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R-Value Calculator

Find the R-value of an insulation layer.

How it is calculated

The tool uses the SI form of thermal resistance: R = L ÷ k and U = 1 ÷ R. L is the layer thickness in metres and k is the thermal conductivity in W/(m·K), so R comes out in m²·K/W and U in W/(m²·K). Both inputs must be above zero. The result is for one homogeneous layer only; a wall with several layers adds the R-values of the layers (and surface films), then inverts the sum to get the overall U. This is the metric R-value. The US figure in ft²·°F·h/BTU is about 5.68 times larger.

Worked example

Take 100 mm of mineral wool, so L = 0.1 m, with k = 0.04 W/(m·K). Then R = 0.1 ÷ 0.04 = 2.500 m²·K/W. The U-value is 1 ÷ 2.5 = 0.400 W/m²·K. The layer has a thermal resistance of 2.500 m²·K/W; doubling the thickness would double R and halve U.

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Common mistakes

  • Entering thickness in millimetres or centimetres. The tool expects metres, so 100 mm must be typed as 0.1, otherwise R is off by a factor of 1,000 or 100.
  • Comparing this R-value directly with US labels such as R-13. The units differ, and the metric value is about 5.68 times smaller for the same insulation.
  • Using the U-value of a single layer for a whole wall. Real walls need every layer added together, since insulation is not the only layer in the path of heat.
  • Using a conductivity measured at a different temperature or moisture state than the actual use. Wet insulation conducts more.

About this calculator

An insulation material's R-value measures its resistance to heat flow: R = thickness ÷ thermal conductivity (k), so thicker or less-conductive materials have higher R-values and insulate better. Its inverse, the U-value (U = 1/R), measures conductance instead — how easily heat passes through — and is the measure more commonly used in Europe for windows and building envelopes. For multiple layers (like a wall assembly), R-values simply add together.

Builders, architects, and energy auditors use R-values to select insulation that meets building-code minimums for a given climate zone, compare products (fiberglass batts, spray foam, rigid foam board all have different R-value-per-inch), and estimate a building's heating/cooling energy needs. Enter thickness and conductivity to find the R-value and U-value.

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