R-Value Calculator

R-value measures how well a layer resists heat flow, so a higher number means better insulation. U-factor is its inverse (U = 1 / R), describing the rate of heat loss per square foot per degree. R-values of layers in series add up, while the U-factor sums the whole-wall heat loss, so a total of R-12 is a U-factor of about 0.083.

Wood studs conduct heat faster than the insulation between them, creating a thermal bridge. This calculator uses the ASHRAE parallel-path method: it finds the R-value through the insulated cavity and through a stud, then area-weights their U-factors by the framing factor. A 2x4 wall with R-13 batts at 16 inches on center (about 25 percent framing) falls from a nominal R-15 to an effective R-12.

The framing factor is the share of the wall area taken up by studs, plates, and headers. Use about 25 percent for standard 2x4 or 2x6 framing at 16 inches on center, around 22 percent for 24 inches on center, and 15 to 18 percent for advanced (optimized) framing. Walls with many windows, corners, or doors sit at the higher end.

When you include air films, the calculator adds the standard ASHRAE surface resistances: R-0.68 for the still interior air and R-0.17 for the exterior surface in a 15 mph winter wind. Together they add R-0.85 to the assembly. Turn them off when you only want the R-value of the solid material layers.

Typical US values are about R-0.61 for vinyl siding, R-0.51 for 7/16 inch OSB sheathing, R-0.45 for 1/2 inch drywall, and roughly R-3.5 per inch for fiberglass batt insulation. A wood stud adds about R-1.25 per inch, so a 3.5 inch 2x4 stud is about R-4.4. Continuous foam sheathing can add R-3 to R-6 and cuts thermal bridging because it covers the studs too.

It is a solid planning estimate, not a stamped energy-code calculation. The parallel-path method reads slightly higher than the more conservative isothermal-planes method that some codes require, and it does not model windows, corners, or metal framing. Use it to compare assemblies and size insulation, then confirm final numbers with a full energy model or your local code official.