R-Value Calculator
Add up the insulation in a wall or roof assembly — layer by layer — to get its total R-value and U-factor, then check it against the code minimum for your climate zone.
Layers (material and thickness)
Code check (optional)
Heat loss (optional)
R-Value, U-Factor, and How They Add Up
R-value measures resistance to heat flow — higher is better insulation. The handy part is that the layers of an assembly add in series: drywall plus cavity insulation plus sheathing plus the air films sum to a total R. The U-factor is just its inverse, and it is what drives heat loss:
Heat loss through the assembly is then Q = U × area × temperature difference. Double the R-value and you roughly halve the heat lost through that surface.
Watch Out for Thermal Bridging
A wall is not all insulation — the studs are a shortcut for heat. A 2×6 cavity filled to R-21 performs more like R-16 to R-18 once you account for the framing, which is why codes increasingly call for continuous exterior insulation that is not interrupted by studs. This calculator gives the center-of-cavity (one-dimensional) R; derate it for framing or add continuous insulation to compensate.
Code Minimums by Climate Zone
Approximate IECC residential prescriptive minimums:
| Climate zone | Wood-frame wall | Ceiling |
|---|---|---|
| 1 – 2 | R-13 | R-30 to R-49 |
| 3 – 4 | R-20 | R-49 to R-60 |
| 5 – 6 | R-20 | R-60 |
| 7 – 8 | R-21 | R-60 |
Frequently Asked Questions
What are air films?
Thin layers of still air clinging to the inside and outside surfaces add real resistance — about R-0.85 combined for a typical wall. The calculator includes them by default; turn them off to compare bare material R only.
R-value or RSI?
R-value is the imperial figure (ft-squared degF hr / BTU); RSI is the metric equivalent (m-squared K / W). RSI = R divided by 5.68. The calculator shows both.
Does more insulation always pay off?
Returns diminish — going from R-10 to R-20 halves the loss, but R-20 to R-30 only cuts it another third. Match the level to your climate zone and the code rather than overbuilding.
Related calculators
- Heat Transfer Calculator — conduction, convection and U-value heat flow.
- Insulation Calculator — how much insulation to buy for the area.
- Thermal Expansion Calculator — movement from temperature change.
- Engineering Unit Converter — energy, power and temperature units.