Insulation Calculator
ENERGY STAR check
Estimate from your measurements. Bag coverage and depth come from the label on the product; the R-value printed there takes priority.
ENERGY STAR recommends R-49 to R-60 in most attics. A 1,000 ft² attic in zone 4 with 3½ in. of old batts has about R-11. Reaching R-60 means adding about R-49, which is 15¼ in. of blown cellulose at R-3.2 per inch. Enter your attic, wall, or floor below for the depth, bags, and cost.
Recommended insulation R-values by climate zone
| Climate zone | Attic, empty | Attic, 3–4 in. there | Floor | Cellulose depth (in.) |
|---|---|---|---|---|
| 1 | R-30 | R-25 | R-13 | 9.4 |
| 2 | R-49 | R-38 | R-13 | 15.3 |
| 3 | R-49 | R-38 | R-19 | 15.3 |
| 4A, 4B | R-60 | R-49 | R-19 | 18.8 |
| 4C, 5, 6 | R-60 | R-49 | R-30 | 18.8 |
| 7, 8 | R-60 | R-49 | R-38 | 18.8 |
How to calculate how much insulation you need
Start from the R-value you want, not the thickness. The thickness follows from it:
- Existing R: depth of what is there × its R-value per inch.
- R to add: target R − existing R.
- Depth to add: R to add ÷ R-value per inch of the new material.
- Bags: area ÷ the coverage per bag the label gives for your R-value.
For a 40 × 25 ft attic (1,000 ft²) in zone 4, ENERGY STAR recommends R-60. The 3½ in. of old fiberglass batts gives about 3.5 × 3.2 = R-11.2. That leaves R-48.8 to add. Blown cellulose settles to about R-3.2 to 3.8 per inch, so at the low end it takes 48.8 ÷ 3.2 = 15¼ in. of new cellulose. The finished depth is about 18¾ in.
What R-value do you need?
The table above is ENERGY STAR's recommendation, based on the 2021 IECC (Table R402.1.3). The calculator fills in the target when you pick your zone:
- Attics: R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8. If you already have 3 to 4 in., ENERGY STAR lists the amount to add: R-25, R-38, or R-49.
- Floors over a crawlspace or garage: R-13 in zones 1 and 2, R-19 in 3, 4A and 4B, R-30 in 4C to 6, and R-38 in 7 and 8.
- Basement walls: R-13 batts or R-5 board in zone 3, R-13 or R-10 in 4A and 4B, and R-19 or R-15 in 4C to 8. ENERGY STAR gives no value for zones 1 and 2.
Climate zones follow county lines. Zone 1 is South Florida and Hawaii, the Gulf Coast is mostly zone 2, and the northern states are zones 5 to 7. Your local building department can confirm yours.
How many inches is R-38, R-49, or R-60?
It depends on the material. The Building America Solution Center at Pacific Northwest National Laboratory gives these R-values per inch:
- Blown cellulose: 3.2 to 3.8 once settled. R-38 is 10 to 12 in., R-49 is 13 to 15¼ in., and R-60 is 16 to 18¾ in.
- Blown fiberglass: 2.2 to 2.7. R-38 is 14 to 17¼ in., R-49 is 18 to 22¼ in., and R-60 is 22¼ to 27¼ in.
- Fiberglass batts: 2.5 to 4, depending on density. Batts are sold by R-value, so buy the R on the label.
- Foam board: about R-3.8 to 4.4 for EPS, R-5 for XPS, and about R-6 for polyiso.
- Spray foam: about R-6 for closed-cell. DOE's Building America guides give about R-3.6 for open-cell.
The calculator starts loose-fill at the low end of each range so that settling doesn't leave you short. If your bag lists a different R per inch, or a settled thickness for your R-value, type that in instead.
How many bags of blown-in insulation?
Every bag of loose-fill has a coverage chart that lists, for each R-value, the square feet a bag covers, the minimum settled thickness, and the bags per 1,000 ft². Read the line for your target R and enter its coverage per bag. The 40 ft² preset is only a starting value. Coverage drops quickly as the R-value rises, so the chart matters more than any rule of thumb. For batts and rolls, enter the square feet printed on the package. Add 5 to 10% for cutting and fitting around framing.
Walls, floors, and basements
- Walls: a wall cavity can only hold its own depth, 3½ in. for 2×4 studs and 5½ in. for 2×6. The calculator shows the R-value that depth gives with your material: about R-9 to R-14 in a 2×4 wall with batts, or R-21 with closed-cell foam. When the siding comes off, ENERGY STAR suggests adding R-5 insulating sheathing in zone 3 and R-5 to R-10 in zones 4 to 8. For the drywall that follows, use the drywall calculator.
- Floors: batts fit between joists from below. Enter the floor area; the joist depth limits how much fits.
- Basement walls: foam board on the concrete, or batts in a framed wall in front of it. Pick the board material to see the thickness for the R-value.
About these figures: R-value targets are ENERGY STAR's recommendations, and the codes in your area may require more or less. R-values per inch are typical ranges from PNNL and DOE, not tested values for a specific product. Air sealing before insulating matters as much as the R-value. The label on your product and your local code take priority.
Frequently asked questions
How much insulation do I need in my attic?
R-30 in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8, according to ENERGY STAR. With 3 to 4 in. already there, it recommends adding R-25, R-38, or R-49 on top.
How many inches of blown-in insulation is R-49?
About 13 to 15¼ in. of settled cellulose, at R-3.2 to 3.8 per inch, or 18 to 22¼ in. of blown fiberglass at R-2.2 to 2.7 per inch. The bag chart lists the minimum settled thickness for your product.
How many inches is R-60?
About 16 to 18¾ in. of blown cellulose or 22¼ to 27¼ in. of blown fiberglass. Existing insulation counts: 3½ in. of old batts is about R-11, so you only add about R-49.
How many bags of blown-in insulation do I need?
Divide the attic area by the coverage per bag the label gives for your R-value. The bag's chart also lists bags per 1,000 ft². Coverage falls as the R-value rises, so use the line for your target, and add 5 to 10%.
Can I put new insulation on top of old insulation?
Usually yes, if the old insulation is dry and free of mold or pests. Count its R-value toward the target, as the calculator does. Seal air leaks first, and don't use a faced batt on top of existing insulation.
What R-value is a 2×4 wall?
About R-9 to R-14 with batts in the 3½ in. cavity, at R-2.5 to 4 per inch, or about R-21 with closed-cell spray foam at about R-6 per inch. When the siding comes off, ENERGY STAR suggests adding R-5 to R-10 insulating sheathing in zones 4 to 8.
What R-value do I need under a floor?
R-13 in zones 1 and 2, R-19 in zone 3 and 4A or 4B, R-30 in 4C to 6, and R-38 in zones 7 and 8, per ENERGY STAR. Batts between the joists are the usual choice; the joist depth limits the thickness.
Which insulation has the highest R-value per inch?
Polyiso board and closed-cell spray foam, at about R-6 per inch, then XPS at R-5 and EPS at about R-3.8 to 4.4. Loose-fill cellulose is R-3.2 to 3.8 and blown fiberglass R-2.2 to 2.7, but those are cheap to pile deep in an open attic.
Related calculators
Sources
- Recommended Home Insulation R-ValuesENERGY STAR (EPA)Attic, floor, wall, and basement R-values by climate zone, based on 2021 IECC Table R402.1.3.
- Typical R-Values and Vapor Retarder Classifications for Common Insulation MaterialsBuilding America Solution Center (PNNL)R per inch: fiberglass 2.5–4, mineral wool board 3–4, EPS 3.8–4.4, XPS 5, polyiso about 6, high-density spray foam about 6.
- Loose-Fill InsulationBuilding America Solution Center (PNNL)Settled R per inch: cellulose 3.2–3.8 (settles 10–20%), fiberglass 2.2–2.7 (settles 2–4%).
- Measure Guideline: Wall Air Sealing and Insulation Methods in Existing HomesU.S. Department of Energy, Building AmericaOpen-cell spray foam at approximately R-3.6 per inch.