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How to Convert R-Values to K-Values

David Robinson

Insulation is an important component, both in new homes and in renovation of existing ones. Effective insulation reduces heating and air conditioning costs, reducing the amount of energy you use and saving you money. It's important to know the effectiveness of insulation, and it is often expressed using one of two values: K-value and R-value. K-values refer to the thermal conductivity of a set thickness of insulation and R-values express the resistance to heat transfer, taking into account the K-value of the material and its thickness.

Insulation is important, but some products are more efficacious than others.
  1. Measure or otherwise ascertain the thickness, in inches, of the material for which you have an R-value. For example, a roll of insulation may have the thickness of 3.5 inches, specified on the packaging. Record this value.

  2. Divide the thickness by the R-value. The result is the K-value. For example: Blown fiberglass has a typical R value of 2.20 per inch thickness. The K-value of a 4-inch thick layer is 1.82 because 4 / 2.2 = 1.8181.

  3. Reverse your calculations to check for errors. Multiply the K-value by the R-value. The result should be the thickness of the material. If your answer differs from the thickness value, there was an error in your calculations. Repeat the math until the result is correct.