Build a multi-layer wall assembly, apply framing correction, and get overall R-value and U-value. Compare against ASHRAE 90.1-2019 prescriptive limits for all 8 climate zones.
U-values from ASHRAE 90.1-2019 Tables 5.5-1 through 5.5-8, nonresidential. “Comply” means your assembly U ≤ the maximum allowed. Consult the actual standard for exceptions and alternate paths.
Inputs, results, and method — opens in Excel or Sheets.
This wall assembly U-value calculator builds a multi-layer wall from a material library, adds the inside and outside air films, and applies a framing correction to return the overall R-value and U-value. Add up to eight layers, set the framing fraction, stud depth, and the stud and cavity insulation R-per-inch, and the tool reports the clear-field path, the framing path, and the combined effective performance.
The overall U-value is what energy codes regulate, so the page also checks your assembly against ASHRAE 90.1-2019 prescriptive limits for all eight climate zones and for wood-frame, metal-frame, and mass-wall categories.
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)
Effective R (parallel)
R‑eff = 1 ÷ [ ff ÷ R‑frame + (1 − ff) ÷ R‑clear ]
Overall U-value
U = 1 ÷ R‑eff
Layer resistances add in series for each heat-flow path; the assembly combines the clear-field and framing paths by the parallel-path (area-weighted) method, where ff is the framing fraction. U-value is the reciprocal of the effective R. Air-film and material resistances follow ASHRAE Fundamentals values, and the climate-zone limits are from ASHRAE 90.1-2019 Tables 5.5-1 through 5.5-8 (nonresidential).
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. 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-frame wall loses roughly 10 to 20 percent of its nominal cavity R to framing.
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-values for assemblies, which is why this tool reports the overall U and compares it to the code limits for each climate zone.
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, so add an inside-film and outside-film layer to your assembly for an accurate result.
This calculator handles one step. AIM Works runs the complete MEP design workflow — thermal load calculations, duct & pipe networks, equipment selection, code compliance, and an AI design assistant — in one tool.
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.