Typical R-value per inch by material
R-value per inch is a published typical (mid-range) figure for each insulation type; the exact value for a specific product should be checked on its data sheet, since density, installation method and aging can shift it.
| Material | Typical R-value per inch |
|---|---|
| Fiberglass batt | ≈3.2 |
| Blown cellulose | ≈3.5 |
| Closed-cell spray foam | ≈6.5 |
| Rigid foam board | ≈5.0 |
- These R-per-inch figures are commonly cited mid-range typical values for each insulation category; actual performance varies by manufacturer, product density and installation quality (gaps and compression reduce effective R-value), and can also change slightly with the insulation's age and moisture exposure.
- Target R-values for attics, walls, floors and other assemblies are typically set by local energy codes and vary by climate zone; check the applicable code (such as the IECC in the US) or an energy auditor's recommendation rather than assuming a single target applies everywhere.
What does an insulation calculator do?
An insulation calculator estimates the thickness of insulation needed to reach a target R-value (a measure of resistance to heat flow) for a given material, using published typical R-value-per-inch figures for common insulation types. R-value per inch varies significantly by material because different insulation types trap air, use different cell structures, or handle heat transfer differently.
The mid-range R-per-inch figures used here — approximately 3.2 for fiberglass batt, 3.5 for blown cellulose, 6.5 for closed-cell spray foam, and 5.0 for rigid foam board — are commonly cited typical values. Actual R-value per inch varies by specific product, density and installation quality, and manufacturers publish the exact tested R-value per inch for their product on the packaging or data sheet.
How to use this insulation calculator
- Enter the area to be insulated, in square meters.
- Enter the target total R-value you want to achieve (check local energy code minimums for your climate zone and application, such as attic, wall or floor).
- Select the insulation material — fiberglass batt, blown cellulose, closed-cell spray foam, or rigid foam board.
- Read the required thickness in inches and millimeters to reach the target R-value with the selected material's typical R-per-inch rating.
The formula behind insulation thickness
Required thickness in inches equals the target R-value divided by the material's R-value per inch. This is then converted to millimeters using the exact conversion 1 inch = 25.4 mm.
Worked example: to reach R-20 with fiberglass batt (R-3.2 per inch), the required thickness is 20 ÷ 3.2 = 6.25 inches (about 159 mm). The same R-20 target with closed-cell spray foam (R-6.5 per inch) needs only 20 ÷ 6.5 ≈ 3.08 inches (about 78 mm), since spray foam has a higher R-value per inch.
Common mistakes
- Using a generic R-per-inch figure instead of the value tested and published for the specific product being installed.
- Compressing batt insulation to fit a shallower cavity than it is rated for, which reduces its actual R-value below the nominal rating.
- Ignoring gaps, compression and thermal bridging at framing members, which can meaningfully lower the effective (whole-assembly) R-value below the material's nominal per-inch rating.
- Assuming the same target R-value applies to every part of the building — attics, walls and floors typically have different code-minimum R-value targets by climate zone.
الأسئلة الشائعة
How thick does insulation need to be for R-20?
It depends on the material's R-value per inch. Fiberglass batt (≈R-3.2/in) needs about 6.25 inches for R-20, while closed-cell spray foam (≈R-6.5/in) needs only about 3.1 inches, because it has a higher R-value per inch.
Which insulation material has the highest R-value per inch?
Of the materials compared here, closed-cell spray foam has the highest typical R-value per inch (around 6.5), followed by rigid foam board (around 5.0), blown cellulose (around 3.5), and fiberglass batt (around 3.2).
What R-value do I need for my attic?
Target R-values for attics, walls and floors are set by local energy codes and vary significantly by climate zone — check the applicable building or energy code (such as the IECC in the US) for the minimum required in your area rather than assuming a single figure.
Does compressing fiberglass batt reduce its R-value?
Yes. Fiberglass batt is rated for a specific uncompressed thickness; compressing it into a shallower cavity than it is designed for reduces its actual installed R-value below the nominal rating printed on the packaging.
Why do different insulation materials have different R-values per inch?
R-value per inch depends on how a material resists heat conduction, convection and, for some products, radiation — factors such as trapped air-cell structure, density and whether the material is open-cell or closed-cell all affect its per-inch performance.
المراجع
- North American Insulation Manufacturers Association (NAIMA) — published typical R-value ranges by insulation material type.
- U.S. Department of Energy (DOE), Energy Saver program — insulation R-value guidance and typical per-inch ranges for common materials.
- International Code Council (ICC) — International Energy Conservation Code (IECC), climate-zone-based minimum R-value requirements for building assemblies.