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Insulation Calculator – Batts, Rolls & R-Value

Quick answer

Insulation calculator sizes batts from area (0.7-0.9 m² each), targets R-13 to R-21 walls and R-38 to R-60 attics at R-3.2/inch fiberglass.

Figures are estimates from this calculator’s standard formula — adjust the inputs in the tool below for your exact number.

How the insulation calculator works

Batts = Area ÷ Batt Coverage; Achieved R = Thickness × R/inch

Insulation quantity is area ÷ coverage per unit, but the real target is R-value: standard fiberglass batts deliver about R-3.2 per inch (R-13 in a 90 mm cavity, R-19 at 140 mm, R-30 at 220 mm), high-density batts R-4.3, and spray foams R-6.0-6.5 per inch. Codes typically demand R-13 to R-21 in walls and R-38 to R-60 in attics. Worked example: a 40 m² attic ceiling taken to R-38 with 240 mm batts (0.8 m² each) needs 50 batts plus 10% waste - order 55. Never compress a batt to fit: squeezing R-19 into a 90 mm stud bay forfeits a third of its rated performance, and gaps around outlets and wiring short-circuit the whole assembly.

  1. 1

    Measure the space

    Record the area, cavity depth and target R at the widest and longest points.

  2. 2

    Enter the values

    Type the measurements into the calculator and select your units.

  3. 3

    Apply the formula

    The tool computes Batts = Area ÷ Batt Coverage; Achieved R = Thickness × R/inch for your inputs instantly.

  4. 4

    Add waste and order

    Review the total with a 5-10% margin and round up to whole units.

Use Cases

Drywall installations

Sheet, screw and mud quantities for insulation rooms.

Insulation upgrades

R-value targets and areas for insulation in walls and roofs.

Window and door retrofits

Rough-opening checks before ordering insulation units.

Common Mistakes

  • Ordering exact sheet count

  • Compressing insulation

  • Vapor barrier on the wrong side

FAQs

How much insulation per m² of wall?

One 90 mm batt covers 0.7-0.9 m²; a 40 m² wall consumes 50-55 batts plus a 10% cutting margin.

R-value per inch of insulation?

Fiberglass R-3.2, rock wool R-3.7, EPS R-4.0, XPS R-5.0, open-cell foam R-3.7, closed-cell R-6.5 per inch.

R-value needed for walls and attics?

R-13 to R-21 walls, R-38 to R-60 attics in most codes - check your climate zone's exact table.

Faced or unfaced batts?

Faced carries a vapor barrier for first layers on walls; unfaced tops up existing insulation without double-vaporlocking.

Air gaps around insulation?

A 4% void in a wall drops its R by half - cut batts 10 mm oversize and split behind wires.

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