FixerCalc

Insulation Calculator

Insulation is specified as R-value over an area. This calculator converts the R you want to add into thickness and a package count, using R-per-inch and coverage you can match to the bag.

Last reviewed 2026-08-25. How we calculate

Results update as you type. Units stay imperial inside the math.

Area
R-value

Attic recommendations vary by IECC climate zone. Look up your zone rather than copying a neighbor.

Product

Bag charts depend on settled R. Replace this with the chart on the bag you will buy. Ignored for batts/foam count.

Results

  • R-value to add

    36

  • Approximate added thickness

    10.3 in

  • Packages (planning)

    20 bags

  • Material (your $/sq ft)

    $1,440

  • Area

    800 sq ft

R per inch used
3.5
Product
blown
  • This does not estimate utility-bill savings. Climate, air leaks, and the rest of the house dominate that math.
  • Do not bury exposed knob-and-tube or non-IC recessed lights. Air sealing and fire rules come first.

How this calculation works

  1. R to add = max(0, target − current). Thickness in inches = R to add ÷ R per inch.
  2. Blown-in bags use your sq ft per bag at the depth you intend. Batt packages use sq ft per bundle. Foam is priced by area × your $/sq ft (install is usually contracted).
  3. Material cost is area × $/sq ft. Labor, two-story access, and air sealing are extra.

ΔR = max(0, R_target − R_now); inches = ΔR / (R/in); bags = ceil(area / coverage) for blown-in

What to do next

  • Air-seal attic penetrations before or with insulation. Adding R on a leaky attic is a weaker project.
  • Match climate-zone attic R from IECC or ENERGY STAR tables, not a single national number.
  • Hire spray-foam work from someone who understands vapor and fire barriers in your assembly.

FAQ

Can I just stack batts to hit R-49?
Sometimes, if depth and ventilation allow and you do not compress them. Compression lowers R. Blown-in is often used over existing batts in attics.
Why don’t you show a percent energy savings?
Because it would be a guess from area alone. A tight house in Minneapolis and a leaky house in San Diego are not the same project.

Sources

  • Added thickness (in) ≈ (target R − current R) / R per inch of the chosen product
  • Default R/in: fiberglass batt ~3.1–3.4, cellulose ~3.2–3.8, closed-cell spray foam ~6–7. Check the label

Cost defaults are typical US planning bands, last reviewed 2026-08-25. Edit the price fields on this page rather than treating them as a local market index.