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Insulation Calculator

Find how many inches of insulation to add over what you have to reach a target R-value, and how many bags, batts, boards or foam kits to buy.

Level: Grades 6–7

Printable worksheet

Insulation Calculator

Know the answer you need? Pick an input to work backward.

Inputs

Recommended levels depend on your climate zone and the part of the house; check your local energy code.

Enter 0 for an empty cavity or a bare attic floor.

Printed on the package for each R-value. For spray foam, enter the board feet per kit instead.

Result

Thickness to add

8.57 in

Add about 8.6 in of blown (loose-fill) cellulose (R-30) over 1,000 ft², roughly 32 packages.

R-value to add
30
Bags, bundles or boards
32
Board feet
8,571.4
Volume of insulation
714.3 ft³
Thickness in centimeters
21.8 cm

See the step-by-step solution ↓

Step-by-step solution

  1. R-values stack, so subtract what you have from the target to see how much R to add (never below zero)

    49−19=30

  2. Divide that by the R each inch gives (3.5 for blown (loose-fill) cellulose) to get the thickness to add

    303.5=8.5714 in

  3. Multiply the area by the thickness for the volume in board feet (1 ft² one inch thick). Divide by 12 for cubic feet

    1,000×8.5714=8,571.43 board ft=714.29 ft³

  4. Add 10% for waste, then divide the area by what one package covers. Round up, since you can't buy part of a package

    1,000×1.135=31.4286=rounded up:32

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how thick your new insulation needs to be, and how many bags or boards to buy. Insulation is rated by R-value, a measure of how well it slows heat from passing through. R-values add up layer by layer. So topping up an attic that already has some insulation only needs the difference. That's the gap between what's there and the level you're aiming for. Each material packs a different amount of R into each inch. That's why the same target means very different depths of loose-fill fiberglass and closed-cell foam.

The calculator subtracts the R-value you already have from your target. It divides the rest by the material's R-value per inch, which gives the depth to add. Then it multiplies by the area to get the volume in board feet and cubic feet. Finally, it adds a waste allowance and divides by the coverage printed on the package. That gives the number of bags, bundles or boards to buy. For spray foam, it divides by the board feet in each kit instead.

Where it's used

Why this math matters outside the classroom:

  • Attic top-ups. Homeowners raising an R-19 attic to R-49 divide the missing R-30 by about 3.5 per inch to plan 8.6 in of cellulose.
  • Wall cavities. Builders see that R-13 batts at about R-3.2 per inch need roughly 4.06 in, which fits a 2 × 4 wall's 3.5 in cavity only with denser batts.
  • Spray foam quotes. Contractors price 2 in of foam over 200 ft² as 400 board feet, the unit foam kits are sold in.
  • Unit rates. Students treat R per inch as a unit rate and divide to find how many inches give a total R-value of 30.
  • Energy audits. Energy auditors measure 6 in of old fiberglass and multiply by its R per inch to estimate the R-value already in an attic.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.b. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Measure the area to insulate in square feet. For an attic floor, that's the length times the width of the space.
  2. Enter the target R-value from your local energy code or a utility recommendation. Then enter the R-value already in place (0 if it's empty).
  3. Choose the material. Or pick Custom and type the R-value per inch from the product label.
  4. Enter the package coverage at your target R-value, from the label. Set a waste allowance, then read the thickness and package count.

What the inputs mean

Target total R-value
Recommended levels depend on your climate zone and the part of the house; check your local energy code.
R-value already in place
Estimate old insulation by its depth. Measure it, then multiply by a typical R per inch for that material. Squashed or wet insulation performs below its rating.
Insulation type
The R per inch values are mid-range figures for planning. Real products vary by brand, density and how they're installed. Loose-fill also settles over time. That's why its labels give both a starting and a settled thickness.
R-value per inch
From the product label or data sheet.
Coverage per bag, bundle or board
Bags of loose-fill list the area each bag covers at several R-values. Batt bundles list the square feet in the bundle, and boards cover their face area. For spray foam, enter the board feet per kit.

Formula and method

R to add = max(0, target R − existing R)
thickness (in) = R to add ÷ R per inch
board feet = area (ft²) × thickness (in);  cubic feet = board feet ÷ 12
packages = area × (1 + waste ÷ 100) ÷ coverage per package, rounded up
spray foam kits = board feet × (1 + waste ÷ 100) ÷ board feet per kit, rounded up

"max(0, …)" means the answer never drops below zero; if you already meet the target, you add nothing. Layers stacked on top of each other add their R-values together. So the layers you install stack on top of what's already there. Divide the R-value you still need by the material's R per inch, and you get the depth. That assumes the material reaches its rated value at that depth.

A board foot is one square foot, one inch thick. It's the unit spray foam is sold in. Twelve board feet make one cubic foot. The waste percentage is turned into a multiplier, so 10% waste becomes × 1.1.

Worked example

A 1,000 ft² attic has R-19 of old insulation and the target is R-49. The owner plans to blow in cellulose at about R-3.5 per inch, from bags that cover 35 ft² each at that depth, with 10% extra.

  1. R to add: 49 − 19 = 30.
  2. Thickness: 30 ÷ 3.5 = 8.57 in.
  3. Volume: 1,000 × 8.5714 = 8,571.4 board feet, or 714.3 ft³.
  4. Bags: 1,000 × 1.1 ÷ 35 = 31.43, rounded up to 32.

Result: Blow in about 8.57 inches of cellulose, roughly 32 bags, to bring the attic to R-49.

Understanding the result

The thickness is the installed depth to aim for. For loose-fill, use the settled thickness on the bag's chart. Mark depth rulers on the attic framing so the layer comes out even.

Order a little extra rather than run short partway through. Most suppliers accept unopened bags back.

Assumptions and limitations

  • The R-value per inch is the same all through the layer. Existing insulation still performs at the R-value you enter.
  • The area is a single flat surface; framing pieces aren't subtracted.
  • Framing (studs and joists) lets heat through more easily. So a wall's whole-assembly R-value is lower than the cavity insulation's rating.
  • It doesn't check vapor retarders, ventilation baffles, clearances around recessed lights or fire-safety rules that apply to installation.
  • Recommended R-values depend on your climate zone and are set by local codes. The target is yours to choose.

Frequently asked questions

Can I put new insulation on top of old insulation?

Usually yes, as long as the old material is dry and free of mold or pests. The R-values add. The new layer is usually unfaced (no paper or foil backing). That way, no second vapor barrier gets trapped between the layers.

Why does loose-fill need to be so much deeper than spray foam?

Closed-cell foam packs roughly R-6 to R-7 into each inch, while loose-fill fiberglass gives roughly R-2.5. So reaching R-30 takes under 5 inches of foam, but about 12 inches of loose-fill.

What R-value should I aim for?

It depends on where you live and which part of the house it is. Attics in cold climates are often recommended R-49 to R-60, and warm climates less. Walls get far less, because the cavity is shallow. Your local energy code or utility gives the figure for your zone.

Does compressing a batt into a thinner space keep its R-value?

No. Squeezing a batt lowers its total R-value, even though the R per inch rises slightly. Use batts made for the cavity depth.

How this calculator is checked

Method: Series thermal resistances add; thickness = required R ÷ R per inch; typical R per inch values are mid-range planning figures and can be overridden. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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