What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Layers: 4 layers.
- Assembly area: 100 sq ft.
- Temperature difference (inside − outside): 50 °F.
What happens, step by step
- Layer 1, board, adds R-0.68. The arrows show heat getting through.
- The insulation adds R-13. Heat arrows shrink; the temperature drops most here.
- Layer 3, board, adds R-0.45. R-values simply add: R-14.13 so far.
- Layer 4, air film, adds R-0.17. R-values simply add: R-14.3 so far.
- Total R-14.3. Heat flow is ΔT × area ÷ R: 350 BTU/h.
The answer: total r-value is 14.3 h·ft²·°F/Btu.
How it’s calculated
- Total R = Σ layer R-values
- U = 1 ÷ R
- Heat flow (Btu/h) = area × ΔT ÷ R
Worked example
Default wall layers, 100 sq ft, 50 °F difference.
Inputs
- Layers
- 0.68; 13; 0.45; 0.17
- Assembly area
- 100 sq ft
- Temperature difference (inside − outside)
- 50 °F
Results
- Total R-value
- 14.3 h·ft²·°F/Btu
- U-factor
- 0.0699 Btu/h·ft²·°F
- Heat flow
- 350 BTU/h
Assumptions and limits
- One-dimensional steady-state conduction through a uniform assembly.
- Framing, gaps and air leakage are not modelled.
- Default layers: inside air film 0.68, R-13 batt, ½ in drywall 0.45, outside air film 0.17.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
Why is my wall R-value lower than the batt rating?
Studs conduct heat far better than insulation. A 2×4 wall with R-13 batts often performs closer to R-10 overall once framing is included.
What is the difference between R-value and U-factor?
R-value is resistance to heat flow; U-factor is its reciprocal. Windows are usually rated by U-factor, walls by R-value.