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U-value & R-value Converter

Convert between U-value and R-value for building insulation. Calculate total thermal resistance for multi-layer assemblies. Free HVAC tool.

The U-value & R-value Converter helps you convert between thermal transmittance (U-value) and thermal resistance (R-value). Calculate total R-value for multi-layer building assemblies including walls, roofs, and floors for accurate energy modeling and code compliance.
U-value and R-value are reciprocals: U = 1/R and R = 1/U Higher R-value = better insulation. Lower U-value = better insulation.

What are U-value and R-value?

U-value (thermal transmittance) measures how easily heat flows through a material or assembly. Lower U-values indicate better insulating performance. R-value (thermal resistance) measures a material's resistance to heat flow. Higher R-values indicate better insulation. They are mathematical reciprocals: U = 1/R. In the US, R-value is more commonly used, while international building codes often use U-value. Understanding both is essential for energy modeling, building envelope design, and code compliance.

U-value and R-value Relationship

U = 1 / R and R = 1 / U

Example: R-20 insulation has U-value = 1/20 = 0.05 Btu/(h·ft²·°F)

Thermal Resistance Formulas

1. Reciprocal Relationship

U-value = 1 / R-value OR R-value = 1 / U-value

2. Total R-value (Series Layers)

Rtotal = R1 + R2 + R3 + ... + Rn

Typical R-values (Imperial)

  • Exterior air film: R-0.17 (15 mph wind)
  • Interior air film: R-0.68 (still air)
  • Fiberglass batt insulation: R-3.1 to R-3.4 per inch
  • Spray foam (closed cell): R-6 to R-7 per inch
  • XPS foam board: R-5 per inch
  • Polyisocyanurate foam: R-5.6 to R-6.5 per inch
  • Wood studs (2x4): R-4.4, (2x6): R-6.9
  • Drywall 1/2": R-0.45, 5/8": R-0.56

Design Tips

  • Don't forget air films - they contribute R-0.85 total (exterior + interior)
  • Thermal bridging through studs reduces effective R-value by 10-25%
  • Continuous insulation (exterior) is more effective than cavity insulation alone
  • In cold climates, focus on ceiling insulation first (R-50+)
  • Air sealing is as important as insulation - don't ignore infiltration
  • Condensation control: May need vapor barriers depending on climate
  • Window/door U-values dominate heat loss in well-insulated homes
  • Use REScheck or COMcheck software to verify energy code compliance
See also
HVAC CalculatorsHVAC CALCULATORS6
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