2021 IECC Climate Zone 4C Building Assembly: Unvented Roof, 2x6 Wall, Vented Crawl Space

Drawing 4C.9: IECC CZ 4C: Unvented Roof, 2x6 Wall, Vented Crawl Space

• This unvented roof assembly should not be used where the ground snow load is greater than 50 lb/ft2 (293 kg/m2). Vented roofs should be used instead to control ice damming.
• A low-permeance roofing underlayment (less than 1 perm) is recommended for this roof type in this climate to reduce water diffusion through the underlayment to the sheathing.

• The rigid foam insulation at the roof deck is required to control condensation and to meet energy efficiency code (2021 IECC). There is no interior vapor barrier – there is a Class III vapor “retarder” (semi-permeable latex paint). The reason that there is no interior vapor barrier is to permit drying to the interior.
• The roof deck rigid foam R-value required for vapor control depends on the R-value of the cavity insulation. In Climate Zone 4, the R-value of the rigid foam should be at least R-15 per Table 806.5 of the 2021 IRC. However, if a higher amount of cavity insulation were to be used, then a higher R-value rigid foam would need to be used to control vapor. DOE-funded research has shown that the R-value of the rigid foam should be at least 20% of the total R-value in Climate Zone 4C. This drawing shows R-20 rigid insulation and R-40 cavity insulation. This more than meets the vapor control requirement: (20 / (40+20) = 33%). Regardless of which type of cavity insulation is used, there must not be a gap between the cavity insulation and the sheathing/rigid foam above it, per Section 806.5 of 2021 IRC. The combined R-value of the rigid foam and the cavity insulation must be at least R-60.
• Note that this assembly does not have an attic (vented or unvented). If the assembly did have an attic space, Section R402.2.1 of the 2021 IECC may apply, depending on the interpretation of the local code official. If so, R-49 could be used over the attic rather than R-60 as long as the R-49 insulation extended over the full width of the exterior wall top plate.
• 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 has therefore been shown to be less expensive in material cost. It has also been proven to be less expensive in labor (25% fewer framing members) and is faster to assemble. This must be balanced with the difference in insulation costs for the two framing approaches.
• The R-5 rigid insulation on the exterior of the wall framing is not only required per 2021 IECC, but it is also recommended to control condensation within the framing cavities as there is no interior vapor barrier – there is a Class III vapor “retarder” (semi-permeable latex paint). The reason that there is no interior vapor barrier is to permit drying to the interior.
• The rigid foam on the exterior of the wall 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.
• The plastic L-bracket at the bottom of the wall’s exterior rigid insulation should be perforated to facilitate drainage.
• The R-10 rigid insulation on the underside of the floor framing is also a vapor barrier that prevents moisture from the vented crawlspace from damaging the floor framing and floor finishes. The protection board is necessary to prevent rodents and other animals from damaging the assembly.

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.