U-Value ↔ R-Value Converter

About the U-Value ↔ R-Value Converter

U-value and R-value are two ways of expressing the same thermal property of a material or assembly. R-value measures thermal resistance — higher is better for insulation. U-value measures thermal transmittance — lower is better because less heat passes through. They are simple reciprocals: U = 1/R and R = 1/U.

R-value is the standard in the US for insulation products (R-13 batts, R-38 attic insulation). U-value (or U-factor) is used for windows, doors, and in building energy codes for wall and roof assemblies. Understanding both is essential for energy modeling and code compliance.

This calculator converts between U-value and R-value in both Imperial (US) and SI (metric) units, making it easy to compare materials and verify energy code requirements.

Understanding this metric in precise terms allows energy managers to evaluate investment options, forecast savings, and build compelling business cases for efficiency upgrades and retrofits. Tracking this metric consistently enables energy professionals and facility managers to identify consumption trends and implement efficiency improvements before costs escalate unnecessarily.

When This Page Helps

Building codes often specify maximum U-values for walls and roofs, while insulation products are sold by R-value. This converter bridges the gap, letting you verify compliance without manual math errors.

How to Use the Inputs

1. Enter either the U-value or R-value.
2. Select whether you're using Imperial or SI units.
3. The calculator quickly shows the converted value.
4. Use the result to compare materials or check code compliance.

Example Calculation

Tips & Best Practices

• U-value includes the entire assembly (insulation + framing + air films), not just the insulation.
• Window U-factors are whole-unit ratings including frame, glass, and spacer.
• Lower U-value = better insulation = higher R-value.
• US energy codes typically specify maximum U-values for assemblies.
• Thermal bridging through framing reduces the effective R-value of a wall assembly.
• To convert Imperial R to SI R, multiply by 0.1761.

When to Use R-Value vs U-Value

Use R-value when comparing insulation products (batts, blown-in, foam) or specifying insulation thickness. Use U-value when evaluating complete assemblies (walls, roofs, windows) or checking energy code compliance.

The Whole-Wall U-Value Problem

A wall's actual U-value is worse than its cavity insulation suggests. Thermal bridges (studs, plates, headers) conduct heat faster than insulation. A nominally R-21 wall (U-0.048) has a whole-wall U-value of about 0.065 (R-15) when accounting for 25% framing factor. Adding continuous exterior insulation dramatically improves whole-wall performance.

International Comparisons

US and European insulation standards use different units. A US R-19 wall equals about SI R-3.3. European Passive House standard requires walls of SI R-6.7 or better (US R-38), which explains why Passive House walls are so much thicker than code-minimum construction.

Frequently Asked Questions

• What is a good U-value for walls?

  Modern energy codes require wall U-values of 0.045–0.060 in cold climates (equivalent to R-17 to R-22). High-performance walls achieve U-values of 0.025–0.035 (R-28 to R-40). The lower the U-value, the better the insulation.

• What is a good U-value for windows?

  ENERGY STAR windows have U-factors of 0.25–0.30 depending on climate zone. High-performance triple-pane windows achieve U-factors of 0.15–0.20. Standard double-pane is typically 0.30–0.35.

• Why do windows use U-value instead of R-value?

  Windows have low R-values (R-2 to R-7), making small differences hard to compare. The U-value scale (0.15–0.50) provides more useful distinctions. A U-0.25 window is clearly better than U-0.35, while "R-4 vs R-2.9" is less intuitive.

• How does thermal bridging affect U-value?

  Wood studs in a 2×6 wall insulated with R-21 batts create thermal bridges. The whole-wall U-value is about 0.065 (R-15.4), not 0.048 (R-21), because studs have only R-6.9. Continuous exterior insulation helps mitigate bridging.

• What are SI vs Imperial units for R-value?

  Imperial R-value uses ft²·°F·hr/BTU (used in the US). SI R-value uses m²·K/W (used internationally). To convert: R_SI = R_Imperial × 0.1761. So R-19 Imperial = R-3.35 SI.

• Can I add U-values together?

  No. U-values are not additive. To combine layers, convert each to R-value, add the R-values, then convert back. R-values of layers in series are additive: R_total = R_1 + R_2 + R_3.