R-Value Calculator — Assembly R & U-Value
Stack up to three layers, get the assembly R-value and its U-value. Layers add in series, like resistors.
Calculator
Adding the layers in series gives R-16.2, a U-value near 0.0617. R/inch bands are labeled published typicals — actual R depends on install quality, thermal bridging, moisture and settling; follow the data sheet.
Insulation is a resistance problem, not a magic number. Every layer resists heat flow, and in a wall or ceiling those resistances stack in series — exactly like resistors in an electrical circuit. Add them up and you have the assembly R-value; flip it and you have the U-value the energy code actually cares about. This tool does the stacking for up to three layers so you can sanity-check a build-up before you buy a single batt.
Enter each layer’s thickness in inches and its R per inch, then decide whether to add the still-air films on the two faces. The default shows a common retrofit: a 3.5″ fiberglass batt (R-3.2/in) plus 1″ of XPS (R-5.0/in).
Formula
Layers add in series:
R_layer = thickness_in × R_per_inchR_assembly = Σ R_layer (+ 0.68 interior film + 0.17 exterior film, optional)U = 1 ÷ R_total
The 0.68 and 0.17 are the labeled still-air films on the inside and outside faces; include them for a whole-assembly number, drop them to compare bare materials.
Worked example
2×6 wall, batt + rigid. Layer 1: 3.5″ fiberglass batt at R-3.2/in = 3.5 × 3.2 = R-11.2. Layer 2: 1″ XPS at R-5.0/in = 1 × 5.0 = R-5.0. Sum = R-16.2. Add both air films (0.68 + 0.17 = 0.85) and the assembly is R-17.05, a U-value of 1 ÷ 17.05 = 0.0587. That is the whole-cavity number — the wood studs are a separate, lower-R parallel path (see the notes).
Background & practice
Studs are a thermal short-circuit. The R-value above is the insulated cavity only. Framing (2×4 or 2×6 lumber runs about R-1.2/in) crosses the wall on ~15–25% of its area, so the whole-wall R is lower than the cavity R. That is exactly why continuous exterior foam — a layer with no studs through it — buys so much: it insulates the framing too.
Compression kills R. An R-19 batt crammed into a 3.5″ 2×4 cavity performs closer to R-13. Match the batt to the cavity depth, and never staple a fat batt into a thin bay.
What to measure first: the true cavity depth (nominal 2×6 = 5.5″, not 6″), each existing layer, and whether there is already a vapor retarder you should not double up. Air films are small but real — leave them out when you just want to compare two materials head-to-head.
Reference table
| Material | R per inch (labeled) | Inches for R-38 | Inches for R-49 | Field note |
|---|---|---|---|---|
| Fiberglass batt | R3.1–R3.4/in | 11.7 in | 15.1 in | Cheapest; watch gaps and compression |
| Blown fiberglass | R2.2–R2.7/in | 15.5 in | 20.0 in | Loose-fill; settles a little over time |
| Cellulose | R3.2–R3.8/in | 10.9 in | 14.0 in | Denser, less air movement; recycled paper |
| Mineral wool (Rockwool) | R3.0–R3.3/in | 12.1 in | 15.6 in | Fire and water resistant; good acoustics |
| Open-cell spray foam | R3.5–R3.7/in | 10.6 in | 13.6 in | Air-seals; needs more depth for R |
| Closed-cell spray foam | R6.0–R7.0/in | 5.8 in | 7.5 in | High R/inch; adds rigidity, low perm |
| Rigid EPS | R3.6–R4.2/in | 9.7 in | 12.6 in | Lowest-cost rigid board |
| Rigid XPS | R4.5–R5.0/in | 8.0 in | 10.3 in | Mid rigid board; moisture resistant |
| Rigid polyiso | R5.6–R6.5/in | 6.3 in | 8.1 in | Highest rigid R; drops in cold temps |
Labeled published planning typicals — confirm the rated R/inch on your product’s data sheet. Depth uses the band midpoint.