Weather-Specific Region 3A Building Assembly:

IECC CZ 3A: Vented Attic, 2x6 Wall, Basement with Interior Insulation

• If a raised-heel roof truss is used to allow R-38 attic insulation over the full width of the exterior wall, then R-38 can be used throughout the attic rather than R-49. See Section R402.2.1 of the 2021 IECC for additional information.
• A class III vapor retarder is used on the interior surface of the walls and ceiling instead of a vapor barrier. This allows drying to the interior. Avoid vinyl wallpaper and oil-based paint or coatings in Climate Zone 3. These wall coverings are vapor impermeable and increase the risk of condensation within the wall.
• The rigid insulation where the basement concrete slab intersects the exterior concrete basement wall has two lines of continuous sealant. The first seals the concrete slab edge to the interior surface of the insulation. The second seals the back surface of the rigid insulation to the concrete foundation wall. The two seals provide air control layer continuity between the concrete slab and the concrete wall to control radon and other soil gases.
• The removable unfaced batt insulation installed in the band joist should be cut to fill the entire perimeter cavity for the full width of the foundation wall. This avoids significant thermal bridging through the foundation wall and sill plate. It should be removable to allow for termite inspections.

Window Detail 4 - Framed Wall with Plywood or OSB Sheathing, a WRB, and Siding (Wood, Fiber Cement, Aluminum or Vinyl, or Stucco)

• Note: Always follow the window manufacturer’s installation guidance. Not following manufacturer guidance may void the warranty.
• This is a “drained wall”.  Drainage occurs between the siding and the water control layer or water resistant barrier (WRB) in the gap created by furring which is installed vertically over the water control layer (WRB) at stud locations. In the case of a stucco wall, the drainage plane is a drainage mat rather than furring gaps. 
• The window openings are drained to the water control layer (WRB).
• The rough opening at the window sill is sloped and flashed with a formable flashing.
• The upper portion of the head trim flashing goes under the furring and is taped directly to the WRB with flashing tape. The furring is “cut through” by the head trim flashing, so that the furring below the flashing (behind the head trim) is separate from the furring above the flashing (behind the siding). During construction, the head trim is installed first on short strips of furring. It is flashed directly to the WRB as described above. The rest of the furring is installed with the siding. In the case of a stucco wall, the drainage mat would be treated in similar fashion to furring: the head trim flashing would go under the drainage mat and be taped directly to the WRB with flashing tape.
• Note the gap between the head trim and the top of the window assembly frame. This gap allows water to drain and allows the bottom of the trim to dry out more easily. If installed without this gap, capillary action can draw water into the tight space between the head trim and the window assembly frame. Note also the gap between the siding and the head trim flashing, which serves the same purposes.
• Consider installing rigid head flashing (rigid head flashing is not shown in the schematic). Rigid head flashing is similar to the head trim flashing shown in the schematic, but it goes over the top of the window frame instead of over the head trim. This is required by some manufacturers. It should be installed against the head nailing flange and over the top of the window frame. The vertical and horizontal portion of the flashing should be sealed directly to the window frame and flange with sealant. The red flashing membrane strip shown overlapping the head nailing flange in the schematic would now overlap the rigid head flashing. Use rigid head flashing with a drip edge to guide water away from the window assembly.
• Backer rod for the interior air sealant should be installed after the window is installed, leveled, and shimmed. Use the correct size backer rod.