How to calculate the R-value of a wall or ceiling
A wall's R-value is just the sum of its layers — the exact same math as resistors in series. Add each layer's thickness × R-per-inch, drop in the air films, and you have the assembly R. Here's how.
Layers add in series
Thermal resistance behaves like electrical resistance: stack layers and their R-values add. So R-assembly = Σ (thickness × R-per-inch) for every layer, plus the two air films that always sit on a wall — the interior surface film (~0.68) and the exterior film (~0.17). Do it layer by layer in the R-value calculator.
Worked example: a 2×6 wall
Take a 2×6 wall with a 3.5-inch fiberglass batt (R-3.2/in) in a 5.5" cavity plus 1 inch of XPS (R-5.0/in) on the outside:
| Layer | Thickness | R/inch | R |
|---|---|---|---|
| Fiberglass batt | 3.5 in | 3.2 | 11.2 |
| XPS rigid foam | 1.0 in | 5.0 | 5.0 |
| Interior air film | — | — | 0.68 |
| Exterior air film | — | — | 0.17 |
| Assembly | R-17.05 |
The insulating layers alone are R-16.2; with both air films it's R-17.05. Its U-value is U = 1 ÷ 17.05 = 0.0587. Air films are small but real — leave them off for a quick core check, include them when you're comparing to a code U-factor.
U-value: the flip side of R
Some codes and window specs are written in U-value (conductance) instead of R. They're reciprocals: U = 1 ÷ R and R = 1 ÷ U. R-20 is U-0.050; a U-0.077 assembly is R-13. Convert either way in the U-value ↔ R-value converter. Lower U = better; higher R = better.
The catch: thermal bridging through the studs
The layer math gives you the R through the insulation. But wood studs are only about R-1.25 per inch, so heat shortcuts through the framing. A wall is roughly 15–25% framing by area, so the whole-wall R is a weighted average of the cavity path and the stud path — usually 10–20% below the center-of-cavity number. This is exactly why continuous exterior foam is so effective: it covers the studs too, so there's no bridge to shortcut through.
Get the R/inch right for each material
Layer math is only as good as your R/inch inputs. Use labeled bands, not memory: fiberglass batt ~3.1–3.4, cellulose ~3.2–3.8, closed-cell foam ~6.0–7.0, EPS ~3.6–4.2, XPS ~4.5–5.0, polyiso ~5.6–6.5. Full list in the R/inch by material reference and the R/inch table. Note polyiso's R/inch drops in cold weather — derate it for exterior walls in cold zones.
What to measure before you calculate
- Actual cavity depth (nominal 2×6 is 5.5", not 6").
- Each layer's real thickness — compressed batts lose R.
- The rated R/inch on each product's data sheet, not a rounded rule of thumb.
- Whether you need center-of-cavity R (for a quick check) or whole-wall R (for code / energy modeling).
Parallel paths: the whole-wall R the right way
Layers in series add; framing runs in parallel with the cavity, and that's a different calculation. To get an area-weighted whole-wall R, work in U (conductance) for each path, blend by area fraction, then flip back. Take a 2×6 wall, ~23% framing: the cavity path is R-21 (U-0.0476), the stud path is roughly 5.5" × 1.25 = R-6.9 plus sheathing/films ≈ R-8.5 (U-0.118). U_wall = 0.77 × 0.0476 + 0.23 × 0.118 = 0.0637, so R_wall = 1 ÷ 0.0637 ≈ R-15.7 — well under the R-21 you'd read at the cavity. That gap is exactly the money continuous exterior foam buys back. Use the U↔R converter to move between the two.
Where each path's numbers come from
Don't forget the layers that aren't insulation but still count: gypsum board ~R-0.45 for 1/2", OSB/plywood sheathing ~R-0.5–0.6, housewrap and siding a little more. They're small individually but real on a bare-stud wall. And the air films flip with orientation: a wall uses the ~0.68 interior / ~0.17 exterior films, but a ceiling with heat flowing up has a smaller interior film (~0.61), and a floor with heat flowing down a larger one (~0.92). For a code U-factor comparison, include the films for the correct orientation; for a quick material-only check, leave them out and note it.
Common R-value mistakes on site
- Reading nominal instead of installed. A high-density R-15 batt in a 2×4 is real R-15; a standard R-19 crammed into the same 3.5" is ~R-13. Buy the batt made for the cavity.
- Trusting polyiso's label in the cold. Its R/inch sags below ~40°F — derate it on cold-side exterior foam.
- Ignoring the framing. Center-of-cavity R flatters the wall by 10–20%. Quote whole-wall R for anything that matters.
- Double-counting air films. Interior and exterior films once each — not per layer.
Quick numbers to leave with
- Series: R-assembly = Σ (thickness × R/inch) + 0.68 interior film + 0.17 exterior film.
- Convert: U = 1 ÷ R; R = 1 ÷ U. R-20 = U-0.050; U-0.077 = R-13.
- Example: 3.5" batt (R-11.2) + 1" XPS (R-5) + films = R-17.05, U-0.0587.
- Non-insulation layers count: 1/2" gypsum ~R-0.45, OSB sheathing ~R-0.5.
- Whole-wall < cavity R by 10–20% from stud bridging — continuous exterior foam closes the gap.
Add the layers, add the films, flip to U if the spec asks for it — that's the whole method.