XPS insulation
R-4.5 to R-5.0 per inch; lambda 0.029-0.032 W/mK. Use XPS for details where compressive strength, water exposure, and board durability are more important than the lowest material cost.
XPS Insulation R-value
XPS insulation R-value per inch
R-4.5 to R-5.0 per inch; lambda 0.029-0.032 W/mK. Use XPS for details where compressive strength, water exposure, and board durability are more important than the lowest material cost.
Assembly thickness
Thickness examples: 2 inch XPS: 2 in * R-5.0 per in = R-10.0; 75 mm foundation board: 2.95 in * R-5.0 per in = R-14.8; R-20 basement target: R-20 / R-5.0 per in = 4.0 in.
U-value
Higher R per inch can reduce required thickness, but the final U-value still depends on every layer, surface resistance, and thermal bridging.
| Material | XPS insulation |
|---|---|
| Category | Moisture-resistant rigid board |
| Lambda | R-4.5 to R-5.0 per inch; lambda 0.029-0.032 W/mK |
| Thickness | Thickness examples: 2 inch XPS: 2 in * R-5.0 per in = R-10.0; 75 mm foundation board: 2.95 in * R-5.0 per in = R-14.8; R-20 basement target: R-20 / R-5.0 per in = 4.0 in. |
Use XPS for details where compressive strength, water exposure, and board durability are more important than the lowest material cost.
Aging method, blowing agent, facer, water exposure, fire detailing, and environmental rules can change the correct specification.
XPS is commonly estimated around R-4.5 to R-5.0 per inch. It is used where rigid-board handling, basement or foundation exposure, and moisture resistance are important, but product variation still matters.
Thickness examples: 2 inch XPS: 2 in * R-5.0 per in = R-10.0; 75 mm foundation board: 2.95 in * R-5.0 per in = R-14.8; R-20 basement target: R-20 / R-5.0 per in = 4.0 in.
Typical values are planning ranges. Check the declared product value, local code, and the complete assembly before construction.
Practical note
How to interpret lambda for XPS insulation
XPS insulation should be assessed through declared lambda, layer thickness, and the conditions it will see inside the assembly. In this database the material is grouped as “Moisture-resistant rigid board”, and the reference value is shown as Lambda R-4.5 to R-5.0 per inch; lambda 0.029-0.032 W/mK. Use it as a comparison value, not as a guarantee for every product sold under a similar name.
Real insulation performance depends on density, moisture, temperature, ageing, compression, and installation quality. Two products with similar commercial names can have different declared values, so current technical data sheets should be checked before purchase or specification.
In a simple calculation, layer resistance is thickness in metres divided by lambda. If a layer is 100 mm and lambda is 0.040 W/mK, resistance is about 2.50 m²K/W. A complete U-value still needs the other layers, surface resistances, and thermal bridge review.
Do not compare the material by lambda alone. Moisture resistance, compressive strength, fire behaviour, acoustic goals, compatibility with adhesives or membranes, and the ability to form a continuous layer can all change the best choice. For final decisions, use current manufacturer data and local requirements.
When comparing materials, check whether the value is a declared design value, a laboratory value, or a calculation value. Different sources may show different numbers for the same product family, especially when density, moisture, or application changes.
A material with a strong lambda value does not guarantee a strong U-value if it is installed too thinly or with thermal bridges. After reading the value, test the full layer build-up in the calculator.
Material selector data
These ranges feed the calculator material lists. Treat them as typical ranges and verify product declarations.
| Id | Material | R per inch | λ range | Note | Page |
|---|---|---|---|---|---|
fiberglass | Fiberglass batt | R 3.0-3.8/in | 0.038-0.046 W/mK | Lightweight option for walls and attics; verify the product declaration. | Open |
mineral-wool | Mineral wool | R 3.7-4.3/in | 0.034-0.041 W/mK | Non-combustible layer for walls, roofs, and acoustic assemblies. | Open |
eps | EPS foam | R 3.6-4.2/in | 0.034-0.040 W/mK | Rigid board for floors, walls, and exterior systems. | Open |
xps | XPS foam | R 4.5-5.0/in | 0.029-0.034 W/mK | Rigid board with good moisture and compressive resistance. | Open |
polyiso | Polyiso board | R 5.6-6.5/in | 0.020-0.026 W/mK | High R per inch at lower thickness; facers change declared values. | Open |
Assumptions and sources
Material values are typical ranges for early comparison. Real values depend on the product, density, moisture, installation, and manufacturer declaration.
- Check the product datasheet before selecting a final lambda or R per inch value.
- Local codes, thermal bridges, and air layers can change the required result.
- For permit or design work, have the assembly reviewed by a qualified professional.
- Last range review: 2026-04-27.
See how formulas, unit conversions, rounding, and limitations are handled. Methodology details.
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Calculator integration
Thickness examples: 2 inch XPS: 2 in * R-5.0 per in = R-10.0; 75 mm foundation board: 2.95 in * R-5.0 per in = R-14.8; R-20 basement target: R-20 / R-5.0 per in = 4.0 in. Higher R per inch can reduce required thickness, but the final U-value still depends on every layer, surface resistance, and thermal bridging.
Material-specific check
Use XPS for details where compressive strength, water exposure, and board durability are more important than the lowest material cost.
Planning limitation
Aging method, blowing agent, facer, water exposure, fire detailing, and environmental rules can change the correct specification.
Worked example 1: 2 inch XPS
2 in * R-5.0 per in = R-10.0. Treat this as a planning result, then confirm the exact product label and installed thickness.
Worked example 2: 75 mm foundation board
2.95 in * R-5.0 per in = R-14.8. Treat this as a planning result, then confirm the exact product label and installed thickness.
Worked example 3: R-20 basement target
R-20 / R-5.0 per in = 4.0 in. Treat this as a planning result, then confirm the exact product label and installed thickness.
Forms and use cases
Use XPS for details where compressive strength, water exposure, and board durability are more important than the lowest material cost.
Limits to check
Aging method, blowing agent, facer, water exposure, fire detailing, and environmental rules can change the correct specification.
Quick answer
XPS is commonly estimated around R-4.5 to R-5.0 per inch. It is used where rigid-board handling, basement or foundation exposure, and moisture resistance are important, but product variation still matters. R-4.5 to R-5.0 per inch; lambda 0.029-0.032 W/mK.
Material R-value FAQ
Use R-4.5 to R-5.0 per inch; lambda 0.029-0.032 W/mK for early comparison, then replace it with the declared value for the exact product.
Use XPS for details where compressive strength, water exposure, and board durability are more important than the lowest material cost.
Aging method, blowing agent, facer, water exposure, fire detailing, and environmental rules can change the correct specification.
Yes. You can print the result or export it to CSV, Excel, or PDF for reports and documentation.
Yes. It is designed for layered assemblies such as external walls, flat roofs, pitched roofs, floors, and slabs. For unusual assemblies, add every relevant layer and treat the result as a planning check before formal verification.
Yes. It is intended for fast concept-stage calculations, insulation comparison, and envelope optimisation before detailed design. It is best used to narrow choices, not to replace a code check or project-specific thermal bridge assessment.
Yes. You can switch between millimeters, centimeters, and inches, and the calculator keeps the values consistent. For fewer mistakes, choose one unit system at the start of a project and review converted thicknesses before export.
Use XPS insulation R-value per inch as a first-pass reference. Before specifying anything, compare the result with the service penetration, actual project dimensions, product data sheet, and local requirements.