Insulation Calculator: R-Value, Thickness and Coverage by Material

1. Enter the area to insulate in square feet. Subtract any window and door openings first.
2. Set the target R-value. Common targets are R-13 for exterior walls, R-19 for floors, and R-30 to R-49 for attics depending on your climate zone.
3. Choose an insulation material from the dropdown. Each option shows its typical R-value per inch.
4. Adjust the waste factor if needed. The default 10% covers normal cutting and fitting waste.
5. Read the thickness, coverage area with waste, and material quantity in batts, bags or boards.

R-value measures how well a material resists heat flow. A higher R-value means better insulation. The R-value of a layer equals the material's R-per-inch multiplied by the thickness in inches.

Building codes set minimum R-values by climate zone and building component. The International Energy Conservation Code (IECC) recommends values from R-13 for mild-climate walls up to R-49 or more for cold-climate attics. The insulation calculator uses these targets so you can find the right thickness for any material.

For other construction estimates, see the construction calculators hub.

The calculator divides your target R-value by the material's R-per-inch to find the thickness, then computes the quantity of material needed to cover the area plus waste.

1. Divide the target R-value by the R-per-inch of the chosen material to get thickness.
2. Multiply the area by (1 + waste/100) to account for cuts and fitting.
3. For batts, divide the adjusted area by the area of one batt (15 in x 93 in = 9.69 sq ft) and round up.
4. For blown-in, compute the volume in cubic feet and divide by coverage per bag (~25 cu ft).
5. For rigid boards, divide by a standard 4 x 8 ft sheet (32 sq ft) and round up.
6. For spray foam, compute board-feet (area x thickness).

Why R-Per-Inch Varies

The R-per-inch values shown are typical industry averages. Actual performance depends on the specific product, density, installation quality and temperature conditions. Always check the manufacturer's data sheet.

Wall Insulation with Fiberglass Batts

Insulate 200 sq ft of exterior wall to R-13 using fiberglass batts (R-3.2 per inch).

Thickness: 13 / 3.2 = 4.06 inches, which fits a standard 2x4 wall cavity.

Area with 10% waste: 200 x 1.10 = 220 sq ft.

Batts needed: 220 / 9.69 = 22.7, rounded up to 23 batts (15 in x 93 in each).

Attic Insulation with Cellulose Blown-in

Insulate 500 sq ft of attic floor to R-38 using cellulose blown-in (R-3.5 per inch).

Thickness: 38 / 3.5 = 10.86 inches (about 11 inches).

Area with 10% waste: 500 x 1.10 = 550 sq ft.

Volume: 550 x 10.86 / 12 = 497.6 cu ft.

Bags: 497.6 / 25 = 19.9, rounded up to 20 bags.

The table below lists typical R-values per inch for common insulation materials. These are industry averages and actual values depend on the specific product and manufacturer.

The International Energy Conservation Code sets minimum insulation levels. These are broad guidelines and local building codes may differ. Check with your local building department.

These targets assume standard construction. An insulation calculator helps you find the thickness and material amount for whichever R-value your zone requires.

The cost of insulating a space varies widely by material. The figures below are typical ranges per square foot of installed insulation and are meant only as a rough guide for comparing options. Always get local quotes.

Higher R-per-inch materials cost more per square foot but may save space in tight cavities. The insulation calculator does not price materials for you. Enter your own costs after getting quotes.

New Construction

Before framing, calculate the thickness for each wall, floor and ceiling assembly to meet code R-values and order the right amount of material.

Retrofit and Renovation

Adding insulation to an existing home starts with knowing how much you already have and how much more you need to reach the target R-value.

Attic Upgrades

Attics are the single biggest opportunity to cut heating and cooling costs. Use the calculator to find how many bags of blown-in insulation will bring your attic to R-38 or R-49.

Comparing Materials

Run the calculator for two or three materials at the same R-value to see which needs less thickness and fewer pieces.

Fiberglass Batts

The most common residential insulation. Sold in pre-cut widths to fit standard stud spacing (15 or 23 inches). Easy to install but must be cut carefully around wiring and plumbing.

Blown-in Cellulose and Fiberglass

Good for attics and retrofitting walls. A machine blows the loose fill into cavities. Cellulose offers a higher R-per-inch than blown fiberglass and settles less when treated with borate.

Mineral Wool

Fire resistant and moisture resistant. Slightly higher R-per-inch than fiberglass batts and stiffer, which makes it easier to friction-fit without sagging.

Spray Foam

Open cell fills cavities and provides an air seal at R-3.7 per inch. Closed cell is denser, acts as a vapor barrier, and delivers R-6.5 per inch but costs more per board-foot.

Rigid Foam Boards

XPS, EPS and polyiso boards are used on exterior walls, foundations and flat roofs. Polyiso offers the highest R-per-inch at R-6.0, but its performance drops in very cold temperatures.

Installation Quality

Gaps, compression and voids reduce effective R-value. A batt that is compressed from 3.5 inches to 2 inches loses much of its thermal resistance.

Moisture

Wet insulation conducts heat much faster. Proper vapor barriers and drainage prevent moisture from degrading performance over time.

Air Sealing

Insulation works best when air movement through it is minimized. Sealing gaps around outlets, pipes and attic penetrations before insulating improves the effective R-value of the assembly.

Settling

Blown-in insulation can settle over time, reducing the thickness and R-value. Cellulose treated with borate and installed at the correct density settles about 20% less than untreated loose fill.

1. Forgetting to Subtract Openings

Windows, doors and other openings reduce the area to insulate. Subtract them from the total wall area before calculating material quantity.

2. Ignoring Waste

Cutting batts and boards always produces some scrap. A 10% waste factor is a reasonable default; complex layouts with many cuts may need 15%.

3. Compressing Batts to Fit

Squeezing R-19 batts into a 2x4 cavity does not give R-19. The compressed batt has a lower R-value because you reduced its thickness.

4. Mixing R-value with Thickness

R-value depends on both the material and the thickness. Two inches of closed-cell spray foam (R-13) outperforms four inches of blown fiberglass (R-10).

5. Skipping the Air Seal

Adding insulation without sealing air leaks is like wearing a sweater with holes. Seal gaps first, then insulate.

The math is exact for the geometry (area, volume, piece count). The R-per-inch values are published typical averages and may differ from a specific product.

It depends on the material and the R-value you need. Divide your target R-value by the R-per-inch of the material. For example, R-13 in fiberglass batt (R-3.2/in) needs about 4 inches, while R-38 in cellulose blown-in (R-3.5/in) needs about 11 inches.

The IECC recommends R-30 to R-49 for attics depending on your climate zone. Colder zones need higher R-values. Check your local building code for the exact requirement.

Exterior walls typically need R-13 to R-20 depending on the climate zone. A standard 2x4 wall fits about R-13 of fiberglass batt insulation.

The calculator computes the volume of insulation (area times thickness divided by 12) and divides by the bag coverage, typically about 25 cubic feet per bag. Enter your area and target R-value to get the exact count.

There is no single best material. Closed-cell spray foam has the highest R-per-inch at R-6.5 but costs the most. Fiberglass batts are the most affordable and widely available. Cellulose is a good middle ground for blown-in applications.

Yes. R-values add together. If your attic has R-19 and you add R-19 on top, the total is R-38. Make sure the new layer is unfaced (no vapor barrier) to avoid trapping moisture.

A 10% waste factor is standard for most projects. For rooms with many cuts around windows, doors and obstacles, consider 15%.

R-value measures thermal resistance (higher is better). U-value measures thermal conductance (lower is better). U = 1 / R. Building codes sometimes express requirements as U-values, especially for windows.

Compressing insulation reduces its thickness and its R-value. A batt rated R-19 at 6.25 inches will have a lower R-value if squeezed into a 3.5-inch cavity. Never compress batts to fit a narrower space.

The thickness and quantity math is exact. The R-per-inch values are published typical averages. Check your chosen product's data sheet for the exact R-per-inch, then re-enter it if it differs.

• DOE/ORNL Insulation Fact Sheet (U.S. Department of Energy; types of insulation, R-values and recommended levels).
• IECC Insulation Requirements (International Energy Conservation Code; minimum R-values by climate zone and building component).
• ORNL Building Envelope Research (Oak Ridge National Laboratory; insulation performance data and research).