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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 (the tool uses a typical per-inch figure for the material you pick — fiberglass, spray foam, cellulose or mineral wool — and takes the thickness in centimetres), 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, spray foam, cellulose, mineral wool) 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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Comments (1)

  • Editorial team

    The figure covers the insulation layer alone, using one fixed value per inch for each of four materials. A real wall or roof also depends on framing, air gaps, moisture and how well the material was installed, and studs can cut the effective value noticeably. Compare the result with the requirement for your own region, and check whether that requirement is quoted as R, RSI or a U-value. Which other insulation materials would you want listed?

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