Answer: Enter your values and the Insulation Calculator returns the exact result instantly — formula, worked example, and a plain-English explanation are included below the tool.
R-value achieved, batts/rolls needed & material cost
Enter the area to insulate (length × width in feet), pick the insulation product and framing spacing, set a waste factor and price, and the calculator returns the R-value achieved, total coverage area, the number of batts, rolls, or 4×8 rigid foam sheets to buy, coverage per package, and a material cost estimate.
Quantity math is package coverage: packages to buy = area × (1 + waste%) ÷ coverage per package, rounded up. Coverage depends on framing: the calculator uses 10.17 sq ft per R-13/R-19 batt at 16-inch on-center framing and 15.33 sq ft at 24-inch; R-30/R-38/R-49 come as rolls at 7.65 sq ft per package at 16-inch or 76.7 sq ft at 24-inch; rockwool R-15 batts cover 11.33/17.0 sq ft; rigid foam is counted in 32 sq ft (4×8) sheets.
R-value measures thermal resistance — higher is better — and R-values add when you layer materials. Fiberglass delivers roughly R-3.1–3.4 per inch, rockwool about R-3.7–4.2 per inch, and rigid foam about R-5 per inch. Compressing a batt thins it and lowers its rated R-value: squeezing an R-19 (6.25-inch) batt into a 3.5-inch 2×4 wall cavity yields closer to R-13.
The product dropdown maps directly to the calculator's coverage values: R-13 and R-19 fiberglass batts (3.5 and 6.25 inches for 2×4 and 2×6 walls), R-30 (9.5-inch floor batts), R-38 and R-49 (12 and 15 inches, attic rolls), rockwool R-15 batts, and R-5-per-inch rigid foam sheets.
A 30 × 24 attic ceiling is 720 sq ft. R-38 at 24-inch on-center framing covers 76.7 sq ft per roll, so 720 × 1.10 (10% waste) ÷ 76.7 = 10.3 → buy 11 rolls. The R-value card shows R-38 at full 12-inch thickness, and at $0.75 per sq ft the material cost is 720 × 0.75 = $540.
Switch the same job to 16-inch framing and roll coverage drops to 7.65 sq ft per package, tripling the package count — an illustration of why framing spacing is an input, not a detail.
| Product | Coverage @ 16" oc | Coverage @ 24" oc | Typical Use |
|---|---|---|---|
| Fiberglass batts R-13 | 10.17 sq ft | 15.33 sq ft | 2×4 walls |
| Fiberglass batts R-19 | 10.17 sq ft | 15.33 sq ft | 2×6 walls |
| Fiberglass rolls R-30/38/49 | 7.65 sq ft | 76.7 sq ft | Floors / attics |
| Rockwool batts R-15 | 11.33 sq ft | 17.0 sq ft | 2×4 walls |
| Rigid foam (4×8 sheet) | 32 sq ft / sheet | 32 sq ft / sheet | Continuous insulation |
Common IECC-based code minimums run R-13 to R-15 for 2×4 exterior walls, R-19 to R-21 for 2×6 walls, R-30 for floors over unconditioned space, and R-38 to R-60 for attics depending on climate zone; check your local amendment because zones differ substantially.
You can stack layers: R-19 plus R-30 gives R-49, and running a second attic layer perpendicular over the joists also cuts thermal bridging through the framing. Use unfaced batts for the second layer so you don't trap a second vapor retarder inside the assembly.
Match the product to the cavity. Batts labeled for 2×4 walls are cut to 15.25 inches wide to friction-fit between studs 16 inches on center (the stud faces leave a 14.5-inch clear cavity); 24-inch framing uses 23-inch-wide batts. Rolls make sense for long open attic joist runs; individual batts save cutting time in walls full of blocking and outlets. Rigid foam shines as continuous insulation over framing, where it covers the thermal bridges that cavity insulation can't reach.
Air sealing belongs in every insulation job and is often the higher-return half. Seal top plates, wire and pipe penetrations, and rim joists with caulk or foam before insulating — fiberglass and rockwool filter air but don't stop it. Manufacturers' coverage figures (which this calculator uses) assume batts split neatly around obstructions; the waste percentage is where you account for real-world cutting losses.
Safety basics: wear gloves, long sleeves, and a dust mask when handling fiberglass; rockwool is less irritating and adds fire resistance. In attics, keep insulation clear of heat-producing fixtures unless they're IC-rated, and maintain clearance at soffit vents so airflow through the attic isn't blocked by new insulation.
What R-value do I need?
Typical code minimums are R-13–15 for 2×4 walls, R-19–21 for 2×6 walls, R-30 for floors, and R-38–60 for attics depending on climate zone. Your local building department's amendment is the controlling number.
Can I layer two layers of insulation?
Yes — R-values simply add, so R-19 over R-30 gives R-49. In attics, run the second layer perpendicular to the joists to reduce thermal bridging, and use unfaced material on top.
Does compressing batts hurt R-value?
Yes. R-value depends on thickness and trapped air; squeezing a 6.25-inch R-19 batt into a 3.5-inch cavity loses roughly a third of its rating. Buy batts sized to the cavity depth.
How much waste should I add?
5–10% for open attics and regular framing; 12–15% for walls crowded with outlets, windows, and odd cavities where offcuts pile up.
How does the calculator count rigid foam?
Rigid foam is counted in 4×8 sheets (32 sq ft each): sheets = area × (1 + waste%) ÷ 32, rounded up. A price entered per sheet is divided by 32 to get the per-sq-ft cost used in the estimate.