Insulation Calculator

Pick a material — fiberglass batt (R-3.7/in), blown cellulose (R-3.2/in), closed-cell spray foam (R-6/in), or rigid foam (R-5/in), all editable — and the tool divides your target R-value by the R-per-inch to get the required depth. Enter the area and package coverage to count batt packages with waste.

Common uses

• Figure out how many inches of blown cellulose reach R-49 in an attic
• Count batt packages for a garage wall project

Example

R-38 target with blown cellulose (R-3.2/in)

11.9 in of settled depth required

Limitations

• R-per-inch figures are typical published values; compression, gaps, and settling reduce real performance
• Doesn't model thermal bridging through studs

How many inches of insulation do I need for R-49?

About 15.3 in of blown cellulose (R-3.2/in), 13.2 in of fiberglass batt (R-3.7/in), 9.8 in of rigid foam (R-5/in), or 8.2 in of closed-cell spray foam (R-6/in). R-49 is a common attic target for cold-climate zones.

What does R-value actually measure?

Resistance to heat flow — the higher the R-value, the slower heat moves through the assembly. It adds up linearly with thickness, which is why this calculator can divide your target R by the material's R-per-inch to get a required depth.

Which insulation has the highest R-value per inch?

Of the materials here, closed-cell spray foam at about R-6 per inch, followed by rigid foam board at R-5. Fiberglass batts (R-3.7) and blown cellulose (R-3.2) need roughly twice the depth for the same R-value but cost far less per R.

Does compressing fiberglass insulation reduce its R-value?

Yes. A batt squeezed into a cavity thinner than its rated loft delivers less than its labeled R-value because the trapped air pockets do the insulating. Gaps, gaps around wiring, and settling in blown products reduce real-world performance the same way.

Is anything I enter here sent over the internet?

No. The thickness and package math runs entirely in your browser — your home's details are never uploaded or stored.