Q: How was the exterior window trim handled on the Passive House featured in “Building a Passive House for the First Time”
A: Farley Pedler, the article’s author and a home builder from Tisbury, Martha’s Vineyard, Mass., responds: Because the wall construction differed on the upper and lower sections of the house, we used two approaches for applying the exterior window trim.
The core structure for the house was 2x6 balloon-framing with a skin of 1/2-inch Zip System sheathing. We used European-style aluminum-clad windows on which the cladding is spaced off the frame by approximately 5/8 inch. On both levels, we installed the windows with the wood frame in plane with the outside of the sheathing.
On the lower level, we built a superinsulated wall by attaching 14-inch-deep trusses over the sheathing and fastening them to the framing behind. This created a deep window well (also clad in sheathing) around each opening. The framing stepped the openings in a few inches on all sides, which allowed us to use more traditional-looking flat casing. After sealing the window frames to the sheathing with vapor-open flashing tape, we had to fill the space created by the aluminum cladding sitting proud of the sheathing. We inserted a layer of rigid foam ripped to a 5/8-inch thickness, which added a layer of insulation over the frame while padding out the wall to keep the casing flat. We finished the opening by returning the sidewall shingles into the opening and butting them against the trim.
The upper windows were much easier. These walls had a double layer of rigid insulation over the wall sheathing, and a layer of sheathing to box in each window opening. After the windows were installed and sealed to the opening, we installed 1 1/2-inch rigid foam that acted essentially as an exterior extension jamb. Because the space between the cladding and the window buck was only an inch wide, we rabbeted the foam so that it could slip past the cladding and be in contact with the window frame. The siding on that level consisted of horizontal 1x4 yellow-cedar boards attached to vertical furring strips over the rigid foam. Instead of casing each window, we simply returned the siding into each opening and let it terminate on the window cladding. Because of the built-up insulation, we attached the siding returns with 3 1/2-inch nails.