Insulation answer guide

How much insulation do I need?

Estimate insulation quantity from area, target R-value, package coverage, attic top-up depth, air sealing, material range, and labor planning.

Reviewed - May 27, 2026

Short answer

Measure the insulated area, choose the target R-value, divide the area by the package coverage, then round up to whole packages. For attic top-ups, subtract the existing R-value first so you only estimate the added insulation.

For a 40 ft by 30 ft area with an R-38 target and 40 sq ft package coverage, the current calculator estimates 1,200 sq ft of insulated area and 30 packages of blown fiberglass insulation.

Package coverage is product-specific. A 28 ft by 28 ft cellulose example with 25 sq ft package coverage estimates 32 packages, so use the coverage printed for the exact material and installed R-value you plan to buy.

Use the insulation calculator

Insulation quantity method

  1. Calculate insulated area: length x width x number of areas.
  2. Choose the insulation type and target R-value for the assembly or attic zone.
  3. For new insulation, package count is insulated area / package coverage, rounded up.
  4. Use the package coverage printed for the chosen material and installed R-value because coverage usually changes as depth increases.
  5. For attic top-up mode, estimate existing R-value from known R-value or existing depth, then subtract it from the target R-value.
  6. Add air-sealing allowance and labor planning ranges separately from the package count.

Quick examples

20 ft by 30 ft area
15 packages
R-30 blown fiberglass, 40 sq ft package coverage
40 ft by 30 ft area
30 packages
R-38 blown fiberglass, 40 sq ft package coverage
Two 45 ft by 22 ft areas
25 packages
R-49 spray foam, 80 sq ft package coverage
28 ft by 28 ft cellulose area
32 packages
R-49 blown cellulose, 25 sq ft package coverage
Attic top-up from R-19
30 packages
R-38 target, known existing R-value

These generated examples use the current U.S. default insulation assumptions: area divided by package coverage, explicit R-30/R-38/R-49 targets, and a basic air-sealing allowance.

Worked example

1 area at 40 ft by 30 ft, target R-38.

Insulated area
1,200 sq ft
R-38 target with 1.18x material factor
Packages
30 packages
40 sq ft per package
Air sealing area
1,200 sq ft
Basic air-sealing allowance included
DIY material total
$785–$3,641
Insulation plus air-sealing allowance
Contractor total
$1,776–$6,048
Materials plus labor

Starter shopping list

  • fiberglass blown 30 packages
  • Air-sealing foam, caulk, weatherstripping, baffles As needed
  • Protective gear Respirator, gloves, eye protection

This example is generated from the same calculator logic used on the Insulation calculator page.

R-value target

  • Use climate-zone guidance, local code, and the assembly type to choose R-30, R-38, R-49, or a higher professional target.
  • Higher R-values increase material depth and cost, so do not compare estimates unless the target R-value matches.

Package coverage

  • Use the coverage printed on the exact batt, bag, or kit you plan to buy because coverage changes by product and installed depth.
  • The calculator rounds packages up after dividing by coverage, but tight spaces, framing, kneewalls, and waste may require extra material.

Material and coverage labels

  • Fiberglass batts, blown fiberglass, cellulose, and spray foam can have different R-value per inch, labor assumptions, and package coverage.
  • Match the package coverage to the R-value you are installing; a bag or batt that covers 40 sq ft at one depth may cover less area at a higher R-value.

Air sealing and prep

  • Seal air leaks before adding insulation, especially around attic bypasses, top plates, plumbing, wiring, and penetrations.
  • Keep ventilation baffles, recessed fixture clearances, knob-and-tube wiring rules, moisture issues, and old insulation removal outside the basic package count.

Common mistakes

  • Using floor area when the insulated surface area is larger or split across multiple zones.
  • Forgetting to round package counts up to whole bags, batts, or kits.
  • Comparing insulation quotes with different target R-values or material types.
  • Using the coverage for a lower installed R-value when the product label lists less coverage at the deeper target.
  • Adding attic insulation before air sealing leaks and protecting ventilation paths.
  • Ignoring old insulation removal, moisture, pest damage, wiring, recessed fixtures, and access limits.

FAQ

How do I calculate how much insulation I need?

Multiply length by width and by the number of areas, choose the target R-value, divide area by the product's package coverage, then round up to whole packages. Use attic top-up mode when existing insulation should be counted.

How many packages of insulation do I need for 1,200 sq ft?

With 40 sq ft package coverage, the generated example estimates 30 packages for 1,200 sq ft of new blown fiberglass insulation at an R-38 target.

Why do insulation package counts change by R-value?

The calculator divides area by the package coverage you enter, but real product labels often show less coverage at higher installed R-values. For example, 28 ft by 28 ft is 784 sq ft; at 25 sq ft per package, the generated example rounds up to 32 packages.

Does R-value change the package count?

The calculator's package count divides area by package coverage, while R-value adjusts material and labor cost factors. In real product labels, coverage can also change by installed R-value, so use the product's stated coverage.

How do I estimate attic top-up insulation?

Measure the attic area, estimate the existing R-value from known R-value or depth, subtract it from the target R-value, then estimate the added depth and package count for the new material.

Should I air seal before adding insulation?

Yes. Air sealing before adding insulation improves performance and prevents covering leaks that will be harder to reach later.

Related calculators

Insulation calculator Roofing calculator Drywall calculator