Weather-Specific Region 3A Building Assembly:

IECC CZ 3A: Unvented Attic, 2x4 Wall with Exterior Insulation, Unvented Crawlspace

• The attic insulation in this unvented attic is located on the underside of the roof sheathing. To prevent moisture issues at the roof deck, follow the requirements of 2021 IRC Section R806.5. One of the following vapor-control approaches can be used:
  • Use air-impermeable insulation such as spray polyurethane foam (open cell or closed cell) or rigid foam board sealed around the edges with spray foam.
  • Install airtight vapor diffusion ports at the ridge, allowing air-permeable insulation such as cellulose or fiberglass to be used. 
  • Install minimum R-5 air-impermeable insulation against the roof deck, allowing air-permeable insulation such as cellulose or fiberglass to be used to make up the rest of the total R-value requirement of R-49. The air-impermeable insulation can be spray polyurethane foam or rigid foam board (with joints sealed), installed against the underside of the roof decking or on the exterior side of the roof decking.
  • Install a vapor-permeable roof underlayment and vapor-permeable roofing (such as tile), allowing air-permeable insulation such as cellulose or fiberglass to be used.
  • Install dehumidification in the attic space.
• Unvented attics are more resistant to wind uplift under hurricane conditions.
• 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.
• The water control layer for this wall assembly is between the rigid foam insulation and the sheathing. The water control layer should be taped at all seams and have all penetrations properly air sealed and flashed. This drawing shows a drainable housewrap used as the water control layer, which allows the assembly to dry more readily if water gets behind the rigid foam. 
• 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 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.
• The rigid insulation on the interior of the masonry foundation wall should provide a view strip at the top of the wall for termite inspections. This area should be filled with removable rigid insulation to allow for continuous insulation. This removable strip can be taped along the seam to create a hinge for easy inspection.