How to calculate insulation R-value
Multiply installed thickness by R-value per inch. In the default example, 6 inches x R-3.5 per inch = about R-21.
Use product data, not a generic promise
R-value per inch changes by material and product. Fiberglass, cellulose, open-cell foam, closed-cell foam, foam board, and ICF systems should be checked against the selected product data sheet.
Installed performance can differ from simple math
Compression, air gaps, thermal bridging, moisture, vapor control, air sealing, and installation quality can all affect real building performance.
R-value is not a complete insulation plan
This calculator estimates a layer value. Whole assemblies also include framing, sheathing, air barriers, ventilation clearances, and local energy-code requirements.
Insulation R-value planning checks
These are editable planning examples. Use product data sheets and local energy-code targets for final decisions.
| Example | Thickness and R per inch | Layer R-value |
|---|---|---|
| Fiberglass or cellulose planning layer | 6 in at R-3.5/in | R-21 |
| Attic loose-fill planning layer | 14 in at R-3.2/in | R-44.8 |
| Open-cell spray foam planning layer | 5.5 in at R-3.8/in | R-20.9 |
| Closed-cell spray foam planning layer | 3 in at R-6.9/in | R-20.7 |
Insulation coverage example checks
Coverage changes by product, R-value, thickness, and package size. Use product labels for final ordering.
| Example | Area | Units with 10% waste |
|---|---|---|
| Small attic zone | 500 sq ft | 13 at 45 sq ft/unit |
| Typical attic | 1,000 sq ft | 25 at 45 sq ft/unit |
| Wall batt area | 480 sq ft | 36 at 15 sq ft/unit |
| Garage walls or ceiling | 600 sq ft | 44 at 15 sq ft/unit |
Before you calculate
- Use installed thickness in inches, not loose package thickness before installation.
- Enter R-value per inch from the selected product data sheet.
- Calculate different layers separately when materials or thicknesses change.
Common mistakes
- Assuming every insulation material has the same R-value per inch.
- Ignoring compression, gaps, framing thermal bridges, air leakage, and vapor-control details.
- Treating layer R-value as a complete local-code or whole-assembly approval.
Formula
R-value = thickness * rValuePerInch
Assumptions
- R-value per inch is an editable planning input, not a code requirement.
- Use the selected product data sheet, installed thickness, and local energy-code target before final decisions.
- Air sealing, vapor control, thermal bridging, compression, gaps, and installation quality can change real performance.
Example
Estimated insulation R-value: 21 R-value
How to calculate insulation R-value
- Enter insulation thickness in inches.
- Enter R-value per inch from the selected product data sheet.
- Multiply thickness by R-value per inch to estimate total R-value.
- Compare the result with the target R-value for the assembly and climate zone.
- Keep air sealing, vapor control, compression, gaps, and code requirements separate from the simple R-value math.
Before you buy materials
- Compare the result with local energy-code targets and product documentation.
- Plan air sealing, ventilation clearances, vapor control, and installation quality separately from the R-value math.
FAQ
What is the example r-value R-value?
Use insulation thickness and R-value per inch, then calculate the insulation R-value. In the default example, the result is 21 R-value.
How do I calculate insulation R-value from thickness?
Multiply installed thickness in inches by the product's R-value per inch. Example: 6 inches at R-3.5 per inch is about R-21.
What is the R-value for 6 inches in this example?
6 inches at R-3.5 per inch is about R-21.
Is R-value per inch the same for every insulation type?
No. R-value per inch varies by material, density, product, and installation. Use the selected product data sheet.
Does this calculate local code compliance?
No. It estimates layer R-value only. Check local energy-code targets, assembly details, and ventilation requirements separately.
How do I calculate total insulation R-value?
Multiply insulation thickness by R-value per inch from product data, then compare the result with the local code target. The default example returns 21 R-value.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.