R-Value Calculator
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Total R = sum of layers; U-factor = 1/R — for walls, attics, and assemblies.
Rates last reviewed: July 2026
Insulation layers (R each)
Total R
18
U-factor: 0.0556
- U-factor (1/R)
- 0.0556
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Thermal resistance
R-value measures thermal resistance — higher R means better insulation. In layered assemblies, total R is approximately the sum of each layer's R (ignoring thermal bridges). U-factor is conductance: U = 1/R_total, in BTU/h·ft²·°F.
Building codes specify minimum R for climate zones. This calculator adds up to four layers you enter — drywall, insulation, sheathing, etc. — for a quick envelope estimate.
Example: R-13 cavity + R-5 continuous → R-18 total, U ≈ 0.056. Compare to code tables for your zone — not a substitute for energy modeling.
Official sources
Rates and formulas in this calculator reference the documentation below. Confirm current numbers on the source site before relying on them. Links do not imply endorsement by those organizations of Calcometry or this tool.
Common questions
Do air gaps count?
Still-air gaps add small R-values, but convection reduces benefit. Use published R for insulated assemblies, not guesses.
Is U the same as R?
U = 1/R. Lower U means less heat transfer. Windows are often rated in U directly.