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Insulation Calculator

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

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

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 inchR-3.2
Total volume781.25 CF
Material cost$1,250
Quantity12 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.