Guides
How to calculate U-value from layers
Add homogeneous layer resistances, account for surface resistances and check the limits of a nominal U-value calculation.
Summing resistances is useful for homogeneous layers when data and scope are explicit. Real assembly details can change heat flow. Check the following before comparing numerical U with a design requirement.
Key points
- Enter each layer once, using d/λ or direct R.
- Enabled surface resistances are fixed values, not automatic selection for every condition.
- Bridges, fixings and cavities need appropriate modeling.
In this guide
Prepare your inputs
- Layer order and thickness from the drawing.
- Design λ or R with scope for each layer.
- Inhomogeneous framing, cavities and heat-flow direction.
1. Choose one basis per layer
Thickness/λ mode calculates R = d/λ; direct-R mode accepts the entered resistance. Do not add another row for the same insulation. Determine whether supplied R includes multiple layers or surfaces before adding it.
Inputs need consistent calculation conditions. Do not invent an air-space λ to obtain a desired result. Its resistance depends on conditions; this tool requires an established appropriate R.
For a documented R, choose SI m²·K/W or US h·ft²·°F/Btu. Illustrative US R-13 is about 2.289 m²·K/W, not 13 m²·K/W. This mode expects the product's total resistance at its thickness; multiply R per inch by thickness in inches first. Unit conversion does not adjust declared conditions to design conditions.
Source: [2] ISO 10456:2007 — official scope · [1] ISO 6946:2017 — official scope · [3] NIST — Guide to the SI, Appendix B.9
2. Check surface resistances
Without surfaces, the result is only the sum of entered layers. Enabling Rsi/Rse adds fixed values of 0.13 and 0.04 m²·K/W in this application. Record the choice with the result.
The tool does not select these constants for heat-flow direction or boundary conditions. Do not add them again when direct R already includes them. This switch does not imply complete implementation of ISO 6946.
Total R = Σlayer R + Rsi + Rse; U = 1 / total R
3. Identify what lies outside the model
Repeating studs, rafters, metal parts and other paths do not become homogeneous insulation. The tool does not account for their shares or spatial bridges. An arbitrary percentage cannot repair that omission.
Openings, ground, ventilated spaces, air flow and fixing corrections may require other methods. Establish assessment scope with the design. ISO 6946's public abstract describes a broader method and exclusions; this simple model does not replace it.
Two homogeneous layers
Hypothetical build-up: 0.1 m / 0.040 W/(m·K) and a direct-R layer of 0.5 m²·K/W. Compare the two surface modes.
- 1Layer 1
- R = 2.5 m²·K/W
- 2Layer 2
- R = 0.5 m²·K/W
- RsiInside surface
- 0.13 m²·K/W
- RseOutside surface
- 0.04 m²·K/W
| Quantity | Result |
|---|---|
| First layer R | 2.5 m²·K/W |
| Material R | 3.0 m²·K/W |
| U without surfaces | ≈ 0.333 W/(m²·K) |
| R with 0.13 + 0.04 | 3.17 m²·K/W |
| U with surfaces | ≈ 0.315 W/(m²·K) |
The difference follows model scope and does not establish compliance of the real assembly.
Bridges, fixings and other corrections are omitted. Inputs are illustrative.
Try this example in the calculatorCommon mistakes
| Mistake | How to avoid it |
|---|---|
| Entering the same layer by λ and again as R. | Use one mode per layer. |
| Adding Rsi/Rse twice. | Check supplied R scope. |
| Applying homogeneous results to stud framing. | Use an appropriate inhomogeneity method. |
Check before using the result
- No duplicated layers.
- The result records surface-mode selection.
- Exclusions are identified before comparing U.
Method limits
- Nominal homogeneous-layer model without complete standard corrections.
- No moisture, bridge or dynamic assessment.
Questions and answers
Does layer order change summed R?
Not in this simple sum. Order can matter for moisture and actual-use conditions, which this tool does not assess.
Does low U prove no condensation?
No. U describes heat transfer within the assumed model. Moisture assessment requires other inputs and methods.
Sources and verification scope
- ISO 6946:2017 — official scopePublic standard abstract
Public abstract: thermal resistance/transmittance methods and their scope. This simplified calculator does not implement the full standard.
Access checked: - ISO 10456:2007 — official scopePublic standard abstract
Public abstract: declared/design thermal values and conversion between conditions. The full paid standard was not reviewed for this article.
Access checked: - NIST — Guide to the SI, Appendix B.9
Conversions for length, area, volume, mass and thermal quantities. These do not convert a material's physical state.
Access checked: