How to Convert R-Value to U-Factor in Insulation

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Tip

Use an online R-value-to-U-factor calculator to simplify the process. Insulation products have their R-value printed on the packaging or in the technical data that accompany the product. Batt insulation is stiff insulating sheeting with an R-value of between 3.7 and 4.3 per inch of thickness.

Effective insulation is an essential element of new buildings.

It makes good sense to properly insulate new walls. Efficient use of insulation keeps a building warmer in winter and cooler in summer, thereby reducing both heating and air-conditioning costs. Many building regulations now specify minimum R-values for insulation products, but when you need to find the overall thermal conductance of a wall or building, you need to calculate the U-factor. Calculating the U-factor is straightforward when you know the R-values of the materials you have used.

Step 1

Determine the R-values of all the materials you have used, and then add them together. For example, a 2-by-4-inch stud wall has an R-value of 5, and a layer of 3-inch batt insulation has an R-value of 11. The stud wall plus the insulation has an R-value of 16 — 5 + 11 = 16.

Step 2

Calculate the U-factor by using this formula: U-factor = 1/Sum of the R-values. Using the previous example with an R-value of 16, the U-factor is one-sixteenth, or 0.0625.

Step 3

Check your calculations by reversing the calculation and determining 1/U-factor. If the result is not the original R-value, then there was an error in your calculations. Repeat them until the correct result is obtained.

references & resources

David Robinson

David Robinson has written professionally since 2000. He is a Fellow of the Royal Geographical Society and the Royal Meteorological Society. He has written for the "Telegraph" and "Guardian" newspapers in the U.K., government publications, websites, magazines and school textbooks. He holds an honors Bachelor of Arts in geography and education and a teaching certificate from Durham University, England.