R-Value Calculator
Total assembly R-value and U-factor from your layers.
Enter your values
ⓘ Live results
Results
- Total R-value
- R-26.35
- U-factor
- 0.038 BTU/hr-ft2-F
- Heat loss
- 1,214 BTU/hr
- Per square foot
- 1.52 BTU/hr-ft2
Working
- 1R 26.35 -> U 0.038 x 800 ft2 x 40 F = 1,214 BTU/hr
Quick answer
R-values of layers in series simply add. U-factor is 1/R total, and heat loss per square foot is U x temperature difference.
R total = sum of layer R; U = 1/R; Q = U x A x deltaT
How to use the R-Value Calculator
- 1List every layer of the assembly and enter its R-value.
- 2Include interior and exterior air films (about 0.85 combined for walls).
- 3Compare the total against your climate zone's code minimum.
Worked example
Using the values the calculator loads with:
Inputs
- Insulation R-value19
- Sheathing R-value5
- Cladding + finishes R-value1.5
- Air films R-value0.85
- Area800 sq ft
- Temperature difference40 F
Results
- Total R-valueR-26.35
- U-factor0.038 BTU/hr-ft2-F
- Heat loss1,214 BTU/hr
- Per square foot1.52 BTU/hr-ft2
- R 26.35 -> U 0.038 x 800 ft2 x 40 F = 1,214 BTU/hr
Frequently asked questions
Do R-values really just add?
In series, yes. But studs bridge the insulation, so a real wall performs 10 to 25% worse than the cavity number unless you add continuous exterior insulation.
What R-value do I need?
Typically R-13 to R-21 walls and R-38 to R-60 attics in the US, depending on climate zone. Check your local energy code.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.