How this works
R-value is the resistance to heat flow of a material or assembly — higher means better insulation. The R-value of a complete wall, roof or floor is the sum of the R-values of each layer in series, from inside to outside. The math is genuinely simple: total R = R₁ + R₂ + ... + Rₙ. The complications are knowing the R-per-inch of each material (which varies by product within a category) and remembering to include the small layers — drywall, sheathing, exterior cladding, even still air films — that add up to a meaningful contribution.
A few common R-per-inch values for residential materials. Fiberglass batt: R-3.1 to R-3.4 per inch (R-13 standard for a 2×4 wall, R-19 or R-21 for a 2×6 wall). Mineral wool batt: R-3.7 to R-4.2 per inch — slightly higher than fiberglass and increasingly popular for fire and acoustic performance. Cellulose blown-in: R-3.6 per inch, common in attics. Open-cell spray foam: R-3.5 to R-3.7 per inch, expands to fill cavities. Closed-cell spray foam: R-6 to R-7 per inch, the highest R-per-inch in common use, also acts as a vapour barrier. Rigid foam boards range from R-3.8 (EPS) to R-6.5 (polyiso) per inch, used as continuous exterior insulation outside the framing layer.
Three practical points. (1) Thermal bridging matters in real walls. The studs in a 2×4 wall are wood at about R-1 per inch, so the stud (R-3.5) is much less insulating than the cavity insulation (R-13 fiberglass). About 25% of the wall area is studs in standard framing, so the "effective" assembly R is about 75% of the cavity-only number. The calculator above gives you the simple sum (clear-wall R, with no thermal bridging); subtract roughly 15-25% for a realistic whole-wall R, depending on your stud spacing and corner/header detailing. Continuous exterior insulation (rigid foam outside the studs) bypasses thermal bridging entirely. (2) Climate-zone codes set minimum R-values. US IRC zones 5-7 (Northeast/Midwest/Mountain) typically require R-20 walls and R-49 attics. Zone 3 (Southern US) requires R-13 walls and R-30 attics. EU codes are more variable and tend to specify U-values rather than R; a typical Passive House standard wants U ≤ 0.15 W/m²·K, equivalent to RSI ≥ 6.7 or imperial R ≥ 38. Check your local building code for the actual minimum — going above is usually a good call given energy prices. (3) R-value is for steady-state conduction; real heat loss includes air leakage, radiation through windows, thermal bridging at joints, and moisture-transport heat losses. R doesn't capture any of those. Air-sealing first, then insulating, gives you far more bang for your buck than chasing the last R-3 of insulation.
The formula
Each row in the calculator above is one layer; pick a material and a thickness, and the calculator computes that layer's R-value from the per-inch value. R-per-inch values used: fiberglass batt 3.2, mineral wool 3.9, cellulose 3.6, open-cell foam 3.6, closed-cell foam 6.5, EPS 3.8, XPS 5.0, polyiso 6.0, plywood 1.25, stud framing 1.0. Drywall (1/2") and a still air film are fixed-R contributions. The result is the "clear wall" R-value — to estimate whole-wall R including thermal bridging at studs, multiply by ~0.75 for typical 2×4/2×6 framing.
Example calculation
- 2×4 wall: 1/2" drywall + R-13 fiberglass batt (3.5") + 1/2" plywood sheathing + still air film.
- Layers: 0.45 + 13 + 0.625 + 0.68 = R-14.76 total clear-wall R.
- U-factor = 1 / 14.76 ≈ 0.068 BTU/hr·ft²·°F. RSI = 14.76 / 5.678 ≈ 2.60 m²·K/W.
- With ~25% thermal bridging at studs, effective whole-wall R ≈ 14.76 × 0.75 ≈ R-11. Below modern code in cold climates.
Frequently asked questions
How do I convert between imperial R and metric RSI?
Divide imperial R by 5.678 to get RSI (m²·K/W); multiply RSI by 5.678 to get imperial R. Quick reference: imperial R-13 ≈ RSI 2.29, R-19 ≈ 3.35, R-30 ≈ 5.28, R-49 ≈ 8.63. Or rounded: imperial R ≈ RSI × 5.7. The factor comes from the unit conversion between BTU/hr·ft²·°F and W/m²·K. Both numbers are physically equivalent — they describe the same insulation, just in different unit systems. The calculator reports both side by side so you don't have to do the conversion mentally.
What R-value do I actually need?
Depends on climate and code. The US IECC/IRC sets minimum R-values by climate zone: Zone 1 (south Florida, Hawaii) needs R-13 walls and R-30 attics; Zone 4 (mid-Atlantic, mid-South) needs R-13 walls and R-49 attics; Zone 5 (Northeast, Midwest) needs R-20 walls and R-60 attics; Zones 6-8 (cold/Alaska) need R-21 to R-30 walls and R-60+ attics. UK Building Regs require U ≤ 0.18 W/m²·K for new walls (≈ RSI 5.6, ≈ R-32). German EnEV/GEG demands similar with stricter recent updates. Passive House targets U ≤ 0.15 (RSI 6.7, R-38). The numbers are minimums; in cold climates with high energy prices, going 50-100% above code typically pays back within 5-10 years on heating/cooling costs. The calculator reports your assembly's R against simple typical-climate benchmarks.
Why does the cavity R not equal the wall R?
Because of thermal bridging through the studs (or rafters in a roof, or joists in a floor). The studs in a wood-framed wall conduct heat much faster than the insulation between them — wood is roughly R-1 per inch, fiberglass batts are R-3.2 per inch. With 16-inch on-centre stud spacing and 3.5-inch deep cavities, about 25% of the wall area is studs at R-3.5 and the rest is insulation at R-13. Combined as parallel paths, the effective R is around R-10 to R-11 — significantly less than the cavity-only R-13. This is why "continuous insulation" outside the studs (rigid foam board) is now standard in code-compliant new construction in cold climates: it bypasses the thermal bridge entirely and adds straight to the cavity R. The simple-sum R the calculator shows is the cavity R; multiply by ~0.75 to estimate the realistic whole-wall effective R for typical wood framing.