How much insulation you need depends on where you live and where it goes. ENERGY STAR recommends R-60 for an uninsulated attic in climate zones 4 to 8, R-49 in zones 2 and 3 and R-30 in zone 1. In fiberglass batts, R-60 over a 1,000 sq ft attic on joists 16 in apart is two layers of R-30, about 414 batts with 10% waste and 19 inches deep.
How the insulation calculator works
Pick where you insulate and your state or climate zone. The calculator looks up the recommended R-value, then sizes the material you choose:
Thickness = target R ÷ R per inch; batts = area × (1 + waste) ÷ (framing spacing × batt length), per layer
Blown-in insulation is counted from the manufacturer's bag chart instead, and rigid foam in 4 × 8 ft boards, one set per layer.
Worked example: attic in zone 4, fiberglass batts
- Target R-60 (ENERGY STAR, zones 4A and 4B)
- No single batt reaches R-60 (Owens Corning EcoTouch goes up to R-49 at 14 in), so the calculator uses two layers: R-30 + R-30, each 9½ in thick
- One R-30 batt fills a 16 in bay 48 in long: 16 × 48 in = 5.33 sq ft
- 1,000 × 1.1 ÷ 5.33 = 206.3, so 207 batts per layer, 414 in all
The second layer goes across the joists so it covers their tops. With R-49 as the target, one layer of R-49 batts does it.
Recommended R-values by climate zone
| Zone | Attic, uninsulated | Attic, 3 to 4 in already | Floor | Wood frame wall (new) | Basement wall (new) |
|---|---|---|---|---|---|
| 1 | R-30 | add R-25 | R-13 | R-13 | none |
| 2 | R-49 | add R-38 | R-13 | R-13 | none |
| 3 | R-49 | add R-38 | R-19 | R-20 or R-13 + R-5 ci | R-13 or R-5 ci |
| 4 (A, B) | R-60 | add R-49 | R-19 | R-20 + R-5 ci or R-30 | R-13 or R-10 ci |
| 4C, 5, 6 | R-60 | add R-49 | R-30 | R-20 + R-5 ci or R-30 | R-19 or R-15 ci |
| 7, 8 | R-60 | add R-49 | R-38 | R-20 + R-5 ci or R-30 | R-19 or R-15 ci |
Attic and floor values are ENERGY STAR's retrofit recommendations, which are based on 2021 IECC Table R402.1.3. Wall and basement values are the code minimums for new homes from that table, as published by the Building America Solution Center (PNNL). "ci" is continuous insulation: rigid foam over the studs. The bar chart under the result draws the column for your location.
Insulation calculator for the attic
Turn on "The attic already has 3 to 4 in of insulation" and the target becomes the R-value to add on top: R-49 in zones 4 to 8. For loose-fill, blow it rather than lay batts; the blown in insulation calculator counts bags from the Owens Corning, Greenfiber and Johns Manville charts and subtracts what is already there.
Walls
A 2x4 stud is 3½ in deep and a 2x6 is 5½ in (ProWood). DOE notes that 2x4 walls hold R-13 or R-15 batts and 2x6 walls R-19 or R-21. That is why the IECC asks for R-20 plus R-5 of foam sheathing in zones 4 and colder: no batt reaches R-20 in a 2x4. The calculator only offers batts that fit the cavity and tells you when the cavity falls short. For 400 sq ft of 2x4 wall with R-13 batts (15 in wide, 93 in long, 10.33 sq ft per bay), that is 43 batts with 10% waste.
Floors and basement walls
Floors over a crawl space or garage get R-19 in zone 4 and R-30 from zone 4C up. For basement walls, rigid foam against the concrete counts as continuous insulation: R-10 is 2 in of XPS at R-5 per inch.
R-value per inch
| Material | R per inch | Source |
|---|---|---|
| Fiberglass batts | 3.2 | UMD Extension |
| Mineral wool | 3.1 | UMD Extension |
| Loose-fill cellulose | 3.7 | UMD Extension |
| Loose-fill fiberglass | 2.2 | UMD Extension |
| EPS board | 4.0 to 4.5 | UMD Extension |
| XPS board | 5.0 to 5.5 | UMD Extension |
| Polyiso board | 5.6 to 8 | DOE |
| Spray foam, open / closed cell | 3.6 / 5.5 to 6.5 | DOE |
The calculator uses the low end of each range for boards, so the thickness is never short. R-49 of blown cellulose is 49 ÷ 3.7 = 13¼ in; in rigid foam, R-10 of XPS is 10 ÷ 5 = 2 in, and a 1,000 sq ft wall takes 35 boards of 4 × 8 ft with 10% waste.
Finding your climate zone
Choosing a state fills in its most common 2021 IECC zone, counted by county from PNNL's "Guide to Determining Climate Zones by County". Many states span several zones (Texas runs from 1 to 4, California from 2 to 6), so the page flags them: check your county in that guide and set the zone directly.
Next steps
For a sprayed air seal on rim joists or a roof deck, the spray foam calculator turns area and thickness into board feet and kits, and the spray foam insulation cost calculator prices it installed. Closing up the walls afterwards? The drywall calculator counts the sheets. Better insulation lowers the heat load: size the system with the BTU calculator. More tools are in drywall, paint and walls.