Insulation R-Value Calculator — Total R, U-Value and RSI

Real-world use cases

• Check a wall assembly against your climate-zone code minimum

  Your code requires R-20 walls. You are building 2x6 framing with 5.5 inches of fiberglass batt and want to know if you clear it. Add a layer for the batt, one for the half-inch gypsum board inside, and one for the half-inch foam sheathing outside. The running total tells you on the spot whether you are over or under R-20, before you order a single bag of insulation.

• Compare two insulation products by R per dollar

  A 2-inch polyiso board claims R-12; a 3.5-inch fiberglass batt claims R-13. They cost different amounts and take different cavity depths. Enter each as a single layer, read the R-value, and pair it with the price you were quoted. Now you can rank them by R per dollar and by R per inch of wall thickness, which matters when cavity depth is fixed by the framing you already have.

• Translate a US product label into a metric RSI for a permit

  You bought insulation rated in US R-value but your local building department works in RSI. Enter the assembly, and the tool shows RSI next to every R figure. A wall at R-22.7 reads as RSI 4.0, which is the number the permit reviewer expects, so you copy it straight into the application instead of fishing for a conversion factor.

• Plan an attic top-up to reach an R-49 target

  You have R-19 of old batt in the attic and the recommendation is R-49. Add the existing R-19 as one layer, then add blown cellulose at about R-3.7 per inch as a second layer and adjust the depth until the total hits R-49. The tool tells you that roughly 8 inches of new cellulose closes the gap, so you know how many bags to buy.

• Teach the R-value and U-value relationship in a class

  Explaining building science to students or a crew? Build a simple two-layer wall live, watch the total R climb as each layer is added, then point at the U-value updating to one over R in real time. The shareable URL lets every student reopen the exact example on their own device and tweak the thickness to see the numbers move.

Common pitfalls

• Multiplying R-values of layers instead of adding them. Layers in series add, they do not multiply. A 0.45 board plus an R-19 batt is R-19.45, not 8.55. The tool sums correctly, but if you check by hand, add every layer including the thin facings.

• Forgetting to convert when a number is actually RSI. A European label saying 4.0 is RSI, which equals about R-22.7 in US units, not R-4. Mixing the two systems makes a well-insulated wall look ten times too weak. Confirm which unit a figure is in before you compare.

• Treating the calculated total as the real whole-wall R-value. The total is the clear-field, center-of-cavity figure. Wood studs bridge heat at only about R-1.25 per inch, so the effective whole-wall R is often 10 to 30 percent lower. For code-critical work, account for framing factor separately.

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