Wall insulation is specified by cavity R-value — R-13 or R-15 in a 2×4 wall, R-19 to R-21 in a 2×6. Those numbers describe the batt. They do not describe the wall.
Framing factor
A typical wood-framed wall is 20 to 25% framing by area once you count studs, plates, headers, corners and window bucks. Wood is about R-1.25 per inch, so a 2×6 stud is roughly R-6.9 — against R-21 of insulation beside it.
Heat takes the easy route. Those studs are a continuous thermal bridge from inside to outside, and they carry far more heat per square foot than the insulated cavity does.
The correct method is the parallel-path calculation from ASHRAE: work out the U-factor of each path, area-weight them, and invert.
| Wall | Cavity R | Effective R at 23% framing |
|---|---|---|
| 2×4, R-13 | 13 | ~10.5 |
| 2×4, R-15 | 15 | ~11.6 |
| 2×6, R-19 | 19 | ~14.6 |
| 2×6, R-21 | 21 | ~15.8 |
| 2×6, R-21 + R-5 continuous | 21 | ~20.5 |
The last row is the point. Adding R-5 of continuous exterior foam to an R-21 wall gains almost 5 effective points, because it covers the studs too. Adding R-4 more inside the cavity would gain about one.
Code, by zone
IECC R402.1.3 wall minimums: Zone 3 — R-20 cavity or R-13 + R-5 continuous. Zone 4 — R-20 or R-13+5. Zone 5 — R-20 or R-13+5. Zones 6–8 — R-20+5 or R-13+10.
Notice the code offers the continuous-insulation option as equivalent to a higher cavity number. That equivalence exists precisely because of the framing factor.
Air sealing still comes first
An R-21 wall with unsealed penetrations loses more heat through air movement than the difference between R-15 and R-21 batts. Seal the bottom plate, the top plate, every wire and pipe penetration, and around window and door bucks before insulating.
Batts have to fill the cavity
Compressed insulation loses R-value roughly in proportion to how much it is squeezed. Batts stuffed behind wiring instead of split around it, or crammed into a narrow bay, deliver less than the label.
Gaps are worse than compression. A cavity 95% filled loses far more than 5% of its performance, because the empty 5% convects.
Work the numbers for your wall on the wall insulation calculator — it does the parallel-path arithmetic rather than quoting the batt.