Thermal Transmittance (U-Value) Calculator

About the Thermal Transmittance (U-Value) Calculator

The total thermal transmittance (U-value) of a wall, roof, or floor depends on all the layers of material it contains, plus the air films on each surface. Heat must pass through every layer in sequence, so the total thermal resistance (R-value) is the sum of all individual layer R-values.

A typical exterior wall includes: interior air film, drywall, insulation, sheathing, house wrap, cladding, and exterior air film. Each layer contributes some R-value. The total U-value is the reciprocal of the total R-value: U = 1 / R_total.

This calculator lets you add up to six material layers plus standard surface air films to determine the complete assembly U-value. It's essential for energy code compliance, comparing wall designs, and understanding where to invest in better insulation.

By calculating this metric accurately, energy analysts gain actionable insights that inform equipment selection, system design, and operational strategies for maximum efficiency and savings. 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.

When This Page Helps

Energy codes specify maximum U-values for walls, roofs, and floors. This calculator lets you model different assembly configurations to find the most cost-effective way to meet code requirements.

How to Use the Inputs

1. Enter the R-value for each material layer in your assembly.
2. Leave unused layers at zero.
3. The calculator adds standard air film R-values automatically.
4. Review the total R-value and U-value.
5. Compare different assembly configurations.

Example Calculation

Tips & Best Practices

• Always include air film R-values — they add R-0.85 total for walls.
• Thermal bridging through studs reduces the actual R-value — this calculator assumes continuous layers.
• Adding continuous exterior insulation has the biggest impact on reducing thermal bridging.
• R-values are additive for layers in series; U-values are not.
• For parallel paths (studs + cavity), calculate a weighted average.
• Verify results against energy code prescriptive R-value or performance U-value requirements.

Building Up an Assembly

Every building assembly is a series of layers, each with a known R-value. By summing all the R-values, you get the total thermal resistance. The reciprocal gives you the U-value that energy codes use for compliance.

Common Material R-Values

Fiberglass batts: R-3.2/inch. Cellulose: R-3.5/inch. Open-cell spray foam: R-3.7/inch. Closed-cell spray foam: R-6.5/inch. XPS rigid foam: R-5.0/inch. Polyiso rigid foam: R-6.0/inch. OSB sheathing: R-0.56 (7/16"). Plywood: R-0.63 (1/2").

Meeting Energy Code with Assembly U-Values

The performance path in energy codes (IECC) specifies maximum assembly U-values. For Climate Zone 5, walls require U-0.060. You can meet this with a 2×6 wall with R-20 batts + R-5 continuous exterior insulation, or a double-stud wall with R-30 dense-pack cellulose. The calculator helps you verify the numbers.

Frequently Asked Questions

• What R-value are air films?

  Interior still air film: R-0.68. Exterior air film (15 mph wind): R-0.17. These are standard ASHRAE values included in most build-up calculations. Together they add R-0.85 to any wall or roof assembly.

• What is the R-value of drywall?

  Standard 1/2-inch drywall has an R-value of about 0.45. 5/8-inch drywall is about 0.56. Double drywall adds correspondingly. Drywall's R-value is small compared to insulation but is included in precise calculations.

• How does thermal bridging affect the calculation?

  Studs create parallel heat paths that bypass cavity insulation. A 2×4 wall at 16" OC has about 25% framing (R-4.4) and 75% cavity (R-13). The effective R-value is about R-10.5, not R-13. This calculator shows continuous layers only.

• What is a good wall U-value?

  Code requirements vary by climate zone. Cold climates require U-0.045–0.060. Moderate climates allow U-0.077–0.082. Passive House standard is U-0.026 or better. Check your local energy code for specific requirements.

• Should I use prescriptive R-value or performance U-value?

  Both paths meet code. The prescriptive path specifies minimum R-values for each component. The performance path specifies maximum assembly U-values, allowing more design flexibility. The U-value path often allows less insulation if other components perform well.

• What about air cavities in the assembly?

  An uninsulated air cavity (3/4"+) adds about R-0.9 to R-1.0 for a vertical wall. Horizontal air spaces (above insulation) add R-0.87 to R-1.02 depending on direction of heat flow. These are worth including in detailed calculations.