2021 IECC Climate Zone 3C Building Assembly: Vented Attic, 2x6 Wall, Monolithic Slab on Grade

Drawing 3C.3: IECC CZ 3C:  Vented Attic, 2x6 Wall, Monolithic Slab on Grade

• 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.
• The rigid foam on the exterior of the wall is not necessary to meet energy efficiency code or to control vapor. It allows construction without using structural sheathing: alternative methods of wall bracing are used instead, and the rigid foam provides a backstop for the cavity insulation as well as some structural support to the siding. Many contractors have found this to be a more economical approach. It is possible to simply use well-installed house wrap instead of the rigid foam, but this would require siding that can span between studs without intermediate backing. Another option for this wall would simply be to use structural sheathing (OSB or plywood) in place of the rigid foam, with house wrap on the exterior of the sheathing.
• The wall is framed with 2x6 24” o.c. advanced framing as it uses less board footage (volume of wood framing) than standard 2x4 16” o.c. framing and therefore is less expensive in material cost. It is also less expensive in labor (25% fewer framing members) and is faster to assemble. Although the code allows for 2x4 framing and lower R-value cavity insulation, the use of advanced framing and the associated cost savings more than offset the increase in the cost of cavity insulation (this of course depends on relative price fluctuations in labor and materials). 
• Although the minimum R-value of the insulation in this wall is R-13 per IECC, fire code may require the entire cavity to be filled, likely resulting in a higher R-value for a 2x6 wall.
• The plastic L-bracket at the bottom of the above-grade wall should be perforated to facilitate drainage.
• A class III vapor retarder (latex paint) 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.
• In high-risk termite regions, installing rigid insulation on the top of the concrete slab prevents migration of termites through perimeter insulation.
• The entire monolithic concrete slab should have a polyethylene vapor barrier wrapping the underside of the slab and footing to control capillary uptake. The polyethylene should extend upward from the bottom of the footing to grade on the exterior.

Window Detail 6 - Framed Wall with Rigid Insulation and Siding (Wood, Fiber Cement, Aluminum or Vinyl)

• Note: Always follow the window manufacturer’s installation guidance. Not following manufacturer guidance may void the warranty.
• This is a “drained wall”. The exterior face of the rigid insulation is carefully taped at all seams, allowing it to act as the water control layer. Drainage occurs between the siding and the exterior face of the rigid insulation. A drainage gap is provided by furring installed vertically over the exterior face of the rigid insulation at stud locations.
• The window openings are drained to the exterior face of the rigid foam since it is acting as the water control layer.
• The rough opening at the windowsill is sloped and flashed to the rigid foam with a formable flashing.
• The upper portion of the head trim flashing goes under the furring and is taped directly to the rigid foam 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 pieces of furring. It is flashed directly to the rigid foam as described above. The rest of the furring is installed with the siding.
• A formable flashing membrane connects the rigid foam to the wood frame. It should extend inwards at least past the point where the interior air seal is located around all sides of the window frame. This creates water-resistant surfaces inside the cavity between the window flanges and the interior air seal, in case any water gets behind the flanges. The head and jamb window flanges are flashed to the rigid foam using a self-adhered flashing membrane. The sill window flange is not flashed, to allow any water that may get behind the flanges to escape.
• This assembly relies heavily on the use of proper tapes, flashings, and membranes that will adhere securely and durably to the rigid foam. Products should only be used if they have been well-established to be appropriate for this application.
• 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.