Convert between U-value and R-value for insulation and building materials. Calculate thermal resistance from thermal transmittance and vice versa.
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.
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. Consistent measurement creates a reliable baseline for tracking energy efficiency improvements and validating the impact of conservation measures and equipment upgrades over time.
U = 1 / R R = 1 / U Imperial: R in ft²·°F·hr/BTU, U in BTU/(hr·ft²·°F) SI: R in m²·K/W, U in W/(m²·K) Conversion: R_SI = R_Imperial × 0.1761
Result: U-value = 0.053
An R-19 wall has a U-value of 1/19 = 0.053 BTU/(hr·ft²·°F). In SI units, R-19 = 3.35 m²·K/W and U = 0.299 W/(m²·K).
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.
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.
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.
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.
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.
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.
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.
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.
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.