Transition points in a building assembly are critical locations to get right. They include foundation to above-grade walls, and above-grade walls to roof. It is at these junctures that the continuity of the four control layers is most subject to failure.
This detail shows the transition from the above-grade wall to the roof/ceiling assembly of a home designed to meet Passive House standards. The wall is framed with 2x6s, 24 in. o.c. and is insulated with 4 in. of polyisocyanurate rigid insulation on the exterior. The roof/cathedral ceiling was built with scissor trusses and includes venting through the eaves.
Bulk water is handled with a 2-ft. eave overhang, which will keep a large portion of the wall relatively dry in a typical rain event. (The overhang-to-head ratio is 2:1.) The wall has a 1×3 wood-furring rainscreen on the exterior to assist with drying. Note the drainage outlet/air inlet at the head trim. The exterior is 1×4 open-board siding, the upper edges of which are sloped 10 degrees to the exterior to move water away from the wall.
The exterior air barrier for the wall system is Zip sheathing, which also provides adequate shear strength in this coastal zone. (It is always a “plus” for a single material to perform multiple duties.) The drywall is the interior air barrier for the sloped ceiling. The challenge here is the transition from the wall to the interior drywall. A strip of self-adhered membrane connects the face of the exterior sheathing to the backside of the 3/4-in. plywood. While this may look difficult to construct, it’s not. The wall is framed and sheathed, the plywood is temporarily tacked to the top of the wall, the adhered membrane is installed and rolled, the trusses are installed, and the plywood is nailed off. The drywall, installed on…
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