Assembly R-Value Calculator — nominal vs effective
See what a framed wall actually delivers once the studs bridge the insulation, using the parallel path method with a framing factor.
Adding up the R-values on the labels gives you the number in the cavity, not the number the wall actually delivers. The studs are a thermal bridge, and this shows what they cost you.
Quick answer: label R-value against effective R
| Wall | Label math | Effective | Lost to studs |
|---|---|---|---|
| R-13 in 2×4, 16 in. o.c. | R-15.3 | R-11.6 | 24 % |
| R-15 in 2×4, 16 in. o.c. | R-17.4 | R-12.4 | 28 % |
| R-21 in 2×6, 16 in. o.c. | R-23.4 | R-16.9 | 28 % |
| R-21 in 2×6, 24 in. o.c. | R-23.4 | R-17.5 | 25 % |
| R-21 in 2×6, 24 in. advanced | R-23.4 | R-18.8 | 20 % |
| R-13 in 2×4 + R-5 continuous | R-20.4 | R-17.2 | 16 % |
Parallel-path method, as the calculator: wood at R-1.25 per inch, air films 0.68 + 0.17, R-1.5 of other layers, framing factors 25 % (16 in.), 22 % (24 in.), 16 % (24 in. advanced). Continuous insulation is the one layer the studs cannot bypass.
| Nominal (label math) | — |
|---|---|
| Path through cavity | — |
| Path through stud | — |
| Effective assembly R | — |
| Assembly U-factor | — |
| Lost to thermal bridging | — |
| Metric (RSI / U) | — |
The code asks for a label R, and the label R is the one the studs bypass. 2018 IRC Table N1102.1.2 (the residential energy chapter, identical to IECC R402.1.2) prescribes wood-frame wall insulation by climate zone; "13 + 5" means R-13 in the cavity plus R-5 continuous. Run R-13 at 16 in. o.c. through the calculator and the assembly delivers about R-11.6; the R-5 continuous layer the colder zones add is the one layer the framing cannot bridge, which is why the table reaches for it before it asks for a thicker cavity.
Prescribed insulation, 2018 IRC Table N1102.1.2
| Climate zone | Wood-frame wall R | Ceiling R |
|---|---|---|
| 1 | 13 | 30 |
| 2 | 13 | 38 |
| 3 | 20 or 13 + 5 | 38 |
| 4 (except Marine) | 20 or 13 + 5 | 49 |
| 5 and Marine 4 | 20 or 13 + 5 | 49 |
| 6 | 20 + 5 or 13 + 10 | 49 |
| 7 and 8 | 20 + 5 or 13 + 10 | 49 |
The table is prescriptive, and this page is not a compliance path. The code also allows a U-factor alternative and whole-house performance; those use their own tested assembly values, not this parallel-path estimate. Use the calculator to see where the label R goes; use the code's own tables, and your jurisdiction's amendments, to file.
What I check on a job
Run the wall twice before quoting it. Before I quote a wall's R-value to anyone, I run it twice: once at the framing factor the wall is drawn to, and once at the worst one in the list. R-21 in a 2×6 comes out at R-18.8 at 16% and R-16.9 at 25%, and that spread is an honest picture of how little I know until the framing is up. Then I look at the actual elevation. Every window brings jacks and a header, so a wall with many openings sits nearer the high factor than its stud spacing suggests; the factors in the list, RESNET 301's 25% included, are whole-wall averages, not a reading of my wall. I also replace the 1.5 in other layers with the sheathing, gypsum board and cladding I am actually specifying, because the default is a placeholder. Last, I type R-5 into the continuous box and keep the result next to the bare wall: the same 2×6 at 25% reads R-22.7 against R-16.9.
Method — parallel path (isothermal planes are not used here).
Uassembly = fcavity×Ucavity + fframing×Ustud,
then Reffective = 1 / Uassembly. Softwood framing is taken at R 1.25 per inch,
so a 5-1/2" stud contributes about R 6.9 where the cavity beside it has R 21. Air films
use the standard still-air interior 0.68 and moving-air exterior 0.17.
Framing factors are approximations for a whole wall including plates, corners and headers —
a real wall varies. Continuous exterior insulation is the one layer the studs cannot bridge, which is
why a small R on the outside often beats a large R in the cavity. Energy code compliance calculations
have their own prescribed methods; use this to understand the assembly, not to submit it.
Worked example
R-21 batts in a 2×6 wall framed at 16" on centre: what does the wall actually deliver?
- Cavity path: R-21 + other layers 1.5 + air films 0.85 = R-23.35.
- Stud path: wood at about R-1.25 per inch over 5.5 in = 6.9, plus the same 1.5 and 0.85 = R-9.2.
- Parallel paths at 25% framing: 1 ÷ (0.75/23.35 + 0.25/9.2) = R-16.9.
R-16.9 effective, U-0.059 — not R-21. Take the framing out (0% factor) and it reads R-23.3; the studs cost the wall a quarter of its label.
6 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
Why is the wall's R-value less than the insulation's?
Because the studs are a parallel path with a lower R-value than the cavity. At 25% framing, R-21 batts in a 2×6 wall deliver about R-16.9 effective.
What is a framing factor?
The share of the wall area that is framing rather than cavity. 25% is typical for 16-inch on-centre standard framing; advanced framing at 24 inches is nearer 16–19%.
What wall insulation does the IRC prescribe for my climate zone?
2018 IRC Table N1102.1.2: R-13 in zones 1 and 2; R-20 or 13 + 5 in zones 3 to 5; R-20 + 5 or 13 + 10 in zones 6 to 8, where "+ 5" is continuous insulation. Ceilings go from R-30 to R-49. It is prescriptive; your jurisdiction's adopted code and amendments govern.
What does R-13 at 16 in. on centre actually deliver?
About R-11.6 for the assembly by the parallel-path method this page uses, because the studs bypass the cavity. That is why the colder zones add R-5 continuous outside: it is the one layer the framing cannot bridge.