Overall R-value and U-factor of a layered wall assembly, with framing and air films.
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Assembly Layers (add up to 8 layers)
Typical ≈25% at 16 in. o.c. incl. plates and headers
Tick Cavity on the layer installed between the studs. Its thickness is the stud depth. Wood framing replaces it with solid wood in the framing path; steel framing derates it to the ASHRAE 90.1 Appendix A effective R.
Assembly Thermal Performance
Clear-Field R-Total
—
hr·ft²·°F/BTU
Framing Path R-Total
—
hr·ft²·°F/BTU
Overall Assembly R
—
hr·ft²·°F/BTU
Overall U-Factor
—
BTU/hr·ft²·°F
Compare With Your Energy Code Maximum
ASHRAE 90.1-2022 sets prescriptive above-grade wall maxima by climate zone (Tables 5.5-0 through 5.5-8) and by construction class — mass, metal building, steel-framed, and wood-framed and other — with separate nonresidential, residential, and semiheated columns; the IECC has its own table (C402.1.4). Enter the value that applies to your project. The assembly U-factor should be built the way the code assumes — 90.1 Appendix A gives its default assemblies, including the steel-stud effective R used here.
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This wall assembly U-value calculator builds a multi-layer wall from a material library, includes the inside and outside air films, and applies a framing correction to return the overall R-value and U-factor. Add up to eight layers, tick the layer that fills the stud cavity, and choose wood studs (with a framing fraction), steel studs (with a stud spacing), or no framing; the tool reports the clear-field path, the framing path or effective cavity R, and the combined effective performance.
The overall U-factor is what energy codes regulate, so you can enter the maximum from your adopted code (ASHRAE 90.1-2022 Tables 5.5-0 through 5.5-8, or the IECC) and see whether the assembly meets it.
Formula & Method
Quantity
Equation
Layer R-value
R‑layer = R-per-inch × thickness (in)
Clear-field R
R‑clear = Σ R‑layer (series, incl. air films)
Wood studs (parallel path)
R‑eff = 1 ÷ [ ff ÷ R‑frame + (1 − ff) ÷ R‑clear ]
Steel studs (90.1 App. A)
R‑eff = R‑clear − R‑cavity + R‑cavity,effective
Overall U-value
U = 1 ÷ R‑eff
Layer resistances add in series for each heat-flow path. For wood studs the framing path swaps the cavity layer for solid wood of the same depth (R/inch you enter, softwood ≈1.25), and the two paths combine by the parallel-path (area-weighted) method of ASHRAE Fundamentals Ch. 27, where ff is the framing fraction. Parallel path is not valid for steel studs, which short-circuit the cavity; for steel the cavity insulation counts at its effective R from ASHRAE 90.1 Appendix A Table A9.2-2 (R-11/13/15 in 3.5 in. studs, R-19/21 in 6 in., R-25 in 8 in., at 16 or 24 in. o.c., interpolated between those batts) and the layers add in series. U is the reciprocal of the effective R. Air films (0.17 outside, 0.68 inside) and material resistances are typical ASHRAE Fundamentals Ch. 26 values; product data governs a submitted design.
Frequently Asked Questions
How is the R-value of a wall assembly calculated?
For a single heat-flow path, the layer R-values add in series: total R is the sum of each layer's R-value, including the inside and outside air films. A material layer's R-value is its R-per-inch times its thickness. The U-value is the reciprocal of the total R. This tool sums the layers you add and also computes a separate R-value for the path through the framing.
Why does framing reduce the wall's effective R-value?
Studs are a thermal bridge: lumber has a much lower R-per-inch than cavity insulation, so heat short-circuits through the framing. For wood studs this tool uses the parallel-path method, combining the clear-field path and the framing path weighted by the framing fraction, so the effective U-value reflects both; a typical wood-stud wall loses roughly 20 to 30 percent of its cavity insulation R to framing at 15 to 25 percent framing. Steel studs conduct so well that the parallel-path method badly overstates R, so for steel the tool uses the ASHRAE 90.1 Appendix A effective R instead, which counts an R-13 batt in 3.5 inch studs at 16 inches on center as about R-6.
What is the difference between R-value and U-value?
R-value measures thermal resistance, so higher is better insulation, while U-value measures thermal conductance, so lower is better. They are reciprocals: U equals 1 divided by R. Energy codes such as ASHRAE 90.1 set maximum U-factors for assemblies by climate zone and construction class, which is why this tool reports the overall U and lets you compare it with the maximum from your adopted code.
Do I need to include air films in the calculation?
Yes. The inside and outside surface air films add real thermal resistance, about 0.68 hr-ft2-F/BTU inside and 0.17 outside for a vertical wall with horizontal heat flow. Including them is standard practice and matches how ASHRAE 90.1 assembly U-factors are derived; the default assembly here includes both film layers, so keep them when you edit the wall.
Connect the calculation to the rest of the project.
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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.