the screws get a 2-inch bite into the studs. Passing as they do through 4 inches of foam board, you might expect them to eventually bend under the weight of the siding and allow it to sag. In fact, though, we used the exact same assembly on another job more than four years ago, and the siding shows no signs of movement — so it appears to work pretty well.
We started the strapping flush with the bottom edge of the sheathing. Lengths of 41/2-inch-wide galvanized metal flashing, overlapping the mudsill and foundation, protect the bottom edge of the foam sheathing from UV degradation, bugs, and sparks. We attached a continuous strip of nylon insect screen to the flashing and let it hang until the strapping was installed. We blocked between the strapping with strips of Cobra plastic-matrix ridge-vent material (973/628-3000, gaf.com), then folded up the screen and tacked it to the strapping. The vent material holds the screen against the back of the siding, helping to keep bees and other bugs from nesting in the vent space. To prevent the flashing from drooping, we screwed it to the strapping ends.
Blocking for trim. At the two-stud outside corners, the 4-inch offset created by the foam board places the strapping conveniently behind the 1x8 corner boards without the need for any supplemental blocking. We preassembled the corner boards from the back using pocket screws and attached them to the strapping with stainless steel trim-head screws.
The strapping installed over the studs on both sides of the windows supports the window trim. But at inside corners and around windows and doors, we needed additional strapping to catch the ends of the siding. In these places, we installed 2x4 ladder blocking in the framing cavities and screwed the added strapping to the blocks through the foam. Because the ladder blocking is installed on the flat in the wall bays, it would eventually be buried in blown-in cellulose insulation and wouldn’t add greatly to the conductive heat loss.
Siding installation. The client wanted a particular look for the siding, one typical of older Cape-style homes: The courses start narrow at the base of the wall, with a 1 7/8-inch exposure, and gradually widen over about 15 courses to the standard 4 1/2-inch face. You can’t effectively overlap 5 1/2-inch-wide fiber-cement siding at those narrow exposures; it stacks up in a thick pile on the wall. So we played with some mockups and found we had to rip each course to maintain the 1 1/4-inch overlap prescribed by the manufacturer — a dusty and time-consuming process. The siding is blind-nailed in this overlap zone, which is pretty tight. We discovered that if we nailed a little too close to the top edge, the material tended to break out, something we haven’t seen when installing Hardiplank. In general, the thinner, less-expensive WeatherBoard required gentler handling than we were used to during installation, and I’m not inclined to choose it again.
We painted both the trim and the siding with two coats of acrylic latex. The vented installation will help ensure a long-lasting, low-maintenance finish.
