Calculatoo

Insulation Calculator: Thickness for a Target R-Value

Choose the R-value you want to reach and the insulation material. The calculator divides by the material's typical R-value per inch to give the thickness, then counts the packs for your area with waste.

Updated

Embed this calculator

Paste this code into your page. The calculator runs inside the frame and links back to the full version.

Keep the credit link in place. Do not resell the calculator.

Formula

  • Thickness (in): target R-value / R-value per inch
  • Thickness (mm): inches x 25.4
  • RSI: R-value x 0.1761101838
  • Area to buy: area x (1 + waste / 100)
  • Packs: area to buy / coverage per pack, rounded up

Assumptions

  • R-value per inch (US units): fiberglass batt 3.2, blown fiberglass 2.5, blown cellulose 3.5, mineral wool 4.0, EPS 4.0, XPS 5.0, polyiso 6.0, open-cell spray foam 3.7, closed-cell spray foam 6.5. These are typical values, not product guarantees.
  • The result is the thickness of one uniform layer. Framing, thermal bridging, compression and air gaps lower the real R-value of a wall.
  • Coverage per pack comes from the product label.
  • Target R-values depend on climate and building rules in your area; enter the one you need.

Worked example

Target R-38 in blown cellulose over 1,000 ft2. Thickness = 38 / 3.5 = 10.86 in (275.8 mm). RSI = 38 x 0.1761 = 6.69. Area with 10 percent waste is 1,100 ft2; at 40 ft2 per bag that is 27.5, so 28 bags.

Frequently asked questions

How thick does insulation need to be for R-38?

Divide the target R-value by the material's R-value per inch. R-38 in blown cellulose at R-3.5 per inch needs 38 / 3.5 = 10.86 inches (276 mm). In polyiso at R-6 per inch it needs 6.3 inches.

What is the difference between R-value and RSI?

R-value uses US units (ft2 F h per BTU) and RSI uses SI units (m2 K per watt). Multiply an R-value by 0.1761 to get RSI, so R-38 is RSI 6.69.

How many packs of insulation do I need?

Add your waste allowance to the area, divide by the coverage printed on one pack, and round up. For 1,000 ft2 plus 10 percent and 40 ft2 per pack that is 1,100 / 40 = 27.5, so 28 packs.

Are the R-values per inch exact?

No. They are typical values and vary by brand, density and how the material is installed; compression reduces R-value. Use the figure on your product's label and the R-value your local building rules call for.

How we calculate

Related calculators