Fiberglass insulation
R-2.9 to R-4.3 per inch; lambda 0.034-0.050 W/mK. Use batts when the cavity depth is regular and compression can be avoided. Use blown fiberglass for attic top-ups or cavities where pieces would leave gaps.
Fiberglass Insulation R-value
Fiberglass insulation R-value per inch
R-2.9 to R-4.3 per inch; lambda 0.034-0.050 W/mK. Use batts when the cavity depth is regular and compression can be avoided. Use blown fiberglass for attic top-ups or cavities where pieces would leave gaps.
Assembly thickness
Thickness examples: R-13 wall cavity: R-13 / R-3.7 per in = 3.5 in; R-19 floor or wall: R-19 / R-3.7 per in = 5.1 in; R-38 attic top-up: R-38 / R-3.2 per in = 11.9 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 | Fiberglass insulation |
|---|---|
| Category | Batt and loose-fill insulation |
| Lambda | R-2.9 to R-4.3 per inch; lambda 0.034-0.050 W/mK |
| Thickness | Thickness examples: R-13 wall cavity: R-13 / R-3.7 per in = 3.5 in; R-19 floor or wall: R-19 / R-3.7 per in = 5.1 in; R-38 attic top-up: R-38 / R-3.2 per in = 11.9 in. |
Use batts when the cavity depth is regular and compression can be avoided. Use blown fiberglass for attic top-ups or cavities where pieces would leave gaps.
Compression, wind washing, moisture, gaps around services, and uneven blown depth can all reduce the installed R-value.
Fiberglass usually sits around R-2.9 to R-4.3 per inch. Batts are fast to place in regular cavities, while blown fiberglass can fill attics and irregular spaces when depth and density are controlled.
Thickness examples: R-13 wall cavity: R-13 / R-3.7 per in = 3.5 in; R-19 floor or wall: R-19 / R-3.7 per in = 5.1 in; R-38 attic top-up: R-38 / R-3.2 per in = 11.9 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 Fiberglass insulation
Fiberglass 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 “Batt and loose-fill insulation”, and the reference value is shown as Lambda R-2.9 to R-4.3 per inch; lambda 0.034-0.050 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: R-13 wall cavity: R-13 / R-3.7 per in = 3.5 in; R-19 floor or wall: R-19 / R-3.7 per in = 5.1 in; R-38 attic top-up: R-38 / R-3.2 per in = 11.9 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 batts when the cavity depth is regular and compression can be avoided. Use blown fiberglass for attic top-ups or cavities where pieces would leave gaps.
Planning limitation
Compression, wind washing, moisture, gaps around services, and uneven blown depth can all reduce the installed R-value.
Worked example 1: R-13 wall cavity
R-13 / R-3.7 per in = 3.5 in. Treat this as a planning result, then confirm the exact product label and installed thickness.
Worked example 2: R-19 floor or wall
R-19 / R-3.7 per in = 5.1 in. Treat this as a planning result, then confirm the exact product label and installed thickness.
Worked example 3: R-38 attic top-up
R-38 / R-3.2 per in = 11.9 in. Treat this as a planning result, then confirm the exact product label and installed thickness.
Forms and use cases
Use batts when the cavity depth is regular and compression can be avoided. Use blown fiberglass for attic top-ups or cavities where pieces would leave gaps.
Limits to check
Compression, wind washing, moisture, gaps around services, and uneven blown depth can all reduce the installed R-value.
Quick answer
Fiberglass usually sits around R-2.9 to R-4.3 per inch. Batts are fast to place in regular cavities, while blown fiberglass can fill attics and irregular spaces when depth and density are controlled. R-2.9 to R-4.3 per inch; lambda 0.034-0.050 W/mK.
Material R-value FAQ
Use R-2.9 to R-4.3 per inch; lambda 0.034-0.050 W/mK for early comparison, then replace it with the declared value for the exact product.
Use batts when the cavity depth is regular and compression can be avoided. Use blown fiberglass for attic top-ups or cavities where pieces would leave gaps.
Compression, wind washing, moisture, gaps around services, and uneven blown depth can all reduce the installed R-value.
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 Fiberglass insulation R-value per inch as a first-pass reference. Before specifying anything, compare the result with the acoustics and fire, actual project dimensions, product data sheet, and local requirements.