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Insulation R-Value Calculator

Find the R-value of insulation.

How it is calculated

Thermal resistance adds up with thickness: R = r × (t / 2.54), where t is the thickness in centimetres, 2.54 converts it to inches, and r is the R-value per inch of the material. The tool uses these per-inch values: fiberglass 3.2, spray foam 6.5, cellulose 3.7, mineral wool 4.2. The result is in US units (R-value, ft²·°F·h/BTU), and a second figure converts it to metric RSI with RSI = R × 0.1761. These are typical textbook values; real products vary, and compressing batts lowers the value. Thickness cannot be negative.

Worked example

15 cm of fiberglass. Convert to inches: 15 / 2.54 ≈ 5.91 in. Multiply by 3.2 per inch: 5.91 × 3.2 ≈ 18.9, shown as R-18.90. In metric that is 18.90 × 0.1761 ≈ 3.33 RSI. Swapping to spray foam at 6.5 per inch would give about R-38.4 for the same thickness.

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Common mistakes

  • Entering the thickness in inches although the field is in centimetres; the result comes out 2.54 times too large.
  • Mixing R and RSI values when comparing to a local code; RSI is about one sixth of R.
  • Assuming the number applies to the whole wall; studs, gaps and thermal bridges lower the real value.
  • Stacking layers of different materials in one entry; calculate each layer and add the R-values.
  • Compressing insulation to fit a thinner cavity, which drops the R-value per inch.

About this calculator

This calculator finds the R-value of an insulation material — its resistance to heat flow — from the material's thickness and its R-value per inch (or per unit thickness), since R-value scales linearly: total R = thickness × R-per-inch. A higher R-value means better insulating performance.

Homeowners and contractors use it to compare insulation options (fiberglass batts, spray foam, rigid foam board) and to check whether a wall, attic, or floor assembly meets local building-code minimums. Energy auditors also use R-value totals when estimating a building's heat loss.

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