Insulation Calculator

Batts, blown-in, or rigid insulation by area and R-value. Estimates only — verify with IRC R402.

Area to insulate (sq ft)

Insulation type

Target R-value

See IRC R402 / 2024 IECC reference card below for recommendations by climate zone.

Material cost per sq ft ($)

Typical range $0.50–$3.00/SF — enter your local supplier price.

IRC R402 / 2024 IECC reference (US)

Ceilings: R-30 (CZ 1–2), R-38 (CZ 3–5), R-49 (CZ 6–8)
Wood-frame walls: R-13 (CZ 1–2), R-20 or R-13+R-5 (CZ 3–8)
Floors: R-13 (CZ 1–2), R-19 (CZ 3–4), R-30 (CZ 5–8)
Basement walls: R-10 continuous or R-13 cavity (CZ 3–8)

Always check local amendments — many jurisdictions exceed IECC minimums.

Thickness Required

9.38 in

to reach R-30 with Fiberglass Batt

R per inch	R-3.2
Total volume	781.25 CF
Material cost	$1,250
Quantity	12 bags of batts

warning

ESTIMATE ONLY — not a stamped engineering design. Verify with a licensed PE before procurement or construction.

Methodology

Thickness (in) = target R-value ÷ R-per-inch of material. Volume (CF) = area × thickness ÷ 12. R-per-inch values from manufacturer specs and ASHRAE Handbook. R-value targets reference IRC R402 / 2024 IECC by climate zone. Bag and board counts use typical manufacturer coverage ratings — always verify against the bag label for the specific product.

Frequently Asked Questions

What R-value do I need for my project?

The 2024 IECC and IRC R402 set minimum R-values by climate zone. For ceilings, expect R-30 in hot zones (FL, TX coast), R-38 in mixed zones (most of the South and Mid-Atlantic), and R-49 in cold zones (Northeast, Upper Midwest, Mountain West). Walls range from R-13 to R-20 plus continuous R-5. Always check your local amendments — California (Title 24), Massachusetts stretch code, and others exceed federal minimums. Pulling the wrong R-value triggers a re-inspection and can void HERS / ENERGY STAR ratings.

Batts vs. blown-in — which should I use?

Batts are faster to install in open stud bays during new construction and require no special equipment. Blown-in cellulose or fiberglass excels in attics (uniform coverage with no gaps), retrofits where the wall cavity is closed, and irregular framing where batts would have to be cut. Blown-in typically achieves a slightly better effective R-value in field conditions because it fills voids that batts can miss around wires, pipes, and irregular framing. Cost is comparable; labor is the bigger swing factor.

What is a thermal bridge and why does it matter?

A thermal bridge is a path of low resistance through the assembly — typically the wood studs themselves, which conduct heat at R-1.25/in versus R-3.2/in for fiberglass batt in the bay. A 2×6 wall with R-19 cavity insulation actually performs around R-14 effective once you account for the studs (about 25% of the wall area at the framing factor). The R-13 + R-5 continuous insulation requirement in modern codes specifically addresses this by adding a layer of rigid foam outside the framing to break the bridge.

When does spray foam pay off?

Closed-cell spray foam at R-6.5/in is the highest-R-per-inch option and acts as a vapor retarder and air seal in one application — it pays off where wall depth is constrained (renovations, vaulted ceilings, rim joists) or where air sealing is the priority. Open-cell at R-3.7/in is cheaper but only suits interior assemblies. Both are 2–4× the material cost of batts, so the math only works when the air-sealing value or the per-inch density is worth it. For standard 2×6 walls in a new build, batts plus continuous exterior rigid foam usually beats spray foam on cost.

How the Insulation Calculator works

Every result starts from one relationship: thickness = target R-value ÷ the material’s R-value per inch. The calculator carries R-per-inch for eight materials — closed-cell spray foam at 6.5, polyiso at 6.0, XPS at 5.0, mineral wool batt at 4.0, open-cell spray foam at 3.7, blown-in cellulose at 3.5, fiberglass batt at 3.2, and blown-in fiberglass at 2.5. Volume is then your area multiplied by that thickness converted to feet.

Material quantity depends on the form. Batts are counted by bag coverage: 88 square feet per bag for fiberglass, 60 for mineral wool. Rigid board is counted in 4 by 8 sheets at 32 square feet each. Blown-in is the one that moves with R-value, because bags are rated at R-1 — coverage at your target is the R-1 rating divided by the target, so cellulose rated 110 square feet at R-1 covers 3.67 square feet at R-30.

Cost is straightforward: your price per square foot multiplied by the area. It is not multiplied by thickness, so if you change the target R-value you should also update the price per square foot to match the deeper assembly.

Worked example: 1,000 square feet of attic at R-30 in fiberglass batt needs 30 ÷ 3.2 = 9.4 inches of depth, which is 781 cubic feet of material. At 88 square feet per bag that is 12 bags. At $1.25 per square foot the material runs $1,250. Switching to closed-cell spray foam would cut the depth to 4.6 inches for the same R-30, which is why it gets specified where rafter or cavity depth is the binding constraint.

Results are an estimate for budgeting and ordering, not a stamped energy design. Code minimums vary by climate zone under IRC R402, so confirm the required R-value for your jurisdiction before ordering.

Frequently Asked Questions

How thick does insulation need to be to hit R-30?

It depends entirely on the material. At R-3.2 per inch, fiberglass batt needs 9.4 inches to reach R-30. Closed-cell spray foam at R-6.5 per inch gets there in 4.6 inches, while blown-in fiberglass at R-2.5 per inch needs 12 inches. The calculator divides your target R-value by the material R-per-inch to give the required thickness.

Which insulation has the highest R-value per inch?

Of the eight materials in the calculator, closed-cell spray foam leads at R-6.5 per inch, followed by polyiso rigid board at R-6.0 and XPS rigid board at R-5.0. Mineral wool batt is R-4.0, open-cell spray foam R-3.7, blown-in cellulose R-3.5, fiberglass batt R-3.2, and blown-in fiberglass R-2.5. Closed-cell and rigid foam buy the most R in the least depth, which matters when cavity depth is fixed.

Why does blown-in coverage change when I raise the R-value?

Blown-in bags are rated by the square footage they cover at R-1. Coverage at your target is the R-1 rating divided by the target R-value, so a cellulose bag rated 110 square feet at R-1 covers only 3.67 square feet at R-30. Raising the target packs more material into each square foot, so the bag count climbs proportionally. Always check the coverage chart printed on the actual bag.

Why does the calculator not give a bag count for spray foam?

Spray foam is sold and installed by the board-foot (one square foot at one inch thick), not in bags or batts, and it is applied by a contractor with dedicated equipment rather than bought off a shelf. The tool reports thickness, volume, and your cost per square foot for spray foam, then directs you to an installer quote for the material figure.