Calculate the total U-value of a multi-layer wall, roof, or floor assembly. Combine R-values of individual material layers to find overall thermal transmittance.
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.
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. Precise quantification supports regulatory compliance and sustainability reporting, ensuring that energy data meets the standards required by auditors and industry certification bodies.
R_total = R_interior_air + R_layer1 + R_layer2 + ... + R_layerN + R_exterior_air U_total = 1 / R_total Air Films: Interior = R-0.68, Exterior = R-0.17
Result: U-value = 0.065
Wall assembly: drywall (R-0.45) + R-13 batt + OSB sheathing (R-0.56) + siding (R-0.5) + air films (R-0.85) = R-15.36 total. U = 1/15.36 = 0.065.
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.
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").
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.
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.
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.
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.
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.
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.
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.