Because of the building’s orientation and shade from neighboring houses, we were limited in the amount of passive heat gain we could collect through the windows. To make the most of what was available, we used Thermotech casements on the south side of the building; these units have a solar heat-gain coefficient of 0.61 and a low U-factor of 0.19 (thermotechfiberglass.com). Most of the other windows came from EcoShield, a small manufacturer in Upstate New York (ecoshieldwindows.com). We also installed one tilt-turn window from German manufacturer Schuco (schueco.com/web/us) in a location where an outswing window would have interfered with the entry stair.
Both the owner’s downstairs apartment and the rental unit upstairs received new high-efficiency windows, dense-pack cellulose in the uninsulated framing cavities, and a thick exterior layer of rigid foam. Airtight drywall and an additional layer of cellulose in the lower unit increased the R-value of the wall system only modestly — but reduced air infiltration to a fraction of that in the unit above.
One way to install “outie” windows is to extend window bucks from the rough openings to the depth of the foam. But this creates a thermal bridge, plus it takes up space within the rough opening. While the space issue is easy to deal with in new construction, we didn’t want to reframe the openings or downsize the windows. So instead, we used a method similar to one common in Europe: We fastened the window frames with Simpson nailing plates, cantilevering them so the nailing flanges would be flush with the strapping on the exterior foam. For added strength we used 1-by furring around the window openings, fastening these boards with long screws through the foam into the framing. We then attached the window flanges to the furring. This method may sound a little shaky if you’re used to fastening windows directly to solid lumber, but once the exterior trim was fastened to the 1-bys and the windows themselves, the assembly was extremely solid. Those windows aren’t going anywhere.
In the upstairs unit — because we wouldn’t be removing the interior trim — we fastened the metal straps to the original window jambs, which were reinforced with blocking in the sash weight cavities.
Sealing and Insulating the Attic
Air-sealing the eaves of an existing structure can be a challenge. One well-known solution — the “chain-saw retrofit” — wasn’t practical with our gambrel roof, because cutting off the rafter tails would have seriously weakened the roof structure. Instead, we did the best we could by cutting and fitting the housewrap around the rafter tails, then sealing the edges with canned spray foam. The steep gambrel rafter bays later received 10 inches of closed-cell foam, completely burying the 2x4 rafters and forming a continuous seal with the edges of the housewrap. (The foam also stiffens the whole roof assembly nicely.)
Most of the attic floor — which lay beneath the less-steeply-sloped upper part of the gambrel roof — got a flash coat of foam for air-sealing, followed by a 20-inch layer of loose-fill cellulose. About a fourth of the attic was partitioned off for use as a small mechanical room, its walls and rafters insulated with spray foam, bringing the space inside the thermal envelope. Access to the mechanicals is through a scuttle with a drop-down ladder. We removed the now-unneeded brick chimney, which left a convenient chase for running water lines, HRV ducts, and electrical conduit to the utility room in the basement.