What you're looking at
A section through a 2x4 wood-framed exterior wall with R-15 batt insulation in the stud cavity plus R-4 continuous insulation on the sheathing side. The cool coastal climate zones (CZ3, CZ5, CZ6, CZ7) use this lighter assembly because their heating + cooling loads are lower than the inland zones.
How this compares to the 2x6 variant
| | This wall | 2x6 variant | |---|---|---| | Stud size | 2x4 @ 16" OC | 2x6 @ 16" OC | | Cavity R | R-15 | R-21 | | CI on sheathing | R-4 | R-5 | | Total R | R-19 | R-26 | | Where it's prescriptive | CZ3, CZ5, CZ6, CZ7 | Everywhere else |
The 2x4 cavity has less depth and lower batt capacity, so the cavity R-value drops from R-21 to R-15. The CI also drops slightly (R-4 vs R-5) because the lower heating load doesn't justify the extra thickness.