Insulation Calculator

The formula

Coverage and yield assumptions

R per inch values are typical: blown fiberglass about R-2.2 to 2.9 (default 2.5), blown cellulose about 3.2–3.8 (default 3.5), fiberglass batts about 2.9–3.8, open-cell spray foam about 3.5–3.8 and closed-cell about 6–7, as summarized by the US Department of Energy. For loose fill, use the bag label coverage chart, which lists bags per 1,000 sq ft at each R-value and accounts for settling.

Recommended R-values

ENERGY STAR recommends attic insulation of R-30 to R-49 in the warmest US climate zones and R-49 to R-60 in most others when starting from little or no insulation. Adding to an existing 3–4 in layer usually targets R-38 to R-49 total. Walls are limited by cavity depth: 2×4 walls take R-13 to R-15 batts, 2×6 walls R-19 to R-21, and continuous exterior foam can add R-5 or more.

Batts vs loose fill

Batts fit standard stud and joist cavities at 16 or 24 in on center and are easy to handle in open walls and floors. Loose fill blown with a rented machine covers attics quickly and fills around wires, pipes and irregular framing. In attics, it is common to blow loose fill over existing batts. Spray foam air-seals and insulates in one step but is usually a professional job.

Air sealing first

The DOE recommends sealing air leaks before insulating: top plates, wiring holes, plumbing chases, recessed lights and the attic hatch. Air carries heat and moisture past insulation, so sealing often saves as much energy as the insulation itself. Keep insulation away from chimneys and non-IC-rated light fixtures, and install baffles at the eaves so soffit vents stay open.

Safety and comfort

• Wear a respirator, goggles, gloves and long sleeves.
• Walk only on joists or boards in the attic.
• Use depth rulers stapled to trusses to blow an even layer.
• Check for rebates from utilities and federal tax credits.

Common mistakes to avoid

• Insulating before sealing air leaks.
• Covering soffit vents, which traps moisture.
• Compressing batts, which lowers their R-value.
• Burying non-IC recessed lights or touching chimneys with insulation.
• Buying bags by area alone instead of the coverage chart for your R-value.

Waste factor

10% covers uneven depth and spillage. Use 5% for batts in open, standard cavities and 15% for loose fill over irregular framing or when you expect settling beyond the label's allowance.

Imperial and metric

Switch the toggle above the inputs to enter ft ↔ m. The page converts metric entries to imperial before calculating, using NIST factors (1 ft = 0.3048 m, 1 in = 2.54 cm), and shows key results in both systems.

Worked examples

Attic to R-49 with cellulose

Inputs: area length 40 ft; area width 30 ft; insulation type: Blown cellulose (~R-3.5/in); target r-value 49; waste factor 10%.

Result: Install about 14 inches over 1,320 sq ft.

• Units to buy (rounded up): 1,320 sq ft
• Area: 1,200 sq ft (111.5 m²)
• Area with waste: 1,320 sq ft (122.6 m²)
• Loose-fill volume: 1,540 cu ft

Attic to R-60 with fiberglass

Inputs: area length 50 ft; area width 28 ft; insulation type: Blown fiberglass (~R-2.5/in); target r-value 60; waste factor 10%; cost per unit $1.1.

Result: Install about 24 inches over 1,540 sq ft.

• Units to buy (rounded up): 1,541 sq ft
• Area: 1,400 sq ft (130.1 m²)
• Area with waste: 1,540 sq ft (143.1 m²)
• Loose-fill volume: 3,080 cu ft
• Estimated material cost: $1,695.10

Rim joist with closed-cell foam R-15

Inputs: area length 150 ft; area width 1 ft; insulation type: Closed-cell spray foam (~R-6.5/in); target r-value 15; waste factor 10%.

Result: Install about 2.3 inches over 165 sq ft.

• Units to buy (rounded up): 165 sq ft
• Area: 150 sq ft (13.9 m²)
• Area with waste: 165 sq ft (15.3 m²)
• Loose-fill volume: 32 cu ft

Quick reference for common sizes

The table repeats the first worked example at smaller and larger sizes, keeping every other setting the same, so you can see how the insulation calculator result scales.

Limitations

Real installed R-value depends on density, settling, compression and air leaks. Use the product's coverage chart for bag counts. Building codes set minimums by climate zone.