Guides
Lambda values for EPS, XPS and mineral wool
Read product lambda, declared and design values. Compare documented EPS, XPS and mineral wool examples and calculate layer R correctly.
Copy thermal values together with units, conditions and product identity. A similar trade name is insufficient, and declared λ is not automatically design λ. Read the right data and check a single-layer result before applying it.
Key points
- λ is conductivity; lower λ gives greater R at the same thickness.
- R belongs to a defined layer; U refers to the modeled assembly.
- Pack coverage is a separate purchasing quantity.
In this guide
Prepare your inputs
- Exact variant, thickness and market label/document.
- Source and conditions for the design value.
- Thickness units and scope: material or complete assembly.
1. Link each value to its product
Record name, code, thickness and document edition. Look for λD, RD and dimensions on an insulation label; do not borrow a better value from another range. The DoP declares properties while instructions describe use.
Separate thermal data from packaging. Pack area affects packs bought, not the resistance of one layer. Changing thickness can change both R and pieces per pack.
Source: [1] ROCKWOOL — product labels and technical documentation
2. Distinguish declared and design values
Declared values follow specified conditions and declaration rules. Design values serve assumed conditions in use. ISO 10456 describes determining and converting these values; this does not supply one universal correction factor.
A design-λ field needs a value appropriate to the calculation rather than an automatic copy of λD. If only a label is available, record the missing information and establish the correct data before using the result in a design. Our calculator does not convert material moisture or temperature conditions.
3. Check units and R scope
With λ in W/(m·K), d in the d/λ formula must be metres: 100 mm is 0.1 m and R is m²·K/W. The material-layer calculator also offers inches and k in Btu·in/(h·ft²·°F), converting valid entered values when you change units. Length preference does not independently change conductivity units.
If a US document gives R per inch, select that entry type in the calculator. Hypothetical R-4 per inch at 5.5 in gives R-22. Total R for a specific batt or board is not R per inch. Scaling assumes the same homogeneous material and conditions; retain the manufacturer's declared R for the chosen thickness rather than replacing it with your extrapolation.
The R/U converter takes the reciprocal of a positive number: U = 1/R. For assembly U, R must cover the appropriate complete build-up, not just an insulation board. US customary R-value has a different unit; convert it explicitly rather than merely changing decimal notation.
Layer R = d [m] / λ [W/(m·K)]
4. EPS, XPS and mineral wool lambda: documented examples
There is no single correct lambda for all EPS, XPS or mineral wool. The examples below show specific products on manufacturers' Polish-market pages, checked on 2 October 2026. They are not a ranking, buying recommendation or automatic design inputs, and should not be transferred to a similarly named product in another market.
For the XPS range, do not choose the lowest number without checking the exact variant and thickness. The current URSA product is named ECO; the older similarly named sheet was unavailable and was not used as evidence for the current range. Checking document versions is part of using material data correctly.
| Family and exact product | Value on the page | Check before calculating |
|---|---|---|
| EPS: Austrotherm EPS 038 FASADA SUPER | λD ≤ 0.038 | Declared limit for this product, not a generic EPS lambda. |
| XPS: URSA XPS ECO N-III-L | 0.031–0.036 | The page gives a range; establish the exact variant and thickness value in its DoP. |
| Stone wool: ROCKWOOL FRONTROCK SUPER | λD = 0.036 | Confirm variant and document; lambda alone does not establish application suitability. |
Source: [4] Austrotherm — EPS 038 FASADA SUPER (Poland) · [5] URSA — XPS ECO N-III-L (Poland) · [6] ROCKWOOL — FRONTROCK SUPER (Poland)
Sensitivity to entered λ
Our comparison uses a 100 mm layer and hypothetical λ values of 0.040 and 0.044 W/(m·K). These are neither recommended product values nor a conversion factor.
| Input | Calculation | R |
|---|---|---|
| λ = 0.040 | 0.1 / 0.040 | 2.5 m²·K/W |
| λ = 0.044 | 0.1 / 0.044 | ≈ 2.273 m²·K/W |
Higher entered λ gives lower layer R. Neither result is the U-value of a complete wall.
The comparison illustrates sensitivity, not selection of a design value.
Try this example in the calculatorCommon mistakes
| Mistake | How to avoid it |
|---|---|
| Using 100 mm as 100 m. | Convert: 100 mm = 0.1 m. |
| Calling board R whole-wall R. | Define the layer scope and missing components. |
| Automatically treating λD as design λ. | Establish design conditions and source. |
Check before using the result
- Variant and thickness match the document.
- λ and R have units and conditions.
- R scope is explicit.
Method limits
- The tool does not choose design conditions or material conversion factors.
- Layer R excludes bridges and surface resistances.
Questions and answers
Is my calculated d/λ the product's declared RD?
Do not present your own calculation as a manufacturer's declaration. Its basis and displayed precision can differ from RD in the product document. Check declared RD for the actual thickness, units and conditions; when recording product data, use the actual declaration.
Does double thickness double R?
In d/λ, yes, when the same design λ remains appropriate for a homogeneous layer. It does not double the entire assembly's R.
Does more pack coverage mean better insulation?
Coverage describes how much area is packaged. Thermal performance comes from material and thickness rather than pack size.
Sources and verification scope
- ROCKWOOL — product labels and technical documentation
Identification, dimensions, declared lambda and declared R on insulation labels. No legal requirements or universal product values are inferred here.
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: - Austrotherm — EPS 038 FASADA SUPER (Poland)
The specific product page states λD ≤ 0.038 W/(m·K). The product page was checked; its linked PDF was unavailable during this review. This is a reading example, not a value for all EPS or a design value.
Access checked: - URSA — XPS ECO N-III-L (Poland)
The current ECO variant page gives conductivity 0.031–0.036 W/(m·K), without mapping one number to each thickness in the accessible text. The range from the unavailable older N-III-L sheet is not transferred to ECO.
Access checked: - ROCKWOOL — FRONTROCK SUPER (Poland)
The specific product table lists declared conductivity 0.036 W/(m·K), referring to EN 12667. This value does not describe all mineral wool or select a suitable application.
Access checked: